Perrin Charles L, Dong Yanmei
Department of Chemistry 0358, University of California-San Diego, La Jolla, California 92093-0358, USA.
J Am Chem Soc. 2008 Aug 20;130(33):11143-8. doi: 10.1021/ja803084w. Epub 2008 Jul 29.
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.
已通过适用于混合物的核磁共振滴定法准确测量了二级氘同位素效应（IEs）对三甲胺同位素异构体碱性的影响。氘化确实会增加碱性，每一个氘的ΔpK约增加0.021。该同位素效应归因于氮孤对电子离域到C-H反键轨道中导致的C-H伸缩频率和零点能的降低。由于这取决于孤对电子与C-H之间的二面角，进一步的结果是倾向于H与孤对电子呈反位交叉且D呈邻位交叉的构象。这导致了同位素效应的预测非加和性，实验证实了这一点。发现每个氘导致的碱性降低随着氘的数量增加而增加。同位素效应的非加和性违反了广泛假设的几何平均规则。
