title: Blue shifts of the C-H stretching vibrations in hydrogen-bonded and protonated trimethylamine. Effect of hyperconjugation on bond properties.
abstract: 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 C-H 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 sigma*(C-H) orbitals of the C-H 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 sigma*(C-H) 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 C-H stretching vibrations involved in the lone pair effect. Smaller blue shifts between 14 and 23 cm(-1) are predicted for the other C-H bonds. In these complexes, a linear correlation is found between the frequency shifts and the elongation of the C-H bonds. Protonation of TMA results in a nearly equalization of all the C-H distances and a blue shift of 180 cm(-1) of the C-H bonds involved in hyperconjugation with the N lone pair.
authors.affiliation: Department of Chemistry, North Eastern Hill University, Shillong 793022, India. akchandra@nehu.ac.in
authors.forename: Asit K
authors.initials: AK
authors.lastname: Chandra
authors.forename: Salma
authors.initials: S
authors.lastname: Parveen
authors.forename: Subojit
authors.initials: S
authors.lastname: Das
authors.forename: Thérèse
authors.initials: T
authors.lastname: Zeegers-Huyskens
mesh_terms: D003198:Computer Simulation; D006860:Hydrogen Bonding; D008744:Methylamines / Q000737:chemistry*; D008956:Models, Chemical; D010636:Phenols / Q000737:chemistry; D011522:Protons; D011789:Quantum Theory; D015203:Reproducibility of Results; D014732:Vibration
chemical_list: D008744:Methylamines; D010636:Phenols; D011522:Protons; C023336:trimethylamine