Semiempirical π-electron models for the calculation of MCD B terms for systems with approximate alternant pairing symmetry. MCD of biphenylene

Abstract

The recent success of the PPP model in predicting MCD B terms for π-electron transitions in conjugated molecules has not been complete. When applied to alternant hydrocarbons, the pairing symmetry contained in the model leads to a prediction of vanishing B terms for all transitions, in disagreement with experimental observations. In order to calculate the MCD for such “soft” chromophores, complete inclusion of overlap between the basis 2p orbitals in π-electron calculations was investigated. The resulting wave functions did not have the perfect pairing symmetry, and the calculated nonzero B terms of benzenoid hydrocarbons were generally in satisfactory agreement with experiment, provided the choice of parameters did not cause large deviation from perfect pairing. The results also provide an empirical justification for a simplified and, strictly speaking, inconsistent π-electron procedure in which non-neighbor matrix elements are kept for the linear momentum operator but set equal to zero for the core Hamiltonian. Experimental MCD and LD spectra of biphenylene, a 4N-electron perimeter, are presented and compared with the theoretical results.