Search

In the upper atmosphere of Titan, C 6 H 5 + is the most important precursor of benzene and it primarily comes from the reaction between C 4 H 2 + and C 2 H 4 . In this paper, the complex doublet potential energy surface for the ion molecule reaction of C 4 H 2 + with C 2 H 4 was investigated at the B3LYP/6-311G(d,p) and CCSD(T)/6-311+G(2df) levels. The initial association between C 4 H 2 + and C 2 H 4 forms two energy-rich addition intermediates 1 (c CH 2 CH 2 CH C 3 H) and 13 (CH 2 CH 2 CHCCCCH), which undergo the subsequent isomerization and decomposition steps. A total of six products, including P 1 ( c - C 6 H 6 + ) , P 2 ( c - C 6 H 5 + α + H ) , P 3 ( C 6 H 5 + β + H ) , P 4 ( C 4 H 4 + + C 2 H 2 ) , P 5 ( C 6 H 4 + + H 2 ) , and P 6 ( C 3 H 3 + + C 3 H 3 ) are obtained. The present study will be helpful for understanding the formation mechanism of benzene in the Titan’s atmosphere.

The structures, energetics, spectroscopies, and isomerization of the doublet Si 2 PO radical are explored at various levels. At the UB3LYP/6-311+G(2df) level, a total of 12 minimum isomers and 15 interconversion transition states are located. Four cyclic isomers are found to be thermodynamically and kinetically stable at the higher UCCSD(T)/6-311++G(2df)//UM062x/6-311+G(2df)+ZPVE and may be experimentally or astrophysically observable. The lowest-lying isomer 1 c-SiOSiP ( 2 B 2 ) (0.0 kcal/mol) corresponds to a rhomboidal-type structure similar to the global state of Si 2 NO, followed by another three-membered ring isomer 3 O-cSiSiP ( 2 A′) (23.1 kcal/mol) which is similar to the global state of [Si, C, P, O]. Another two stable isomers are the three-membered ring isomers 11 P-cSiSiO (35.0 kcal/mol) and 6 O-cPSiSi (47.5 kcal/mol), respectively. The bonding nature of the four isomers is analyzed. Meanwhile, the calculated results are compared with its analogs SiCPO, Si 2 NO, and Si 2 PS. 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 2 PO in the laboratory and space.

The reaction of NCO with CH 3 is studied at B3LYP/6-11G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels. On the singlet potential energy surface (PES), the most feasible channel is direct radical–radical recombination leading to isomers OCNCH 3 and NCOCH 3 . Further isomerization and dissociation of OCNCH 3 and NCOCH 3 to yield the dissociation products need to surmount the energy barrier at least 4.7 kcal/mol with respect to reactant, which indicate that these processes are kinetically unfeasible at normal temperatures and may become feasible only at high temperatures. Compared with the singlet pathways, the triplet pathways may have less contribution to the title reaction.

The reaction mechanism of CN radicals with ClO radicals has been studied theoretically using ab initio and density functional theory (DFT). The result shows that the main reaction path is the O atom in radical ClO attacks the C atom in radical CN to compose the intermediate 1 ClOCN. Three thermodynamically accessible prodncts, P1 (CO+ClN), P3 (NO+CCl), and P4 (ClNCO), were obtained from intermediate 1 through isomerization and decomposition reactions. P4 is the primary product, and P1 and P3 are the secondary product. Compared with the singlet potential energy surface, the contribution of the triplet potential energy surface can be ignored.

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.

The radical-molecule reaction of C2Cl3 with NO2 is explored at the B3LYP/6-311G(d,p) and CCSD(T)/6-311+G(d,p) (single-point) levels. On the singlet potential energy surface (PES), the association between C2Cl3 and NO2 is found to be carbon-to-nitrogen attack forming the adduct C2Cl3NO2 (1) without any encounter barrier, followed by isomerization to C2Cl3ONO (2). Starting from 2, the most feasible pathway is the N–O1 bond cleavage which lead to P 1 (C2Cl3O + NO). Much less competitively, 2 transforms to the three-membered ring isomer c-OCCl2C–ClNO (4 a ) which can easily interconvert to c-OCCl2C–ClNO 4 b . Then 4 (4 a , 4 b ) takes direct C1–C2 and C2–O1 bonds cleavage to give P 2 (COCl2 + ClCNO). The lesser competitive channel is the 4 a isomerizes to the four-membered ring intermediate O-c-CNClOCCl2 (5) followed by dissociation to P3 (CO + ClNOCCl2). The concerted 1,2-Cl shift along with C1–O1 bond rupture of 4 b to form ONC(O)CCl3 (6) followed by dissociation to P 4 (ClNO + OCCCl2) is even much less feasible. Moreover, some of P 3 and P 4 can further dissociate to P 5 (ClNO + CO + CCl2). Compared with the singlet pathways, the triplet pathways may have less contribution to the title reaction. Our results are in marked difference from previous theoretical studies which showed that two initial adducts C2Cl3–NO2 and C2Cl3–ONO are obtained. Moreover, in the present paper we focus our main attentions on the cyclic isomers in view of only the chain-like isomers are considered by previous studies. The present study may be helpful for understanding the halogenated vinyl chemistry.

The complex doublet potential energy surface for the ion-molecule reaction of HCN(+) with C(2)H(4) is investigated at the B3LYP/6-311G(d,p) and CCSD(T)/6-311++G(3df,2pd) (single-point) levels. The initial association between HCN(+) and C(2)H(4) forms three energy-rich addition intermediates, 1 (HCNCH(2)CH(2)(+)), 2 (HC-cNCH(2)CH(2)(+)), and 3 (N-cCHCH(2)CH(2)(+)), which are predicted to undergo subsequent isomerization and decomposition steps. A total of nine kinds of dissociation products, including P(1) (HCN + C(2)H(4)(+)), P(2) (HCNCHCH(2)(+) + H), P(3) (NCCH(2) + CH(3)(+)), P(4) (CN + C(2)H(5)(+)), P(5) (NCCHCH(2)(+) + H(2)), P(6) (HNCCHCH(2)(+) + H), P(7) (c-CHCCH(2)N(+) + H(2)), P(8) (c-NHCCH(2)C(+) + H(2)), and P(9) (HNCCCH(+) + H(2) + H), are obtained. Among the nine products, P(1) is the most abundant product. P(2) is the second feasible product but is much less competitive than P(1). P(3), P(4), P(5), and P(6) may have the lowest yields observed. Other products, P(7), P(8), and P(9), may become feasible at high temperature. Because the intermediates and transition states involved in the most favorable pathway all lie below the reactant, the HCN(+) + C(2)H(4) reaction is expected to be rapid, which is confirmed by experiment. The present calculation results may provide a useful guide for understanding the mechanism of HCN(+) toward other unsaturated hydrocarbons.

The structures, energetics, spectral parameters and stability of the doublet [Si, C, P, S] radical are explored at the density functional theory and ab initio levels. Sixteen isomers connected by 23 interconversion transition states are located on the PES. The structures of the stable isomers and their 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 a bent SSiCP 1 (0.0 kcal/mol) with considerable isomerization barriers (the lowest barrier is 12.6 kcal/mol). In addition, the bent isomer SiCSP 5 (57.3 kcal/mol) and the cyclic species S-cCSiP 6 (2.7 kcal/mol) also possess considerable isomerization barriers (more than 10.0 kcal/mol). The valence bond structures of the three isomers 1 , 5 and 6 are analyzed. The calculated results are compared with those of analogous molecules C 2 PS and [Si, C, N, O]. Implications in laboratory and interstellar space are also discussed.

New species with molecular formula Si 2 NS , 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 Si 2 NS 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 Si 2 NO . 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 C 2 NO , C 2 NS , SiCNO , and Si 2 NO is also carried out.

The radical–molecule reaction mechanism of CH2Cl with NO2 has been explored theoretically at the B3LYP/6–311G(d,p) and MC–QCISD (single-point) levels of theory. Our results indicate that the title reaction proceeds mostly through singlet pathways, less go through triplet pathways. The initial association between CH2Cl and NO2 is found to be the carbon-to-nitrogen attack forming the adduct a H2ClCNO2 with no barrier, followed by isomerization to b1 H2ClCONO-trans which can easily convert to b2 H2ClCONO-cis. Subsequently, the most feasible pathway is the C–Cl and O–N bonds cleavage along with the N–Cl bond formation of b (b1, b2) leading to product P1 CH2O + ClNO, which can further dissociate to give P5 CH2O + Cl + NO. The second competitive pathway is the 1,3-H-shift associated with O–N bond rupture of b1 to form P2 CHClO + HNO. Because the intermediates and transition states involved in the above two favorable channels all lie below the reactants, the CH2Cl+NO2 reaction is expected to be rapid, as is confirmed by experiment. The present results can lead us to understand deeply the mechanism of the title reaction and may be helpful for further experimental investigation of the reaction.

The DFT/B3LYP/6-311G(d) and CCSD(T)/6-311G(2df) (single-point) methods have been used to investigate the potential energy surface of SiC2O. A total of 11 isomers are located, which are connected by 13 interconversion transition states. The global minimum is the linear SiCCO with 1Σ+ electronic state whose structure can be described as cumulenic double bonding form :Si C C O:. The isomer O-cSiCC possessing SiCC three-membered ring with an exocyclic OSi bond also shows considerable stability. Both SiCCO and O-cSiCC are kinetically stable towards isomerization and dissociation. Further calculations are performed at the CCSD(T)/6-311G(2df)//QCISD/6-311G(2d) level to obtain more reliable energies, geometrical parameters, harmonic vibrational frequencies and infrared intensities of the isomers. The energies and geometrical parameters are also calculated with the CASPT2//CASSCF methods.

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-pVQZ 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.

The structural, spectroscopic and kinetic properties of small molecules composed of second-row elements such as Si and P are vital to understanding the growing process of relevant semiconductors and functional films as well as to predicting new species to be detected in space. In this paper, a detailed computational study is performed on the singlet and triplet [C,Si,N,P] potential energy surfaces at the DFT/B3LYP, QCISD and CCSD(T) (single-point) levels. A total of 38 (15 for singlet and 23 for triplet) minimum isomers and 37 (19 for singlet and 18 for triplet) interconversion transition states are located. Generally, the triplet species lie energetically higher than the singlet ones. At the CCSD(T)/6-311G(2df)//B3LYP/6-311G(d) + ZPVE level, the lowest-lying isomer is SiNCP( 1 Σ) 1 1 (0.0 kcal/mol) followed by NCPSi( 1 A′) 1 2 (8.9), NCSiP( 1 Σ) 1 3 (19.7), PCNSi( 1 A′) 1 4 (19.8), CNSiP( 1 Σ) 1 5 (26.5), SiCNP 1 6 ( 1 Σ) (31.1) and CNPSi( 1 A′) 1 7 (32.2). Except for the isomer 1 4 with very low isomerization barrier, the remaining six ones have reasonable kinetic stabilities. In addition, although the linear isomers NSiCP 1 8 (63.8) and SiCPN 1 10 (73.5) are energetically very high-lying, they contain very large isomerization barriers to allow for existence either in laboratory or in space. By means of structural and bonding analysis, the interesting Si- or P-related triple-bonding are revealed among species like 1 1 (C P), 1 3 (Si P), 1 5 (Si P), 1 6 (Si C, N P). Since up to now, stable typical Si P and Si N triply-bonded species are still experimentally unknown, isomers 1 1 , 1 3 , 1 5 and 1 6 may represent such good examples. Moreover, the intramolecular donor–acceptor mechanism from the exocyclic π-bonding to the vacant orbital of one ring-atom is proposed to account for the unique three-membered ring-like structure of the chainlike isomers 1 2 , 1 4 and 1 7 . Finally, the similarities and discrepancies of CSiNP molecule are compared to the analogous 18-valence-electrons species AA′BB′ (A, A′ are group IV elements and B, B′ are group V elements) such as C 2 N 2 , C 2 NP, CSiN 2 , Si 2 N 2 , Si 2 NP, and Si 2 P 2 .

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.

The structures and isomerization pathways of singlet and triplet PC3P isomers are investigated at the DFT/B3LYP/6-311G(d) level. Among the 23 triplet and 18 singlet isomers, the global minimum is PCCCP 31 with ∑ g − 3 electronic state. It can be viewed as resonance structures among |P C–C C– P| , |P C C C P| , and |P C– C –C P|. Additionally, chainlike isomer 1PCPCC, ring isomers 1PC–cCCP, 1CC–cCPP, c1PCPCC, c1PCCCP, and cage-like isomers cage-1PPCCC and cage-1PCPCC also posses considerable kinetic stability. These eight isomers are predicted to be possible candidates for future experimental and astrophysical detection. Furthermore, the QCISD/6-311G(d) and DFT/B3LYP/cc-pVTZ calculations are performed on the structures and harmonic vibrational frequencies of the relevant species so as to aid their future laboratory and interstellar detection.

The structures, energetics, spectroscopies, and isomerization of various doublet Si2CP species are explored theoretically. In contrast to the previously studied SiC2N and SiC2P radicals that have linear SiCCN and SiCCP ground states, the title Si2CP radical has a four-membered-ring form cSiSiPC 1 (0.0 kcal/mol) with Si-C cross-bonding as the ground-state isomer at the CCSD(T)/6-311G(2df)//B3LYP/6-311G(d)+ZPVE level, similar to the Si2CN radical. The second low-lying isomer 2 at 11.6 kcal/mol has a SiCSiP four-membered ring with C-P cross-bonding, yet it is kinetically quite unstable toward conversion to 1 with a barrier of 3.5 kcal/mol. In addition, three cyclic species with divalent carbene character, i.e., cSiSiCP 7, 7' with C-P cross-bonding and cSiCSiP 8 with Si-Si cross-bonding, are found to possess considerable kinetic stability, although they are energetically high lying at 44.4, 46.5, and 41.4 kcal/mol, respectively. Moreover, a linear isomer SiCSiP 5 at 44.3 kcal/mol also has considerable kinetic stability and predominantly features the interesting cumulenic /Si=C=Si=P/* form with a slight contribution from the silicon-phosphorus triply bonded form /Si=C*-Si[triple bond]P/. The silicon-carbon triply bonded form *Si[triple bond]C-Si[triple bond]P/ has negligible contribution. All five isomers are expected to be observable in low-temperature environments. Their bonding nature and possible formation strategies are discussed. For relevant species, the QCISD/6-311G(d) and CCSD(T)/6-311+G(2df) (single-point) calculations are performed to provide more reliable results. The calculated results are compared to those of the analogous C3N, C3P, SiC2N, and Si2CN radicals with 17 valence electrons. Implications in interstellar space and P-doped SiC vaporization processes are also discussed.

Various levels of calculations are applied to obtain the structures, energies, dipole moments, vibrational spectra, rotational constants, and isomerization of SiC3P species. A total of 27 minima which are connected by 40 interconversion transition states on the potential energy surface are located at the DFT/B3LYP/6-311G(d) level. The global minimum is found to be a linear SiCCCP of the 2Π electronic state. Besides the three-membered-ring isomer CC-cCPSi (36.2 kcal/mol), the four-membered-ring isomers P-cCCCSi (31.2 kcal/mol) and P-cSiCCC (79.1 kcal/mol), the five-membered-ring isomer cCPCCSi (46.6 kcal/mol), and the cagelike isomer pPSiCCC (56.8 kcal/mol) also possess great kinetic stability (more than 10.0 kcal/mol). The bonding natures of the relevant species are analyzed. The calculated results may be helpful for understanding the P-doped SiC vaporization process. The structures, energies, and bonding properties of the relevant species are compared with those of the SiC2N, SiC2P, and SiC3N analogues.

A detailed isomerization reaction potential energy surface of HPS3 system is investigated by means of density functional method (B3LYP) employing 6-311++G(3df,3pd) basis set. The results indicate that planar isomer HSP(S)S is thermodynamically and kinetically the most stable species. Other several isomers, (cis–cis)-HSSPS (Cs), trans-HSP(S2) (C1), cis-HSP(S2) (Cs), and HP(S2)S (Cs), are also considered as kinetically stable isomers, and may be experimentally observable provided that precise precursors are available. Furthermore, the structural nature of these kinetically stable isomers are compared with its analogues and discussed by comparison with the standard bond length and bond angle parameters in relevant compounds.

The structures and isomerization of Si(2)CN species are explored at density functional theory and ab initio levels. Fourteen minimum isomers are located connected by 23 interconversion transition states. At the coupled-cluster single double (CCSD)(T)/6-311+G(2df)//QCISD/6-311G(d) +zero-point vibrational energies level, the thermodynamically most stable isomer is a four-membered ring form cSiSiCN 1 with Si-C cross bonding. Isomer 1 has very strong C-N multiple bonding characters, formally suggestive of a radical adduct between Si(2) and CN. Such a highly pi-electron localization can effectively stabilize isomer 1 to be the ground state. The second low-lying isomer is a linear form SiCNSi 5 (9.8 kcal/mol above 1) with resonating structure among [Si=C-*N=Si], *[Si=C=N=Si], and [Si=C=N-Si*]* with the former two bearing more weight. The species 1 and 5 have very high kinetic stability stabilized by the barriers of at least 25 kcal/mol. Both isomers should be experimentally or astrophysically observable. In light of the fact that no cyclic nitrogen-containing species have been detected in space, the cyclic species 1 could be a very promising candidate. The calculated results are compared to those of the analogous molecules C(3)N, C(3)P, SiC(2)N, and SiC(2)P. Implications of Si(2)CN in interstellar and N-doped SiC vaporization processes are also discussed.

The structures, energetics, spectroscopies, and isomerization of singlet and triplet SiC2S isomeric species are explored at various levels. At the B3LYP/6-311G(d) level, a total of 29 minimum isome...

Calculations at the QCISD(T)/6-311+G(d,p)//B3LYP/6-31G(d) level indicate that the participation of water can effectively lower the isomerization barrier from linear propargylium (H2CCCH+) to cyclop...

A singlet potential energy surface (PES) of the [Si,N,C,S]+ system is investigated by means of MP2 and QCISD(T) (single-point) methods. Four linear isomers of [Si,N,C,S]+ system are found to be kinetically stable and should be experimentally observable. The linear isomer SiNCS+ is the thermodynamically most stable, followed by SSiCN+, SSiNC+ and SiCNS+. No cyclic kinetically stable isomers are found. For the isomer SiNCS+, several isomerizations can very favorably compete with its direct dissociation, and thus, the very recent mass spectrometric experiment can reasonably be interpreted by analyzing the structures and bonding.

The structures and isomerization pathways of various HC2P isomers in both singlet and triplet states are investigated at the B3LYP/6-311G(d,p), QCISD/6-311G(d,p) (for isomers only) and single-point CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) levels. At the CCSD(T)/6-311G(d,p)//B3LYP/6-311G(d,p) level, the lowest-lying isomer is a linear HCCP structure 31 in the 3∑− state. The second low-lying isomer has a CPC ring with exocyclic CH bonding 15 in a singlet state at 10.5 kcal/mol. The following third and fourth low-lying isomers are a singlet bent HCCP structure 11 at 20.9 kcal/mol and a bent singlet HPCC structure 13 at 35.8 kcal/mol, respectively. Investigation of the HC2P potential-energy surface indicates that in addition to the experimentally known isomer 31, the other isomers 11, 13 and 15 also have considerable kinetic stability and may thus be observable. However, the singlet and triplet bent isomers HCPC 12 and 32 as well as the triplet bent isomer HPCC 33 are not only high-lying but are also kinetically unstable, in sharp contrast to the situation of the analogous HCNC and HNCC species that are both kinetically stable and that have been observed experimentally. Furthermore, the reactivity of various HC2P isomers towards oxygen atoms is briefly discussed. The results presented here may be useful for future identification of the completely unknown yet kinetically stable HC2P isomers 11, 13 and 15 either in the laboratory or in interstellar space.

The B3LYP/6-311G(d) and CCSD(T)/6-311G(2df) (single-point) methods have been used to investigate the singlet potential energy surface of C2NP, in which seven stationary isomers and seventeen interconversion transition states are involved. At the final CCSD(T)/6-311G(2df)//B3LYP6-311G(d) level with zero-point vibrational energy correction the lowest-lying isomer is a linear NCCP followed by two linear CNCP isomers at 23.9 and CCNP at 65.8 kcal mol−1, respectively. The three isomers are kinetically very stable towards both isomerization and dissociation, and CCNP is even more kinetically stable than CNCP – by 14.3 kcal mol−1 despite its high energy. Further comparative calculations were performed at the QCISD and QCISD(T) levels with the 6-311G(d) and 6-311G(2d) basis sets to obtain more reliable structures and spectroscopy for the three isomers. The calculated bond lengths, rotational constant, and dipole moment for NCCP were in close agreement with the experimentally determined values. Finally, similarities and discrepancies between the potential energy surface of C2NP and those of the analogous species C2N2 and C2P2 were compared. The results presented in this paper might be helpful for future identification of the two still unknown yet kinetically very stable isomers CNCP and CCNP, both in the laboratory and in interstellar space.

A detailed singlet potential energy surface (PES) of the [Si, N, C, O]+ system including 13 isomers and 15 transition states is investigated by means of the MP2 and QCISD(T) (single-point) methods. At the final QCISD(T)/6-311+G(3df)//MP2/6-311G(d) level with zero-point energy inclusion, SiNCO+ 1 is found to be thermodynamically the most stable isomer followed by SiOCN+ 2, OSiCN+ 3, OSiNC+ 4 and SiCNO+ 5 at 40.90, 70.00, 70.43 and 87.72 kcal mol−1, respectively. For isomer 1, the isomerization to the species, 2, 3 and 4 can very favorably compete with its direct dissociation, and thus, together with the structural and bonding analysis, the very recent mass spectrometric experiment can reasonably be interpreted. Furthermore, our results indicate that while 1 is kinetically the most stable, the species, 3, 4 and 5 are also shown to reside in very deep potential wells. Therefore, in addition to the mass spectroscopically characterized isomers 1 and 3, the species 4 and 5 should also be experimentally observable. However, observation of the second low-lying isomer 2 seems less likely due to its easy conversion to isomer 1 via a four-membered ring Si-shift process.

The [C3H6N]+ potential energy surface is investigated in detail at the QCISD(T)/6-311+G(d,p)//B3LYP/6-31G(d,p) level with a focus to discuss the possible association and dissociation mechanism of the CH3++CH3CN and HCNH++C2H4 reactions. For the former reaction, the most feasible dissociation pathways are proceeded via a stable four-membered ring isomer cCH2CH2CHNH+ 18 leading to the lowest-lying product P1 HCNH++C2H4, part of which has been previously studied. In addition, several new energetically accessible pathways are found to be responsible for the formation of P1, either via the stable chain-like isomer CH3CHNCH2+ 7 or via the direct dissociation of the stable chain-like isomer CH2CHNHCH2+ 3 (3′). Formation of the products P2 C2H5++HCN and P3 C2H5++HNC is reasonably interpreted by isomerization and dissociation mechanism rather than the previously proposed proton-transfer process from P1. Moreover, other possible dissociation channels such as C3H3++NH3 and C2H2N++CH4 are considered. For the HCNH++C2H4 reaction, which has been considered as a precursor to the interstellar molecule C2H5CN via the deprotonation of C2H5CNH+ 14, our calculations indicate that only the stable isomer C2H5NCH+ 4 instead of isomer 14 can be barrierlessly formed, in accordance with the experimental finding. However, our work shows that another stable isomer CH2CHCHNH2+ 15 (15′), which has been previously proposed as a possible product, cannot be obtained in low-temperature interstellar clouds due to substantial barriers as for 14. Yet, formation of both the isomers 14 and 15 (15′) may be possible in hot region in interstellar medium. The results presented in this paper may be useful for understanding the interstellar chemistry in which the C3H6N+ ion is involved.

The detailed potential energy surface of [HC2N2]+ is theoretically investigated at the B3LYP/6-311G(d,p) and CCSD(T)/6-311G(3df,2p) (single-point) levels. The linear isomer HCCNN+ 5 is predicted to lie just 28.4 kcal/mol higher than the lowest-lying isomer HNCCN+ 1 [the neutral species CCNN lies 82.7 kcal/mol above the lowest-energy NCCN (Ding et al., J. Chem. Phys. 2000, 113, 1745)]. Moreover, HCCNN+ 5 resides in a very deep potential well stabilized by the barrier of 55.4 kcal/mol either toward dissociation or isomerization (the smallest barrier for CCNN is 42.1 kcal/mol). The calculated C-proton affinity of CCNN is very large as 211.2 kcal/mol. Therefore, CCNN may be significantly stabilized upon protonation at the terminal carbon both thermodynamically and kinetically, a phenomenon characteristic of carbene protonation. In addition, the nitrogen-protonated isomer HNNCC+ 6 at 107.3 kcal/mol also possesses high kinetic stability of 51.9 kcal/mol. Our calculations indicate that while the neutral CCNN, th...

To predict potentially stable molecules with Si≡C triple bonding, theoretical calculations at the B3LYP/6-311G(d) and CCSD(T)/6-311G(2df) (single-point) levels were employed to study the structures, energetics, and isomerization of various SiCN2 isomers. A schematic potential energy surface (PES) of SiCN2 was established to discuss the kinetic stability of the isomers. A new isomer SiCNN was found to possess a typical Si≡C triple bond, as confirmed by comparative calculations at the B3LYP, QCISD, QCISD(T), CCSD, and CCSD(T) levels on the bond lengths of SiCNN and other experimentally or theoretically known species of RSiCH (R=H, F, Cl, OH). Moreover, SiCNN resides in a very deep potential; the stabilization barrier is at least 53.2 kcal mol−1. Thus, SiCNN may be considered as the most kinetically stable isomer with Si≡C triple bonding known to date, and it may represent a very promising molecule for future experimental characterization. In addition, the stability of the other isomers, such as the four linear species SiNCN, SiNNC, NSiCN and NSiNC, a three-membered NNC ring isomer with exocyclic C−Si bonding, and a four-membered SiCNN ring isomer is discussed and compared with SiCNN.

The potential-energy surface of the interstellar molecule C4N is explored at the B3LYP/6-311G(d) level of theory. Thirteen isomers including the linear, three-membered ring, four-membered ring, A-like, Y-like, and cage-like structures are located as minima connected by 23 interconversion transition states. The structures of the most relevant isomers and transition states are further optimized at the QCISD/6-311G(d) level followed by single-point energy calculations at the MP4SDTQ, CCSD(T), and QCISD(T) levels with the 6-311G(2df) basis set. At the CCSD(T)/6-311G(2df)//QCISD/6-311G(d) level, the lowest-lying isomer is a linear structure CCCCN 1 followed by a CCC three-membered ring structure 4 with exocyclic CCN bonding that lies only 2.8 kcal/mol higher. The third and fourth low-lying isomers possess a CCC three-membered ring structure 5 with exocyclic CNC bonding at 21.4 kcal/mol and a linear structure CCCNC 2 at 23.4 kcal/mol, respectively. All the four isomers 1, 2, 4, and 5 and another high-lying isom...

The singlet potential energy surface of the [CFNO2] system is investigated at the B3LYP and CCSD(T) (single-point) levels to explore the possible reaction mechanism of CF radical with NO2. The top attack of C-atom of CF radical at the N-atom of NO2 molecule first forms the adduct isomer FCNO21 followed by oxygen-shift to give trans-OC(F)NO 2 and then to cis-OC(F)NO 3. Subsequently, the most favorable channel is a direct dissociation of 2 and 3 to product P1 FCO+NO. The second and third less favorable channels are direct dissociation of 3 to product P2 FNO+CO and isomerization of 3 to a complex NOFCO 4, which can easily dissociate to product P3 FON+CO, respectively. The large exothermicity released in these processes further drives most of the three products P1, P2, and P3 to take secondary dissociation to the final product P12 F+CO+NO. Another energetically allowed channel is formation of product P41NF+CO2, yet it is much less competitive than P1, P2, P3, and P12. The present calculations can well interpret one recent experimental fact that the title reaction is quite fast yet still much slower than the analogous reaction CH+NO2. Also, the results presented in this article may be useful for future product distribution analysis of the title reaction as well as for the analogous CCl and CBr reactions. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1907–1919, 2001

The structures, energetics, spectroscopies, and isomerization of possible low-lying Si2P2 isomers in both singlet and triplet states are theoretically investigated at the B3LYP/6-311G(d) and CCSD(T)/6-311+G(2df) (single-point) levels. At the final CCSD(T)/6-311+G(2df)//B3LYP/6-311G(d) level, the lowest energy isomer is a singlet butterfly-like SiPSiP structure 11 with P−P cross bonding followed by a singlet rhombic SiPSiP isomer 12 with Si−Si cross bonding, whereas the cyanogen analogue PSiSiP 15 is the highest lying of all the singlet isomers. The singlet potential energy surface of Si2P2 indicates that the rhombic isomer 12 is kinetically much more stable than the butterfly-like isomer 11, although isomer 12 is 3.2 kcal/mol higher in energy than isomer 11, while other isomers are kinetically unstable toward isomerization to isomer 11 or 12. It is also shown that the triplet Si2P2 isomers are energetically higher than all the single species except 15. Furthermore, for the most relevant singlet Si2P2 isom...

The dissociation and isomerization stability of the linear cyanogen isomer CCNN, a formally charge-transfer species comprising C2− and N2+, is theoretically investigated by means of ab initio methods, including the Hartree–Fock (HF), the Mo/ller–Plesset perturbation theory from second through fourth order (MP2, MP4SDQ, MP4SDTQ), the configuration interaction with singles and doubles (CISD), the quadratic configuration interaction with singles and doubles (QCISD) as well as with triples [QCISD(T)], and the density functional theory (DFT) including Beck’s three parameter hybrid methods with the Lee–Yang–Parr correlation functional (B3LYP) and with the Perdew–Wang 91 correlation functional (B3PW91) methods. At the QCISD(T)/6-311G(3df)//QCISD(T)/6-311G(d) level with QCISD/6-311G(d) zero-point vibrational energy (ZPVE) correction, the barriers from CCNN to the NNC three-membered ring structure with exocyclic C–C bonding and to the dissociation products C2 and N2 are predicted to be 42.1 and 51.8 kcal/mol, resp...

The complex triplet potential energy surface for the reaction of ground-state carbon atom C((3)P) with trans-C(4)H(8) is theoretically investigated at the B3LYP/6-311G(d,p) and G3B3(single-point) levels. Various possible isomerization and dissociation pathways are probed. The initial association between C((3)P) and trans-C(4)H(8) is found to be the C((3)P) addition to the C=C bond of trans-C(4)H(8) to barrierlessly generate the three-membered cyclic isomer 1 CH(3)-cCHCCH-CH(3). Subsequently, 1 undergoes a ring-opening process to form the chainlike isomer 3a cis-trans-CH(3)CHCCHCH(3), which can either lead to P(6)((2)CH(3)CHCCCH(3) + (2)H) via the C-H bond cleavage or to P(7)((2)CH(3)CHCCH + (2)CH(3)) via C-C bond rupture. These two paths are the most favorable channels of the title reaction. Other channels leading to products P(1)((2)CH(3)-cCHCCH + (2)CH(3)), P(2)((2)CH(3)-cCHCC-CH(3) + (2)H), P(3)(trans-(2)CH(3)CHCH + (2)C(2)H(3)), P(4)(cis-(2)CH(3)CHCH + (2)C(2)H(3)), P(5)((3)CH(3)CH + (1)CH(3)CCH), P(8)(cis-(2)CH(3)CHCHCCH(2) + (2)H), P(9)(trans-(2)CH(3)CHCHCCH(2) + (2)H), P(10)((2)CH(3)CCCH(2) + (2)CH(3)), and P(11)((2)CH(3)CHCCHCH(2) + (2)H), however, are much less competitive due to either kinetic or thermodynamic factors. Because the intermediates and transition states involved in the C((3)P) + trans-C(4)H(8) reaction all lie below the reactant, the title reaction is expected to be rapid, as is consistent with the measured large rate constant. Our results may be helpful for future experimental investigation of the title reaction.

All the 924 classical isomers of fullerene C(56) have been investigated by PM3, and some most stable isomers are refined with HCTH/3-21G and B3LYP6-31G(d) methods. D(2):003 with the least number of adjacent pentagons is predicted to be the most stable isomer at B3LYP/6-31G(d) level, while C(s):022 and C(2):049 possess nearly degenerate energies with relative energies of 0.03 and 3.90 kcal/mol, respectively. However, as to dianionic C(56)(2-) fullerene, C(2v):011 is predicted to be the most stable isomer. Investigations also show that the encapsulation of Ca atom in C(56) fullerene is exothermic and the metallofullerenes Ca@C(56) can be described as Ca(2+)@C(56)(2-). The computed relative stabilities show that the D(2):003 behaves more thermodynamically stable than other isomers in a wide temperature interval, and C(2v):011 should also be an important component. The electronic isomerization of C(56) (C(2v):011) and C(50) (D(5h):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 C(60).

Despite the importance of the Fluoromethyl radicals in combustion chemistry, very little experimental information on their reactions toward stable molecules is available in the literature. Motivated by recent laboratory characterization about the reaction kinetics of Chloromethyl radicals with NO 2 , we carried out a detailed potential energy survey on the CH 2 F + NO 2 reaction at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels as an attempt toward understanding the CH 2 F + NO 2 reaction mechanism. It is shown that the CH 2 F radical can react with NO 2 to barrierlessly generate adduct a (H 2 FCNO 2 ), followed by isomerization to b 1 (H 2 FCONO-trans) which can easily interconvert to b 2 (H 2 FCONO-cis). Subsequently, Starting from b (b 1 , b 2 ), the most feasible pathway is the C-F and N-O1 bonds cleavage along with N-F bond formation of b (b 1 , b 2 ) leading to P 1 (CH 2 O + FNO), or the direct N-O1 weak-bond fission of b (b 1 , b 2 ) to give P 2 (CH 2 FO + NO), or the 1,3-H-shift associated with N-O1 bond rupture of b 1 to form P 3 (CHFO + HNO), all of which may have comparable contribution to the reaction CH 2 F + NO 2 . Much less competitively, b 2 either take the 1,4-H-shift and O1-N bond cleavage to form product P 4 (CHFO + HON) or undergo a concerted H-shift to isomer c 2 (HFCONOH), followed by dissociation to P 4 . Because the rate-determining transition state (TSab 1 ) in the most competitive channels is only 0.3 kcal/mol higher than the reactants in energy, the CH 2 F + NO 2 reaction is expected to be rapid, and may thus be expected to significantly contribute to elimination of nitrogen dioxide pollutants. The similarities and discrepancies among the CH 2 X + NO 2 (X = H, F, and Cl) reactions are discussed in terms of the electronegativity of halogen atom. The present article may assist in future experimental identification of the product distributions for the title reaction, and may be helpful for understanding the halogenated methyl chemistry.

The radical-molecule reaction mechanism of CHCl(2) and CCl(3) with NO(2) have been explored theoretically at the B3LYP/6-311G(d,p) and MC-QCISD (single-point) levels. For the singlet potential energy surface (PES) of CHCl(2) + NO(2) reaction, the association of CHCl(2) with NO(2) was found to be a barrierless carbon-to-nitrogen approach forming an energy-rich adduct a (HCl(2)CNO(2)) followed by isomerization to b(1) (trans-cis-HCl(2)CONO), which can easily interconvert to b(2), b(3), and b(4). Subsequently, the most feasible pathway is the 1,3-chlorine migration associated with N-O1 bond cleavage of b(1) leading to P(1) (CHClO + ClNO). The second competitive pathway is the 1,4-chlorine migration along with N-O1 bond rupture of b(4) giving rise to P(2) (CHClO + ClON). Moreover, some of P(1) and P(2) can further dissociate to give P(6) (CHClO + Cl + NO). The lesser followed competitive channel is the 1,3-H-shift from C to N atom along with N-O1 bond rupture of b(1) to form P(3) (CCl(2)O + HNO). The concerted 1,4-H-shift accompanied by N-O1 bond fission of b(3) to product P(4) (CCl(2)O + HON) is even much less feasible. For the singlet PES of CCl(3) + NO(2) reaction, the only primary product is found to be P(1) (CCl(2)O + ClNO), which can lead to P(2) (CCl(2)O + Cl + NO) via dissociation of ClNO. The obtained major products CHClO and CCl(2)O for CHCl(2) + NO(2) and CCl(3) + NO(2) reactions, respectively, are in good agreement with kinetic detection in experiment. Compared with the singlet pathways, the triplet pathways may have less contributions to both reactions. Because the rate-determining transition state involved in the feasible pathways lie above the reactants R, the title reactions may be important in high-temperature processes. The similarities and discrepancies among the CH(n)Cl(3-n) + NO(2) (n == 0-2) reactions are discussed in terms of the substitution effect. The present study may be helpful for further experimental investigation of the title reactions.

The radical-molecule reaction mechanism of CH3 with NOx (x = 1, 2) has been explored theoretically at the B3LYP/6-311Gd,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-O bond cleavage of b (b1, b2) leading to P1 (CH3O + NO) or the 1,3-H-shift and N-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-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.

The complex doublet potential energy surface of the CHClNO system, including 31 minimum isomers and 84 transition states, is investigated at the QCISD(T)/6-311G(d, p)//B3LYP/6-31G(d, p) level in order to explore the possible reaction mechanism of the singlet CHCl with NO. Various possible isomerization and dissociation channels are probed. The initial association between 1CHCl and NO at the terminal N-site can almost barrierlessly lead to the chainlike adducts HClCNO a (a1, a2) followed by the direct Cl-extrusion to product P9 Cl + HCNO, which is the most feasible channel. Much less competitively, a (a1, a2) undergoes a ring-closure leading to the cyclic isomer c-C(HCl)NO d followed by a concerted Cl-shift and N-O cleavage of d to form the branched isomers ClNC(H)O f (f1, f2). Eventually, f (f1, f2) may take a direct H-extrusion to produce P7 H + ClNCO or a concerted 1,2-H-shift and Cl-extrusion to form P1 Cl + HNCO. The low-lying products P2 HCl + NCO, P3 Cl + HOCN, P14 HCO + 3NCl, P6 ClO + HCN, and P13 ClNC + OH may have the lowest yields observed. Our calculations show that the product distributions of the title reaction are quite different from those of the analogous 1CHF + NO reaction, yet are similar to those of another analogous 3CH2 + NO reaction. The similarities and discrepancies among the three reactions are discussed in terms of the substitution effect. The present article may assist in future experimental identification of the product distributions for the title reaction and may be helpful for understanding the halogenated carbene chemistry.

The structures, energetics, spectroscopies, and isomerization of possible low-lying Si2P2 isomers in both singlet and triplet states are theoretically investigated at the B3LYP/6-311G(d) and CCSD(T)/6-311+G(2df) (single-point) levels. At the final CCSD(T)/6-311+G(2df)//B3LYP/6-311G(d) level, the lowest energy isomer is a singlet butterfly-like SiPSiP structure 11 with P−P cross bonding followed by a singlet rhombic SiPSiP isomer 12 with Si−Si cross bonding, whereas the cyanogen analogue PSiSiP 15 is the highest lying of all the singlet isomers. The singlet potential energy surface of Si2P2 indicates that the rhombic isomer 12 is kinetically much more stable than the butterfly-like isomer 11, although isomer 12 is 3.2 kcal/mol higher in energy than isomer 11, while other isomers are kinetically unstable toward isomerization to isomer 11 or 12. It is also shown that the triplet Si2P2 isomers are energetically higher than all the single species except 15. Furthermore, for the most relevant singlet Si2P2 isom...

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 P(0) 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.