Conformation of 1,3-dimethoxypropane in the gas phase and in solution: rotational isomeric state simulation of NMR vicinal coupling constants

Abstract

Conformation of 1,3-dimethoxypropane has been studied in the gas phase (approximately 120–180°C). Vicinal coupling constants 3 J HH and 3 J CH determined by 1H and 13C 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 1HCC 1H, and J G = 2.0 Hz and J T + J′ G = 16.0 Hz for 13COC 1H. The conformational energy parameters of the chain were conventionally defined as Eσ and Eϱ for the gauche state around the CH 2CH 2 and OCH 2 bond, respectively, and Eω for the repulsive second-order interaction (g ±g ∓) for the OCH 2CH 2CH 2O fragment. The experimental values of the coupling constants were found to be reproduced by a variety of combinations of Eσ and Eω, Eϱ being kept at 1.03 kcal mol −1. Adoption of Eω = 1.97 kcal mol −1, as calculated by a molecular mechanics method, led to Eσ = −0.84 kcal mol −1. Alternatively, by setting Eσ = −0.58 kcal mol −1 as calculated, Eω = 0.40 kcal mol −1. Assuming Eω > 1 kcal mol −1 for the gas phase, a value of Eσ 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.