Density functional theory calculations of δ(13 C) and δ(1 H) chemical shifts and 3 J(13 COO1 H) coupling constants as structural and analytical tools in hydroperoxides: Prospects and limitations of 1 H13 C heteronuclear multiple bond correlation experiments

Abstract

Density functional theory (DFT) calculations of δ(¹³ C) and δ(¹ H) chemical shifts and ³ J(¹³ COO¹ H) coupling constants of three model hydroperoxides of the naturally occurring cis-11-OOH and trans-9-OOH isomers of oleate and 9-cis, 11-trans-16-OOH endo hydroperoxide of methyl linolenate are reported. The computational δ(OOH) for various functionals and basis sets were found to be nearly identical for the cis/trans geometric isomers. The chemical shifts of the methine CHOOH protons and carbons, on the contrary, are highly diagnostic for the identification of cis/trans geometric isomerism. The chemical shifts of the olefinic protons and carbons strongly depend on the orientation of the hydroperoxide unit relative to the double bond and, thus, of importance in conformational analysis. The results are in very good agreement with the available experimental data. For the various diastereomeric pairs of the model endo-hydroperoxide, the strongly deshielded OOH resonances, due to the presence of an intramolecular hydrogen bond between the hydroperoxide proton and an oxygen of the endo-peroxide ring, along with the δ(CHOOH), are highly diagnostic for identification and structure elucidation of complex erythro- and threo- diastereomeric pairs of endo-hydroperoxides; the computational results are in very good agreement with the available experimental data. The ³ J(¹³ COO¹ H) coupling constants were found to be < $\left(2\right)$ Hz for the cis-trans geometric models and < $\left(0.5\right)$ Hz for the endo-hydroperoxide and, thus, unimportant in stereochemical analysis. Sharp resonances of the hydroperoxide protons, with Δν_1/2 < 3 Hz, are required for the successful implementation of the ¹ H¹³ C heteronuclear multiple bond correlation (HMBC) technique.

Keywords: 1H13C HMBC; DFT; chemical shifts; coupling constants; hydroperoxides.