SCF MO calculations of polyene spectra

Abstract

A computationally simple "variable fl" procedure within the Pariser-ParrPople formalism has been applied with success to the calculation of aromatic hydrocarbon spectra [1]. The method has now been extended to encompass linear polyenes and cr co-diphenylpolyenes. In the variable/7 approach the precise molecular geometry need not be specified and it is not necessary to assume bond alternation. Bond lengths were all set at 1.40 tix and bond angles at 120 ~ The core resonance integrals,/~ ---Alp + Ao, were adjusted at each iteration but the twocenter repulsion integrals, ypq = 14.394/(1.294 + rpq) eV were kept fixed. A 1 --0.51 eV is appropriate for all C-C bonds [1] and a good fit for the trans-butadiene spectrum is obtained with A 0 = -2 . t0 eV. Since this latter quanti ty is close to the value used for benzene, it was employed for all bonds in the polyenes and diphcnylpolyenes. PLATT has classified the polyene transitions by means of configuration interaction t rea tment [2]. The transitions to 1B, ~D, and 1F (PLATT'S 1D2) are allowed. Labeling the occupied orbitals l, 2, 3 . . . (in order of decreasing energy) and the vacant orbitals t ' , 2', 3' . . . (in order of increasing energy), 1B +1A is essentially I -~ t ' , but I -+ 2' and 2 -+ 1', degenerate in the zeroth order, are mixed by configuration interaction to form the IC and 1L c states. Transitions to both of these states arc forbidden in the all-trans isomers, but 1C +1A becomes allowed when the center of symmetry is destroyed (cis band) I f the SCF MO method with extensive configuration interaction is to be useful for routine computations, the results cannot be greatly dependent on the amount of configuration interaction. To obtain the higher transition energies it is useful to include the nine configurations arising from the set 3, 2, t --, t ' , 2', 3'. For convenience in machine calculations additional configurations were included. This only results in an increase in the number of low-lying states of 1L character, to which transitions are forbidden in all configurations, but the energies and intensities of the allowed transitions are essentially unaltered. The linear polyene results are compared with experiment in Tab. 1. This comparison is complicated by solvent effects [3], but the results are comparable to those obtained by assuming bond alternation [3, 4]. The oscillator strengths calculated by a "molecules in molecules" method [3] were smaller than those observed, but in the present work the calculated f nos. exceed experiment, a result