Chemistry of Vibronic Coupling
Vibronic Interactions: Jahn-Teller Effect in Crystals and Molecules
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 ÏÏ for heteronuclear AB species) and Ðrst excited singlet or triplet states ("" covalent ÏÏ). 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
... en sought, with the aim of Ðnding "" a chemistry of vibronic coupling ÏÏ. 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 deÐned 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 r* MO (occupied by one electron in the excited state) seem to provide higher values of VCC than di †used s orbitals. The energy of the singletÈtriplet 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.