All valency electron molecular orbital calculations

Publisher Website
Google Scholar
Abstract
The approximate self-consistent molecular orbital method CNDO/2 is applied to calculations on all the valency electrons of some fluorobenzenes and fluoronitrobenzenes. The charge distribution, which shows σ-electron acceptance and π-electron donation by the fluorine atoms, is in accordance with general chemical ideas. It leads to satisfactory values for the dipole moments of the fluorobenzenes and slightly less good values for the fluoronitrobenzenes. The spin densities on the carbon, nitrogen and oxygen atoms obtained for the anions of the fluoronitrobenzenes by the unrestricted form of the method give a satisfactory representation of the experimental hyperfine splitting constants. The elements of the bond order matrix are used with the average energy approximation and theoretical values for r -3 integrals to obtain relative shielding constants for the fluorine nuclei. The assumption made by other workers that the σ-electron terms are effectively constant for the fluorobenzenes is found to be justified. With the average energy as an empirical parameter good agreement is obtained with the experimental chemical shifts, and evidence for an ortho effect is found. The results suggest that the overall electronic structure obtained for these molecules by the CNDO/2 method is satisfactory; but that some modification of the values for the π-electron resonance integrals may be desirable.