Chemistry of Vibronic Coupling How To Maximize the Dynamic Diagonal and Off-Diagonal Vibronic Coupling Constants?

摘要

We investigate 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, I). 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. We look for qualitative trends in the VCC within the families of systems studied, 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 with increasing sum of electronegativities of A and B elements, as well as with decreasing AB bond length. We are led to define a parameter f as a sum of electronegativities of A and B elements divided by AB bond length. This leads to 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 M_1M_2 (M = alkali metal), interhalogen ones X_1X_2 (X = halogen) and salt-like ones MX. We also investigate computationally vibronic coupling for inter-valence charge-transfer states in linear symmetric ABA molecules (A, B = s―, p― or d-block element). In particular we examine vibronic coupling as a function of the s, p, d ― block nature of the A and B constituent elements. Based on density-functional theory computations for 395 triatomic molecules, we construct a map of a vibronic stability parameter G (defined as the ratio of asymmetric to symmetric stretching force constants) across the periodic table. Usually, the larger the sum of electronegativities, and the shorter the AB bond, the larger the vibronic instability. The largest vibronic instability thus occurs for interhalogen compounds. Molecules containing d-block elements exhibit trends similar to those of molecules built of p-block elements with similar electronegativities, although the latter are usually more unstable.