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The hydrogen abstraction reactions of fluoroalkane isomers CF2HCFHCF2H and CF3CFHCFH2 with the OH radicals have been studied theoretically by a dual-level direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path are obtained at the BB1K/6-31+G(d,p) level of theory, and then the energy profiles are refined at G3(MP2) level of theory. Using the improved canonical variational transition-state theory (ICVT) with the small-curvature tunneling correction (SCT), the rate constants for each channel are calculated over a wide temperature range of 200–1000 K. Our results show that the tunneling correction plays an important role in the rate constant calculation in the low temperature range. The calculated ICVT/SCT rate constants are consistent with available experimental data. Our calculations indicate the contribution of the abstraction from the –CFH– group of isomeric compounds CF2HCFHCF2H and CF3CFHCFH2 to the overall reactions is quite different over the whole temperature range due to the effect of different groups at both sides of –CFH– group. Furthermore, to further reveal the thermodynamic properties, the enthalpies of formation of the two reactants CF2HCFHCF2H, and CF3CFHCFH2, and the product radicals CF2HCFCF2H, CF2HCFHCF2, CF3CFCFH2, and CF3CFHCFH are obtained by using isodesmic reactions. [ABSTRACT FROM AUTHOR]

A detailed theoretical study is performed on the hitherto unknown germanium cyanide radical and its ions. The 2Π state GeCN lies 5.0 kcal/mol lower than the 2Π state GeNC at the coupled-cluster theory including single and double excitations and perturbative inclusion of triple excitations [CCSD(T)]/6-311++G(3df)//quadratic configuration interaction with single and double excitations (QCISD)/6-311G(d)+zero-point vibrational energy (ZPVE) level. For interconversion between them, two electronic state pathways 2A′ and 2A″ are located, with the latter being 0.7 kcal/mol more favorable than the former. On the 2A″ path, the GeCN→GeNC and GeNC→GeCN conversion barriers are 14.5 and 9.5 kcal/mol, respectively. The detailed singlet and triplet potential-energy surfaces of both the cationic and anionic GeCN species are also investigated. On the ground-state electronic hypersurface, singlet GeNC+ is 4.6 kcal/mol more stable than singlet GeCN+, whereas triplet GeNC- is 10.0 kcal/mol less stable than triplet GeCN-. The relative energy difference between the GeCN0,± and GeNC0,± can be well correlated with the number of vacant orbitals on the Ge atom. The stability of the neutral and ionic CGeN and cyclic cGeCN is also discussed. The predicted structures, spectroscopies, ionization, and affinity energies as well as the Renner–Teller properties are expected to provide reliable estimates for future characterization of the potential GeCN and GeNC radicals as well as their ionic counterparts both in the laboratory and in the interstellar space. [ABSTRACT FROM AUTHOR]

The title unknown reaction is theoretically studied at various levels to probe the interaction mechanism between the ethynyl radical (HC=C·) and formaldehyde (H2C==O). The most feasible pathway is a barrier-free direct H-abstraction process leading to acetylene and formyl radical (C2H2+HCO) via a weakly bound complex, and then the product can take secondary dissociation to the final product C2H2+CO+H. The C-addition channel leading to propynal plus H-atom (HCCCHO+H) has the barrier of only 3.6, 2.9, and 2.1 kcal/mol at the CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p)+ZPVE, CCSD(T)/6-311+G(3df,2p)//QCISD/6-311G(d,p)+ZPVE, and G3//MP2 levels, respectively [CCSD(T)—coupled cluster with single, double, and triple excitations; ZPVE—zero-point vibrational energy; QCISD—quadratic configuration interaction with single and double excitations; G3//MP2—Gaussian-3 based on Moller–Plesset geometry]. The O addition also leading to propynal plus H atom needs to overcome a higher barrier of 5.3, 8.7, and 3.0 kcal/mol at the three corresponding levels. The title no-barrier reaction presents a new efficient route to remove the pollutant H2CO, and should be included in the combustion models of hydrocarbons. It may also represent the fastest radical-H2CO reaction among the available theoretical data. Moreover, it could play an important role in the interstellar chemistry where the zero- or minute-barrier reactions are generally favored. Discussions are also made on the possible formation of the intriguing propynal in space via the title reaction on ice surface. [ABSTRACT FROM AUTHOR]

The gas-phase ion-molecule reactions play very important roles in interstellar and in plasma chemistry. Motivated by recent astrophysical detection of the SiCN/SiNC radicals and laboratory characterization of some SiCN-containing species, we carried out a detailed potential energy survey on the SiCN+/SiNC++H2O reaction at the Becke's three-parameter Lee–Yang–Parr-B3LYP/6-311G(d,p) and coupled cluster with single, double, and triple excitations-CCSD(T)/6-311+G(2df,p) (single-point) levels as an attempt towards understanding the SiCN+/SiNC+ reaction mechanisms. In contrast to the carbene-featured analogous CCN+/CNC++H2X (X=O,S) reactions, the title reaction SiCN+/SiNC++H2O are not associated with any competitive silylene-insertion characters. Moreover, the –CN↔–NC interconversion has a low barrier and plays an important role in determining the final product distributions. This is also in marked difference from the CCN+/CNC+ reaction. It is shown that the isomeric sila-cations SiCN+ and SiNC+ can both react with H2O to barrierlessly generate the major product P1 HOSi++HCN and the minor one P3 HOSi++HNC, whereas other low-lying products such as P2 SiNCO++H2, and P0 H2NSi++CO are kinetically unfeasible. The high efficiency of the SiCN+/SiNC+ reaction towards H2O and the potential importance of SiCN+/SiNC+ ion chemistry in interstellar and SiCN-based microelectric and photoelectric processes strongly appeals for future laboratory investigations on the SiCN+/SiNC+ chemical reactivity. [ABSTRACT FROM AUTHOR]

Geometries of 10, 7, and 6 low-lying states of the HCCN neutral radical, its anion and cation, were optimized by using the complete active space self-consistent field (CASSCF) method in conjunction with the aug-cc-pVTZ basis set, respectively. Taking the further correlation effects into account, the second-order perturbations (CASPT2) were carried out for the energetic correction. Vertical excitation energies (Tv) at the ground state geometry of the HCCN neutral radical were calculated for 11 states. The results of our calculations suggest that the spin-allowed transitions of HCCN at 4.179, 4.395, 4.579, 4.727 and 5.506 eV can be attributed to X3A′′ → 23A′′, X3A′′ → 3A′, X3A → 33A′′, X3A′′ → 23A′, and X3A′′ → 33A′, respectively. The singlet−triplet splitting gap of HCCN is calculated to be 0.738 eV. The vertical and adiabatic ionization energies were obtained to compare with the PES data. The results we obtained were consistent with the available experiment results. [ABSTRACT FROM AUTHOR]

The adsorption of poly (vinyl alcohol) with different chain lengths on a hydroxylated β-cristobalite (100) surface is studied via molecular simulations. The relative dielectric constant is selected to be 1 and 78 to mimic in vacuum and in solution environment, respectively. Different configurations and dynamic properties are found in the two absolutely different environments, showing that the solvent condition plays an important role in the process of chain adsorption and diffusion on a surface. In equilibrium, the chain possesses (partial) ordered configuration in vacuum condition. [ABSTRACT FROM AUTHOR]

Currently, the molecular assembly and growth from a small building block to the bulk compounds have become a focus in various fields. Ever being chemical curiosities, the “anti-van’t Hoff/Le Bel” realm that is associated with tetracoordinate or hypercoordiate planar centers has made vast progress. Being important in the fundamental research areas, the ptC species have potential applications in materials science. The existence of ptC in a divanadium complex and a large number of organometallic compounds have since been reported to possess ptC and these provide us with great hope that many more compounds with ptC building blocks may be synthesized in future. Herein, we report the assembly and stabilization of CAl3Si− in both the “ homo-decked sandwich” and “ hetero-decked sandwich” schemes at the B3LYP/6-311+G(d) level. We show that while the Si-doped indeed introduces much complexity during assembly, the electronic and structural integrity feature of CAl3Si− is well conserved during cluster-assembly, characteristic of a “superatom”. This study should be helpful in understanding the hetero-doped assembly mechanism of the ptC chemistry. Moreover, the present results are expected to enrich the flat carbon chemistry, superatom chemistry, metallocenes and combinational chemistry. [ABSTRACT FROM AUTHOR]

A general strategy for designing stable hypercarbon CB 62−-based cluster-assembled compounds CpM(CB 6) q− (M = Li, Na, K, Be, Mg, and Ca) is proposed. This approach takes advantage of (1) the extra stability of a rigid organic aromatic Cp −deck, (2) the electrostatic interactions, (3) charge transfer based on orbital interaction, and (4) the good electronic match between the CB 62−and CpM fragments. The nature and origin of various sandwich-type interactions and the requirements and technologies of the sandwiching decks’ assembly and stabilization are generally classified and comprehensively revealed. The quite crucial to perform fusion stability investigation is performed by comprehensive thermodynamic stability calculations. The electrostatic and orbital interactions and charge transfer in our designed hypercarbon complexes CpM(CB 6) q− are revealed by detailed NBO analysis, orbital characterization, and frontier molecular orbital theory. Detailed analysis and comparison of the intrinsic difference of cluster-assembled compounds between hypercarbon phC CB 62−and pentaatomic ptC CAl 42−are performed. [ABSTRACT FROM AUTHOR]

Complete active space self-consistent field and second-order multiconfigurational perturbation theory methods have been performed to investigate the quartet excited state $${\tilde{a}}^{4}{A^{\prime\prime}}$$ potential energy surface of HCNN radical. Two located minima with respective cis and trans structures could easily dissociate to CH $$({\tilde{a}}^{4}\Sigma^{-})$$ and $$N_{2} ({\tilde{X}}^{1}\Sigma_{\rm g}^{+})$$ products with similar barrier of about 16.0 kcal/mol. In addition, four minimum energy crossing points on a surface of intersection between $${\tilde{a}}^{4}A^{\prime\prime}$$ and X ( $$X={\tilde{X}}^{2}A^{\prime\prime}$$ and $${\tilde{A}}^{2}A^{\prime}$$ ) states are located near to the minima. However, the intersystem crossing $${\tilde{a}}^{4}A^{\prime\prime} \rightarrow X$$ is weak due to the vanishingly small spin–orbit interactions. It further indicates that the direct dissociation on the $${\tilde{a}}^{4}{A^{\prime\prime}}$$ state is more favored. This information combined with the comparison with isoelectronic HCCO provides an indirect support to the recent experimental proposal of photodissociation mechanism of HCNN. [ABSTRACT FROM AUTHOR]

We report the first attempt to assemble the magic cluster Al4N-on the basis of the density functional theory calculations on a series of -stacked dimers (Al4N-)2, sandwich-like compounds D(Al4N)Mq-(where D Al4N-, Cp-(C5H5-); M Li, Na, K, Be, Mg, Ca) and extended compounds (Cp-)m(Li)n(Al4N-)o(where m, n, and oare integers). For the six metals, the magic Al4N-can only be assembled and grow up in our newly proposed “hetero-decked sandwich” scheme (e.g., CpM(Al4N)q-) so as to avoid cluster fusion. The ground-state hetero-decked sandwich species (Cp-)(M)q(Al4N)-(M Li, Na, K, q1; M Be, Mg, Ca, q2) and the extended sandwich species (Cp-)m(Li)n(Al4N-)oare mainly ionically bonded, cluster-assembled “polyatomic molecules”, grown from the combination of Cp-, M-atoms, and Al4N-. As a prototype for ionic bonding involving intact Al4N-subunits, CpM(Al4N)q-may be a stepping stone toward forming ionic, cluster-assembled AlN compounds. [ABSTRACT FROM AUTHOR]

Ever being a large curiosity, a series of simple "planar tetracoordinate carbon (ptC)" molecules have been recently characterized by experiments. Incorporation of such exotic ptC units into the assembled molecular materials, which will bridge the isolated clusters in molecular beams and the potential solid materials, is very challenging. In this paper, we described the first attempt on how to assemble the fewest-number ptC unit CAl42- into molecular materials in sandwich forms on the basis of the density functional theory calculations on a series of model compounds [D(CAl4)M]q- as well as the saturated compounds [D(CAl4)Mq-] ((D = CAl42-, Cp(C5H5); M = Li, Na, K, Be, Mg, Ca). For M = Li, Be, Mg, and Ca, the ptC unit CAl42- can only be assembled in our newly proposed "heterodecked sandwich" scheme (e.g., [Cp(CAl4)M]q-. (M = Li, Na, K, q = 2; M = Be, Mg, Ca, q = 1)) so as to avoid cluster fusion. For M = Na and K, the ptC unit CAl42- can be assembled in both the traditional "homodecked sandwich" [(CAl4)2M]q- (M = Li, Na, K, q = 3; M = Be, Mg, Ca, q = 2) and the novel heterodecked sandwich schemes. Moreover, the counterions were found to have an important role in determining the type of the ground structures for the homodecked sandwich. Various assembled species in extended frameworks were designed. Notably, among all the designed sandwich species, the ptC unit CAl42- generally prefers to interact with the partner deck at the side (Al-Al bond) or corner (Al atom) site. This has not been reported in the sandwich complexes on the basis of the known decks such as Cp-, P5-, N42-, and Al42-, for which only the traditional face-face interaction type was considered. Our results for the first time showed that the ptC unit CAl42- can act as a new type of "superatom". The present results are expected to enrich the flat carbon chemistry, superatom chemistry, metallocenes, and combinational chemistry. [ABSTRACT FROM AUTHOR]

The cyanomethylidyne (CCN) has been the long-standing subject of extensive theoretical and experimental studies on its structures and spectroscopies. However, there are few investigations on its reactivity. Our very recent theoretical work indicated that even with the simplest methane, the CCN reaction faces almost zero barriers following the carbyne mechanism as CH does. This was suggestive of the powerfulness of the nonatomic and nonhydrogenated CCN radical in depleting old molecules and synthesizing new cyanogen-containing molecules in either combustion or interstellar processes. In this paper, a detailed mechanistic study at the CCSD(T)/6-311+G(2df,p)//B3LYP/6-311g(d,p) and G2M(CC1)//B3LYP/6-311G(d,p) computational levels is reported for the reactions of CCN with a series of σ-bonded molecules of the second row HnX (X,n) = (P,3), (S,2), and (Cl,1). The carbenoid insertion is confirmed as the most favored entrance channel, forming Hn-1XC(H)CN. Subsequently, Hn-1XC(H)CN will predominantly lead to product Hn-2XC(H)CN+H via the H-extrusion processes (except X = Cl). Yet, the CCN+HX (X = Cl) reaction is the exception because XC(H)CN intrinsically has no H-atoms at X for extrusion or migration. At G2M(CC1)//B3LYP/6-311G(d,p) computational level, ClC(H)CN can only dissociate back to the reactant or be stabilized with its isomers upon sufficient collisions or radiation. The carbyne character confirmed in this paper provides a useful base for future experimental and theoretical study on the chemistry of this nonatomic and nonhydrogenated reactive radical. In addition, interestingly, the complexes HnX-CCN (X,n) = (P,3) and (S,2) formed in the reactions are found not to be the simple (loosely bound) donor–accepter complexes as those formed in the CCN insertions into other hydrides (NH3, H2O, HF, HCl).On the basis of the comparison with the qualitative features of typical ylides, H3P&bond;CCN and H2S&bond;CCN are considered to be similar to the ylides in nature, being “ylide-like radicals.” They might be observed in some experiments, since they are in deep potential wells on the energy surface. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2006 [ABSTRACT FROM AUTHOR]

Complete active space (CAS) calculations with 6-311g(3df,3pd) basis sets were performed for a large number of electronic states of the nitrate free radical (CH3N/CH3CH2N) and their positive and negative ions. All calculated states are valence states, and their characters are discussed in detail. To investigate the Jahn−Teller effect on the CH3N radical, Cssymmetry was used for both CH3N and CH3CH2N in calculations. The results (CASPT2 adiabatic excitation energies and CASSI oscillator strengths) suggest that the calculated transitions of CH3N at 32172 and 32139 cm-1are attributed to the 23A‘ ‘ → 13A‘ ‘ and 13A‘ → 13A‘ ‘, respectively, which is in accordance with the Ã3E → 3A2emission spectrum at T0 31 817 cm-1. The calculated transitions of CH3CH2N at 334 nm are attributed to the 13A‘ ‘ → 23A‘ ‘ and 13A‘ ‘ → 13A‘, respectively, which is in accordance with the UV absorption spectrum of a series of 11 bands beginning at 335 nm. The vertical and adiabatic ionization energies were obtained to compare with the PES data. These results are in agreement with previous experimental data, which is discussed in detail. [ABSTRACT FROM AUTHOR]

The reaction of the ketenyl radical (HCCO) with acetylene (C2H2) is very relevant to the oxygen−acetylene flames and fuel-rich combustion process for nitrogen-containing compounds. Unfortunately, except for several rate constant measurements, the mechanism is completely unknown for this reaction. In this paper, detailed theoretical investigations are performed for the HCCO + C2H2 reaction at the G3B3 level using the B3LYP/6-31G(d), B3LYP/6-311++G(d,p), and QCISD/6-31G(d) geometries. The exclusive fragmentation channel is the formation of the cyclopropenyl radical (c-C3H3) and carbon monoxide (CO) via the chainlike OCCHCHCH and three-membered ring OC−cCHCHCH intermediates. Thus, the mass spectroscopic peak of C3H3+ in a previous experiment can be explained. The calculated overall reaction barrier is 4.4, 4.4, and 5.3 kcal/mol at the G3B3//B3LYP/6-31G(d), G3B3//B3LYP/6-311++G(d,p), and G3B3//QCISD/6-31G(d) levels, respectively. The title reaction may provide an effective route for generating the long-sought cyclopropenyl radical in the laboratory, which has been the long-standing subject of numerous theoretical studies as the simplest cyclic conjugate radical, and its bulky derivatives were already known. Future experimental investigations for the HCCO + C2H2 reaction are greatly desired to test the predicted fragmentation channel. The implication of the present study in combustion and interstellar processes is discussed. [ABSTRACT FROM AUTHOR]

The known reactions between a neutral radical and a neutral closed-shell σ-type molecule are generally associated with the atom/group-abstraction (e.g., CN, OH reactions) or the carbenoid insertion processes (e.g., CH reactions). In this article, we describe a new type of neutral radical–molecule reaction that feature the nucleophilic addition and elimination mechanism based on the systematic Gaussian-3//MP2(full)/6-31G(d) investigations on the PH3CH+HX model reactions (X = CH3, NH2, OH, F, CH2F, CHF2, CF3, NHCH3/CH2NH2, OCH3/CH2OH). The novel properties are found to benefit both from the radical and ylidic characters of PH3CH, and the type of reagent HX determines the nucleophilic addition reactivity. The present work is the first study on the chemical reactivity of the ylidic radicals. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 545–551, 2006 [ABSTRACT FROM AUTHOR]

Although a number of hydrocarbon radicals including the heavier C3-radicals C3H3 and C3H5 have been experimentally shown to deplete NO effectively, no theoretical or experimental attempts have been made on the reactivity of the simplest C3-radical towards NO. In this article, we report our detailed mechanistic study on the C3H+NO reaction at the Gussian-3//B3LYP/6-31G(d) level by constructing the singlet and triplet electronic state [H,C3,N,O] potential energy surfaces (PESs). The l-C3H+NO reaction is shown to barrierlessly form the entrance isomer HCCCNO followed by the direct O-elimination leading to HCCCN+3O on triplet PES, or by successive O-transfer, N-insertion, and CN bond-rupture to generate the product 1HCCN+CO on singlet PES. The possible singlet–triplet intersystem crossings are also discussed. Thus, the novel reaction l-C3H+NO can proceed effectively even at low temperatures and is expected to play an important role in both combustion and interstellar processes. For the c-C3H+NO reaction, the initially formed H&bond;cCCC&bond;NO can most favorably isomerize to HCCCNO, and further evolution follows that of the l-C3H+NO reaction. Quantitatively, the c-C3H+NO reaction can take place barrierlessly on singlet PES, yet it faces a small barrier 2.7 kcal/mol on triplet PES. The results will enrich our understanding of the chemistry of the simplest C3-radical in both combustion and interstellar processes, which to date have received little attention despite their importance and available abundant studies on its structural and spectroscopic properties. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 641–660, 2006 [ABSTRACT FROM AUTHOR]

A detailed computational study is performed on the unknown radical−molecule reactions between HCO/HOC and acetylene (C2H2) at the CCSD(T)/6-311G(2d,p)//B3LYP/6-311G(d,p)+ZPVE, Gaussian-3//B3LYP/6-31G(d), and Gaussian-3//MP2(full)/6-31G(d) levels. For the HCO + C2H2 reaction, the most favorable pathway is direct C-addition forming the intermediate HC=CHCH=O followed by a 1,3-H-shift leading to H2C=CHC=O, which finally dissociates to the product C2H3 + CO. The overall reaction barrier is 13.8, 10.5, and 11.3 kcal/mol, respectively, at the three levels. The quasi-direct H-donation process to produce C2H3 + CO with barriers of 14.0, 14.1, and 14.1 kcal/mol is less competitive. Thus only at higher temperatures could the HCO + C2H2 reaction play a role. In contrast, the HOC + C2H2 reaction can barrierlessly generate C2H3 + CO via the quasi-direct H-donation mechanism proceeding via a prereactive complex with OH···C2 hydrogen bonding. This is suggestive of the potential importance of the HOC + C2H2 reaction in both combustion and interstellar processes. However, the direct C-addition channel is much less competitive. For both reactions, the possible formation of the intriguing interstellar molecules propadiene and propynal is also discussed. The present theoretical study represents the first attempt to probe the reaction mechanism between HOC and π-systems. Future laboratory investigations on both reactions (particularly HOC + C2H2) are recommended. [ABSTRACT FROM AUTHOR]

The three structures of rare gas inserted fluorohydrides HRgF (Rg=He, Ar, and Kr) with all real frequencies are obtained at the QCISD(T)/aug-cc-pVTZ level. The static first hyperpolarizabilities (β0) at the QCISD/aug-cc-pVQZ level are 8 a.u. (HF), 384 a.u. (HHeF), 737 a.u. (HArF), and 465 a.u. (HKrF). The β0 value remarkably increases by about 50–90 times from 8 a.u. (HF) to 384–737 a.u. (HRgF) due to the inserted rare gas (Rg). The Rg atomic number dependence of β0 for HRgF (Rg=He, Ar, and Kr) is found at the first time. The order of β0 is unmonotonic to be HHeFHKrF, but not monotonic (HHeF

The geometrical structure, electronic structure, one-photon absorption (OPA) and two-photon absorption (TPA) properties of the perylene tetracarboxylic derivatives (PTCDs) were studied theoretically by using density functional theory (DFT) and Zerner’s intermediate neglect of differential overlap (ZINDO) methods. The results revealed that increasing the number of naphthalene nucleus, extending the conjugated length on long axis, increasing the strength of donor group on lateral side, decreasing the ΔEH-L (energy gap between the highest occupied orbital and the lowest unoccupied orbital) and keeping the conjugation effect and inductive effect along the same molecular axis are the efficient ways to enlarge TPA cross section of PTCDs compounds. The results that PTCDs compounds exhibited extremely large TPA cross section of around 800–1100 nm (near infrared region) shed light into the significance of the PTCDs compounds for applications in TPA labeling materials in vivo. [ABSTRACT FROM AUTHOR]

All the 924 classical isomers of fullerene C56 have been investigated by PM3, and some most stable isomers are refined with HCTH/3-21G and B3LYP/6-31G(d) methods. D2:003 with the least number of adjacent pentagons is predicted to be the most stable isomer at B3LYP/6-31G(d) level, while Cs:022 and C2:049 possess nearly degenerate energies with relative energies of 0.03 and 3.90 kcal/mol, respectively. However, as to dianionic C562- fullerene, C2v:011 is predicted to be the most stable isomer. Investigations also show that the encapsulation of Ca atom in C56 fullerene is exothermic and the metallofullerenes Ca@C56 can be described as Ca2+@C562-. The computed relative stabilities show that the D2:003 behaves more thermodynamically stable than other isomers in a wide temperature interval, and C2v:011 should also be an important component. The electronic isomerization of C56 (C2v:011) and C50 (D5h:002) indicates that this phenomenon might be rather general in fullerenes and causes different properties, thus bringing about new possible applications of fullerenes. The static second-order hyperpolarizabilities of the three most stable isomers are slightly larger than that of C60. [ABSTRACT FROM AUTHOR]

The 437 classical isomers of fullerene C52 have been studied by PM3, HCTH/3-21G, and B3LYP/6-31G(d). C2:029 with the least number of adjacent pentagons is predicted to be the most stable isomer. The investigations show that both the number of adjacent pentagons and the degree of aromaticity play important roles in the relative stabilities of fullerene isomers. To clarify the relative stabilities of the C52 isomers in a wide range of temperatures, the entropy contributions are taken into account on the basis of the Gibbs energy at the B3LYP/6-31G(d) level. C2:029 prevails in a wide temperature range. In addition, the electronic spectra and second-order hyperpolarizabilities are determined by means of ZINDO and sum-over-states model. The static second-order hyperpolarizability of C2:029 is 51% larger than that of C60. Furthermore, intensity-dependent refractive index γ (-ω;ω,ω,-ω) (ω=1.1653 eV) of C2:029 is 13 times larger than that of C60. The encapsulation of Ca atom in C52 fullerene is exothermic and the metallofullerene Ca-C52 is described as Ca2+-C522-. [ABSTRACT FROM AUTHOR]

Polycoordinate planar B compounds BXn (X=B, Al, C, N and Si; n=3–8) are optimized at B3LYP/6-311++G (3df,p) theoretical level. For X=B, center B atom can coordinate three to eight atoms, while for X=Al, C, Si, and N, it can only coordinate three to five atoms. The natural bond orbital analysis shows that the center B atom does not violate the octet rule, though the numbers of coordinated atom even reach 8. According to molecular orbital analysis and nucleus independent chemical shift value calculation, it seems that these polycoordinate planar B compounds BXn (X=B, Al, C, N, and Si; n=3–8) hold twofold (α and π) aromatic, which play an important role in their stability and keeping all atoms in one plane. [ABSTRACT FROM AUTHOR]

A new kind of single-electron lithium bonding complexes H3C...LiY (Y=H, F, OH, CN, NC, and CCH) was predicted and characterized in the present paper. Their geometries (C3v) with all real harmonic vibrational frequencies were obtained at the MP2/aug-cc-pVTZ level. For each H3C...LiY complex, single-electron Li bond is formed between the unpaired electron of CH3 radical and positively charged Li atom of LiY molecule. Due to the formation of the single-electron Li bond, the C–H bonds of the CH3 radical bend opposite to the LiY molecule and the Li–Y bond elongates. Abnormally, the three H3C...LiY (Y=CN, NC, and CCH) complexes exhibit blueshifted Li–Y stretching frequencies along with the elongated Li–Y bonds. Natural bond orbital analyses suggest ca. 0.02 electron transfer from the methyl radical (CH3) to the LiY moiety. In the single occupied molecular orbitals of the H3C...LiY complexes, it is also seen that the electron could of the CH3 radical approaches the Li atom. The single-electron Li bond energies are 5.20–6.94 kcal/mol for the H3C...LiY complexes at the CCSD(T)/aug-cc-pVDZ+BF (bond functions) level with counterpoise procedure. By comparisons with some related systems, it is concluded that the single-electron Li bonds are stronger than single-electron H bonds, and weaker than conventional Li bonds and π-Li bonds. [ABSTRACT FROM AUTHOR]

In the present study, an attempt is made to reveal the main mechanism of photodissociation on the lowest-lying Rydberg state 1B1 of ketene, referred to as the second singlet excited state S2, by means of the complete active space self-consistent field and the second-order multiconfigurational perturbation theory methods. The located S2/S1/T1 three-surface intersection plays an important role in the dissociation process. It is shown that the intersection permits an efficient internal conversion from S2 to S1 state, but prohibits the intersystem crossing from S2 to T1 state because of the small spin-orbital coupling value of 0.136 cm-1. The main photodissociation process could be described as follows: after one photon absorption to the S2 state, ketene preferentially relaxes to the minimum S2_C2v, and undergoes a transition state S2_TS with small potential barrier along the Cs-I (out-of-plane bent) symmetry, and passes through the S2/S1/T1 intersection to reach S1 surface, then arrives at the transition state S1_TS along the minimum energy path. As is well known, S1→S0 internal conversion around the Franck-Condon region is expected to be very efficient, and eventually the hot S0 molecule has accumulated enough energy to yield the CH2 (ã 1A1) and CO (X 1Σ+) products. [ABSTRACT FROM AUTHOR]

The electronic structures and stabilities of cationic MPb12+ clusters (M=B, Al, Ga, In, and Tl) with 50 valence electrons are investigated within density functional theory. It is shown that, at the B3LYP/cc-pVDZ(-PP) and BPW91/cc-pVDZ(-PP) levels of theory, the structures of MPb12+ with icosahedra (Ih) symmetry are energetically favorable, and their high stabilities may arise from the closed-shell nature of the π subsystems which are subject to the 2(Nπ+1)2 rule with Nπ=1. In addition, the possessing of large nucleus-independent chemical shifts of the five kinds of clusters reflects the common aromatic character of these clusters. From the comparison of our studies on the binding energies and the highest occupied molecular orbital and the lowest unoccupied molecular orbital energy gaps, the cluster AlPb12+ has higher stability than the others and this is consistent with the recent mass-spectrometric discovery of Al-doped Pbn+ clusters, in which AlPb12+ is highly abundant. The same methods are used to search for the structures of the neutral MPb12 clusters. The calculations reveal that the most stable geometries of the BPb12 and GaPb12 clusters have Ih symmetry, the AlPb12 and InPb12 clusters have Th symmetry, and the TlPb12 cluster has C5v symmetry. Furthermore, the vertical ionization potentials of the neutral MPb12 clusters are smaller than that of some alkali atoms, indicating that the neutral MPb12 clusters possess superalkali character. [ABSTRACT FROM AUTHOR]

Theoretical studies of the solvated electrons (HCN)n- (n=3, 4) reveal a variety of electron trapping possibilities in the (HCN)n (n=3, 4) clusters. Two isomers for (HCN)3- and four isomers for (HCN)4- are obtained at the MP2/aug-cc-pVDZ+dBF (diffusive bond functions) level of theory. In view of vertical electron detachment energies (VDEs) at the CCSD(T) level, the excess electron always “prefers” locating in the center of the system, i.e., the isomer with higher coordination number shows larger VDE value. However, the most stable isomers of the solvated electron state (HCN)3- and (HCN)4- are found to be the linear C∞ν and D∞h structures, respectively, but not the fullyl symmetric structures which have the largest VDE values. [ABSTRACT FROM AUTHOR]

Using the dissipative particle dynamics simulation technique, we have studied the influence of the molecule rigidity on the nanostructures of the A2(B4)2-type miktoarm block copolymers. A typical spherical micellar ordered structure is obtained for a coil-coil miktoarm block copolymer in melt. By introducing a bond angle potential in our model to enhance the molecule rigidity systematically, we find, respectively, a hexagonal cylindrical structure and a parallel ellipsoid in lamellae structure which is discovered for the first time. [ABSTRACT FROM AUTHOR]

The D3d and D2d isomers of the Li3–O–Li3 molecule are metal-nonmetal-metal sandwichlike structures that contain two Li3 superalkali atoms. Their geometries and the real frequencies are obtained at the CCSD(T)/aug-cc-pVDZ level. They are different from the traditional types of the nonmetal-metal-nonmetal sandwich compounds. The natural bond orbital calculation and the topological property ∇2ρ(r) calculation indicate that they are typical ionic compounds. In two isomers, the O2- anion is sandwiched in between two Li3+ cation rings. However, the different orientations of two Li3+ planes give the D3d isomer its own special characteristics. Under the action of the O2- anion in the center, the valence electrons of the D3d isomer are pushed out from two Li3+ triangle rings. This special interaction causes three phenomena. First, the valence electron clouds are distended. Second, the vertical ionization energy of the D3d isomer is considerably low, 4.39 eV, so that it may also be viewed as a superalkali atom. Third, we find that the D3d isomer owns the out-of-plane aromaticity and the largest negative nucleus-independent chemical shift value (-10.8 ppm) exists at 2.5 Å above the center of the Li3+ ring, not at the center of the Li3+ ring like the isolated aromatic Li3+ cation. [ABSTRACT FROM AUTHOR]

By the counterpoise geometry optimization at the level of CCSD(T)/aug-cc-pVDZ, the asymmetrical linear structures with all the real frequencies were obtained for the triatomic cations of (ABA)+ type: Ne3+, (He–Ne–He)+, (Ar–Ne–Ar)+, (Ar–He–Ar)+, (He–O–He)+, and (Ar–O–Ar)+. The validity of this optimization method is confirmed by comparing with the method of the potential-energy surface for the calculations of Ne3+ and (He–Ne–He)+. Using the molecular-orbital theory, it is found that the interaction within the triatomic cations is dominated by the contribution from the first two atoms while the contribution from the third atom is small. This result is justified as a direct consequence of forming an asymmetrical linear structure. Specifically, four types of interaction within the triatomic cations are identified: three-electron σ-type hemibond, three-electron π-type hemibond, two-electron σ bond, and the attraction between cation and atoms. For Ne3+, (He–Ne–He)+, and (He–O–He)+ clusters, it is shown that the electron correlation effect supports the asymmetry. [ABSTRACT FROM AUTHOR]

Dissipative particle dynamics, a simulation technique appropriate at mesoscopic scales, has been applied to investigate the interfaces in immiscible binary A/B homopolymer blends and in the ternary systems with their block copolymers. For the binary blends, the interfacial tension increases and the interface thickness decreases with increasing Flory–Huggins interaction parameter χ while the homopolymer chain length is fixed. However, when the χ parameter and one of the homopolymer chain length is fixed, increasing another homopolymer chain length will induce only a small increase on interfacial tension and slight decrease on interface thickness. For the ternary blends, adding the A-b-B block copolymer will reduce the interfacial tension. When the mole number of the block copolymer is fixed, longer block chains have higher efficiency on reducing the interfacial tension than the shorter ones. But for the block copolymers with fixed volume fraction, shorter chains will be more efficient than the longer ones on reducing the interfacial tension. Increasing the block copolymer concentration reduces interfacial tension. This effect is more prominent for shorter block copolymer chains. [ABSTRACT FROM AUTHOR]

Lowe–Andersen (LA) temperature controlling method [C. P. Lowe, Europhys. Lett. 47, 145 (1999)] is applied in a series of mesoscopic polymer simulations to test its validity and efficiency. The method is an alternative for dissipative particle dynamics simulation (DPD) technique which is also Galilean invariant. It shows excellent temperature control and gives correct radial distribution function as that from DPD simulation. The efficiency of LA method is compared with other typical DPD integration schemes and is proved to be moderately efficient. Moreover, we apply this approach to diblock copolymer microphase separation simulations. With LA method, we are able to reproduce all the results from the conventional DPD simulations. The calculated structure factors of the microphases are consistent with the experiments. We also study the microphase evolution dynamics with increasing χN and find that the bath collision frequency Γ does not affect the order of appearing phases. Although the thermostat does not affect the surface tension, the order-disorder transition (ODT) is somewhat sensitive to the values of Γ, i.e., the ODT is nonmonotonic with increasing Γ. The dynamic scaling law is also tested, showing that the relation obeys the Rouse theory with various Γ. [ABSTRACT FROM AUTHOR]

This work predicts the extraordinary hyperpolarizability of inorganic clusters: two water trimer anions. The first hyperpolarizabilities of (H2O)3- are considerable, β0=1.715×107 a.u. for configuration A and β0=1.129×107 a.u. for configuration B at MP2/d-aug-cc-pVDZ+x level. The first hyperpolarizabilities of (H2O)3- (configuration A) and related systems [(H2O)3 and (H2O)3F-] are compared at the MP2/d-aug-cc-pVDZ+x level. These results are β0=1.715×107 a.u. for (H2O)3-, β0=35 a.u. for (H2O)3 [the neutral core of (H2O)3-], and β0=46 a.u. for (H2O)3F-. Comparing the β0 values of related systems, we find that the dipole-bound excess electron is the key factor in the extraordinary first hyperpolarizability of (H2O)3- species. It will provide a future in the development of some materials with the excess electron (e.g., electrides) that exhibit large nonlinear optical response. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By the counterpoise-correlated potential energy surface method (interaction energy optimization), five structures of the C2H4-nFn-HF (n=0,1,2) dimers with all real frequencies have been obtained at MP2/aug-cc-pVDZ level. The influence of F substituent effect on the structure and π-hydrogen bond of dimer has been discussed. For C2H4-nFn-HF (n=1,2), the π-hydrogen bonds are elongated comparing with that for C2H4-HF. For C2H3F-HF, g-C2H2F2-HF, cis-C2H2F2-HF, the π-hydrogen bonds are further deformed. These changes (elongate, shift, and deformation) of π-hydrogen bond mainly come from deformation of π-electron cloud of C==C bond. The π-electron cloud is pushed towards the one C atom, the π H-bond shift also to the C direction. Since the two lobes of π-electron cloud have deviated slightly from the molecular vertical plane passing through C==C bond, the π-hydrogen bond is sloped. Intermolecular interaction energies of the dimers are calculated to be -3.9 for C2H4-HF, -2.8 for C2H3F-HF, -2.1 for g-C2H2F2-HF, -1.6 for cis-C2H2F2-HF, -1.3 kcal/mol for trans-C2H2F2-HF, at CCSD(T)/aug-cc-pVDZ level. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A dual-level direct dynamics study has been carried out for the two hydrogen abstraction reactions CF[sub 3]CHCl[sub 2]+Cl and CF[sub 3]CHFCl+Cl. The geometries and frequencies of the stationary points are optimized at the BHLYP/6-311G(d,p), B3LYP/6-311G(d,p), and MP2/6-31G(d) levels, respectively, with single-point calculations for energy at the BHLYP/6-311++G(3df,2p), G3(MP2), and QCISD(T)/6-311G(d,p) levels. The enthalpies of formation for the species CF[sub 3]CHCl[sub 2], CF[sub 3]CHFCl, CF[sub 3]CCl[sub 2], and CF[sub 3]CFCl are evaluated at higher levels. With the information of the potential energy surface at BHLYP/6-311++G(3df,2p)//6-311G(d,p) level, we employ canonical variational transition-state theory with small-curvature tunneling correction to calculate the rate constants. The agreement between theoretical and experimental rate constants is good in the measured temperature range 276–382 K. The effect of fluorine substitution on reactivity of the C–H bond is discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

By the counterpoise-correlated potential energy surface method (interaction energy optimization), the structure of the π H-bond complex FH...C[sub 4]H[sub 4]...HF has been obtained at the second-order Møller–Plesset perturbation theory (MP2/aug-cc-pVDZ) level. Intermolecular interaction energy of the complex is calculated to be -7.8 kcal/mol at the coupled-cluster theory with single, double substitutions and perturbatively linked triple excitations CCSD (T)/aug-cc-pVDZ level. The optimized structure is a “wheel with a pair of pedals” shaped (1|1) structure in which both HF molecules almost lie on either vertical line passing through the middle-point of the C==C bond on either side of the horizontal plane of the C[sub 4] ring for cyclobutadiene. In the structure, an antiaromatic ring π-dihydrogen bond is found, in which the proton acceptor is antiaromatic 4 electron and 4 center π bond and the donors are both acidic H atoms of HF molecules. In accompanying with the π-dihydrogen bond, two secondary interactions are exposed. The first is a repulsive interaction between an H atom of HF and a near pair of H atoms of C[sub 4]H[sub 4] ring. The second is the double π-type H bond between two lone pairs on a F atom and a far pair of H atoms. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ab initio direct dynamics methods have been used to study the title reaction. The electronic structure information including geometries, gradients and force constants (Hessians) is calculated at the QCISD/6-31+G(d) level. With the aid of intrinsic reaction coordinate theory, the minimum energy path (MEP) is obtained at the same level, and the energies along the MEP are further refined by performing the single-point calculations at the PMP4/6-311++G(3df,3pd) level. For this reaction, the theoretical rate constants by using improved canonical variational transition state theory incorporating small-curvature tunneling correction are in excellent agreement with the more recent experimental values. Our calculated results show that for the CH[sub 3]+HBr reaction, the rate constants have a strong nonlinear Arrhenius behavior, i.e., the CH[sub 3]+HBr reaction has negative temperature dependence at T<536 K, but clearly shows positive temperature dependence at T>536 K. The current work predicts that the kinetic isotope effect for the title reaction is normal in the temperature range 200–482 K, i.e., k[sub HBr]/k[sub DBr]>1. This result is in good agreement with the experimental one. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The potential energy surface of the CH[sub 2]O+CH[sub 3] reaction is explored at the MP2/6-311++G(d,p), MP4SDQ/6-311G(d,p), and QCISD(T)/6-311+G(3df,2p) (single point) levels of theory. Theoretical calculations suggest that the major product channel (R1) is the hydrogen abstraction leading to the product P[sub 1] CHO+CH[sub 4] (R1), while the addition process leading to P[sub 2]H+CH[sub 3]CHO (R2) appears to be negligibly small. The calculated enthalpies and dissociation activation energies for CH[sub 3]CH[sub 2]O and CH[sub 3]OCH[sub 2] radicals involved in the reaction are in line with the experimental values. Dual-level dynamics calculation is carried out for the direct hydrogen abstraction channel. The energy profile of (R1) is refined with the interpolated single-point energies (ISPE) method at the QCISD(T)//MP2 level. The rate constants, which are evaluated by canonical variational transition-state theory (CVT) including small-curvature tunneling (SCT) correction, are in good agreement with the available experimental data. It is shown that tunneling effect plays a significant role in the rate constant calculation; and as a result, the CVT/SCT rate constants exhibit typical non-Arrhenius behavior over a wide temperature range 300–2000 K. The three parameter expression is k=6.35×10[sup -26] T[sup 4.4] exp(-2450/T) cm[sup 3] molecule[sup -1] s[sup -1]. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The article presents information on a study which describes the first attempt to assemble and stabilize a neutral planar tetracoordinated carbon (ptC) radical CAI3Si via a newly proposed heterodecked sandwich scheme. The scheme has been testified to be an effective way to capture planar radicals and to design spin-based molecular materials.

Kynurenine aminotransferase III (KAT III) is a novel member of the kynurenine amino-transferase enzyme family. Its active site topology and structure characteristics have not been established. In this study, with extensive computational simulations, including homology modeling and molecular dynamics simulations, a 3D structure model of human KAT III dimer was created and refined. Furthermore, CDOCKER approach was employed to dock two ligands (L-methionine and L-tryptophan) into the active sites of human KAT III dimer and uncover the ligand-binding modes. The complexes were subjected to 5 ns MD simulation, and the results indicate that TYR119 and TRP13 might be the key residues as they have the large contri-butions to the binding affinity, which is in good agreement with the experimental results. Moreover, another two residues (ASP120 and TYR57) are also found that their strong inter-actions stabilize the whole system. The structural and biochemical insights obtained from the present study will be helpful for designing the specific inhibitors of human KAT III. [ABSTRACT FROM AUTHOR]

We propose a multiscale simulation strategy to study the interplay between diffusion and curing reaction on the network formation and the corresponding mechanical properties of epoxy resins. Atomistic molecular dynamics simulations are first used to estimate the parameters that will be used in coarse-grained simulations. Then a dissipative particle dynamics coupled with curing reaction model is developed and adopted to simulate the cross-linking process of the system to form an epoxy network structure. We find that, during the curing process, to which extent that the components can diffuse between each other greatly influences the generated network structure. Finally, the reverse mapping of the coarse-grained structure to atomistic representation is carried out to analyze the mechanical properties and the Tgof the epoxy resin system. [ABSTRACT FROM AUTHOR]

The methane (CH4) hydrogen abstraction reaction by linear butadiynyl radical C4H (CCCCH) has been investigated by direct ab initio dynamics over a wide temperature range of 100 - 3000 K, theoretically. The potential energy surfaces (PESs) have been constructed at the CCSD(T)/aug-cc-pVTZ//BB1K/6-311G(d,p) levels of theory. Two different hydrogen abstraction channels by C1 and C4 of C4H (C1C2C3C4H) have been considered. The results indicate that the C1 position of C4H is a more reactive site. The electron transfer behaviors of two possible channels are also analyzed by quasi-restricted orbital (QRO) in detail. The rate constants calculated bvy canonical variational transition-state theory (CVT) with the small-curvature tunneling correction (SCT) are in excellent agreement with available experimental values. The normal and three-parameter expressions of Arrhenius rate constants are also provided within 100-3000 K. It is expected to be helpful and further studies on the reaction dynamics behaviors over a wide temperature range where no experimental data is available so far. [ABSTRACT FROM AUTHOR]

The fullerene [] can bind a variety of metal to form exohedral metallofullerenes with special chemical and physical properties. However, how about the structure and properties of the perfluorinated exohedral metallofullerene with excess electron? The structures of K···CF ( C), K···CF ( C), and K···CF ( C) with all real frequencies are represented at the B3LYP/6-31G(d) theory level. The large ionization potentials of 7.066 ~ 7.422 eV suggest the large stabilities of them. Owing to the interpolarization between K and CF ( n = 20 or 60) cage, an electron transfers from K atom to the perfluorinated cage (CF or CF) to form excess electron and long K···CF ionic bond (length > 2.9 Å) with interaction energies of −78.24 ~ −93.72 kcal/mol. Comparing to the solvated electron e@CF ( n = 20 or 60) with interior state, under the effect of counterion K, partial excess electron is pulled from the interior to the surface of the cage to form partial interior and surface excess electron state. It is found that cage size and shape influence excess electron absorption spectrum, which may be important for the design of the new optical and photoelectric materials or devices with good performances. Graphical abstract: Existence of the cage and counterion cause the formation of partial interior and surface excess electron state.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

To realize the chemistry of a multicage organic molecule with excess electron, as a model, by confining an excess electron inside a double-cage single molecule, the structures of e−@C24F22(NH)2C20F18(e−@AB) and e−@C20F18(NH)2C20F18(e−@BB′) are obtained at the B3LYP/6-31G(d) 4s4p theory level. It is confirmed that the excess electron is mainly confined inside one cage with larger interior electronic attractive potential (A for e−@AB and B for e−@BB′) in the ground state, while the electron is localized in the other one in the first excited state. Owing to such excess electron localizations, an interesting intercage excess electron transfer transition takes places. This intercage excess electron transfer transition exhibits five characteristics: (1) the excess electron transfer from one cage to another (A → B for e−@AB and B → B′ for e−@BB′); (2) the transition is between the ground and first excited state; (3) the wavelength and strength are the largest; (4) the transition accompanies a significant charge transfer (Δq> 0.8) and molecular dipole moment change (Δμ > 20 D); (5) the transition corresponds to SOMO → LUMO. For the transition, the oscillator strength is larger and the wavelength is shorter for the asymmetric structure (e−@AB) than for the symmetric one (e−@BB′), which indicates that the intercage excess electron transfer transition may be regulated by changing the size of cage. This work is useful for the designs of organic electronic sponges (porous organic electrides), organic conductor with excess electrons, and photoelectric and nanoelectronic devices. [ABSTRACT FROM AUTHOR]

The effect of substitution and cooperativity on the blue shift of Cl-F stretch vibration in H3C ··· ClF complex has been studied with quantum chemical calculations at the UMP2(Full)/aug-cc-pVTZ level. The electron-withdrawing group (F atom) in the electron donor decreases the blue shift, whereas the electron-donating group (methyl group) in the electron donor cause it to increase. The cooperativity between two different types of halogen bonds in H3C ··· ClF ··· ClF complex enhances the strength of single-electron halogen bond and the blue shift. The natural bond orbital (NBO) and atoms in molecules (AIM) analyses have been performed for the halogen-bonded complexes. [ABSTRACT FROM AUTHOR]

The reaction of CH3SH with Cl atoms has been theoretically studied using dual-level direct dynamics method. Three reaction channels, two H-abstraction (from the –SH and –CH3 groups) and one substitution channel, have been found. The optimized geometries and frequencies of all the stationary points are calculated at the MP2/6-311+G(d,p) level, and the potential-energy profile along each reaction path is refined by the single-point calculation at the CCSD(T)/6-311+G(d,p) level. The rate constants for the two H-abstraction channels are evaluated by the improved canonical variational transition-state theory (ICVT) with the small-curvature tunneling (SCT) correction over a wide temperature range of 193–1000 K. The calculated ICVT/SCT rate constants are in good agreement with available experimental values and exhibit negative temperature dependence below 500 K. The present calculations indicate that the H-abstraction from the –SH group is the major channel for the title reaction, while the H-abstraction from the –CH3 group should be taken into account only at high temperatures. [ABSTRACT FROM AUTHOR]

The complete active space self-consistent field and multiconfigurational second-order perturbation theory methods have been used to investigate the low-lying electronic states of the HCCO radical and its anion. The calculated geometrical structure and harmonic vibrational frequencies for the X2A″ state of HCCO are in good agreement with experimental values. The barrier to 12П (12A′) is estimated to be 0.069 eV (554 cm−1), which is also close to experimental result (540 cm−1). Moreover, we find that the 22A′ state is bent, while 22П (22A″) is linear. By comparing the oscillator strengths and adiabatic excitation energies, we show that the 22П (22A″) ← X2A″ transition is the most intense among these transitions. The unpaired electrons mainly locate on the C atoms in the 22A′ and 22П (22A″) states according to the Mulliken spin populations. On the other hand, for HCCO−, the first adiabatic and vertical detachment energies are 2.210 and 2.362 eV, respectively, which reasonably agree with experimental value of 2.338 ± 0.008 eV. Remarkably, we explore several higher excited states of the HCCO radical (14A′, 24A″ and cis-14A′) and its anion (13A″, 11A″ and 13A′), which have not been reported in previous studies. [ABSTRACT FROM AUTHOR]

A direct dynamics study was carried out for the multichannel reaction of CH3NHNH2 with OH radical. Two stable Conformers (I, II) of CH3NHNH2 are identified by the rotation of the &bond;CH3 group. For each conformer, five hydrogen-abstraction channels are found. The reaction mechanisms of product radicals (CH3NNH2 and CH3NHNH) with OH radical are also investigated theoretically. The electronic structure information on the potential energy surface is obtained at the B3LYP/6-311G(d,p) level and the energetics along the reaction path is refined by the BMC-CCSD method. Hydrogen-bonded complexes are presented at both the reactant and product sides of the five channels, indicating that the reaction may proceed via an indirect mechanism. The influence of the basis set superposition error (BSSE) on the energies of all the complexes is discussed by means of the CBS-QB3 method. The rate constants of CH3NHNH2 + OH are calculated using canonical variational transition-state theory with the small-curvature tunneling correction (CVT/SCT) in the temperature range of 200–1000 K. Slightly negative temperature dependence of rate constant is found in the temperature range from 200 to 345 K. The agreement between the theoretical and experimental results is good. It is shown that for Conformer I, hydrogen-abstraction from &bond;NH&bond; position is the primary pathway at low temperature; the hydrogen-abstraction from &bond;NH2 is a competitive pathway as the temperature increases. A similar case can be concluded for Conformer II. The overall rate constant is evaluated by considering the weight factors of each conformer from the Boltzmann distribution function, and the three-term Arrhenius expressions are fitted to be kT = 1.6 × 10-24T4.03exp (1411.5/T) cm3 molecule-1 s-1 between 200–1000 K. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 [ABSTRACT FROM AUTHOR]

A detailed theoretical study is carried out at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels as an attempt to investigate the mechanism of the little understand ion–molecule reaction between HCN+ and NH3. Various possible reaction pathways are considered. It is shown that six dissociation products P 1(NH3 + + HCN), P 2(NH4 + + CN), P 3(NH3 + + HNC), P 9(HCNH+ + NH2) P 10(NCNH3 + + H), and P 12(HNCNH2 + + H) are both thermodynamically and kinetically feasible. Among these products, P 1 is the most competitive product with predominant abundance. P 3 and P 9 may be the second feasible products with comparable yields. P 12 may be the least possible product followed by the almost negligible P 2 and P 10. Because the isomers and transition states involved in the HCN+ + NH3 reaction all lie below the reactant, the title reaction is expected to be rapid, which is consistent with the measured large rate constant in experiment. The title reaction may have a potential relevance in Titan’s atmosphere, where the temperature is very low. Furthermore, our calculated results are compared with the previous experimental findings. [ABSTRACT FROM AUTHOR]

Equilibrium geometries of low-lying electronic states of cyanosilylene (HSiCN), isocyanosilylene (HSiNC), and their ions have been investigated using the complete active space self-consistent field (CASSCF) approach. The harmonic vibrational frequencies on the optimized geometries were calculated using the multiconfiguration linear response (MCLR) method. Taking the further correlation effects into account, the complete active space perturbation theory of second-order (CASPT2) was carried out for the energetic correction. The CASPT2 calculations have been performed to obtain the vertical excitation energies of selected low-lying excited states of HSiCN and HSiNC. Computed results show that the singlet-triplet splittings are calculated to be 0.99 and 1.30 eV for HSiCN and HSiNC, respectively. The vertical excitation energies of the lowest singlet and triplet excited states in HSiCN are lower than those in HSiNC. The first vertical ionization energy of HSiCN (10.04 eV) is higher than that of HSiNC (9.97 eV). The ground-state adiabatic electron affinities are found to be rather high, and the value of HSiCN (1.85 eV) higher than that of HSiNC (1.52 eV). The existences of dipole-bound excited negative ion states have been discovered within HSiCN and HSiNC. [ABSTRACT FROM AUTHOR]

The complex triplet potential energy surface for the reaction of HCNO with NH is investigated at the G3B3 level using the B3LYP/6-311++G(d,p), and QCISD/6-311++G(d,p) geometries. Various possible isomerization and dissociation pathways are probed. The initial association between HCNO and NH is found to be carbon to nitrogen attack leading to HNCHNO 2a, which can convert to 2b, 2c, and 2d. Subsequently, 1,4-H-shift of 2a to form NCHNOH 3a followed by dissociation to P 2(1HCN + 3HON) is the most feasible pathway. Much less competitively, 2d undergoes successive 1,3-H-shift and C-N cleavage to form HNCNOH 8b, and then to product P 3(1HNC + 3HON), the second feasible pathway. 8b can alternatively isomerize to 8c followed by N–O bond rupture to generate P 6(2OH + 2HNCN), the lesser followed feasible pathway. In addition, 2b takes continuously 1,3- and 1,2-H-shift to form NC(H)NHO 6a, then to ONHCNH 7a which can convert to 7b. Eventually, 7b may take C-N bond fission to produce P 5(1HNC + 3HNO), the least feasible pathway. The present paper may be helpful for future experimental identification of the product distributions for the title reaction, and may be helpful to deeply understand the mechanism of the title reaction. [ABSTRACT FROM AUTHOR]