Comparison of the molecular structure and spectra of benzene and borazine

Abstract

Ab initio SCF MO and CI calculations for two different gaussian basis sets are carried out for the isoelectronic molecules benzene C6H6 and borazine B3N3H3 in order to investigate the effect of increasing the flexibility in the representation of their respective π systems. In the process it is found from comparison of orbital charge density contour diagrams and inner shell orbital energies of borazine with analogous data for other systems that the BN bonds of this compound are considerably less polar (B+N−) than that of ammonia borane BNH6 (B−N+). CI calculations employing the larger basis set produce generally better agreement with the experimental transition energies of benzene than do those using the smaller basis. In both treatments 1 E 2g is predicted to lie lower than 1 E 1u , in contrast to single excitation results; an analogous inversion in the calculated order of states is not observed in the study of borazine but the effect of double and triple excitation configurations is also found to be quite important for this system. For both benzene and borazine the CI calculations suggest in addition that transitions to σ→π * states occur at relatively low energy, perhaps within 0.5 eV of the strongest π→π * absorption in each case.