DFT Calculations of ¹ H and ¹³ C NMR Chemical Shifts of Hydroxy Secondary Oxidation Products of Geometric Isomers of Conjugated Linoleic Acid Methyl Esters: Structures in Solution and Revision of NMR Assignments

Detailed DFT studies of ¹H and ¹³C NMR chemical shifts of hydroxy secondary oxidation products of various geometric isomers of conjugated linolenic acids methyl esters are presented. Several low energy conformers were identified for model compounds of the central dienenol OH moiety, which were found to be practically independent on the various functionals and basis sets used. This greatly facilitated the minimization process of the geometric isomers of conjugated linolenic acids methyl esters. Several regularities of the literature experimental ¹H and ¹³C chemical shifts were reproduced very accurately with the computational chemical shifts of the Gibbs low energy DFT optimized conformers, after a Boltzmann analysis. δ(¹³C) and δ(¹H) of the methine CH-OH group are highly diagnostic for the trans/trans and cis/trans geometric isomerism of the adjacent double bond. δ(¹³C) of the -CH₂- group adjacent to the terminal double bond of the conjugated system strongly depend on the cis/trans geometric isomerism of this bond and, thus, could be of importance in structural analysis. Ambiguities in the reported literature resonance assignments of olefinic carbons had been resolved. Computational δ(¹H) and δ(¹³C) can be utilized for the identification of geometric isomerism and structural and conformational elucidation of hydroxy derivatives of conjugated linoleic acids and their ester derivatives.