Ab initio fragment orbital‐based theory

A new formulation of ab initio theory is presented that treats a large molecule in terms of wave functions of its constituent molecular subunits (to be called fragments). The method aims to achieve near conventional ab initio accuracy but using a truncated set of fragment orbitals with a consequent drastic reduction of computing time and storage requirement. Illustrative calculations are presented for the molecule amino‐nitro‐stilbene. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Ab initio chemical kinetics for the unimolecular decomposition of Si2H5 radical and related reverse bimolecular reactions

For plasma enhanced and catalytic chemical vapor deposition (PECVD and Cat‐CVD) processes using small silanes as precursors, disilanyl radical (Si₂H₅) is a potential reactive intermediate involved in various chemical reactions. For modeling and optimization of homogeneous a‐Si:H film growth on large‐area substrates, we have investigated the kinetics and mechanisms for the thermal decomposition of Si₂H₅ producing smaller silicon hydrides including SiH, SiH₂, SiH_3, and Si₂H₄, and the related reverse reactions involving these species by using ab initio molecular‐orbital calculations. The results show that the lowest energy path is the production of SiH + SiH₄ that proceeds via a transition state with a barrier of 33.4 kcal/mol relative to Si₂H₅. Additionally, the dissociation energies for breaking the Si? Si and H? SiH₂ bonds were predicted to be 53.4 and 61.4 kcal/mol, respectively. To validate the predicted enthalpies of reaction, we have evaluated the enthalpies of formation for SiH, SiH₂, HSiSiH₂, and Si₂H₄(C_2h) at 0 K by using the isodesmic reactions, such as ²HSiSiH₂ + ¹C₂H₆→¹Si₂H₆ + ²HCCH₂ and ¹Si₂H₄(C_2h) + ¹C₂H₆ → ¹Si₂H₆ + ¹C₂H₄. The results of SiH (87.2 kcal/mol), SiH₂ (64.9 kcal/mol), HSiSiH₂ (98.0 kcal/mol), and Si₂H₄ (68.9 kcal/mol) agree reasonably well previous published data. Furthermore, the rate constants for the decomposition of Si₂H₅ and the related bimolecular reverse reactions have been predicted and tabulated for different T, P‐conditions with variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory by solving the master equation. The result indicates that the formation of SiH + SiH₄ product pair is most favored in the decomposition as well as in the bimolecular reactions of SiH₂ + SiH₃, HSiSiH₂ + H₂, and Si₂H₄(C_2h) + H under T, P‐conditions typically used in PECVD and Cat‐CVD. © 2013 Wiley Periodicals, Inc.     

Ab initio molecular dynamics simulations study on initial decompositions of β‐HMX at high temperature coupled with high pressures

We performed ab initio molecular dynamics simulations to investigate initial decomposition mechanisms and subsequent chemical processes of β‐HMX (cyclotetramethylene tetranitramine) (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) crystals at high temperature coupled with high pressures. It was found that the initial decomposition step is the simultaneous C–H and N–NO₂ bond cleavage at 3,500 K. When the pressure (1–10 GPa) is applied, the first reaction steps are primarily the C–N and C–H bond fission at 3,500 K. The C–H bond cleavage is a triggering decomposition step of the HMX crystals at 3,500 K coupled with 16 GPa. This indicates that the C–H bonds are much easier to be broken and the hydrogen radicals are much more active. The applied pressures (1–10 GPa) accelerate the decompositions of HMX at 3,500 K. The decomposition pathways and time evolution of the main chemical species demonstrate that the temperature is the foremost factor that affects the decomposition at high pressures (1–10 GPa). However, the decomposition of HMX is dependent on both the temperature (3,500 K) and the pressure (16 GPa). This work will enrich the knowledge of the decompositions of condensed energetic materials under extreme conditions.     

第一激发态HOOO裂解反应的理论研究

利用量子化学方法对第一激发态HOOO的裂解反应进行了理论研究.所有驻点(反应物、产物和过渡态)的几何结构优化和振动分析都是在CASSCF/6-31 G(d,p)理论水平下进行的.反应路径上的选择点及驻点都在CASPT2/6-31 G(d,p)//CASSCF(19,13)/6-31 G(d,p)和MRCI/6-31 G(d,p)//CASSCF(19,13)/6-31 G(d,p)理论水平下进行单点能量校正.CASSCF,CASPT2和MRCI水平的理论计算结果显示,第一激发态HOOO的裂解反应包含一个对称性变化的过程.它首先通过了一个Cs对称性的过渡态,然后逐渐变化为线性结构,最终生成产物O2(3Σg-)和OH(2Π).     

Theoretical investigation of unimolecular decomposition channels of furan4

Density functional theory and high-level ab initio calculations were carried out to investigate three unimolecular decomposition channels of furan. All equilibrium and transition state structures along the proposed decomposition channels are fully optimized by B3LYP/6-31G** and characterized at the same level of theory by vibrational and intrinsic reaction coordinate analyses. Relative energies of the optimized structures were evaluated at theoretical levels up to QCISD(T)/6-311++G**. The theoretical results suggest that the unimolecular decomposition channel of isoxazole, proposed in an experimental study and implied to be the main decomposition channel of furan, is responsible only for the formation of HC(TRIPLE BOND)CH and H₂O(DOUBLE BOND)C(DOUBLE BOND)O, minor products of furan thermal decomposition. A new decomposition mechanism, proposed in the present study, is shown to be more likely responsible for the formation of CH₃C(TRIPLE BOND)CH and CO, major products of furan thermal decomposition. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 240–249, 1998     

Ab initio study on the photochemical behavior of styrene

Ab initio complete active space self‐consistent field (CASSCF) and the second order multireference Møller‐Plesset calculations have been performed to examine the photochemical behavior of styrene upon the strong S₀‐S₂ electronic excitation in the low‐lying excited states. The optimized structure at the S₂/S₁ conical intersection (CIX) is characterized by a quinoid structure. The transition state (TS) in S₁ is in the vicinity of the S₂/S₁‐CIX. At the S₁‐TS, two reaction paths branch. One is the relaxation into the stable structure in S₁ and then emission into S₀. The other is the radiationless decay through the S₁/S₀‐CIX. © 2002 Wiley Periodicals, Inc. J Comput Chem 10: 950–956, 2002     

Ab initio chemical kinetics for reactions of H atoms with SiHx (x = 1–3) radicals and related unimolecular decomposition processes

Hydrogen atoms and SiH_x (x = 1–3) radicals coexist during the chemical vapor deposition (CVD) of hydrogenated amorphous silicon (a‐Si:H) thin films for Si‐solar cell fabrication, a technology necessitated recently by the need for energy and material conservation. The kinetics and mechanisms for H‐atom reactions with SiH_x radicals and the thermal decomposition of their intermediates have been investigated by using a high high‐level ab initio molecular‐orbital CCSD (Coupled Cluster with Single and Double)(T)/CBS (complete basis set extrapolation) method. These reactions occurring primarily by association producing excited intermediates, ¹SiH₂, ³SiH₂, SiH₃, and SiH₄, with no intrinsic barriers were computed to have 75.6, 55.0, 68.5, and 90.2 kcal/mol association energies for x = 1–3, respectively, based on the computed heats of formation of these radicals. The excited intermediates can further fragment by H₂ elimination with 62.5, 44.3, 47.5, and 56.7 kcal/mol barriers giving ¹Si, ³Si, SiH, and ¹SiH₂ from the above respective intermediates. The predicted heats of reaction and enthalpies of formation of the radicals at 0 K, including the latter evaluated by the isodesmic reactions, SiH_x + CH₄ = SiH₄ + CH_x, are in good agreement with available experimental data within reported errors. Furthermore, the rate constants for the forward and unimolecular reactions have been predicted with tunneling corrections using transition state theory (for direct abstraction) and variational Rice–Ramsperger–Kassel–Marcus theory (for association/decomposition) by solving the master equation covering the P,T‐conditions commonly employed used in industrial CVD processes. The predicted results compare well experimental and/or computational data available in the literature. © 2013 Wiley Periodicals, Inc.     

五氧化二氮和水反应的从头算研究

在臭氧空洞形成过程中,极地云是一个重要的条件。为了理解极地云的形成和性质,采用不同水平的从头计算方法对N~2O~5+H~2O→2HNO~3反应进行了理论研究。在QCISD(T)//MP2/6-311G(d,p)理论水平上,该气相反应的焓变△H^0=-39.5kJ·mol^-^1,自由能变化△G^0=-36.1kJ/mol,活化能E~a=88.2kJ·mol^-^1(298K)。计算的结果与五氧化二氮易潮解的性质相符。     

β‐Aminoacrolein: An ab initio,AIM and NBO study

The molecular structure and the intramolecular hydrogen bonding of β‐aminoacrolein and its simple derivatives were investigated at the MP2 and B3LYP levels of theory using the standard 6‐311++G(d, p) basis set. The “atoms in molecules” or AIM theory of Bader which is based on topological properties of the electron density (ρ), was used. Additionally, an analysis of the critical points was performed to study the nature hydrogen bonding in these systems. Natural bond orbital (NBO) analysis was also carried out for to better comprehend the nature of the intramolecular interactions in β‐aminoacrolein and its derivatives. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Ab initio prediction of the vibrational spectra of the hydrogen‐bonded complexes between CO and HONO2

The vibrational characteristics (vibrational frequencies and infrared intensities) for free and complexed CO and HONO₂ have been predicted using ab initio calculations at SCF and MP2 levels with different basis sets and B3LYP/6?31G(d,p) calculations. The ab initio calculations show that the complexation between HONO₂ and CO leads to two stable structures: CO … HONO₂ (1A) and OC … HONO₂ (1B). The changes in the vibrational characteristics from free monomers to complexes have been estimated. It was established that the most sensitive to the complexation is the stretching O? H vibration. In agreement with the experiment, its vibrational frequency in the complexes is shifted to lower frequency (Δν = ?123 cm^?1). The magnitude of the wave number shift is indicative of relatively strong hydrogen‐bonded interaction. The ab initio calculations at different levels predict an increase of the infrared intensity of the stretching O? H vibration for structure 1A more than five times and for structure 1B more than nine times. The most consistent agreement between the computed values of the frequency shifts for structure 1B and those experimentally observed suggests that this structure is preferred. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Single‐ and double‐electron reductions of CO2 by using superalkalis: An ab initio study

CO₂, a major contributor to global warming, can be balanced by converting it into fuels. The reduction of CO₂ has been difficult due to its extremely high stability. Recently, single‐electron reduction of CO₂ by superalkalis has been proposed using quantum chemical methods. Herein, we report a systematic study on the single‐reduction of CO₂ by using typical superalkalis. Superalkalis are hypervalent species possessing lower ionization energies than alkali atoms. We have studied the interaction of CO₂ with FLi₂, OLi₃, and NLi₄ superalkalis using ab initio MP2 calculations. We notice that this interaction leads to stable superalkali‐CO₂ complexes in which the structure of CO₂ is bent due to electron transfer from superalkalis. This clearly reveals that the CO₂ can successfully be reduced to the anion. It has been also noticed that the size of superalkalis plays a crucial in the single‐electron reduction of CO₂. For instance, the binding energy of superalkali‐CO₂ complex and charge transfer to CO₂ decreases monotonically with the increase in the size of superalkali. We have also proposed that CO₂ can be further reduced to in case of the anionic complex such as (FLi₂ CO₂)‾. Thus, FLi₂ superalkali is also capable of double‐electron reduction of CO₂. These findings should provide new insights into CO₂‐activation as well as motivate further research in this direction.     

Spectroscopic constants and molecular properties of rare‐gas diatomic molecule in Lennard‐Jones potential: Ab initio and density functional study

Ab initio and density functional theory (DFT) are applied to study the spectroscopic constants, molecular properties, and nature of force between two rare gas atoms of the weakly bound diatomic molecules He₂, Ne₂, Ar₂, HeNe, and HeAr in the Lennard‐Jones potential. A simple method is developed to calculate the spectroscopic constants of these molecules. The calculated spectroscopic constants and molecular properties agree very well with the experimental and theoretical results wherever available. Most of the spectroscopic constants and molecular properties are reported for the first time. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

四唑及其衍生物的理论研究 2: 氯代四唑负离子的从头算

用从头计算法, 取6-21G基组, 在MP2水平上, 计算研究了1H-和2H-四唑一氯取代物三种负离子的全优化几何构型和电子结构, 比较讨论了它们的芳香性和稳定性。发现三者均取平面构型, 其芳香性和稳定性次序为: 5-氯四唑负离子>2-氯四唑负离子>1-氯四唑负离子。预示了形成金属配合物时5-氯四唑作为配体的重要性和配位方式。     

Ab initio study of the transition-metal carbene cations

The geometries and bonding characteristics of the first-row transition-metal carbene cations MCH_2~ were investigated by ab initio molecular orbital theory (HF/LANL2DZ). All of MCH_2~ are coplanar. In the closed shell structures the C bonds to M with double bonds; while in the open shell structures the partial double bonds are formed, because one of the σ and π orbitals is singly occupied. It is mainly the π-type overlap between the 2p_x orbital of C and 4p_x, 3d_(xz), orbitals of M~ that forms the π orbitals. The dissociation energies of C—M bond appear in periodic trend from Sc to Cu. Most of the calculated bond dissociation energies are close to the experimental ones.     

Photodissociation of 1,2‐Dibromoethylene at 248 nm: Br2 Molecular Elimination Probed by Cavity Ring‐Down Absorption Spectroscopy

The Br₂ elimination channel is probed for 1,2‐C₂H₂Br₂ in the B –X transition upon irradiation at 248 nm by using cavity ring‐down absorption spectroscopy (CRDS). The nascent vibrational population ratio of Br₂(v=1)/Br₂(v=0) is obtained to be 0.7±0.2, thus indicating that the Br₂ fragment is produced in hot vibrational states. The obtained Br₂ products are anticipated to result primarily from photodissociation of the ground‐state cis isomer via four‐center elimination or from cis/trans isomers via three‐center elimination, each mechanism involving a transition state that has a Br? Br distance much larger than that of ground‐state Br₂. According to ab initio potential energy calculations, the pathways that lead to Br₂ elimination may proceed either through the electronic ground state by internal conversion or through the triplet state by intersystem crossing. Temperature‐dependence measurements are examined, thereby supporting the pathway that involves internal conversion—which was excluded previously by using product translational spectroscopy (PTS). The quantum yield for the Br₂ elimination reaction is determined to be 0.12±0.1, being substantially contributed by the ground‐state Br₂ product. The discrepancy of this value from that (of 0.2) obtained by PTS may rise from the lack of measurements in probing the triplet‐state Br₂ product.     

DFT study of the gas‐phase thermal decomposition kinetics of 2‐ethoxypyridine into 2‐pyridone

The mechanism of the gas‐phase elimination kinetics of 2‐ethoxypyridine has been studied through the electronic structure calculations using density functional methods: B3LYP/6‐31G(d,p), B3LYP/6‐31++G(d,p), B3PW91/6‐31G(d,p), B3PW91/6‐31++G(d,p), MPW1PW91/6‐31G(d,p), MPW1PW91/6‐31++G(d,p), PBEPBE/6‐31G(d,p), PBEPBE/6‐31++G(d,p), PBE1PBE1/6‐31G(d,p), and PBE1PBE1/6‐31++G(d,p). The elimination reaction of 2‐ethoxypyridine occurs through a six‐centered transition state geometry involving the pyridine nitrogen, the substituted carbon of the aromatic ring, the ethoxy oxygen, two carbons of the ethoxy group, and a hydrogen atom, which migrates from the ethoxy group to the nitrogen to give 2‐pyridone and ethylene. The reaction mechanism appears to occur with the participation of π‐electrons, similar to alkyl vinyl ether elimination reaction, with simultaneous ethylene formation and hydrogen migration to the pyridine nitrogen producing 2‐pyridone. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Ab initio study of sphere-like mesogen properties

Sphere-like compound C8H17Si(OPhC12H25)3N (1) forms mesophases. In order to investigate the relationship between molecular structure and liquid crystal properties, structural studies are carried out on the model molecules of compound 1 and its substituted derivatives using ah initio calculations. The results show that the cyano or chloro substituted tribenzosilatrane compounds R1Si(OPhR2 )3N (R1 R2 = CN or Cl) have much bigger dipole moments or anisotropy of polarizability and more like sphere than the corresponding alkyl substituted compounds. Cyano or chloro substituted tribenzosilatranes would be better candidates for sphere-like meso-gens.     

Ab initio study on the electronic structures of styrene in the Franck-Condon region

The electronic structures of styrene in the Franck‐Condon region have been theoretically examined by means of ab initio complete active space self‐consistent field (CASSCF) and the second order multireference Møller‐Plesset calculations. The optimized structure of styrene in S₀ is planar but the torsional motion of the phenyl group is very floppy. The S₁ state is assigned to the local π–π* excitation within the benzene ring. On the other hand, S₂, above S₁ by 0.561 eV, is assigned to a state that resembles the so‐called V‐state of ethylene. The transition intensity of S₀–S₁ is weak, while that of S₀–S₂ is strong. This is in good agreement with the experimental absorption spectrum where the S₀–S₁ and S₀–S₂ transitions are in the energy range of 290–220 nm. The optimized geometry of S₁, characterized by an enlarged benzene ring and its vibrational analyses, further justifies the assignment of the S₁ state. © 2002 Wiley Periodicals, Inc. J Comput Chem 9: 928–937, 2002     

Ab initio and semiempirical computational studies on 1‐{(2E)‐2‐[(aminocarbonothioyl)hydrazono]‐2‐(3‐mesityl‐3‐methylcyclobutyl)ethyl}‐pyrrolidine‐2,5‐dione

The molecular geometry, vibrational frequencies, and gauge including atomic orbital (GIAO) ¹H‐ and ¹³C NMR chemical shift values of the title compound in the ground state have been calculated using the Hartree‐Fock (HF) and density functional theory (DFT) methods with 6‐31G(d) basis sets, and compared with the experimental data. The calculated results show that the optimized geometries can well reproduce the crystal structural parameters and the theoretical vibrational frequencies, and ¹H‐ and ¹³C NMR chemical shift values show good agreement with experimental data. To determine conformational flexibility, the molecular energy profile of the title compound was obtained by semiempirical (AM1) calculations with respect to the selected torsion angle, which was varied from ?180° to +180° in steps of 10°. The energetic behavior of the title compound in solvent media was examined using the B3LYP method with the 6‐31G(d) basis set by applying the Onsager and the polarizable continuum model (PCM). The results obtained with these methods reveal that the PCM method provided more stable structure than Qnsager's method. By using TD‐DFT method, electronic absorption spectra of the title compound have been predicted and a good agreement with the TD‐DFT method and the experimental one is determined. The predicted nonlinear optical properties of the title compound are much greater than ones of urea. In addition, the molecular electrostatic potential (MEP), frontier molecular orbitals (FMO) analysis, NBO analysis and thermodynamic properties of the title compound were investigated using theoretical calculations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Geometrical consequences of resonance-assisted intramolecular hydrogen-bond formation from Ab initio MO calculations on 2-nitroresorcinol

Ab initio orbital calculations on phenol, nitrobenzene, and 2-nitroresorcinol have been performed with the GAUSSIAN 92 series of programs. Initial RHF/6-31G^* and RHF/6-31G^** optimizations were followed by second-order MØller-Plesset MP2(FC)/6-31G^* optimizations. The general geometrical features of these molecules, and, in particular, the characteristic changes as going from phenol to 2-nitroresorcinol and from nitrobenzene to 2-nitroresorcinol are in good agreement with recent gas-phase electron diffraction studies and with the notion of resonance-assisted intramolecular hydrogen-bond formation in 2-nitroresorcinol.