TORSIONAL EFFECTS ON THE ONE-BOND C-13-C-13 SPIN COUPLING-CONSTANT IN ETHYLENE-GLYCOL - INSIGHTS INTO THE BEHAVIOR OF (1)J(CC) IN CARBOHYDRATES

Recent NMR studies of a series of C-13-enriched carbohydrates and their derivatives have revealed that 1J(CC) in OH-C-C-OH fragments is affected by the C-C dihedral angle. This and other related correlations are explored in detail in this study by measuring and computing 1J(CC) values in simple model compounds. The ab initio computational methods used are validated through a comparison of absolute values and trends observed for a variety of calculated and experimental 1J(CC). Good agreement with experiment is found when electron correlation is thoroughly treated at the sophisticated QCISD(T) level. Although 1J(CC) values computed at the SCF level are much larger than those observed experimentally, the electron correlation corrections remain relatively independent of conformation, so that SCF calculations are very useful for examining trends. Results for ethane, ethanol, ethylene glycol, and glycolaldehyde hydrate indicate that 1J(CC) increases with the number of hydroxyl substituents on the CC fragment. Calculations of the dependence of 1J(CC) on C-C torsion in ethylene glycol agree with experimental data for carbohydrates, indicating that the coupling is largest when the hydroxyl substituents are trans and smaller for gauche geometries. A significant new finding in this study is that 1J(CC) in ethylene glycol fragments depends to an even larger extent on the C-O torsions, reaching a maximum when the hydroxyl proton is anti to a carbon and a minimum in gauche configurations. Thus, in addition to the relative C-C torsion, it is also important to consider the conformational behavior of the C-O bonds. This observation imposes limitations on the use of 1J(CC) as a structural probe, as in some cases information about C-C and C-O torsions will not be available. In situations where one of these variables is known (more likely the C-C torsion), 1J(CC) may be useful to probe the remaining variable (e.g., hydroxyl proton orientation in solution). The behavior of 1J(CC) in (OH)2-C-C-OH fragments such as those found in aldofuranosyl and aldopyranosyl rings was also examined, using the trihydroxyl compound glycolaldehyde hydrate and D-mannopyranose as model systems. Results indicate that the correlations found between 1J(CC) and C-C and C-O torsions in ethylene glycol are also maintained in these systems.