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

Recent NMR studies of a series of C-13-enriched carbohydrates and thei r derivatives have revealed that 1J(CC) in OH-C-C-OH fragments is affe cted by the C-C dihedral angle. This and other related correlations ar e explored in detail in this study by measuring and computing 1J(CC) v alues in simple model compounds. The ab initio computational methods u sed are validated through a comparison of absolute values and trends o bserved for a variety of calculated and experimental 1J(CC). Good agre ement with experiment is found when electron correlation is thoroughly treated at the sophisticated QCISD(T) level. Although 1J(CC) values c omputed at the SCF level are much larger than those observed experimen tally, the electron correlation corrections remain relatively independ ent of conformation, so that SCF calculations are very useful for exam ining trends. Results for ethane, ethanol, ethylene glycol, and glycol aldehyde hydrate indicate that 1J(CC) increases with the number of hyd roxyl substituents on the CC fragment. Calculations of the dependence of 1J(CC) on C-C torsion in ethylene glycol agree with experimental da ta 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 glyco l fragments depends to an even larger extent on the C-O torsions, reac hing a maximum when the hydroxyl proton is anti to a carbon and a mini mum in gauche configurations. Thus, in addition to the relative C-C to rsion, 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 v ariables 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 th ose found in aldofuranosyl and aldopyranosyl rings was also examined, using the trihydroxyl compound glycolaldehyde hydrate and D-mannopyran ose as model systems. Results indicate that the correlations found bet ween 1J(CC) and C-C and C-O torsions in ethylene glycol are also maint ained in these systems.