Ab initio study of excited state protonation of monosubstituted benzenes

Vertical (PA(v)(B*)) and the adiabatic (PA(a)(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 So 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. (C) 2003 Elsevier B.V. All rights reserved.