engleski

Ab initio study of excited state protonation of monosubstituted benzenes

Vertical (PAv(B*)) and the adiabatic (PAa(B*)) proton affinities of the lowest excited pi, pi* singlet state are calculated for a series of monosubstituted benzenes Ph-X (X=CHO, (1) ; X=COCH3, (2) ; X=NO2, (3) ; X=CN, (4) ; X=NH2, (5) and X=NO, (6)) by applying CASPT2/6-31+G*//HF/6-31G* and CASPT2/6-31+G*//CIS/6-31+G* methods. Excited state proton affinities of 1, 2, 3 and 6 are found to be larger by 20 - 30 kcal/mol than in the S0 states, while PAs of 4 and 5 appear to be of the same order of magnitude in both states. Geometrical relaxation leads in most cases to a slight increase in the PA(B*) values and does not influence relative basicity ordering across the series. Results are in reasonable agreement with the experimental data obtained by ICR technique.

excited state proton affinities; substituted benzenes; CASPT2-CASSCF calculations; CIS geometries

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