丙烯和甲苯的超共轭效应的理论研究

用从头算对丙烯和甲苯2个超共轭体系进行计算,结果表明甲基上氢原子参与超共轭时,其碳氢键键长增大,氢上集居数减少,丙烯、甲苯的甲基旋转势垒为7.61及0.096 kJ/mol.超共轭基作用相当于一带有孤对电子参与共轭之杂原子,超共轭体系不同构型稳定性可用M=sum from i=1 to 3(sinθ_i[sin(θ_i-α)+sinα])来表征。     

The hyperconjugation effect on the graphene counterparts based on silicon and germanium

Bending of the A = A (A of the group IVA) double bond neighboring is rationalized by the hyperconjugation phenomenon analysis. The bending is also observed for the high sized linear, cyclic or graphene-like compounds that imply the conjugated double bonds. The electronic delocalization takes place between occupied σ(π) and unoccupied π*(σ*) orbitals especially for compound implying Si and Ge atoms. Leading to rippled structure, this phenomenon affects the silicene and germane thickness sheets and probably would have some consequences on the properties of such compounds when they will be involved in the industries in the future. However we introduce a new parameter to assess the thickness of graphenic structures when the hyperconjugation takes place in the bonding framework. The study has been undertaken at high levels of theory like B3LYP/6-311 + G(3df,2p).     

A Selenium‐Nitrogen Chain with Selenium in Different Oxidation States

The reaction of tBuNH₂ with a mixture of SeCl₂ and SeOCl₂ in a 6:2:1 molar ratio produces the novel selenium‐nitrogen chain ClSeN(tBu)Se(O)Cl ( 4 ), in which the selenium atoms are in two different oxidation states, Se^II and Se^IV. The crystal structure of 4 is compared with that of the related Se^II/Se^II system ClSeN(tBu)SeCl ( 1 ) and differences are attributed to hyperconjugative effects. The energetics of the formation of 4 via two different routes are elucidated by PBE0/def2‐TZVPP calculations.     

超共轭与立体电子效应

回顾了超共轭概念的起源与发展,简述了超共轭效应在立体电子效应中的重要地位及其对于分子构象和反应活性的重要影响,并对乙烷的优势构象成因进行了讨论。     

Polar substituent effect of the ester group on conformational equilibria of O-mono-substituted cyclohexanes—the para-substituent effect in cyclohexyl benzoates

Together with the nonsubstituted reference compound, para-methoxy- and para-nitro cyclohexyl benzoates have been synthesized and their conformational equilibria studied by low temperature NMR spectroscopy and theoretical DFT calculations. The free energy differences ΔG° between axial and equatorial conformers were examined with respect to polar substituent influences on the conformational equilibrium of O-mono-substituted cyclohexane.     

Molecular and electronic structures, infrared spectra, and vibrational assignment for ap and sc conformers of hexafluoro-iso-propanol

Comprehensive studies of the molecular structures, vibrational frequencies and infrared intensities of the antiperiplanar (ap) and synclinal (sc) conformers of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) have been performed by the density functional (B3LYP) method using the extended 6-311++G(df,pd) basis set. The detailed natural bond orbital (NBO) analysis has revealed the nature of the hyperconjugative interactions, which stabilize each conformer, in the gas phase. The mid-infrared spectra of HFIP in carbon tetrachloride solution were measured, and the experimental intensities of each conformer were obtained by the curve–resolution procedure. The relative abundance of the two conformers, calculated from the relative intensities, shows nearly equimolar ratio (N_sc/N_ap ≈ 1), in this solution. The DFT-predicted frequencies show very good agreement with the experimental data. The clear-cut vibrational assignment for each conformer is reported on the basis of the calculated potential energy distributions. Several controversies in an earlier assignment of HFIP have been elucidated.     

An ab initio study of the keto-enol tautomerism

The keto-enol tautomerism is studied using an approximative HF method outlined in the appendix. The following results are obtained: (1) The experimentally observed alternance of G in acyclic monoketones could not be reproduced. (2) The stabilization of C=C double bonds, especially of conjugated double bonds, by CH₃- or -CH₂- groups is responsible for the observed difference between acyclic and cyclic 1.2-diketones, e.g. for the different enol content of diacetyl and cyclopentane-1.2-dione. (3) The enols of 1.2-diketones contain a hydrogen bond which differs from the hydrogen bond in enols of 1.3-diketones. (4) A system of two conjugated C=O double bonds is not favoured compared to a system of two C=O bonds which are separated by one (or more) -CH₂- group. (5) 5-ring enols with a C=C double bond in the ring are more stable than one would expect by an energy estimation from acyclic compounds.     

超声分子束中中性态和离子态吡咯烷的红外光谱

本文应用真空紫外光电离结合红外光解离以及飞行时间质谱技术获得超声分子束中吡咯烷单体在中性态和离子态的红外光谱. 研究发现，吡咯烷在中性态和离子态的CH伸缩振动均有不同程度的红移现象. 在中性态吡咯烷中，由于NH的电偶极矩较小，没有观察到NH伸缩振动峰. 而电离后的吡咯烷NH伸缩振动峰有很大的增强，并与其他实验相比出现红移现象. 通过理论计算发现中性态和离子态的CH伸缩振动的红移分别是由反超共轭和正超共轭现象引起的. 而NH峰的增强和红移则是由于N原子上的电子被电离所引起的. 通过计算分子的气态酸碱度发现，在离子态中，CH的酸性比NH的酸性略强.     

Theoretical Studies of Inorganic Compounds. 341) Energy Decomposition Analysis of E–E Bonding in Planar and Perpendicular X2E−EX2 (E = B,Al, Ga,In, Tl; X = H,F, Cl,Br, I)

The nature of E–E bonding in group 13 compounds X₂E–EX₂ (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I) has been investigated by means of an energy decomposition analysis (EDA) at the BP86/TZ2P level of theory. The calculated equilibrium geometries of all molecules B₂X₄?Tl₂X₄ have a perpendicular (D_2d) geometry. The largest energy barriers for rotation about the E‐E bond are predicted for the hydrogen species B₂H₄?Tl₂H₄. The EDA shows that the rotational barriers of B₂X₄?Tl₂X₄ may not be used for an estimate of the hyperconjugative strength in the D_2d structures except for the tetrahydrides. The values for the planar (D_2h) transition states reveals that π conjugation of the halogen lone‐pair electrons stabilizes the transition states. The bonding analysis shows that hyperconjugation in B₂I₄ is stronger than in B₂H₄ although the latter compound has a higher rotational barrier than the former. In B₂F₄, hyperconjugative stabilization of the perpendicular structure and conjugative stabilization of the planar structure nearly cancel each other yielding a nearly vanishing rotational barrier. The heavier analogues Al₂X₄?Tl₂X₄ have low rotational barriers and rather weak hyperconjugative interactions. The larger rotational barriers of the hydrogen systems Al₂H₄?Tl₂H₄ compared with the tetrahalogen compounds is explained with the cooperation of the relatively large hyperconjugation in the perpendicular form and the relatively weak conjugation in the planar transition structures. The EDA also indicates that the electrostatic (ΔE_elstat) and molecular orbital (ΔE_orb) components of the E–E bonding are similar in magnitude.Thecalculated B‐B bond dissociation energies of B₂X₄ (D_e = 93.0–108.4 kcal/mol) show that the bonds are rather strong. The heavier analogues Al₂X₄?Tl₂X₄ have weaker bonds (D_e = 16.6–61.7 kcal/mol). In general, the X₂E‐EX₂ bond dissociation energies follow the trend for atoms E: B ? Al > Ga > In > Tl and for atoms X: H > F > Cl > I.     

Infrared Spectroscopy of Neutral and Cationic Pyrrolidine Monomer in Supersonic Jet

Pyrrolidine, a five membered heterocyclic molecule, is widely existing in organism. Herein, infrared spectra of pyrrolidine monomer in neutral and cationic states were obtained by vacuum ultraviolet ionization, infrared photodissociation and time of flight mass spectrometry. Both in neutral and cationic states, it is found that their CH stretching vibration bands are red shifted. In the IR spectrum of neutral pyrrolidine, because the electric dipole moment of NH is small, we have not observed the NH stretching vibration bands. However, the NH stretching vibration band of pyrrolidine is greatly enhanced after ionization, and this band red-shifts compared with the previous experiment. The red shifts of CH stretching vibrations in neutral and cationic states are caused by the negative and positive hyperconjugation, respectively. The enhancement and red shift of the NH stretching band are owing to the ejection of the electrons on the N atom after ionization. Through the calculations, it is found that the acidity of the CH bond is a little stronger than that of NH bond. These kinds of studies would be helpful to understand the intrinsic properties of biomolecules in neutral and cationic states, and to provide reference for the further study of living organic macromolecules.