Endo Tomoya, Matsuda Yoshiyuki, Moriyama Shohei, Fujii Asuka
Department of Chemistry, Graduate School of Science , Tohoku University , 6-3, Aramaki-Aza-Aoba , Aoba-ku, Sendai 980-8578 , Miyagi , Japan.
J Phys Chem A. 2019 Jul 18;123(28):5945-5950. doi: 10.1021/acs.jpca.9b01261. Epub 2019 Jul 8.
Barrierless intermolecular proton transfer from a CH bond has recently been reported in the vertical ionization of the trimethyl amine (TMA) dimer. This result indicates the remarkable enhancement of the proton-donating ability of the CH bond in its cationic state. In the present study, we have carried out an infrared spectroscopy of the neutral and cationic TMA in the CH stretch region and their theoretical calculations to investigate the mechanism of enhancement of the proton-donating ability in the cationic state. In the spectrum of the cation, the CH stretch band shows a long tail of up to 2600 cm. This tail component is attributed to the CH bond hyperconjugated with the nonbonding orbital at the nitrogen atom through geometry deformation (excitation of molecular vibrations) with the excess energy upon photoionization. This hyperconjugation causes the delocalization of the σ electron of the CH bond to the singly occupied nonbonding orbital so that the proton-donating ability of the CH is enhanced. It is shown that the excitation of the CN stretching vibration is especially effective in promoting the hyperconjugation.
最近有报道称，在三甲胺（TMA）二聚体的垂直电离过程中发生了无势垒的分子间质子从C-H键转移的现象。这一结果表明，处于阳离子状态的C-H键的质子供体能力显著增强。在本研究中，我们对中性和阳离子态的TMA在C-H伸缩振动区域进行了红外光谱分析，并进行了理论计算，以研究阳离子状态下质子供体能力增强的机制。在阳离子的光谱中，C-H伸缩振动带显示出长达2600 cm的长尾巴。这个尾部成分归因于在光致电离时，C-H键通过几何变形（分子振动激发）与氮原子上的非键轨道发生超共轭作用，并带有多余的能量。这种超共轭作用导致C-H键的σ电子离域到单占据的非键轨道上，从而增强了C-H的质子供体能力。结果表明，C-N伸缩振动的激发在促进超共轭作用方面特别有效。
