Molecular Orbital Perturbation Research Articles

Thiacyclobutadiene and thiabenzene: A comparative theoretical analysis

The two title compounds have been investigated theoretically using an ab initio SCF-MO treatment at the minimal STO-3G level. It has been found that in the most stable conformation thiacyclobutadiene has all atoms approximately in the same plane except for the H atom bonded to sulphur (i.e. a pyramidal sulphur). On the other hand, in the most stable conformation of thiabenzene, the sulphur atom not only is pyramidal, but also ~10° out of the plane containing the carbon system. The barriers to pyramidal inversion at the sulphur centres are ~48 and ~56 kcal mol for thiacyclobutadiene and thiabenzene, respectively. The properties of these molecules are rationalized by means of Perturbation Molecular Orbital (PMO) theory.

Perturbational molecular orbital (PMO) theory of homoaromaticity

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPerturbational molecular orbital (PMO) theory of homoaromaticityRobert C. HaddonCite this: J. Am. Chem. Soc. 1975, 97, 13, 3608–3615Publication Date (Print):June 1, 1975Publication History Published online1 May 2002Published inissue 1 June 1975https://doi.org/10.1021/ja00846a009RIGHTS & PERMISSIONSArticle Views246Altmetric-Citations33LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (856 KB) Get e-Alerts Get e-Alerts

Concerning periselectivity. A method for predicting the major route in thermal intermolecular two component cycloaddition reactions having manysymmetry-favoured pathways

The application of the Woodward-Hoffmann rules to thermal, intermolecular two-component cycloaddition reactions generates a set of 'symmetry favoured' pericyclic reactions. The problem of choosing, within this set, the most favoured reaction (i.e. periselectivity) is discussed. It has been found that a second order perturbational molecular orbital (PMO) treatment, employing Huckel eigenvectors and eigenvalues, and which is restricted to include only those terms containing highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, gives results which are in satisfactory agreement with the available experimental data. The treatment has been extended to cover the effect of substituents on the preferred mode of cycloaddition.

Double perturbation calculation of the Jgem nuclear spin-spin coupling constant of methane

A nonempirical treatment of the nuclear spin-spin coupling constant J gem of methane using a molecular orbital perturbation method is presented. Starting with the best LCAO-SCF MO's corresponding to a minimal basis set of Slater-type AO's, the value calculated for J gem by the usual second-order perturbation treatment is found to be exceedingly small (−1.27 cps). Agreement with experiment is recovered (−15.02 cps; exptl. −12.4 cps) if the third-order contributions coming from correlation effects are included by a double perturbation treatment with respect to the configuration interaction and the contact term. The same calculations have been repeated with equivalent quasilocalized orbitals, in order to analyze the various contributions in terms of bond excitations. A possible extension of these results to the problem of coupling constants between more distant nuclei is considered by means of a diagrammatic method.

Valency electron molecular orbital calculations

A self-consistent molecular orbital perturbation method suitable for use with the CNDO/2 approximations is formulated. The first-order perturbed equation is solved with an electric field perturbation for several molecules, and the polarizabilities are obtained. The results are too low, particularly for polarizabilities along short molecular axes, with a valency orbital basis set. The inclusion of 2p orbitals on hydrogen atoms gives improved results; but it is concluded that additional basis orbitals on other atoms are required for a valid interpretation of polarizabilities.

The Colour of Organic Compounds: Applications of the Perturbational Molecular Orbital (PMO) Method*

The perturbational molecular orbital (PMO) method is outlined and several applications of the technique to a study of the colour of organic molecules are discussed. Particular reference is made to steric effects in conjugated systems.

Theory of Rydberg Series in Polyatomic Molecules: H2O

The characteristic features of the Rydberg transitions in a polyatomic molecule are examined with emphasis on their extravalence-shell nature. It is made plausible that in molecular orbital perturbation theory the corresponding Hartree—Fock atomic orbital of principal quantum number n is a zero-order approx-imation to the Rydberg orbital. Calculations without integral approximations are carried out for the low-lying states of H2O. The ASP—SCF—LCAO—MO treatment of the H2O ground state provides the core. The mean singlet—triplet absorption spectrum of H2O is fairly well reproduced by the calculation. The calculated oscillator strength of B1(3s)←A1, 1×10—2, and the calculated symmetry-induced splitting of the 3p Hartree—Fock atomic orbital in C2v symmetry into B1 and A1 components, 2200 cm—1, are also in satisfactory agreement with experiment, 3×10—2 and 1500 cm—1, respectively.

Study of the H6 Ring in the Molecular Orbital and Alternant Molecular Orbital Approximation

A configuration interaction calculation in a molecular orbital basis has been carried out on a system of six hydrogen atoms arranged in a regular hexagon. The alternant molecular orbital and second-order perturbation theory approximations have been investigated. All multicentered integrals have been accurately computed.

Molecular orbital perturbation theory II. Charge displacement and stabilization in conjugated molecules

The general perturbation method outlined in part I is used to study energy and charge distribution changes caused by inductive and electromeric-type perturbations in conjugated molecules. Self-consistent orbitals for the unperturbed systems are found by approximating electron interaction integrals and are used as a basis for the perturbed systems. It is found that the change of charge distribution caused by an inductive substituent in benzene is similar to that predicted by the independent-electron model. Second-order perturbation theory is used to discuss electromeric displacements in which lone pair electrons take part in conjugation.

Molecular orbital perturbation theory II. Charge displacement and stabilization in conjugated molecules

This paper develops a method of investigating changes in the electronic structure of a molecule caused by a perturbation such as a substituent or the field of another particle. The method is based on a Hamiltonian including electron interaction. The total wave function for the perturbed molecule is written as a linear combination of approximate self-consistent molecular orbital functions for the ground and excited states of the unperturbed system . First- and second-order expressions for changes in energy and charge distribution are given which are generalizations of results previously obtained using an independent-electron model.