THE TORSIONAL - WAGGING BAND STRUCTURE IN THE n $\rightarrow$ $\pi^{*}$, $S_{0}$ $\rightarrow$ $S_{1}$ ELECTRONIC SPECTRUM OF FORMIC ACID

A fluorescence excitation spectrum of formic acid monomer HCOOH, has been recorded in the 278 - 246 nm region and has been attributed to n $\\rightarrow$ $\\pi^{*}$ electron promotion in the anti conformer. Four vibrational modes $v_{3}$ (C=O), $V_{7}$ (O-C=O), $v_{8}$(C-$H_{eld}$.) and $v_{9}$(O-$H_{hyd}$.) were observed in the spectrum. The activity of the antisymmetric aldehyde wagging and hydroxyl torsional modes in forming progressions leads to the conclusion that the two hydrogens are Distorted from the plane in the upper $S_{1}$ state. Ab initio molecular calculations show that the OH and CH bonds are twisted with respect to the O-C=O plane by 63.7 and 45.8 degrees. The calculations predict a second (syn) rotamer which is $338 cm^{-1}$ above the equilibrium position.