NMR J(13C,13C) spin-spin coupling constants in pyridine-carboxaldehydes. Experimental and DFT-B3LYP studies

J(13C,13C) spin-spin couplings were measured at natural abundance in five pyridine aldehyde derivatives. Such couplings were also calculated at the DFT-B3LYP-6-311G**/EPR-III level of theory where special attention was paid to their possible stereospecific behaviors. Dielectric solvent effects were calculated on all four isotropic contributions to J(13C,13C) couplings using the polarization continuum model, PCM. It is observed that the inclusion of dielectric solvent effects in general leads to a better agreement between calculated and experimental couplings. 2J(13CC,13CA) couplings were observed to be sensitive to the aldehyde side-chain conformation (coupling pathway: CC-Ci-CA, where CC stands for the carbonyl-, Ci is the aromatic ipso- and CA is an adjacent aromatic-C atom). This is a positive coupling and becomes unusually large for a trans configuration between the carbonyl CO and the CC-Ci bonds. In this conformation, there is an enhancement of conjugative interactions involving the π(CO) and π(CCCi) bonding and antibonding orbitals.