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A DFT study of the 1 H NMR chemical shifts, δ( 1 H), of geometric isomers of 18:3 conjugated linolenic acids (CLnAs), hexadecatrienyl pheromones, and model triene-containing compounds is presented, using standard functionals (B3LYP and PBE0) as well as corrections for dispersion interactions (B3LYP-D3, APFD, M06-2X and ωB97XD). The results are compared with literature experimental δ( 1 H) data in solution. The closely spaced "inside" olefinic protons are significantly more deshielded due to short-range through-space H … H steric interactions and appear close to or even beyond δ-values of aromatic systems. Several regularities of the computational δ( 1 H) of the olefinic protons of the conjugated double bonds are reproduced very accurately for the lowest-energy DFT-optimized single conformer for all functionals used and are in very good agreement with experimental δ( 1 H) in solution. Examples are provided of literature studies in which experimental resonance assignments deviate significantly from DFT predictions and, thus, should be revised. We conclude that DFT calculations of 1 H chemical shifts of trienyl compounds are powerful tools (i) for the accurate prediction of δ( 1 H) even with less demanding functionals and basis sets; (ii) for the unequivocal identification of geometric isomerism of conjugated trienyl systems that occur in nature; (iii) for tackling complex problems of experimental resonance assignments due to extensive signal overlap; and (iv) for structure elucidation in solution.

molecules/molecules online/molecules annual

DFT Calculations of 1H NMR Chemical Shifts of Geometric Isomers of Conjugated Linolenic Acids, Hexadecatrienyl Pheromones, and Model Triene-Containing Compounds: Structures in Solution and Revision of NMR Assignments