DFT Calculations of 1H- and 13C-NMR Chemical Shifts of Geometric Isomers of Conjugated Linoleic Acid (18:2 ω-7) and Model Compounds in Solution

Abstract

A density functional theory (DFT) study of the 1H- and 13C-NMR chemical shifts of the geometric isomers of 18:2 ω-7 conjugated linoleic acid (CLA) and nine model compounds is presented, using five functionals and two basis sets. The results are compared with available experimental data from solution high resolution nuclear magnetic resonance (NMR). The experimental 1H chemical shifts exhibit highly diagnostic resonances due to the olefinic protons of the conjugated double bonds. The “inside” olefinic protons of the conjugated double bonds are deshielded than those of the “outside” protons. Furthermore, in the cis/trans isomers, the signals of the cis bonds are more deshielded than those of the trans bonds. These regularities of the experimental 1H chemical shifts of the olefinic protons of the conjugated double bonds are reproduced very accurately for the lowest energy DFT optimized single conformer, for all functionals and basis sets used. The other low energy conformers have negligible effects on the computational 1H-NMR chemical shifts. We conclude that proton NMR chemical shifts are more discriminating than carbon, and DFT calculations can provide a valuable tool for (i) the accurate prediction of 1H-NMR chemical shifts even with less demanding functionals and basis sets; (ii) the unequivocal identification of geometric isomerism of CLAs that occur in nature, and (iii) to derive high resolution structures in solution.

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

“…According to the semi empirical model of Cheney [ 51 ], a much better agreement with the experimental deshielding effect was obtained by correlation with f ( r i , θ i ) than with the exponential exp(− ar i ) term, where r i is the distance between the interacting pair of hydrogens and θ i is the angle between H … H internuclear line and the H–C bond of interest. The above results clearly demonstrate that the deshielding effect due to non-bonded H … H repulsive interaction is a primary factor affecting δ( 1 H) in the trienyl systems reported therein, as in the case of simple alkenes, conjugated dienyl, and aromatic systems [ 47 , 50 , 51 , 57 ].…”

“…The optimized geometries were verified by performing frequency calculation at the same level (zero imaginary frequencies). The scanning of torsional angles was performed using the redundant coordinates in Gaussian 09 [ 47 ]. Δ G values were calculated from the thermochemistry results, either between the stable conformers or between the stable conformers and the corresponding transition states.…”

“…. H repulsive interaction is a primary factor affecting δ( 1 H) in the trienyl systems reported therein, as in the case of simple alkenes, conjugated dienyl, and aromatic systems [47,50,51,57]. 14) carbons, respectively, is to adopt a low-energy gauche conformation, also known as skew (S = 120 • ) or skew (S = −120 • ).…”

“…Several studies have been published that combine experimental NMR chemical shifts with computations for tackling the complex problems of resonance assignment, resonance reassignment, and structural revision [ 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ] and for investigating high-resolution structures in solution [ 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 ]. DFT calculations of NMR chemical shifts in conjugated systems, however, are limited to 1 H and 13 C NMR chemical shifts of retinal isomers [ 48 ], and geometric isomers of diene-containing compounds [ 47 ]. Since no X-ray structures of conjugated linolenic acids (CLnAs) and hexadecatrienyl pheromones have so far been published, it would be of interest to use quantum chemical calculations of δ( 1 Η) for structure elucidation in solution.…”

Versatile Production of Novel PNP Based Metal Complexes Applicable as Water Reduction Catalysts Showing CH/M as Well as CH/π Interactions

“…Despite the fact that DFT is not always the ideal choice for the treatment of non‐covalent interactions, the B3LYP functional has been successfully employed in numerous computational studies of NMR properties reported by Gerothanassis and co‐workers [ 29 ] as well as other spectroscopic investigations such as the impact of non‐covalent interactions in infrared and near‐infrared spectroscopy. [ 30 ] In both cases it was shown that the use of the B3LYP level resulted in a good description of non‐covalent solute‐solvent contributions.…”