New theoretical insight into the thermal cis-trans isomerization of azo compounds: Protonation lowers the activation barrier

The detailed mechanism of thermal cis-trans inversion of protonated diazene s and azobenzenes has been investigated for the first time using ab initio Hartree-Fock (HF), density functional theory (DFT), second order Moller-Ple sset (MP2), and complete active space self consistent field (CASSCF) approa ches. Protonation of one of the nitrogens at the N=N bond leads to a consid erable (5-20 kcal/mol) reduction of the activation barrier. The magnitude o f this effect depends on the electron withdrawing nature of the substituent s and the differential stabilization of the transition state by positively charged functional groups. In the case of 4-phenylazopirydine protonated at pyridine nitrogen, the barrier is dramatically reduced to just 7 kcal/mol. This result establishes the lowest (close to an experimental estimate) ab initio value for the cis-trans isomerization for a substituted azobenzene. The structures of neutral and protonated transition states are similar, and the correlation effects for molecular parameters as well as the energy are negligible for extended basis set calculations. (C) 2001 American Institut e of Physics.