CONFORMATIONAL-ANALYSIS .21. ETHANE-1,2-DIOL - AN ELECTRON-DIFFRACTION INVESTIGATION, AUGMENTED BY ROTATIONAL-CONSTANTS AND AB-INITIO CALCULATIONS, OF THE MOLECULAR-STRUCTURE, CONFORMATIONAL COMPOSITION, SQM VIBRATIONAL FORCE-FIELD, AND ANTI-GAUCHE ENERGY DIFFERENCE WITH IMPLICATIONS FOR INTERNAL HYDROGEN-BONDING

The gas-phase molecular structure and anti-gauche composition of ethan e-1,2-diol (ethylene glycol) at 376 and 733 K has been analyzed from e lectron-diffraction data augmented by rotational constants for eight i sotopic species and with the help of results from ab initio calculatio ns. The system model consisted of three conformers, g(-)Ga, g(-)Gg, an d aAa, which were found to provide a good representation of the 10 pos sible different conformations. Scaled quantum-mechanical (SQM) force f ields in internal coordinates were also evaluated for these three conf ormers using a set of 13 scale constants determined by a least-squares fitting of the theoretical (HF/6-31G) force constants to observed wa ve numbers for the most abundant conformer, g(-)Ga. The mole fraction of conformers with anti vs gauche OCCO torsion angles (i.e. aAa vs. g( -)Ga + g(-)Gg) was found to be 0.08 (2 sigma = 7) at 376 K and 0.18 (2 sigma = 11) at 733 K, under conditions in which the mole ratios of g( -)Ga to g(-)Gg were maintained at the theoretical predictions of 0.58- 0.34 (376 K) and 0.40-0.42 (733 K). An estimate of the internal energy difference is Delta E degrees = 1.4 (5) kcal/mol for the reaction G - -> A and may also be taken as a rough value for the energy of the O-H ... O internal hydrogen bond. The value is consistent with estimates m ade in like fashion for other types of hydrogen bonds in 1,2-disubstit uted ethanes. Average values of some of the more important parameters (r(alpha)(0)/Angstrom; angle(alpha)/deg) with estimated 2 sigma uncert ainties at 376 K for the conformers g(-)Ga, g(-)Gg, and aAa, respectiv ely, are r(C-C) = {1.517, 1.521, 1.517} (5); r(C-O) = {1.424, 1.424, 1 .424} (1); r(O-H) = {0.961, 0.962, 0.960} (8), r(C-H) = {1.118, 1.117, 1.118} (6); angle CCO = 109.3 (4), 111.2 (7), 110.5 (30); angle COH = 105.8 (27), 108.2 (21), 109.6 (21); [angle CCH,OCH] = 109.1 (10), 108 .0 (11), 110.5 (21); angle OCCO = 60.7 (18), 57.5 (30), [180]. Values In curly brackets were refined as groups with differences between memb ers frozen.