Long-Range Bonding/Nonbonding Interactions: A Donor Acceptor Resonance Studied by Dynamic NMR
Long-range bonding interactions were evaluated using variable-temperature NMR spectroscopy and suitable 2'-CH2X-substituted phenylpyridines (X = Me, NMe2, OMe, F). It was found that the arylpyridyl rotational barriers were lower when electronegative atoms were bound to the alpha carbon of the 2' moiety. This effect was ascribed to a stabilizing interaction in the transition state due to the lone pair of the heterocyclic nitrogen with the alpha carbon. Computational support for this hypothesis came from CCSD(T)/6-31+G(d) calculations. Steric effects of the X moiety were ruled out by comparison of the rotational barriers of analogous biphenyls.