LCAO MO Calculations Research Articles

Self-consistent Field Molecular Orbital Treatment of Benzenium Ion for a Simplified Model of Hyperconjugation

Abstract Semi-empirical SCF LCAO MO calculations of benzenium ion was undertaken for the model of localization as well as hyperconjugation of the H2–C bond constructed at the position of proton attack. A simplified model of hyperconjugation in the semi-empirical SCF MO method was extended. Calculated results for the hyperconjugation model agreed satisfactorily with the experimental electronic spectra. Superiority of the SCF MO method to the Hückel MO or ASMO method was indicated in connection with the ordinarily accepted spectral shift through hyperconjugation. Further, some modifications were tried, for the bond localization model, which would have to be taken in the Hückel MO as well as ASMO calculations. The proton magnetic resonance signals due to methyl substituents in the carbonium ions of methylbenzenes were discussed from charge densities of benzenium ion.

On the non-orthogonality problem in the semi-empirical MO-LCAO method

An explicit correlation between semi-empirical MO-LCAO calculations forπ-systems including and not including overlap is derived and discussed under the assumption that overlap integrals, when taken into account, are proportional to the corresponding bond integrals. It is shown that, so far as atomic charges are concerned, the introduction of overlap makes practically no difference in the results. On the contrary all other quantities undergo sometimes serious changes. This is shown by means of a few numerical examples; it gives some support to the idea that the practice of neglecting overlap may not be so sound as has been believed.

Zur Deutung der UV.‐Absorptionsspektren von β,γ‐ungesättigten Ketonen

AbstractThe UV‐spectrum of β,γ‐unsaturated ketones has been studied on the basis of simple LCAO‐MO calculations and an interpolation between the UV.‐spectra of substances with isolated (CO)‐ and (CC)‐groups and of α,β‐unsaturated ketones.

THE LOWER EXCITED STATES OF SOME SIMPLE MOLECULES

The present and prospective rapidly increasing usefulness of good LCAO-MO calculations on the electronic states of simple molecules in the interpretation of observed excited states is pointed out. As examples, the observed and predicted states of, in particular, the π3π configurations of N2 and CO are compared with those of C2H2 and HCN and with those of CO2 and CS2. Further, the results of LCAO-SCF calculations on CO2 and on O3 are surveyed, and it is shown how these can be helpful in interpreting and understanding the ground and excited states of AB2 (expecially AO2) molecules in general. A new interpretation of the so-called d3П state of CO as a case b πu3πg, 3Δu state is proposed. Tentative interpretations of some of the ultraviolet absorption spectra of C2H2, HCN, and of a number of AB2 molecules are reviewed or suggested, including some discussion of the shapes of excited states. The AB2 discussion is a revision of one given earlier. Finally, following up a suggestion of Coon, it is pointed out that there exists strong evidence for slightly unequal A—O distances in certain excited states of C1O2 and SO2.

Hyperconjugation. I. General Theory and Its Applications to Simple LCAO MO Calculations

Abstract The theory of hyperconjugation has been extended and applied to computations of the electronic structures of methyl and ethylsubstituted acetylenes and benzenes using the empirical LCAO MO method not including the overlap integrals. The common empirical parameters introduced from experimental aspects were used consistently through the calculations. Charge distributions of the π-electrons, bond orders, resonance moments, relative shifts of near ultraviolet absorption spectra and oscillator strengths were calculated, and fairly satisfactory results, compared with the observed values, were obtained. Because of the lack of appropriate absorption data, the near ultraviolet absorption spectra of alkylbenzenes in normal hexane were obtained. The calculations show that when the methyl group is substituted for a hydrogen atom of the methyl group, the hyperconjugative effect is recognized as same as before.

LCAO MO Study of the Phenomenon of Thermochromism

An LCAO MO calculation of the energies of the ground, planar triplet, and perpendicular triplet states of bianthrone and bixanthylene indicates that the thermally excited state in the phenomenon of thermochromism is the perpendicular triplet. Because of the uncertainty of the correct values of the parameters used, the best correspondence between the calculated and observed excitation energies that could be expected is that of order of magnitude. This is shown to be the case. The nature of the variation of the calculated values with the choice of parameters is indicated.