ISSN:0021-2148    

DOI:10.1002/ijch.198300001

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractTheoretical predictions that alkenes and carbonyls pyramidalize in asymmetric environments were made before any conclusive experimental evidence for this phenomenon had been noted. In the last two years, several such pyramidalizations have been discovered. This review summarizes the predictions, experimental discoveries, and theoretical explanations of pyramidalizations of alkene and carbonyl carbons and describes the evidence indicating that both pyramidalization and stereoselectivity in reactions of such molecules arise from torsional (closed‐shell repulsion) effect

ISSN:0021-2148    

DOI:10.1002/ijch.198300002

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY

ISSN:0021-2148    

DOI:10.1002/ijch.198300003

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractAb initiomolecular orbital theory has been used to study the ionization of methane, ethane, propane,n‐butane, isobutane, cyclopropane and cyclobutane. For methane radical cation, the preferred structure hasC20symmetry and a pair of long C—H bonds, resembling a complex of CH+2with H2. This appears to be the only significant minimum on the CH+4surface. For the remaining acyclic systems, the calculated structures of lowest energy (e.g., CH3‐CH2—CH+3, CH3‐CH2—CH2‐CH+3) are characterized by the extreme lengthening of a single C—C bond. However, alternative structures with two or more elongated C—C bonds have very low relative energies. The main structural conclusion for the alkane and cycloalkane radical cations is that the potential energy surface connecting various possible isomeric structures is generally very flat. Facile wide amplitude distortions and scrambling processes are therefore expected in these ions. The large structural changes which accompany ionization are manifested in large differences between vertical and adiabatic ioni

ISSN:0021-2148    

DOI:10.1002/ijch.198300004

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractThe electronic structures of organic ion‐radical salts, molecular conductors, and charge‐transfer complexes are described by solid‐state models with one valence state per site. Similar models occur in π‐electron theories of conjugated molecules or in spin Hamiltonians for magnetic insulators. A localized or valence bond (VB) representation is qualitatively adequate in narrow band systems. Diagrammatic VB methods in finite models yield convenient exact solutions to the resulting configuration interaction (CI) problem as correlated states based on linear combinations of VB diagrams. Also found are exact charges, bond orders, ionicity, correlation functions, transition moments or other properties of correlated electronic states. Particular attention is given to qualitative failures of conventional one‐electron descriptions, which often reflect degeneracies lifted by correlations. Correlated states give the correct ordering of the 21Agand 11Bustates of trans‐trans octatetraene, clarify the midgap absorption in polyacetylene, rationalize the partial ionicity and electrostatic energy of ion‐radical organic complexes, demonstrate an exactly soluble magnetic analog to the Mott transition, and quantitatively fit the thermodynamics of several random‐exchange Heisenberg antiferromagnets. Correlated states remain central in simultaneously modeling optical, magnetic, and electrical excitations in

ISSN:0021-2148    

DOI:10.1002/ijch.198300005

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractPairwise orbital transformations in reagents and in reactants indicate which parts of molecules participate actively in chemical reactions. Similar orbitals are obtained for different molecules, demonstrating common features of the relevant reaction, and different orbitals are obtained for the same molecule, reflecting multiple types of reactivities of organic compounds. The possibility of analyzing the roles of orbital interactions along a reaction pathway is demonstrated.

ISSN:0021-2148    

DOI:10.1002/ijch.198300006

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractThe Molecular Orbital‐Valence Bond (MOVB) method is briefly outlined and it is subsequently applied to the problem of geometric isomerism of molecules having eight and ten sigma valence electrons such as bent HC≡CH and HN=≡NH. It is shown that orbital symmetry dictates a trans bending tendency for HCCH and in the hypothetical absence of nonbonded repulsion, a cis geometry for XNNY and

ISSN:0021-2148    

DOI:10.1002/ijch.198300007

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractThe method of local pseudopotentials is applied to the calculation of several ground‐ and excited‐state properties of selected silicon compounds. The calculated ground‐state geometries and ionization potentials of the silicon hydrides: SiH, SiH2, SiH3and SiH4and of the ions SiH+, SiH−, SiH+3and SiH−3are in good agreement with the best theoretical and experimental results — whenever these are available. All the singly excited states of SiH2originating from a given occupied orbital are evaluated in a single calculation. For those few valence states for which reliable experimental and theoretical data exist, agreement is good. The conceptual and computational advantages of transforming the basis set of silicon into a set of atomic pseudoorbitals are demonstrated by repeating the latter computations with increasingly large sets of atomic orbitals, ranging from [3s, 3p] to [3s, 3p, 3d, 4s, 4p]. The utility of this procedure for reliable and economical calculations on organosilicon compounds is illustrated with a similar set of computations on the ground‐state properties and vertical excitation energies of Si(CH3)2. The effect of methyl substitution on the ground‐state properties of silylene and on the energetic ordering of its excited stat

ISSN:0021-2148    

DOI:10.1002/ijch.198300008

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractStructure‐resonance theory can be used to derive charge distributions and bond orders in ferrocenes, open ferrocenes and α‐ferrocenylalkyl cations. These species are of interest because they are unusually accessible and stable and understanding the electronic origin of this stabilization would aid in developing bonding models for organometallic systems. The methods for carrying out the calculations will be outlined. They involve shortcuts and algorithms for counting structures. The choice of structure basis, limited to conventional principal resonance structures, will be discussed. Charges and bond orders will be compared with NMR chemical shift and X‐ray data. The unusual stabilities of the ions and the role of the metal in stabilizing positive charge will be understandable within the conventional ideas of structural organic chemistry and resonance theory. These metal‐organic systems are highly delocalized and consequently resonance stabilized to a significant degree. They can be characterized as three‐dimensional aromatic

ISSN:0021-2148    

DOI:10.1002/ijch.198300009

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY

摘要:

AbstractStructural, conformational and energetic properties of cyclopentane (1) and the seven oxolanes monoxolane (2), 1,3‐dioxolane (3), 1,2‐dioxolane (4), 1,2,4‐trioxolane (5), 1,2,3‐trioxolane (6), tetroxolane (7), and pentoxolane (8) are investigated employing the polarized 6‐31G* basis set at the Hartree‐Fock level of theory. Extensive geometry optimization is carried out within the model of the semirigid pseudorotor. The conformational potentialsVof compounds 1–8 are evaluated as a function of the puckering amplitudeqand the pseudorotation phase angle Φ. Ring molecules 1 and 8 are free pseudorotors, while pseudorotation is hindered by barriers ≤ 3.3 kcal/mol for oxolanes 2–7. Puckering and inversion barriers increase with the number of O‐O bonds but decrease with the number of ether bridges. PuckeredC2‐symmetrical twist forms are the most stable conformations for compounds 2–7 but 6, where highest stability is found for theC2‐symmetrical envelope forms. At room temperature a multitude of conformers of 1–8 coexists either because of free pseudorotation (barriers

ISSN:0021-2148    

DOI:10.1002/ijch.198300010

出版商:WILEY‐VCH Verlag    

年代:1983

数据来源: WILEY