ISSN:0021-2148    

DOI:10.1002/ijch.199100031

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe origin of barriers to internal rotation in ethane‐like molecules has been investigated with ab initio molecular orbital theory and natural bond orbital (NBO) analysis. The “hyperconjugative” (vicinal σ → σ*, n → σ*) nature of these interactions, as previously suggested on the basis of semiempirical INDO studies, is confirmed at higher 4–31G, 6–31G* levels of theory. Wave functions lacking such hyperconjugative interactions, but satisfying the Pauli principle, are shown to generally exhibit unphysical barrier behavior (i.e., wrongsign), contrary to the implication of the “Pauli‐repulsions” model of Sovers et al. Eachtransvicinal σ → σ* interaction is found to contribute about 0.7 kcal/mol to stabilization of the staggered conformer. Geminal interactions and Rydberg orbital contributions are found to be relatively unimportant. The consistency of NBO analysis is exhibited for the methyl series CH3‐X (X = CH3, NH2, OH, F) and the family of fluoromethanes, to demonstrate the essential continuity that relates internal rotation barrier forces to static and kinetic anomeric effects,gaucheeffects, anionic hyperconjugation, bond strengthening in polyhalogenated methanes, (e, 2e) spectroscopic evidence of lone pair delocalization, and othe

ISSN:0021-2148    

DOI:10.1002/ijch.199100032

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe present status of the efforts to develop a closed‐form expression for the product of arbitrary class‐sums in the symmetric group is critically reviewed. The relevant data base is extended by determining the full [(7)]N·[(1)11(2)11…]nproduct. The extended data base is used to formulate a conjectured recurrence relation for reduced class coefficients of the general form C〈(r1+ r2)A·(r1+ r2+ … + r1)B〉 in terms of class coefficients in whichr1andr2a

ISSN:0021-2148    

DOI:10.1002/ijch.199100033

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe mirror theorem in matrix theory is briefly reviewed. It says that ifRandSare matrices of the orderm×nandnxm, then the two productsRSandSRhave the same nonvanishing eigenvalues with the same multiplicities and canonical structures. The mirror theorem is used to show that ifa={ak}andb={bl}are two sets of orbitals of ordermandn, respectively, then there exist two unitary transformationsUandVsuch that the orbitalsa′ =aUandb′ =bVhave the special property= λkδkl, i.e., they are all orthogonal to each other unless they belong to the same pair havingk = l.Thispairing theoremis of fundamental importance in treating the Coulomb correlation in molecular calculations by means of the so‐called Alternant Molecular Orbital (AMO) method, which uses for the occupied orbitals different orbitals for different spins. A simple extension of this approach to the virtual orbitals i

ISSN:0021-2148    

DOI:10.1002/ijch.199100034

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThree single‐reference coupled‐cluster single and double (CCSD) substitution models, CCSD + T(CCSD), CCSDT‐1, and CCSD(T), exact through the fourth order of perturbation theory, are tested on the triplet ground‐state dissociation potential of O2. For the unrestricted Hartree‐Fock (UHF) initial approximation, the interatomic distance regionR˜ 2.4–2.7 a.u. is problematic due to large multireference character, and beyondR˜ 3.0 a.u. spin contamination causes difficulties. To a large extent these deficiencies are cured by the coupled‐cluster treatments. Fourth‐order triples as well as higher‐order singles and doubles play a significant role. A more flexible basis set than DZP is important, too. None of the methods tried reproduce the experimental potential curve to the desired accuracy (2 kcal/mol). However, in a 6–311 + + G(2df, 2p) basis, the CCSD(T) model does give spectroscopic constants that are correct within 0.2 eV for the dissociation energyDe, 70 cm−1for the harmonic vibrational frequencyve, and 0.5 cm−1for the first anharmonicity constantveXe. This model also reproduces the potential throughout the dissociative region to better than 0.2 eV. Overall CCSD + T(CCSD) is considerably less satisfactory than CCSD(T); CCSDT‐1 is similar to CCSD(T) in a DZP basis, but large basis calculations prov

ISSN:0021-2148    

DOI:10.1002/ijch.199100035

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe Complex Coordinate Method (CCM) is reviewed, emphasizing the theoretical aspects and properties of the complex‐scaled Hamiltonian. It is shown that the CCM not only enables the calculation of resonance positions and widths by computational techniques that had originally been developed for bound states, but also can be used as a powerful tool in the calculation of scattering cross sections and partial width

ISSN:0021-2148    

DOI:10.1002/ijch.199100036

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractA theory of photodissociation with coherent pulsed lasers is developed with special emphasis on the nature of the prepared state under various modes of excitation. Stationary and time‐dependent formulations are treated on an equal footing. Detuning of the transition frequencies with respect to the pulse center is shown to have a profound effect on the time evolution of the systemduringthe pulse. The connection between the laser phase and its location and the nature of the prepared photodissociation state is made explicit. Applications to the femtosecond transition‐state spectra of the NaI system are presen

ISSN:0021-2148    

DOI:10.1002/ijch.199100037

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractMultiply charged ions show novel energetic features which can potentially have useful photophysical properties. In order to investigate these interesting type systems, energy interaction curves for the ground and excited electronic states of O2+2, SO2+, and S2+2have been generated by the ab initio complete active space multiconfiguration self‐consistent field (CAS‐MCSCF) method. The calculations were carried out in a triple‐zeta sp plus double‐zeta d Gaussian function basis set using compact effective potentials to replace the core electrons. In order to gauge the accuracy of the results, analogous calculations were carried out on the valence isoelectronic N2and NO+systems for which experimental information is available for comparison of geometric and spectroscopic properties. Diverse high energy spectroscopy experiments on O2+2are interpreted and reconciled using the calculated ground and excited state energy interaction curves. Special attention is paid to the3Σ+ustate, whose location and character have been particularly puzzling. Almost all the dication molecular states studied show the characteristic avoided crossing of diabatic states which gives a trapped equilibrium structure and a barrier to dissociation. These features make the dication kinetically stable, but thermodynamically unstable with an exothermic dissociation energy. The spectroscopy experiments on O2+2show that these states are experimentally at

ISSN:0021-2148    

DOI:10.1002/ijch.199100038

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractThe open‐shell coupled‐cluster method is used to calculate the potential functions of the five lowest states of Na2, at internuclear separation of 3.5 bohr ≤R≤ 10 bohr, with an 80—function contracted Gaussian basis. The usual complete model space calculation diverges, and a general, incomplete model space is used. Core (2s, 2p) correlation plays an important role. Very good agreement with experimentally known molecular constants is obtained when the 2s, 2p, and 3s electrons are c

ISSN:0021-2148    

DOI:10.1002/ijch.199100039

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY

摘要:

AbstractRelying on a semiempirical Pariser‐Parr‐Pople (PPP) model, it is shown that classical valence bond (VB) theory, using only a few covalent structures, can provide highly correlated energies and wave functions and thus a useful description of a molecular electronic structure, assuming that it is based on appropriate overlap‐enhanced atomic orbitals. Approximating the σ‐energy contribution by a quasi‐harmonic empirical potential, this model provides a faithful yet simple and transparent explanation of the interplay between electron derealization or resonance effects and the π‐electron localization tendency leading to bond length alternation in various π‐electron systems possessing aromatic, nonaromatic, or antiaro

ISSN:0021-2148    

DOI:10.1002/ijch.199100040

出版商:WILEY‐VCH Verlag    

年代:1991

数据来源: WILEY