Ab Initio Study on the Potential Energy Surfaces of B4 Cluster

The singlet and triplet potential energy surfaces (PES) for the isomerization and dissociation reactions of B₄ isomers have been investigated using ab initio methods. Ten B4 isomers have been identified and of these 10 species, 4 have not been reported previously. The singlet rhombic structure ¹1 is found to be the most stable on the B₄ surface, in agreement with the results of previous reports. Several isomerization and dissociation pathways have been found. On the singlet PES, the linear ¹3b can rearrange to rhombus ¹1 directly, while ¹3c rearranges to ¹1 through two‐step reactions involving a cyclic intermediate. On the triplet PES, the capped triangle structure ³2 undergoes ring opening to the linear isomer ³3b with a barrier of 34.8 kcal/mol and 44.9 kcal/mol, and the latter undergoes ring closure to the square structure ³1 with a barrier of 30.4 kcal/mol and 33.0 kcal/mol at the MP4/6–311+G(3df)//MP2/6–311G(d) and CCSD/aug‐cc‐pVTZ//MP2/6–311G(d) levels of theory, respectively. The direct decomposition of singlet B₄ yielding to B₃+B is shown to have a large endothermicity of 87.3 kcal/mol (CCSD), and that producing 2B₂ to have activation energy of 133.4 kcal/mol (CCSD).     

Theoretical study on structures and stability of triplet SiC<Subscript>3</Subscript>O isomers

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 . 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. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Theoretical study on the potential energy surface of NC<Subscript>3</Subscript>P isomers

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.     

Theoretical study on structures and stability of HC2P isomers

 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 ³ 1 in the ³∑⁻ state. The second low-lying isomer has a CPC ring with exocyclic CH bonding ¹ 5 in a singlet state at 10.5 kcal/mol. The following third and fourth low-lying isomers are a singlet bent HCCP structure ¹ 1 at 20.9 kcal/mol and a bent singlet HPCC structure ¹ 3 at 35.8 kcal/mol, respectively. Investigation of the HC₂P potential-energy surface indicates that in addition to the experimentally known isomer ³ 1, the other isomers ¹ 1, ¹ 3 and ¹ 5 also have considerable kinetic stability and may thus be observable. However, the singlet and triplet bent isomers HCPC ¹ 2 and ³ 2 as well as the triplet bent isomer HPCC ³ 3 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 ¹ 1, ¹ 3 and ¹ 5 either in the laboratory or in interstellar space. Received: 5 November 2000 / Accepted: 25 November 2001 / Published online: 8 April 2002     

Theoretical study of the Si2NO potential energy surface

The structures, spectroscopies, and stabilities of the doublet Si₂NO radical are explored at the density functional theory (DFT) and ab initio levels. Seventeen isomers are located, connected by 26 interconversion transition states. At the CCSD(T)/6‐311+G(2df)//QCISD/6‐311G(d)+ZPVE level, three low‐lying isomers are predicted, that is, one bent species SiNSiO 3 (5.1 kcal/mol) containing the important Si?N triple bonding and two four‐membered ring isomers including cyclic cSiNSiO 1 (0.0) with Si? Si cross‐bonding with C_2v symmetry and puckered cSiNSiO 1′ (11.9) with divalent carbene character. Three low‐lying isomers 1, 1′, and 3 have reasonable kinetic stabilities and might be observable either experimentally or astrophysically. The possible formation strategies of 1, 1′, and 3 in laboratory and in space are discussed in detail. The calculated vibrational frequencies and possible formation processes of 3 are consistent with recent experimental observations. In light of the fact that no cyclic nitrogen‐containing species have been detected in space, two cyclic isomers 1 and 1′ could be promising candidates. Furthermore, the bonding nature of three isomers 1, 1′, and 3 is analyzed. The calculated results are also compared with those of the analogue C₂NO radical. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Structure and Stability of Interstellar Molecule C_3S

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Ab initio MO study of potential energy surface of NH2 with CN reaction

An extensive quantum chemical study of the potential energy surfaces (PES) for the association reaction of NH₂ with CN and the subsequent isomerization and dissociation reactions has been carried out using density functional theory (DFT)/B3LYP/6‐311++G(3df,2p) level of theory on both singlet and triplet states. The reaction mechanism on the triplet surface is more complicated than that on the singlet surface. A total of 19 isomers and 46 transition states have been identified and characterized on the triplet PES. Among them, IM2 (IM2a), IM3 (IM3a, IM3b), and IM10 are the lowest‐lying isomers with thermodynamic stability. Twenty available dissociation channels, depending on the different initial isomers, have been identified. On the singlet surface, only 12 isomers and 16 transition states have been found, and among them IM1(S) and IM2(S) are the lowest‐lying isomers. The higher isomerization and dissociation barriers on the singlet surface indicate that the addition and the subsequent reactions of NH₂+CN are most likely to occur on the triplet PES because of the lower barriers. A prediction can be made for the possible mechanism explaining the production of H+HNCN. Besides HNCN, other major products are NH+HCN and NH+HNC, which are produced by direct dissociation reactions from triplet IM2 and IM3, respectively. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Theoretical study on the structures, isomerization and stability of SiC3H isomers

The calculations of the geometry optimizations, energies, dipole moments, vibrational spectra, rotational constants, and isomerization of doublet SiC₃H species were performed using density functional theory and ab initio methods. Four types of isomers, a total of 18 minima, connected by 16 interconversion transition states, were located on the potential energy surface (PES) at the B3LYP/6-311G (d, p) level. More accurate energies were obtained at the CCSD(T)/6-311G(2df, 2p), and G3(MP2) levels. With the highest isomerization barrier, the lowest lying structure, linear A1 possesses the largest kinetic stability. Besides, the isomerization barriers of A2, A4, C2, F1, F4 and F5 are over 10 kcal/mol, and these isomers are also considered to be higher kinetically stable. Other isomers cannot be kinetically stabilized with considerably low isomerization barriers. Investigation on the bonding properties and the computations of vibrational spectra, dipole moments, and rotational constants for SiC₃H isomers are helpful for understanding their structures and also valuable for their detections in the interstellar space and laboratory.     

Theoretical study on the reaction mechanisms of CH2SH + NO reaction

The mechanisms for the CH₂SH + NO reaction were investigated on both of the singlet and triplet PES at the BMC-CCSD//B3LYP/6-311+G(d,p) level. The results indicate that the singlet PES is much lower than the triplet PES energetically; therefore, the reaction occurs on the singlet PES dominantly. The most favorable channel on the singlet PES takes place by a barrierless addition of N atom to CH₂SH radical to form HSCH₂NO. Subsequently, the rearrangement of the initial adduct HSCH₂NO (IM1) to form another intermediate IM3 via a four-center transition state, followed by the C–O bond fission in IM3 leading to the major product CH₂S + HNO. Due to high barriers, other product including HC(N)SH + HO, HON + CH₂S, and HNO + CHSH could be negligible. The direct abstraction channel was also determined to yield CH₂S + HON. With high barrier (33.3 kcal/mol), it is not competitive with the addition channel, in which all stationary points are lower than reactant energetically. While on the triplet PES, with the lowest barrier height (18.8 kcal/mol), the direct N-abstracted channel to form CH₂S + HNO is dominant. However, it is not competitive with the channels on the singlet PES. Our results are in good accordance with experimental conclusions that the reaction proceeds via addition mechanism.     

Isomerization study of C5H5NO molecules

The complex potential energy surface (PES) for the isomerization of C₅H₅NO species, including 18 isomers and 23 interconversion transition states, is probed theoretically at the B3LYP/6‐311++G(d,p) and MP2//B3LYP/6‐311++G(d,p) levels of theory. The geometries and relative energies for various stationary points were determined. The zero‐point vibrational energy (ZPVE) corrections have been made to calculate the reliable energy. We predicted a six‐membered ring structure as a global minima isomer I, which is 118.49 and 131.48 kcal · mol^?1 more stable than the least stable, four‐ and three‐membered ring isomer VIII at B3LYP and MP2//B3LYP levels of theory, respectively. The isomers and interconversion transition states have verified by frequency calculation. The intrinsic reaction coordinates (IRC) calculations have been performed to confirm that each transition state is linked by the desired reactants and products. The isomer stability has been studied using relative energies, chemical hardness, and chemical potential. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Mechanism and rate constants for ammonia photochemical oxidation with O2 on the singlet and triplet potential energy surfaces

The mechanism and thermodynamic of NH₃ + O₂ reaction on the singlet and triplet potential energy surfaces (PES), were carried out using the RMP2 and CCSD (T)//RMP2 theoretical approaches in connection with the 6-311++G(d, p) basis set. Three pre-reactive complexes, ¹C1, ¹C2, and ³C1 on the singlet and triplet PES were formed between ammonia and molecular oxygen. With variety of pre-reactive complexes, six types of products are obtained, of which two types are found to be thermodynamically stable. The mechanistic properties of all products channels are discussed. Results show that production of HONO + H₂ and HN(OH)₂ are the main reaction channels in thermodynamic viewpoint with the Gibbs free energy of ?G° = ?34.681 and ?27.153 kcal/mol, respectively. Rate constants of the title reaction over the temperature range of (200–1000 K) show kinetic products are different from thermodynamic products.     

Structural and energetic exploration of a boron‐rich sulfide cluster B6S

Boron and mixed‐boron clusters have received considerable attention because of their wide applications and their essential roles in advancing chemical bonding models. Bearing the bright prospects as building blocks to form novel polymeric materials, the sulfur‐rich boron sulfides have been greatly studied. However, the knowledge of the boron‐rich boron sulfides is much rare. In this article, we report an extensive theoretical study on the structural, energetic, and stability features of a hitherto unknown septa‐atomic cluster B₆S at the CCSD(T)/6‐311+G(2df)//B3LYP/6‐311+G(d) level. The local minimum isomers were obtained through our recently developed program “grid‐based comprehensive isomeric search algorithm.” The results show that the planar knife‐like isomer B₅(?BS) 01 (0.0 kcal/mol) containing the ?BS moiety is the lowest energy, followed by the quasi‐planar belt‐like isomer B₆(>S) 02 (6.7 kcal/mol) and the pyramid‐like isomer B₆(>S) 03 (8.4 kcal/mol). Notably, the three singlet isomers all have good kinetic stability on the basis of the potential energy surface analysis. The B/S‐centered wheel‐like isomers are unfavorable in thermodynamics and kinetics. The triplet state structures generally can not compete with the singlet ones. The results are compared to the analogous and isoelectronic cluster B₆O. Our work is expected to provide useful information for understanding the structures and stability of boron sulfides. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Theoretical study of [Si,O,C,O] species: Prediction of new species on triplet potential energy surface

The intermediates [Si,O,C,O] of the Si + CO₂ reaction have been studied in detail using high level ab iniitio methods. Both singlet and triplet [Si,O,C,O] species are characterized structurally and energetically. On the singlet potential energy surface (PES), the vdw‐OSi–CO isomer and in the triplet PES, the bent‐SiOCO isomer is found to be thermodynamically as well as kinetically most stable species. All possible isomerization transition states (TS) are located on both singlet and triplet potential surfaces. On the triplet surface, the stability of the trans‐OSiCO isomer is comparable with that of the bent‐SiOCO isomer. A non‐planar cis‐SiOCO isomer is located on the triplet PES, which is predicted for the first time. Heats of formation at 0 K (Δ_fH°, 0 K) for all singlet and triplet species are computed using G3B3, G3MP2, and CBS‐Q theories. The discrepancy between G3B3 and the other two methods for the heat of formation value for triplet trans‐OSiCO is discussed. The PESs for singlet as well as triplet species with their dissociation asymptotes are explored at the CCSD(T)/6‐311G(d,p)//MP2/6‐311G(d,p) level of theory. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Halogen switching of azacarbenes C2NH ground states at ab initio and DFT levels

Relative stabilities and singlet–triplet energy differences are calculated for 24 C₂NX azacarbenes (where X is H, F, Cl, and Br). Three skeletal arrangements are employed including azacyclopropenylidene, [(imino)methylene]carbene, and cyanocarbene. Halogens appear to alternate the electronic ground states of C₂NH azacarbenes, from triplet to singlet states, at MP3/6‐311++G**, B1LYP/6‐311++G**, B3LYP/6‐311++G**, MP2/6‐311++G**, MP4(SDTQ)/6‐311++G**, QCISD(T)/6‐311++G**, CCSD(T)/6‐311++G**, CCSD(T)/cc‐pVTZ, G1, and G2 levels of theory. The aromatic characters of singlet cyclic azacyclopropenylidenes are measured using GIAO–NICS calculations. Linear correlations are found between the B3LYP/6‐311++G** calculated LUMO–HOMO energy gaps (ΔE_{HOMO ‐ LUMO}) of the singlet carbenes versus their corresponding singlet–triplet energy separations (ΔE). Electrophilic characters are found for all singlet azacarbenes in their addition reactions to alkenes with the highest electrophilicity being exhibited for X = F. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:377–388, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20442     

A theoretical comparison of the lowest-lying singlet and triplet states of H2CBe and of HCBeH

The low-lying singlet and triplet states of H₂CBe and HCBeH are examined using ab inito molecular orbital theory. In agreement with earlier results, the lowest-lying structure of H₂CBe has C_2v symmetry and is a triplet with one π electron (³ B₁). The results presented here suggest that the lowest-energy singlet structure is the (¹B₁) open-shell singlet, also with C_2v symmetry, at least 2.5 kcal/mol higher in energy. The singlet C_2v structure with two π electrons (¹A₁) is 15.9 kcal/mol higher than ³B₁. All of these structures are bound with respect to the ground state of methylene and the beryllium atom. In HCBeH, linear equilibrium geometries are found for the triplet (³Σ) and singlet (¹Δ) states. The triplet is more stable than the singlet (¹Δ) by 35.4 kcal/mol, and is only 2.9 kcal/mol higher in energy than triplet H₂ CBe. Since the transition structure connecting these two triplet molecules is found to be 50.2 kcal/mol higher in energy than H₂ CBe, both triplet equilibrium species might exist independently. The harmonic vibrational frequencies of all structures are also reported.     

The DFT study on mechanisms for NCO with C2H5 reaction

A detailed investigation has been performed at the QCISD(T)/6‐311++G(d,p)//B3LYP/6‐311+G(d,p) level for the reaction of NCO with C₂H₅ by constructing singlet and triplet potential energy surfaces (PES). The results show that the title reaction is more favorable on the singlet PES than on the triplet PES. On the singlet PES, the initial addition processes are barrierless and release lots of energy. The dominant channel occurs via the fragmentations of the initial adduct C₂H₅NCO and C₂H₅OCN to form C₂H₄ + HNCO and HOCN, respectively. With higher barrier heights, other products such as CH₄ + HNC + CO, CH₃CHNH + CO, CH₃CH + HNCO, and CH₃CN + H₂ + CO are less competitive. On the triplet PES, the entrance reactions surpass significant barriers; therefore, it could be negligible at the normal atmospheric condition. However, the most feasible channel on the triplet PES is the direct hydrogen abstraction channel to form CH₂CH₂ + HNCO. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Theoretical Studies on Reaction Mechanism of CH3SO with NO

The potential energy surface （PES） of CH3SO radical with NO reaction has been studied at MP2/6-311G（2df, p） and QCISD/6-311G（2df, p） levels. Geometries of the reactants, transition states （TS） and products were optimized at B3LYP/6-311G （d,p） level. The geometries of the transition states were found for the first time. The calculated results show that the reaction can proceed via singlet-state or triplet-state PES. Because of the high energy barrier of triplet surface, the singlet surface reactions are dominant. The topological analysis of electron density shows that there are two kinds of structaral transition states （the bifurcation-type ring structure transition state and the T-shaped conflict structure transition state） in the titled reaction. The total electronic density of the reactants, TS and products and the spin electronic density on the triplet surface were also discussed in this paper.     

Theoretical study on the structures, isomerization and stability of SiC4 isomers

The structures, energetics, dipole moments, vibrational spectra, rotational constants, and isomerization of singlet SiC₄ isomers were explored using ab initio methods. Five types of isomers, a total of 11 minima, connected by 11 interconversion transition states, were located on the potential energy surface at the MP2/6-311G(d, p) level. More accurate energies were obtained at the G3(MP2) level. With the highest isomerization barrier, a C2v tetra-angular cone possesses the largest kinetic stability. The lowest-lying structure, linear SiCCCC is also highly kinetically stabilized. Besides, D2d bicyclic c-Si(CC)₂, C2v five-membered ring c-SiCCCC, another C2v tetra-angular cone isomer and C3v trigonal bipyramid isomer are also considered to be kinetically stable, because their isomerization barriers are all over 10 kcal/mol. Other isomers cannot be kinetically stabilized with considerably low isomerization barriers. Investigation on the vibrational spectra, dipole moments, and rotational constants for SiC₄ isomers are valuable for their detections in the interstellar space and laboratory.     

Computational mechanistic study of methanol and molecular oxygen reaction on the triplet and singlet potential energy surfaces

The reaction of CH₃OH with the O₂ on the triplet and singlet potential energy surfaces (PES) was carried out using the B3LYP, MP2, and CCSD(T)//B3LYP theoretical approaches in connection with the 6-311++G(3df–3pd) basis set. Three pre-reactive complexes, ¹C1, ¹C2, and ³C1, on the singlet and triplet PES were formed between methanol and molecular oxygen. From a variety of the complexes, seven types of products are obtained, of which four types are found to be thermodynamically stable. Results reveal that there exists one intersystem crossing between triplet and singlet PES. For P4 adduct that is the main and kinetically the most favorable product, the rate constants are calculated in the temperature range of 200–1,000 K in the reliable pathway.     

Computational mechanistic investigation of the gas phase C2H4 + CO reaction on the singlet and triplet potential energy surfaces

Reaction pathways of ethylene and carbon monoxide on the singlet and triplet potential energy surfaces (PESs) have been calculated at B3LYP/6-311++G (3df, 3dp), G3B3 and CCSD(T)//B3LYP levels. Reaction mechanisms have been investigated by analysis of various structures. Suggested reaction mechanisms reveal that ³P3(CH₂CHCHO) and ³P4(CH₃CCHO) are thermodynamically stable adducts with the negative value in Gibbs free energies on the triplet PES. In addition, results show that one intersystem crossing exists between triplet and singlet PESs, which are obtained by scanning of the C–C bond length in ¹IN3 and ³IN7 species.