Effect of solvent fluctuation on the electronic transitions of formaldehyde in aqueous solution

The absorption spectra of n pi* and pi pi* transitions in formaldehyde aqueous solution were studied by the reference interaction site model self-consistent-field (RISM-SCF) method. The electrostatic potential fluctuations acting on the solute sites originating from the solvent fluctuations were obtained by calculating the derivative of the solute-solvent radial distribution function analytically, and these were utilized to estimate the spectral bandwidths. The contribution from the solute vibrations was also examined. As a result, a blue shift of 1998 with bandwidth of 2987 cm(-1) was obtained for then n pi* transition. The pi pi* transition, on the other hand, showed a redshift of 1598 with the bandwidth of 5474 cm(-1). The solvent fluctuation effect contributes to the bandwidths by 617 and 137 cm(-1) for then pi* and pi pi* transition, respectively. We further analyzed the simulated absorption band shapes using effective charges on the atoms and the charge response kernel calculated for each state. (C) 1999 American Institute of Physics. [S0021-9606(99)50607-7].