1H NMR spectral analysis and conformational behavior of n‐alkanes in different chemical environments

Alkyl chains are common structural units, for example in lipids, and their 1H NMR spectral parameters offer valuable information about their conformational behavior in solvent environment. Even the spectra of short n‐alkanes are complex, which is obviously a reason why their accurate spectral analyses have not been reported before. The present study reports the quantum mechanical analysis of 1H NMR spectra of n‐butane, n‐pentane, n‐hexane, and n‐heptane. The spectral parameters were used to characterize the conformational behavior of n‐alkanes. The temperature dependence analysis of coupling constants suggests that the enthalpy difference between the gauche (g) and trans (t) conformations (ΔHg) of n‐butane in chloroform is 2.55–2.85 kJ mol−1. The difference between the trans–gauche (tg) and all‐trans (tt) conformers of n‐pentane (ΔHtg) seems to be 0.1–0.2 kJ mol−1 higher. The coupling constant information shows that the tn conformations become more favored with longer chains, although not only for energetic reasons but also partly because the g+g‐ arrangements become sterically unfavorable, which decreases the number of favorable gn‐type conformations. The analysis of the 1H NMR spectra of n‐pentane and n‐hexane in solvents representing different chemical environments indicates that polar and spherical dimethyl sulfoxide favors clearly the g conformations, whereas n‐hexane‐d14 favors slightly the extended tn conformation. In addition to the intrinsic scientific importance for NMR spectral parameter prediction and molecular modeling in solution, the results provide some insights to behavior of hydrocarbon chains and their spectra in different chemical environments. Copyright © 2012 John Wiley & Sons, Ltd.

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