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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]

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]

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]

The reaction of N (4S) radical with NCO (X2Π) radical has been studied theoretically using density functional theory and ab initio quantum chemistry method. The triplet electronic state [N2CO] potential energy surface (PES) is calculated at the G3B3 and CCSD(T)/aug-cc-pVDZ//B3LYP/6-311++G(d,p) levels of theory. All the energies of the transition states and isomers in the pathway RP1 are lower than that of the reactants; the rate of this pathway should be very fast. Thus, the novel reaction N + NCO can proceed effectively even at low temperatures and it is expected to play a role in both combustion and interstellar processes. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction. [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]

New species with molecular formula Si2NS, not yet observed experimentally, are described theoretically for the first time. Various levels of calculations are applied to obtain the structures, energies, dipole moments, vibrational spectra, rotational constants, and isomerization of Si2NS species. A total of 15 minima which are connected by 21 interconversion transition states on the potential energy surface are located at the DFT/B3LYP/6-311G(d) level of theory. And at the higher (single point) CCSD(T)/6-311+G(2d)//QCISD/6-311G(2d)+ZPVE level of theory, the global state of 1 (at 0.0 kcal/mol) corresponds to a nonlinear SiNSiS with a 2A′ electronic state. The next most stable isomer of 2 (only 3.7 kcal/mol higher 1) possesses a rhomboidal-type structure similar to the global state of Si2NO. This is followed by another local minimum of 6 (at 43.8 kcal/mol above 1) which is a nonlinear SiNSSi form with a 2A′ electronic state. Note that both 1 and 6 have the important Si≡N triple bonding. Too, the analog of (NCCS) NSiSiS is unstable due to the higher energy of its own. The bonding natures of the relevant species are also discussed. An analysis of the bonding properties and structural similarities and differences between C2NO, C2NS, SiCNO, and Si2NO is also carried out. [ABSTRACT FROM AUTHOR]

The structures, energetics, spectroscopies and stability of the doublet [Si, C, P, O] radical are explored at the density functional theory and ab initio levels. Eighteen isomers connected by 22 interconversion transition states are located at the DFT/B3LYP/6-311G(d) level. The structures of the kinetically stable isomers and the relevant transition states are further optimized at the QCISD/6-311G(d) level followed by CCSD(T)/6-311 + G(2df) single-point energy calculations. At the QCISD/6-311G(d) level, the lowest-lying isomer is the cyclic O-cCSiP 8 (0.0 kcal/mol) with considerable kinetic stability of 22.0 kcal/mol. In addition, two bent isomers OSiCP 1 (7.3 kcal/mol) and SiCPO 3 (34.7 kcal/mol) also possess considerable kinetic stability (more than 10.0 kcal/mol). As a result, three isomers 1, 3 and 8 are predicted to be possible candidates for future experimental and astrophysical detection. The bonding nature of the three isomers is analysed. The calculated results are compared with those of the analogous radical [Si, C, N, O]. Implications in the laboratory and interstellar space are also discussed. The predicted structures and spectroscopic properties are expected to be informative for the identification of [Si, C, P, O] in the laboratory and space. [ABSTRACT FROM AUTHOR]

Various levels of calculations are carried out~for exploring the potential energy surface (PES) of triplet SiC3O, a molecule of potential interest in interstellar chemistry. A total of 38 isomers are located on the PES including chain-like, cyclic and cage-like structures, which are connected by 87 interconversion transition states at the DFT/B3LYP/6-311G(d) level. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by CCSD(T)/6-311+G(2df) single-point energy calculations. At the QCISD level, the lowest lying isomer is a linear SiCCCO 1 (0.0 kcal/mol) with the 3 ∑ electronic state, which possesses great kinetic stability of 59.5 kcal/mol and predominant resonant structure [MediaObject not available: see fulltext.]. In addition, the bent isomers CSiCCO 2 (68.3 kcal/mol) and OSiCCC 5 (60.1 kcal/mol) with considerable kinetic stability are also predicted to be candidates for future experimental and astrophysical detection. The bond natures and possible formation pathways in interstellar space of the three stable isomers are discussed. The predicted structures and spectroscopic properties for the relevant isomers are expected to be informative for the identification of SiC3O and even larger SiC n O species in laboratory and interstellar medium. [ABSTRACT FROM AUTHOR]

The reaction of 3C2 (a3Π) radical with O2 (X3Σ) molecule has been studied theoretically using ab initio Quantum Chemistry method. Both singlet and triplet potential energy surfaces (PES) are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-311+G(d) + ZPE and G3B3 levels of theory. On the singlet PES of the title reaction, it is shown that the most feasible pathway should be the O-atom of O2 attacking the C-atom of the 3C2 molecule first to form the adduct 1 CCOO, followed by the O-shift to give intermediate 2 CC(OO), and then to the major products P1 (2CO). Alternatively, 1 can be directly dissociated to P1 via transition state TS1-P1. The other reaction pathways are less competitive due to thermodynamical or kinetic factors. On the other hand, the pathways on the triplet PES are less competitive than those on the singlet PES in low temperature range, whereas it is not the case in high temperature ranges. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction. The reaction heats of formation calculated are in good agreement with that obtained experimentally. [ABSTRACT FROM AUTHOR]

Calculations with B3LYP within quantum chemical density functional theory have been carried out for 1-H-phosphininium cation and a series of 1-R-phosphininium molecules (R = cyclopentadiene, α and β pyrroles, α and β phosphole, C5BH5&bond; and CH2&bond;). The negative nuclear-independent chemical shift values and the positive aromatic stabilization energies confirm that they are aromatic compounds. In particular, the 1-H-phosphininium cation even exhibits stronger aromatic character than the well-known aromatic phosphinine. The aromatic substituents have strong capability to attract electrons. It is the conjugation and aromaticity that keeps the stability and conformations of the molecules investigated. Owing to the perturbation of the aromatic substituted groups, the predicted large T values and the enlarged HOMO-LUMO gap of the phosphininium cation indicate that these compounds are expected in experiment. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [ABSTRACT FROM AUTHOR]

Various levels of calculations are carried out to explore the potential energy surfaces (PES) of singlet and triplet SiC3S, a molecule of potential interest in interstellar chemistry. At the DFT/B3LYP/6-311G(d) level, a total of 57 minimum isomers and 92 interconversion transition states are located. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by CCSD(T)/6-311 + G(2df) single-point energy calculations. At the QCISD level, the lowest-lying isomer is the chain-like SiCCCS 31 (0.0 kcal/mol) with a great kinetic stability of 54.1 kcal/mol. In addition, ring isomers CC-cCSSi 19 (19.8 kcal/mol), S-cCCCSi 112 (30.4 kcal/mol), S-cCCSiC 118 (9.4 kcal/mol), S-cSiCCC 121 (34.4 kcal/mol) and cage-like isomer cage-SiSCCC 123 (51.8 kcal/mol) also possess considerable kinetic stability (more than 10.0 kcal/mol). As a result, these six isomers are predicted to be possible candidates for future experimental and astrophysical detection. The bond natures and possible formation pathways in interstellar space of the SiCCCS are discussed. The predicted structure and spectroscopic properties for it are expected to be informative for the identification of SiC3S and even larger SiCnS species either in laboratory or in space. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations have been used to study the isomerization process in the NC3P system. At the DFT/B3LYP/6-311G(d) level, 28 triplet and 28 singlet minima were obtained on their respective potential energy surfaces. The linear triplet 3NCCCP is the lowest-energy structure among the isomers. On the triplet PES, only linear isomers 3NCCCP, 3CNCCP, 3CCCNP, and 3CCNCP possess great kinetic and thermodynamic stabilities to exist under low-temperature conditions (such as in the dense interstellar clouds). At the same time, one chain-like and four three-membered-ring isomers on the singlet PES have been located with high kinetic and thermodynamic stabilities. Further CCSD(T)/6-311G(2df)//QCISD/6-311G(d), CCSD(T)/cc-pVTZ//DFT/B3LYP/cc-pVTZ, and CASPT2(14,12)/cc-pVQZ//CASSCF(14,12)/cc-p VQZ calculations are performed on the structures, frequencies, and energies of the relevant species. The bonding natures were analyzed and the results were compared with the analogous NC3N and NC2P molecules so as to aid their future experimental or astrophysical detection. [ABSTRACT FROM AUTHOR]

Radical–radical reactions involving chlorinated methyl radicals are particularly important in the mechanism of combustion of chlorinated hydrocarbons. Yet, they are usually difficult to study experimentally. In this paper, four chloride-related radical–radical reactions, i.e., CH3+CH3-nCln (n = 1, 2, 3) and CH3+CCl2, are theoretically studied for the first time by means of the Gaussian-3//B3LYP potential energy surface survey combined with the master equation study over a wide range of temperatures and pressures. Our calculated results show that the three CH3+CH3-nCln reactions can barrierlessly generate the former two kinetically allowed products P1 H2C&dbond;C(H)3-nCln-1+HCl and P2 CH3CH3-nCln-1+Cl with the very high predominance of P1 over P2. For the CH3 reaction with the biradical CCl2, which inevitably takes place during the CH3+CCl3 reaction and yet has never been studied experimentally or theoretically, H2C&dbond;CCl2+H and H2C&dbond;C(H)Cl+Cl are predicted to be the respective major and minor products. The results are compared with the recent laser photolysis/photoionization mass spectroscopy study on the CH3+CH3-nCln (n = 1, 2, 3) reactions. The predicted rate constants and product branching ratios of the CH3+CCl2 reaction await future experimental verification. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [ABSTRACT FROM AUTHOR]

The reaction of atomic radical F with propyne has been studied theoretically using ab initio quantum chemistry methods and transition state theory. The potential energy surface was calculated at the CCSD(T)/aug-cc-pVDZ (single-point) level using the UMP2/6-311++G(d,p) optimized structures. Two reaction mechanisms including the addition–isomerization–elimination reaction mechanism and the directed hydrogen abstraction reaction mechanism are considered. For the hydrogen abstraction reactions, i.e., the most probable evolution pathway in the title reaction, the HF formation occurs via direct abstraction mechanism dominantly and the H atom picked up by the atomic radical F should come mostly from the methyl group of normal propyne. On the other hand, for the addition–isomerization–elimination mechanism, the most feasible pathway should be the atomic radical F attacking on the C≡C triple bond in propyne (CH3C≡CH) to form a weakly-bound adduct A1 with no barrier, followed by F addition to the C≡C triple bond to form the low-lying intermediate isomer 5. Subsequently, isomer 5 directly dissociates to P3 H2CCCHF + H via transition state TS5/P3. The other reaction pathways on the doublet PES are less competitive due to thermodynamical or kinetic factors. Furthermore, based on the analysis of the kinetics of all channels through which the addition and abstraction reaction proceed, we expect that the competitive power of reaction channels may vary with experimental conditions for the title reaction. The present work will provide useful information for understanding the processes of atomic radical F reaction with other unsaturated hydrocarbons. [ABSTRACT FROM AUTHOR]

DFT/B3LYP/6-311G(d) and CCSD(T)/6-311G(2d) single-point calculations are carried out for exploring the doublet potential energy surface (PES) of PC3O, a molecule of potential interest in interstellar chemistry. A total of 29 minima connected by 65 interconversion transition states are located. The structures of the most relevant isomers and transition states are further optimized at the QCISD level followed by CCSD(T) single-point energy calculations. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d)+ZPVE level, the global minimum is the quasi-linear structure PCCCO 1 (0.0 kcal/mol) with a great kinetic stability of 47.9 kcal/mol, and the cumulenic form $${\mathop P\limits_ - ^ \bullet}=C=C=C={\mathop O\limits_ - ^ -}$$ features largely in its resonance structures. Moreover, the chainlike isomer OPCCC 3 (64.5) and five-membered-ring species cPCCCO 19 (77.8) possess considerable kinetic stability of about 18.0 kcal/mol. All these three isomers are very promising candidates for future experimental and astrophysical detection. Additionally, a three-membered-ring isomer CC-cCOP 10 (69.6) has slightly lower kinetic stability of around 15 kcal/mol and may also be experimentally observable. Possible formation mechanisms of the four stable isomers in interstellar space are discussed. The present research is the first attempt to study the isomerization and dissociation mechanisms of PC n O series. The predicted spectroscopic properties, including harmonic vibrational frequencies, dipole moments and rotational constants for the relevant isomers, are expected to be informative for the identification of PC3O in laboratory and interstellar medium. [ABSTRACT FROM AUTHOR]

The radical-molecule reaction mechanism of CH3 with NOx (x = 1, 2) has been explored theoretically at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels of theory. For the singlet potential energy surface (PES) of the CH3 + NO2 reaction, it is found that the carbon to middle nitrogen attack between CH3 and NO2 can form energy-rich adduct a (H3CNO2) with no barrier followed by isomerization to b1 (CH3ONO-trans), which can easily convert to b2 (CH3ONO-cis). Subsequently, starting from b (b1, b2), the most feasible pathway is the direct N&bond;O bond cleavage of b (b1, b2) leading to P1 (CH3O + NO) or the 1,3-H-shift and N&bond;O bond rupture of b1 to form P2 (CH2O + HNO), both of which may have comparable contribution to the reaction CH3 + NO2. Much less competitively, b2 can take a concerted H-shift and N&bond;O bond cleavage to form product P3 (CH2O + HON). Because the intermediates and transition states involved in the above three channels are all lower than the reactants in energy, the CH3 + NO2 reaction is expected to be rapid, as is consistent with the experimental measurement in quality. For the singlet PES of the CH3 + NO reaction, the major product is found to be P1 (HCN + H2O), whereas the minor products are P2 (HNCO + H2) and P3 (HNC +H2O). The CH3 + NO reaction is predicted to be only of significance at high temperatures because the transition states involved in the most feasible pathways lie almost above the reactants. Compared with the singlet pathways, the triplet pathways may have less contributions to both reactions. The present study may be helpful for further experimental investigation of the title reactions. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 807–817, 2005 [ABSTRACT FROM AUTHOR]

Stimulated by recent preparation and characterization of the first C58F18fullerene derivative, with a heptagon in the framework (Science, 2005, 309, 278), we have performed systematic density functional studies on the stabilities and electronic properties of two different structures C58X18(A) and C58X18(B), where X H, F, and Cl. The large energy gaps between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals (between 2.64 and 3.45 eV) and the aromatic character (with nucleus independent chemical shifts from −10.0 to −13.9 ppm) of C58X18(A) and C58X18(B) indicate that they possess high stabilities. Further investigations show that the heats of formation of C58X18fullerene derivatives are highly exothermic, suggesting that adding nine X2's releases much of the strain of pure C58fullerene and leads to stabilities of the derivatives. Lower in energy and stronger in aromatic character than C58F18(B), which has been experimentally characterized, C58F18(A) should also be isolated. In addition, C58F18and C58Cl18are predicted to possess large electron affinities, especially for C58F18(B) and C58Cl18(B) with values of 3.00 and 3.06 eV, respectively, even larger than that (2.50 eV) of C60F18. Hence, C58F18and C58Cl18can serve as good electron-acceptors with possible photonic/photovoltaic application. The IR spectra of C58X18are simulated to facilitate identification of different isomers experimentally. In addition, the electronic spectra and second-order hyperpolarizabilities of C58X18are predicted by ZINDO and sum-over-states model. With the addition of 9X2, both the static and frequency-dependent second-order hyperopolarizabilities of C58X18greatly decrease compared to those of C58. [ABSTRACT FROM AUTHOR]

Radical–radical reactions involving chlorinated methyl radicals are particularly important in the mechanism of combustion of chlorinated hydrocarbons. Yet, they are usually difficult to study experimentally. In this paper, four chloride-related radical–radical reactions, i.e., CH3+CH3-nCln (n = 1, 2, 3) and CH3+CCl2, are theoretically studied for the first time by means of the Gaussian-3//B3LYP potential energy surface survey combined with the master equation study over a wide range of temperatures and pressures. Our calculated results show that the three CH3+CH3-nCln reactions can barrierlessly generate the former two kinetically allowed products P1 H2C&dbond;C(H)3-nCln-1+HCl and P2 CH3CH3-nCln-1+Cl with the very high predominance of P1 over P2. For the CH3 reaction with the biradical CCl2, which inevitably takes place during the CH3+CCl3 reaction and yet has never been studied experimentally or theoretically, H2C&dbond;CCl2+H and H2C&dbond;C(H)Cl+Cl are predicted to be the respective major and minor products. The results are compared with the recent laser photolysis/photoionization mass spectroscopy study on the CH3+CH3-nCln (n = 1, 2, 3) reactions. The predicted rate constants and product branching ratios of the CH3+CCl2 reaction await future experimental verification. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [ABSTRACT FROM AUTHOR]

The complex singlet potential energy surface for the reaction of CH2OH with NO2, including 14 minimum isomers and 28 transition states, is explored theoretically at the B3LYP/6-311G(d,p) and Gaussian-3 (single-point) levels. The initial association between CH2OH and NO2 is found to be the carbon-to-nitrogen approach forming an adduct HOCH2NO2 (1) with no barrier, followed by C−N bond rupture along with a concerted H-shift leading to product P1 (CH2O + trans-HONO), which is the most abundant. Much less competitively, 1 can undergo the C−O bond formation along with C−N bond rupture to isomer HOCH2ONO (2), which will take subsequent cis−trans conversion and dissociation to P2 (HOCHO + HNO), P3 (CH2O + HNO2), and P4 (CH2O + cis-HONO) with comparable yields. The obtained species CH2O in primary product P1 is in good agreement with kinetic detection in experiment. Because the intermediate and transition state involved in the most favorable pathway all lie blow the reactants, the CH2OH + NO2 reaction is expected to be rapid, as is confirmed by experiment. These calculations indicate that the title reaction proceeds mostly through singlet pathways; less go through triplet pathways. In addition, a mechanistic comparison is made with the reactions CH3 + NO2 and CH3O + NO2. The present results can lead us to deeply understand the mechanism of the title reaction and may be helpful for understanding NO2-combustion chemistry. [ABSTRACT FROM AUTHOR]