Benzylic coupling constants in toluene derivatives by J doubling in the frequency domain and DFT calculations

The recently modified J doubling in the frequency domain method (MJDFDM) allowed the determination of ^{4, 5, 6} J_{H, CH3} in toluene, a series of 4-mono- and 3, 5-disubstituted toluene derivatives, as well as in 4-picoline. The methyl and aromatic signals were subjected to successive deconvolution processes, which at the end led to singlet signals and afforded the corresponding coupling constant values with a high degree of accuracy. Density functional theory calculation of benzylic coupling constants by addition of the Fermi contact, the spin-dipole, the diamagnetic spin-orbit, and the paramagnetic spin-orbit terms revealed good agreement between predicted and measured values when the B3LYP/aug-cc-pVTZ level of theory was used. Evaluation of the substituent effect over the coupling constant was made for all studied compounds and some limitations of the methodology were evidenced.