Ab Initio Study of Aromatic Side Chains of Amino Acids in Gas Phase and Solution

The vertical transition properties have been calculated for the valence electronic excited ππ* singlet states 1 L b, 1 L a, 1 B b, and 1 B a of the chromophores benzene, phenol, and indole in bulk solvent. The polarizable continuum model was employed in the framework of a complete active space self-consistent reaction field with use of an atomic natural orbital basis set. Dynamical electron correlation was accounted for through the use of second-order multiconfigurational perturbation theory. The predicted solvatochromic shifts are compared with previous experimental and computational results. We find general agreement, in particular for the 1 L b state of each species, where a red-shift is observed in bulk solvent. To our knowledge this is the first ab initio self-consistent reaction field study of the high-lying 1 B b and 1 B a states of benzene, phenol, and indole in bulk solvent. At the correlated level, the 1 B b and 1 B a states undergo red-shifts for benzene, shifts to the red and to the blue respectively for phenol, and shifts to the blue for indole.