Entropy/information bond indices of molecular fragments

The communication channels of the disconnected (mutually non-bonded, closed) parts of the molecule are investigated. The entropy/information indices of such subsystems are proposed as measures of the intra-fragment (internal) information bond-order and its covalent/ionic components. The molecular fragment bond-order conservation and a competition between its ionic and covalent contributions are examined. An approximate scheme in the spirit of the grouping theorem of the Information Theory (IT), for combining the subsystem entropy/information data into the corresponding global quantities describing the system as a whole, is derived and tested. It uses the independent subsystem approximation to estimate the entropy/information indices of the inter-fragment (external) chemical interactions in the molecule. The applications to simple orbital models, including the three-orbital model of the transition state complex and the π bond systems (butadiene and benzene) in the Hückel theory approximation, are used to illustrate the concepts proposed.     

Localization function study of excitation processes in a set of small isoelectronic molecules

Electron localization function (ELF) theory is used to characterize changes that occur upon excitation from ground singlet to first excited triplet states in a series of isoelectronic 16‐electron molecules including H₂CCH₂, HNCH₂, H₂CO, HNNH, HNO, and O₂ (ground triplet to excited singlet). ELF allows one to visualize lone pair or nonbonding electrons, and in these cases the π→π* or n→π excitation processes involved lead to an effective 90° rotation of the electronic structure about one heavy atom center and consequent distortion towards pyramidal symmetry about both heavy atom centers. The heavy atom bond lengths change very little in those cases where effectively two‐center three‐electron bonds can be formed (HNNH, HNO, and O₂) while a significant lengthening occurs in those cases where hydrogen atoms prevent such interactions (H₂CCH₂, HNCH₂, and H₂CO). It is shown that both ELF basin populations and atoms‐in‐molecules (AIM) delocalization indices reflect expected bond orders for conventional single and double bonds provided one compares the ratio of the molecular quantities rather than their absolute magnitudes. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1702–1711, 2001     

Ni原子活化氨分子理论研究

在UB3LYP/6-311＋＋G(3df,3pd)水平下, 详细研究了Ni活化NH₃分子的单重态和三重态势能面, 并用分子中的原子量子理论(Quantum Theory of Atom-in-Molecular, QTAIM)计算了势能面上所有驻点的性质. 计算结果表明, 单重态势能面有两条反应途径, 而三重态势能面仅有一条反应途径. 第一个N—H断开的活化能较低, 为99.96 kJ/mol, 活化自由能为100.86 kJ/mol,在常温下就可以进行; 第二个N—H键断裂所需能量高达200 kJ/mol, 不容易进行. 在合适温度下, Ni可以活化NH₃得到三重态HNiNH₂, 这表明Ni可以作为活化NH₃分子的良好催化剂.     

C-Br Bond Dissociation Mechanisms of 2-Bromothiophene and 3-Bromothiophene at 267 nm

C-Br bond dissociation mechanisms of 2-bromothiophene and 3-bromothiophene at 267 nm were investigated using ion velocity imaging technique. Translational energy distributions and angular distributions of the photoproducts, Br(²P_3/2) and Br^*(²P_½), were obtained and the possible dissociation channels were analyzed. For these two bromothiophenes, the Br fragments were produced via three channels: (i) the fast predissociation following the intersystem crossing from the excited singlet state to repulsive triplet state; (ii) the hot dissociation on highly vibrational ground state following the internal conversion of the excited singlet state; and (iii) the dissociation following the multiphoton ionization of the parent molecules. Similar channels are involved for photoproduct Br^* of the 2-bromothiophene dissociation at 267 nm; whereas for the photoproduct Br^* of 3-bromothiophene, the dissociation channel via internal conversion from the excited singlet state to highly vibrational ground state became dominating and the fast predissociation channel via the excited triplet state almost disappeared. Informations about the relative contribution, energy disposal, and the anisotropy of each channel were quantitatively given. It was found that with the position of Br atom in thienyl being far from S atom, the relative ratios of products from channels (i) and (ii) decreased obviously and the anisotropies corresponding to each channel became weaker.     

Theoretical study on the atmospheric reaction of CH3O2 with OH

A quantum chemical investigation on the reaction mechanism of CH₃O₂ with OH has been performed. Based on B3LYP and QCISD(T) calculations, seven possible singlet pathways and seven possible triplet pathways have been found. On the singlet potential energy surface (PES), the most favorable channel starts with a barrierless addition of O atom to CH₃O₂ leading to CH₃OOOH and then the O? O bond dissociates to give out CH₃O + HO₂. On the triplet PES, the calculations indicate that the dominant products should be ³CH₂O₂ + H₂O with an energy barrier of 29.95 kJ/mol. The results obtained in this work enrich the theoretical information of the title reaction and provide guidance for analogous atmospheric chemistry reactions. © 2015 Wiley Periodicals, Inc.     

RN (R＝CH3, CH3CH2)异构化及脱氢反应的量子拓扑研究

利用量子化学从头算CASSCF方法在6-311＋G (d, p)基组水平上对单线态和三线态RN (R＝CH₃, CH₃CH₂)异构化反应及RN脱氢反应的微观机理进行了理论研究. 在MP2/6-311＋G (d, p)和CCSD/6-311＋G (d, p)水平上进行了单点能校正. 单态和三态势能面的交叉点(ISC)的存在清楚地说明了基态反应物³RN异构化为基态产物¹R'NH (R'＝CH₂, CH₃CH)的过程. 电子密度拓扑分析显示在整个异构化过程中有两种类型的结构过渡态: 单态反应通道为T型过渡态, 三态反应通道为环状过渡态. 单线态RN脱氢反应通道中“原子-分子键”的存在说明两个H原子是以H₂的形式从RN中脱去的.     

Singlet-triplet splitting of divalent five-membered fused rings C<Subscript>12</Subscript>H<Subscript>8</Subscript>M (M = C,Si, Ge,Sn, and Pb): Application in nanocomplex synthesis

Energy differences, ΔX _s−t (X = E, H, and G) (ΔX _s−t = X(singlet) − X(triplet)) between singlet (s) and triplet (t) states of C₁₂H₈M were calculated at B3LYP/6-311+G*. The DFT calculations indicated that the ΔG _s−t between singlet (s) and triplet (t) states of C₁₂H₈M were increased from M = C to M = Pb. The ΔG _s−t of C₁₂H₈M was compared with its analogue C₄H₄M through replacement of heavy atoms from M = C to M = Pb. Configurations of the electrons in orbitals (σ² or π²) for the singlet state of C₁₂H₈M were discussed.     

Photodissociation of Dibromobenzenes at 266 nm by the Velocity Imaging Technique

A velocity imaging technique combined with (2+1) resonance‐enhanced multiphoton ionization (REMPI) is used to detect the primary Br(²P_3/2) fragment in the photodissociation of o‐, m‐, and p‐dibromobenzene at 266 nm. The obtained translational energy distributions suggest that the Br fragments are produced via two dissociation channels. For o‐ and m‐dibromobenzene, the slow channel that yields an anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (π,π*) state followed by predissociation along a repulsive triplet (n,σ*) state localized on the C? Br bond. The fast channel that gives rise to an anisotropy parameter of 0.53–0.73 is attributed to a bound triplet state with smaller dissociation barrier. For p‐dibromobenzene, the dissociation rates are reversed, because the barrier for the bound triplet state becomes higher than the singlet–triplet crossing energy. The fractions of translational energy release are determined to be 6–8 and 29–40 % for the slow and fast channels, respectively; the quantum yields are 0.2 and 0.8, and are insensitive to the position of the substituent. The Br fragmentation from bromobenzene and bromofluorobenzenes at the same photolyzing wavelength is also compared to understand the effect of the number of halogen atoms on the phenyl ring.     

The valence bond calculations for conjugated hydrocarbons having 24–28 π‐electrons

A novel algorithm is introduced for coding all Slater determinants in the covalent space with conserved S_Z, the z component of total spin S for a classical valence bond (VB) model. It effectively minimizes the search time and the storing space in the central memory of the computer. In cooperation with symmetry reductions based on molecular point group and spin inversion, the VB calculations have been extended to benzenoid hydrocarbons of up to 28 π‐electrons that have 4×10⁷ configurations. The low‐lying states of benzenoids with 24, 26, and 28 π‐electrons have been obtained for 62 species. To rationalize the aromaticity of benzenoids in a VB scheme, the resonance energy per hexagon (REPH) is defined. A linear correlation between the REPH and the energy gap of the ground (singlet) state and the first excited (triplet) state for 89 benzenoids is established. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 856–869, 2000     

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.     

Theoretical studies of cyclopropylsilylenes: The structures and stability of cyclopropylsilylene C3H5SiH

Ab initio molecular orbital calculations at the G2(MP2) level have been carried out on cyclopropylsilylene C₃H₅SiH. Four equilibrium structures were located. Like H₂Si, the ground state of C₃H₅SiH is singlet and the triplet is the low‐lying excited state. The singlet–triplet separation energy is 127.9 kJ/mol. The cis‐trans isomerization path of singlet cyclopropylsilylene was investigated by intrinsic reaction coordinate (IRC) calculations. The calculations show that no gauche conformers exist along the potential energy curve of the cis‐trans isomerization and the isomerization happens with a barrier of 30.1 kJ/mol. Changes (ΔH and ΔG) in thermodynamic functions, equilibrium constant K(T), and A factor and reaction rate constant k(T) in Eyring transition state theory of the cis‐trans isomerization were also calculated. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001