Theory versus Experiment in Jet Spectroscopy: Glycolic Acid

Ab initio MP2/6-31G(d,p) methods have been used to examine in detail the structures and relative energies of the conformers of glycolic acid and to investigate the pathways and energy barriers involved in possible conformational interconversions. From the results of these calculations it has been shown that if an adequate volatilization temperature is used then two glycolic acid species, not necessarily the two of lowest energy, should be detectable via free jet expansion microwave spectroscopy. We tested this prediction experimentally. The spectrum of a new species ( y ) was detected and shown via comparison of the predicted with observed rotational constants, planar moments of inertia, and isotopic substitution coordinates to be most consistent with the trans -carboxyl theoretical conformer { 13, 18 } (see Figure 2). Previous microwave spectroscopy studies had found only one glycolic acid conformer ( x ). The detection of both ( x ) and ( y ) is fully consistent with the relative energy and conformer relaxation predictions from the ab initio calculations. Conformer ( y ) is similar to a species previously reported from matrix isolation infrared spectroscopy.