CONFORMATION OF 1,2-DIMETHOXYETHANE IN THE GAS-PHASE - A ROTATIONAL ISOMERIC STATE SIMULATION OF NMR VICINAL COUPLING-CONSTANTS

Conformational characteristics of 1,2-dimethoxyethane (DME) in the gas phase have been studied by the NMR method. Observed H-1-H-1 and (C-H)-C-13-H-1 NMR vicinal coupling constants are compared with those previously determined in nonpolar solvents. The values observed in the gas phase and in solution do not exhibit any appreciable discontinuity at the transition. Rotational isomeric state simulations of these vicinal coupling constants yielded conformational energies of the gauche state relative to the trans in the gas phase: E(rho) = 1.0 kcal mol-1 for the C-O bond and E(sigma) = -0.4 kcal mol-1 for the C-C bond. In these treatments, the neighbor-dependent character of the bond rotation has been rigorously taken into account. The conformational energies thus determined were found to be nearly identical with those previously derived from the measurement in nonpolar solvents such as c-C6D12. Accordingly, it has been confirmed that the gauche oxygen effect, i.e., the gauche preference for the OC-CO bond, is not a feature caused by the surrounding solvent molecules in solution. This is contrary to the expectation emerging from the classical consideration of Coulombic interactions between partial charges residing on the oxygen atoms. The value of E(sigma) estimated above is inconsistent with the results of recent MO calculations (0.5 kcal mol-1) reported by Barzaghi et al. The discrepancy between the calculated and observed values amounts to about 1 kcal mol-1. Finally, the fractions of the conformer estimated in this work are compared with those derived from the electron diffraction measurement by Astrup. The agreement was found to be only qualitative.