CONFORMATION OF 1,3-DIMETHOXYPROPANE IN THE GAS-PHASE AND IN SOLUTION - ROTATIONAL ISOMERIC STATE SIMULATION OF NMR VICINAL COUPLING-CONSTANTS

Conformation of 1,3-dimethoxypropane has been studied in the gas phase (approximately 120-180 degrees C). Vicinal coupling constants (3)J(HH) and (3)J(CH) determined by H-1 and C-13 NMR were analysed within the rotational isomeric state approximation. The trans and gauche couplings required in this analysis were set equal to J(G) = 2.8 Hz and J(T) = 13.7 Hz for H-1-C-C-H-1, and J(G) = 2.0 Hz and J(T) + J'(G) = 16.0 Hz for C-13-O-C-H-1. The conformational energy parameters of the chain were conventionally defined as E sigma and E rho for the gauche state around the CH2-CH2 and OCH2 bond, respectively, and E omega for the repulsive second-order interaction (g(+/-)g(-/+)) for the OCH2-CH2-CH2O fragment. The experimental values of the coupling constants were found to be reproduced by a variety of combinations of E sigma and E omega, E rho being kept at 1.03 kcal mol(-1). Adoption of E omega = 1.97 kcal mol(-1) as calculated by a molecular mechanics method, led to E sigma = -0.84 kcal mol(-1). Alternatively, by setting E sigma = -0.58 kcal mol(-1) as calculated, E omega = 0.40 kcal mol(-1). Assuming E omega > 1 kcal mol(-1) for the gas phase, a value of E sigma from -0.7 to approximately -0.9 kcal mol(-1) was proposed. The conformation, and thus conformational energies, varied quite sensitively with the polarity of the medium. The results were compared with those observed in solution.