LASER SPECTROSCOPIC AND QUANTUM CHEMICAL STUDIES OF THE LOWEST EXCITED-STATES OF FORMIC-ACID

A fluorescence excitation spectrum of formic acid monomer (HCOOH) has been recorded in the 268-257-nm region and has been attributed to an n → π* electron promotion in the anti conformer. The S0 ← S1 electronic origins of the HCOOH/HCOOD/DCOOH/DCOOD isotopomers were assigned to weak bands observed at 37431.5/37461.5/ 37445.5/37479.3 cm-1. Four vibrational modes, v3(C=O), v7(O-C=O), v8(C-Hald), and v9(O-Hhyd), were observed in the spectrum. The activity of the antisymmetric aldehyde wagging and hydroxyl torsional modes in forming progressions is central to the analysis, leading to the conclusion that the two hydrogens are distorted from the O-C=O molecular plane in the upper S1 state. The optimized structures and the geometries of formic acid in the S0 and T1 states were explored by ab initio SCF theory using the Gaussian 86 package of MO programs. In the upper T1 state, the potential surface that describes the OH torsion and the CH wagging motions was found to be complex. The OH and CH bonds were calculated to be twisted with respect to the O-C=O plane of the molecule by 67.99° and 45.87°, respectively. The calculations predicted a second stable rotamer which is 464 cm-1 above the equilibrium configuration with OH and CH angles displaced from the plane by -58.29° and 40.71°. The low-frequency motion of the excited electronic state is determined by two large-amplitude floppy-type motions. The first of these, v8, can be described as a molecular inversion of the CH aldehyde group while the second, v9, is a twisting motion of the OH bond with respect to the frame of the molecule. © 1990 American Chemical Society.