The ultraviolet absorption spectrum of oxalic acid vapor

The ultraviolet absorption spectrum of oxalic acid vapor has been studied for the first time. Low-resolution spectra show a moderately weak (εmax = 25 M−1 cm−1 at 277 nm) banded absorption system beginning at about 310 nm and extending to 250 nm where a stronger absorption begins. Medium-resolution spectra show a complex vibrational fine-structure at wavelengths above about 287 nm, with many sharp line-like R-branch heads, each corresponding to a different vibrational transition. A vibrational analysis was completed for 12C2O4H2, 12C2O4D2, and 13C2O4H2, and electronic origins were estimated to be at 31903.2, 31970.2, and 31949.4 cm−1, respectively (about 313 nm). Strong features were assigned to progressions in [Formula: see text] the C=O and C—O stretching vibrations, with prominent sequences in ν10, the C—C torsional vibration, running to the blue, implying that [Formula: see text].This absorption system has been assigned to the first excited singlet state, designated Ã, corresponding to a π* ← n+, 1Au ← 1Ag transition of planar trans-oxalic acid. High-resolution spectra of several bands show rotational fine-structure, and a partial analysis was carried out on the [Formula: see text], and [Formula: see text] bands of 12C2O4H2, based on apparently-simple diverging P-branches.