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     

PH~3...H~2O体系分子间相互作用的从头计算MO研究

在MP2/6-311++G(3d,3p)水平,对PH~3...H~2O体系可能存在的氢键复合物进行了全自由度能量梯度优化,发现PH~3...H~2O体系存在三个能量极小结构A,B和C。其中结构A和B以H~2O作为质子授体,结构C以PH~3作为质子授体,结构A较结构B和C分别稳定6.52kJ/mol和8.18kJ/mol。结构A具有C~s对称性,其结构中P原子和O原子间距离为354.78nm,键角OHP为171.35ⅲ,属于接近于直线的传统型氢键结构。进一步在高级电子相关校正的MP4SDTQ下,用6-311++G(3df,3pd)基加上键函数{3s3p2d1f},通过BSSE校正,精确计算了结构A的结合能为-10.84kJ/mol。     

Ab initio study of the solvent H-bonding effect on ESIPT reaction and electronic transitions of 3-hydroxychromone derivatives

The electronic transitions occurring in 4-(N,N-dimethylamino)-3-hydroxyflavone (DMAF) and 2-furanyl-3-hydroxychromone (FHC) were investigated using the TDDFT method in aprotic and protic solvents. The solvent effect was incorporated into the calculations via the PCM formalism. The H-bonding between solute and protic solvent was taken into account by considering a molecular complex between these molecules. To examine the effect of the H-bond on the ESIPT reaction, the absorption and emission wavelengths as well as the energies of the different states that intervene during these electronic transitions were calculated in acetonitrile, ethanol and methanol. The calculated positions of the absorption and emission wavelengths in various solvents were in excellent agreement with the experimental spectra, validating our approach. We found that in DMAF, the hydrogen bonding with protic solvents makes the ESIPT reaction energetically unfavourable, which explains the absence of the ESIPT tautomer emission in protic solvents. In contrast, the excited tautomer state of FHC remains energetically favourable in both aprotic and protic solvents. Comparing our calculations with the previously reported time-resolved fluorescence data, the ESIPT reaction of DMAF in aprotic solvents is reversible because the emitting states are energetically close, whereas in FHC, ESIPT is irreversible because the tautomer state is below the corresponding normal state. Therefore, the ESIPT reaction in DMAF is controlled by the relative energies of the excited states (thermodynamic control), while in FHC the ESIPT is controlled probably by the energetic barrier (kinetic control).     

Ab initio study on the reaction of uranium with oxygen

The primary reaction products and reaction mechanism of uranium with oxygen were discussed from MP2 method with the relativistic core potential of SDD basis set for U and 6-311+G* for O. The molecular geometries, electronic structure and energies of uranium oxides were obtained. The inspection on the three-dimensional potential energy surfaces of the U–O₂ interaction suggested that the abstraction and insertion mechanism were responsible for the studied reactions. The abstraction reaction channel resulting in the formation of UO and O is favored because the energy barrier is remarkably smaller than the one of the insertion channel resulting in the linear OUO product directly.     

Ab initio study of n-tetrasilane cation and anion radicals as models of doped linear polysilanes

The geometric and electronic structures of n-tetrasilane cation and anion radicals as models of doped linear polysilanes are studied theoretically using an ab initio molecular orbital method at the UMP2/6–31 + G(d, p) level of calculations. It is found that the trans-conformations in these molecules are the most stable structures in each ground state and that the energy differences between the cis- and trans-conformations are 3 kcal/mol for the cation radical and 11 kcal/mol for the anion radical. There exists no stable gauche-conformation in these molecules in contrast to neutral n-tetrasilane. It seems that the weakening of the central silicon-silicon bond on doping is connected to the concentration of the charge distributions on central silicon atoms with the change from trans- to cis-conformations in both n-tetrasilane cation and anion radicals. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62 : 393–401, 1997     

Ab initio study on the mechanism of reaction HNCO+NH_2

Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that the reactions have two product channels: NH2+ HNCO→NH3+NCO (1) and NH2+HNCO-N2H3+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48 kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300-2700 K, transition theory rate constant for reaction (1) is 1.68 × 1011- 3.29 × 1011 mL · mol-1· s-1, which is close to the experimental one of 5.0 ×1011 mL× mol-1· s-1 or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes, cis and trans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (for cis-mode) and 147.43 kJ/mol (for trans-mode), respectively, which is much higher than     

用从头算研究Shilov反应的机理

Shilov反应在CH~4活化中占有中心地位,它有氧化加成和σ迁移两种可能的机理。本文用较大基组的从头算研究了这两种机理的反应过程,认为Shilov反应应按氧化加成机理进行。     

Ab initio study on the reaction mechanism of ozone with bromine atom

Ab initio calculations of the potential energy surface (PES) for the Br+O₃ reaction have been performed using the MP2, CCSD(T), and QCISD(T) methods with 6‐31G(d), 6‐311G(d), and 6‐311+G(3df). The reaction begins with a transition state (TS) when the Br atom attacks a terminal oxygen of ozone, producing an intermediate, the bromine trioxide (M), which immediately dissociates to BrO+O₂. The geometry optimizations of the reactants, products, and intermediate and transition states are carried out at the MP2/6‐31G(d) level. The reaction potential barrier is 3.09 kcal/mol at the CCSD(T)/6‐311+G(3df)//MP2 level, which shows that the bromine atom trends intensively to react with the ozone. The comparison of the Br+O₃ reaction with the F+O₃ and Cl+O₃ reactions indicates that the reactions of ozone with the halogen atoms have the similar reaction mechanism. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

An ab initio MO study on the thymine dimer and its radical cation

Ab initio SCF calculations with the 6-31G basis set for the thymine dimer (cys-syn form) and the thymine dimer radical cation are reported. The fusion of the thymine bases at the C5 and C6 positions involves the formation of a cyclobutane ring with puckering. The puckering causes a notable difference in the electronic structures of the two bases of the thymine dimer. The density of the HOMO orbital of the thymine dimer is localized on the O2, N1, and C6 atoms of both thymine rings, with the higher density on one of the rings. The HOMO orbital has a bonding character on the C6(SINGLEBOND)C6 bond. In the thymine dimer radical cation, the unpaired electron is localized mainly on the lengthened C6(SINGLEBOND)C6 bond with the higher density on one of the C6 atoms and to a lesser extent on the N1 atoms of both rings. © 1996 John Wiley & Sons, Inc.     

Ab initio direct classical trajectory investigation on the SN2 reaction of F- with NH2F: nonstatistical central barrier recrossing dynamics

The bimolecular nucleophilic substitution (S(N)2) reaction of F(a)(-) with NH(2)F(b) has been investigated with the ab initio direct classical trajectory method. According to our trajectory calculations, a dynamic behavior of nonstatistical central barrier recrossing is revealed. Among the 64 trajectories calculated in this work, 45 trajectories follow the dynamic reaction pathways as assumed by statistical theory and other 19 trajectories with central barrier recrossings are nonstatistical. For the nonstatistical trajectories, the central barrier recrossings may originate from the inefficient kinetic energy transfer from the intramolecular modes of the NH(2)F(a) moiety in the dynamic F(b)(-)…H-NH-F(a) complex to the intermolecular modes of the dynamic F(b)(-)…H-NH-F(a) complex on the exit-channel potential energy surface. With respect to the dynamic behavior of the nonstatistical central barrier recrossing, the statistical theories such as the Rice-Ramsperger-Kassel-Marcus and transition state theories without further corrections cannot be used to model the reaction kinetics for this S(N)2 reaction.     

Ab initio study on mechanisms of singlet oxygen reaction with ethylene

The reaction of singlet oxygen with ethylene has been studied at the ab initio level with both HF/3-21G and HF/6-31G basis sets, fully optimizing the geometries of the critical points. The transition state leading to the intermediate peroxirane from the initial reactants is found,Iocated at 81.71 kJ/mol above the dissociation limit. The vibrational analysis is done with two basis sets. From the results it can be seen that the frequencies have not been made an improvement obviously with augmented 6-31G compared to 3-21G basis; it follows that main reason for too high HF/3-21G frequency could mainly be the vibrational anharmonicity. The eigenvector corresponding to the single imaginary vibrational frequency is dominated by the larger O-O distance. The finding of the transition state confirms that the peroxirane minimum can be reachable passing through a peroxirane-like saddle point. In addition , the mechanisms of the reaction forming oxirane are discussed as well.     

Ab initio SCRF study of solvent effect on the nonlinear polarizabilities of different intramolecular charge-transfer molecules

The macroscopic solvent effect on static nonlinear polarizabilities of a number of intramolecular charge-transfer (ICT) molecules have been studied by using the self-consistent-reaction field (SCRF) model in the framework of ab initio time-dependent-HF (TDHF) method using 3-21G basis set. The two-state model of static β and <γ> have been used to obtain their functional dependence on the ground-state hardness parameter, dipole moment and solvation energy. The methyl substituted 4-quinopyran and other zwitterionic molecules are found to exhibit strong diminution of both quadratic and cubic polarizabilities at higher solvent reaction field due to negative solvatochromic effect. However, molecules showing positive solvatochromism lead to strong enhancement of the NLO response on increase in solvent polarity. The evolution pattern of the solvent modulated static β and <γ> of 4-quinopyran (4QP) obtained for varying inter-ring torsion angle differ strikingly from that shown by p-amino p'-nitro biphenyl (ANB).     

Ab initio SCF MO study of the hydrogen-bonded dimers of cyanogen with HF and HCl

Large-scale gaussian orbital SCF MO calculations are presented for the hydrogen-bonded complexes NCCN…HF and NCCN…HCl. Calculated equilibrium geometries, hydrogen-bond dissociation energies and selected one-electron properties are given to supplement available experimental data. Changes of electron distribution on complex formation are discussed in terms of Mulliken population indices.     

Ab initio study of the reaction of H2 with an AuPt3 cluster

The study of the interaction of a pyramidal tetramer of AuPt3 with H2 is carried out by means of Hartree-Fock self-consistent field (SCF) calculations using relativistic effective core potentials and multiconfigurational SCF plus multireference variational and perturbational on second-order Moller-Plesset configuration interaction calculations. The AuPt3-H2 interaction was carried out in C(s) symmetry. The three lowest electronic states X 2A", A 2A', and a 4A' of the bare cluster were considered in order to study this interaction. The AuPt3+H2 reaction by a Pt vertex shows that AuPt3 cluster in the three lowest-lying electronic states can spontaneously capture and dissociate the H2 molecule. While, by the AuPt2 face side, the AuPt3 cluster only in the A 2A' electronic state can capture and dissociate the H2 molecule after surmounting a small energy barrier. For the Au vertex, this cluster in the three electronic states can also spontaneously capture and dissociate the H2 molecule. On the other hand, by the Pt3 face side, the AuPt3 cluster is able to capture and dissociate the H2 molecule after surmounting energy barriers, where the AuPt3 (X 2A" and 4A'-H2 adsorption are slightly activated.     

Trinitromethyl anion. An ab initio MO study of its structure

Several conformers of nitroform anion are studied with ab initio MO methods.     

Ab initio and RRKM study of photodissociation of azulene cation

The ab initio/Rice-Ramsperger-Kassel-Marcus (RRKM) approach has been applied to investigate the photodissociation mechanism of the azulene cation at different values of the photon energy. Reaction pathways leading to various decomposition products have been mapped out at the G3(MP2,CC)//B3LYP level and then the RRKM and microcanonical variational transition state theories have been applied to compute rate constants for individual reaction steps. Relative product yields (branching ratios) for the dissociation products have been calculated using the steady-state approach. The results show that a photoexcited azulene cation can readily isomerize to a naphthalene cation. The major dissociation channels are elimination of atomic hydrogen, an H2 molecule, and acetylene. The branching ratio of the H elimination channel decreases with an increase of the photon energy. The branching ratio of the acetylene elimination as well as that of the H2 elimination rise as the photon energy increases. The main C8H6+ fragment at all photon energies considered is a pentalene cation, and its yield decreases slightly with increasing excitation energy, whereas the branching ratios of the other C8H6+ fragments, phenylacetylene and benzocyclobutadiene cations, grow.     

Ab initio study of dehalohydrogenation reaction of 2-halo-2,3-dihydrophosphinine

Decomposition of 2-fluoro-2,3-dihydrophosphinine (1), 2-chloro-2,3-dihydrophosphinine (3), 2-bromo-2,3-dihydrophosphinine (5) to phosphinine was investigated using Molecular orbital and density functional theory. Study on the B3LYP/6-311+G** level of theory revealed that the required energy for the decomposition of compounds 1, 3, and 5 to phosphinine is 30.56 kcal·mol^?1, 28.23 kcal·mol^?1, and 24.03 kcal·mol^?1, respectively. HF/6-311+G**//B3LYP/6-311+G** calculated barrier height for the decomposition of compound 1, 3, and 5 to phosphinine is 57.56 kcal·mol^?1, 37.26 kcal·mol^?1, and 30.77 kcal·mol^?1, respectively. Also, MP2/6-311+G**//B3LYP/6-311+G** results indicated that the barrier height for the decomposition of compound 1, 3, and 5 to phosphinine is 46.59 kcal·mol^?1, 47.28 kcal·mol^?1, and 42.57 kcal·mol^?1, respectively. Natural bond orbital (NBO) population analysis and nuclear independent chemical shift (NICS) results showed that, reactants are non-aromatic but products of elimination reaction are aromatic, C-H and C-X bonds are broken and H-X bond is appear.     

Ab initio MO study of dipole—dipole interactions in acetonitrile dimers

MO STO-3G ab initio calculations have been carried out for the antiparallel dipole and the head-to-tail dipole model of acetonitrile dimers. The optimized interaction enthalpy is about half of the lowest experimental estimate. The calculated interaction distance for the antiparallel dipole model is very close to the sum of intermolecular radii of N and C; the distance for the head-to-tail model is about 20% higher than the sum of N and H intermolecular radii. The discussion of the interaction in terms of the supermolecule MO's suggests for both models a bonding of mainly electrostatic character. The shortcomings of the STO-3G basis set in dealing with this problem are compared with those reported in the literature. The influence of the basis set on the calculated electron distribution in acetonitrile monomer was examined as a preliminary part of the present study, and is also reported in the paper.