Chemistry of vibronic coupling. Part 2. How to maximize the dynamic diagonal vibronic coupling constant for T1 states in AB systems (A, B = H, Li, Na, K, Rb, Cs, F, Cl, Br or I)?

Abstract

The dynamic diagonal vibronic coupling constant (VCC) in several series of AB and AA molecules (A, B = H, Li, Na, K, Rb, Cs, F, Cl, Br or I) has been investigated. The electronic states considered are the singlet ground state (“ionic ” for heteronuclear AB species) and first excited singlet or triplet states (“covalent”). The VCC is thus studied for a charge transfer lowest lying triplet state. Qualitative trends in the VCC within the families of systems studied have been sought, with the aim of finding “ a chemistry of vibronic coupling”. Two interesting correlations emerge: the VCC for the charge transfer states in an AB system grows as the sum of the electronegativities of the A and B elements increases, as well as with decreasing AB bond length. A parameter f was defined as the sum of the electronegativities of the A and B elements divided by the AB bond length. This leads to a nearly monotonic correlation between computed values of VCC and f for 55 molecules originating from three distinct classes with a formal single bond: intermetallic species M¹M² (M = alkali metal), interhalogens X¹X² (X = halogen) and salt-like compounds MX. It emerges that contracted p-type orbitals making up the σ* MO (occupied by one electron in the excited state) seem to provide higher values of VCC than diffused s orbitals. The energy of the singlet–triplet gap is also correlated with the sum of the electronegativities of the A and B elements within two families of diatomics. Quantitative explanations of these two trends are still sought.