Ab initio study of the n-pi(*) electronic transition in acetone: Symmetry-forbidden vibronic spectra

Ab initio calculations of geometry and vibrational frequencies of the first singlet excited (1)A(2)((1)A ") state of acetone corresponding to the n-pi * electronic transition have been carried out at the CASSCF/6-311G** level. The major geometry changes in this state as compared to the ground state i nvolve CO out-of-plane wagging, CO stretch and torsion of the methyl groups , and the molecular symmetry changes from C-2v to C-s. The most pronounced frequency changes in the (1)A " state are the decrease of the CO stretch fr equency v(3) by almost 500 cm(-1) and the increase of the CH3 torsion frequ ency v(12) from 22 to 170 cm(-1). The optimized geometries and normal modes are used to compute the normal mode displacements which are applied for ca lculations of Franck-Condon factors. Transition matrix elements over the on e-electron electric field operator at various atomic centers calculated at the state-average CASSCF/6-311+G** level are used to compute vibronic coupl ings between the ground (1)A(1), (1)A(2), and Rydberg B-1(2)(n-3s), 2 (1)A( 1)(n-3p(y)), 2 (1)A(2)(n-3p(x)), 2 B-1(2)(n-3p(z)), and B-1(1)(n-3d(xy)) el ectronic states, and the Herzberg-Teller expansion of the electronic wave f unction is applied to derive the transition dipole moment for (1)A(1)-->(1) A(2) as a function of normal coordinates. The results show that the intensi ty for this transition is mostly borrowed from the allowed (1)A(1)-B-1(2)(n -3s) transition due to vibronic coupling between (1)A(2) and B-1(2) through normal modes Q(20), Q(22), and Q(23) and, to some extent, from the (1)A(1) -B-1(1) transition due to Q(19) (CO in-plane bend) which couples (1)A(2) wi th B-1(1)(n-3d(xy)). The calculated total oscillator strength for the n-pi( *) transition through the intensity-borrowing mechanism, 3.62x10(-4), is in close agreement with the experimental value of 4.14x10(-4). Ninety-four pe rcent of the oscillator strength comes from the perpendicular component (b( 1) inducing modes) and 6% from the parallel component (b(2) modes). Calcula ted spectral origin, 30 115 cm(-1) at the MRCI/6-311G** level, underestimat es the experimental value by similar to 300 cm(-1). Calculated positions of the most intense peaks in the spectra also reasonably agree with the exper imental band maximum. The presence of numerous weak vibronic peaks densely covering a broad energy range (similar to 12 000 cm(-1)) explains the diffu se character of the experimental n-pi(*) band. Most of the bands observed i n fluorescence excitation spectra [Baba and Hanazaki, Chem. Phys. Lett. 103 , 93 (1983); Baba, Hanazaki, and Nagashima, J. Chem. Phys. 82, 3938 (1985)] can be assigned based on the computed spectrum. (C) 1999 American Institut e of Physics. [S0021-9606(99)30324-X].