title: Nonadditivity of secondary deuterium isotope effects on basicity of trimethylamine.
abstract: Secondary deuterium isotope effects (IEs) on basicities of isotopologues of trimethylamine have been accurately measured by an NMR titration method applicable to a mixture. Deuteration definitely increases the basicity, by approximately 0.021 in the Delta pK per D. The IE is attributed to the lowering of the CH stretching frequency and zero-point energy by delocalization of the nitrogen lone pair into the C-H antibonding orbital. Because this depends on the dihedral angle between the lone pair and the C-H, a further consequence is a preference for conformations with H antiperiplanar to the lone pair and D gauche. This leads to a predicted nonadditivity of IEs, which is confirmed experimentally. It is found that the decrease in basicity, per deuterium, increases with the number of deuteriums. The nonadditivity of IEs is a violation of the widely assumed Rule of the Geometric Mean.
authors.affiliation: Department of Chemistry 0358, University of California-San Diego, La Jolla, California 92093-0358, USA. cperrin@ucsd.edu
authors.forename: Charles L
authors.initials: CL
authors.lastname: Perrin
authors.forename: Yanmei
authors.initials: Y
authors.lastname: Dong
mesh_terms: D003903:Deuterium / Q000737:chemistry*; D017666:Deuterium Oxide / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy / Q000379:methods / Q000592:standards; D008744:Methylamines / Q000138:chemical synthesis / Q000737:chemistry*; D012015:Reference Standards; D012680:Sensitivity and Specificity
chemical_list: D008744:Methylamines; D003903:Deuterium; D017666:Deuterium Oxide; C023336:trimethylamine