Chemistry of vibronic coupling. Part 2. How to maximize the dynamic diagonal vibronic coupling constant for T-1 states in AB systems (A, B=H, Li, Na,K, Rb, Cs, F, Cl, Br or I)?

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 in vestigated. 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 v ibronic coupling''. Two interesting correlations emerge: the VCC for the ch arge transfer states in an AB system grows as the sum of the electronegativ ities 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 molecule s originating from three distinct classes with a formal single bond: interm etallic species (MM2)-M-1 (M=alkali metal), interhalogens (XX2)-X-1 (X=halo gen) and salt-like compounds MX. It emerges that contracted p-type orbitals making up the sigma* MO (occupied by one electron in the excited state) se em 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 electronegat ivities of the A and B elements within two families of diatomics. Quantitat ive explanations of these two trends are still sought.