NMR conformational analysis of chain molecules using the local interaction model. I. 2,4,-disubstituted pentanes

Abstract

The practical usefulness of the local interaction model in the NMR conformational analysis of chain molecules is demonstrated for 2,4-disubstituted pentanes, CH3CHRCH2CHRCH3, which are model compounds of vinyl polymers. In the present treatment based on this model, the relative energies of all the possible conformers are expressed as sums of the five kinds of intramolecular interaction energy parameters, which are determined to reproduce the observed temperature dependence of the vicinal spin-spin coupling constants between the methine and methylene protons. The method was applied to the analyses of pentane-2,4-diol, 2,4-dichloropentane and 2,4-diphenylpentane on the basis of the NMR data in the literature. The energy parameters, the conformer energies, and the populations were determined. The observed solvent effect in pentane-2,4-diol can be interpreted directly as the variation in the intramolecular interaction energy between the two hydroxyl groups, −1.0 to 0.8 kcal mol−1. In the analysis of this solvent effect, it is shown that the trans and gauche coupling constants, Jt and Jg, which are essential in NMR conformational analysis, can be obtained experimentally as Jt=11.5 ± 1.0 and Jg=2.1 ± 1.0 Hz. The energy parameters obtained in the present method may be used in calculations of the configurational statistics for polymer chains.