Blue shifts of the CH stretching vibrations in hydrogen‐bonded and protonated trimethylamine. Effect of hyperconjugation on bond properties

The optimized geometry of isolated trimethylamine (TMA), its hydrogen bond complexes with phenol derivatives and protonated TMA is calculated at the B3LYP/6‐31++G(d,p) level. A natural bond orbital (NBO) analysis on these systems is carried out at the same level of theory. In isolated TMA, one of the CH bond in each of the three CH 3 groups is more elongated than the two other ones. As revealed by the NBO data, this results from a hyperconjugative interaction from the N lone pair to the σ*(CH) orbitals of the CH bonds being in a transoid position with respect to the N lone pair. The formation of an intermolecular OH···N hydrogen bond with phenols results in a decrease of the lone pair effect. A linear correlation is found between the decrease in occupation of the σ*(CH) orbitals and the decrease in the hyperconjugative interaction energy in the complexes and isolated TMA. Complex formation with phenols results in a blue shift of 55–74 cm −1 of the CH stretching vibrations involved in the lone pair effect. Smaller blue shifts between 14 and 23 cm −1 are predicted for the other CH bonds. In these complexes, a linear correlation is found between the frequency shifts and the elongation of the CH bonds. Protonation of TMA results in a nearly equalization of all the CH distances and a blue shift of 180 cm −1 of the CH bonds involved in hyperconjugation with the N lone pair. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008