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

摘要

J(C-13,C-13) 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(C-13, C-13) 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. (2)J(C-13(C),C-13,) couplings were observed to be sensitive to the aldehyde side-chain conformation (coupling pathway: C-C-C-i-C-A, where C-C stands for the carbonyl-, C-i is the aromatic ipso- and C-A is an adjacent aromatic-C atom). This is a positive coupling and becomes unusually large for a trans configuration between the carbonyl C=O and the C-C-Ci bonds. In this conformation, there is an enhancement of conjugative interactions involving the pi(C=O) and pi(C-C=C-i) bonding and antibonding orbitals. (c) 2005 Elsevier B.V. All rights reserved.