Effect of semi‐empirical parameters on the triplet energy levels and triplet–triplet transition in benzene

Abstract

AbstractsTriplet energy levels and triplet-triplet transition in benzene have been calculated semi-empirically by considering CI up to and including doubly excited configurations and using various values of the core resonance and electron repulsion integrals. The usual method of calibrating j3 (core resonance integral) from one of the observed transitions in the semi-empirical methods is critically examined with respect to the triplet levels of this molecule. Semi-empirical parameters are found to have a pronounced effect on the energy of the triplets and the triplet-triplet transitions; but the intensities of such transitions are quite insensitive to the choice of such parameters.Numerical results show several 3B,, + 3E,, transitions of various intensities. Out of these, the transitions which correspond energetically to the observed triplet-triplet bands are found to have low intensity. Some strong triplet-triplet bands are predicted in the far ultra violet region.On a calcult des niveaux d'tnergies de triplets et des transitions triplet-triplet pour le benzhe avec des mtthodes semi-empiriques. Les configurations deux fois excittes ont ttk inclues et des valeurs difftrentes des inttgrales de rtsonance et de rkpulsion ont t t t essaytes. La mtthode ordinaire pour calibrer j3 de l'une des transitions observh a t t t examinke critiquement par rapport aux niveaux triplets de la moltcule. L'hergie des triplets et les difftrences d'tnergie sont t r h sensibles aux choix des param&res, ce qui n'est pas le cas pour les intensit& des transitions.Les rtsultats indiquent plusieurs transitions 3 4 , --+ 3E2, d'intensites difftrentes. Parmi celles-ci, les transitions qui correspondent du point de vue de l'tnergie aux bandes observtes, ont une intensitt faible. Quelques bandes fortes dans la rtgion ultraviolette lointaines sont prtdites.

Some remarks on the core resonance integral as used in the semi‐empirical π‐electron calculations

“…Only the first 'A level of trans-butadiene and the second 3E,, level of benzene are quite far from their experimental values. But the second 3B,, + 3E2, transition of benzene at (9.89 -4.49) eV = 5.4 eV agrees well with the experimental value of 4.89 eV [8]. Moreover, in view of the strong mixing of the E,, configurations in benzene it has been emphasized [8] that the calculation of the triplet-triplet absorption in this molecule should take the doubly excited configurations also into account.…”

“…Bond lengths numbered 2, 3 and 5 in Table I have been taken for r(C = C) of ethylene and benzene and r(C = 0) of formaldehyde respectively, and those numbered 1 and 4 in Table I are [7]. The reason for this particular choice has already been described [8]. Only the ground and the singly excited configurations have been taken into account in all the calculations and the simplest symmetry orbitals of the molecules have been taken as the starting MO'S for carrying out the configuration interaction calculation.…”