Abstract:
A fluorescence excitation spectrum of formic acid monomer
(HCOOH) , has been recorded in the 278-246 nm region and has
been attributed to an n >7r* electron promotion in the anti
conformer. The S^< S^ 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'''. From
a band contour analysis of the 0°^ band of HCOOH, the
rotational constants for the excited state were estimated:
A'=1.8619, B'=0.4073, and C'=0.3730 cm'\ Four vibrational
modes, 1/3(0=0), j/^(0-C=0) , J/g(C-H^^^) and i/,(0-H^yJ 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
molecular plane, 0-C=0, in the upper S. state.
Ab initio calculations were performed at the 6-3 IG* SCF
level using the Gaussian 86 system of programs to aid in the
vibrational assignments. The computations show that the
potential surface which describes the low frequency OH torsion
(twisting motion) and the CH wagging (molecular inversion)
motions is complex in the S^ excited electronic state. The
OH and CH bonds were calculated to be twisted with respect to
the 0-C=0 molecular frame by 63.66 and 4 5.76 degrees,
respectively. The calculations predicted the existence of the second (syn) rotamer which is 338 cm'^ above the equilibrium
configuration with OH and CH angles displaced from the plane
by 47.91 and 41.32 degrees.
