Zhang Yu, Xie Min, Sun Fufei, Zhang Zhaoli, Nie Wuyi, Sun Xiaonan, Hu Yongjun
MOE Key Laboratory of Laser Life Science & Guangdong Provincial Key Laboratory of Laser Life Science, Guangzhou Key Laboratory of Spectral Analysis and Functional Probes, College of Biophotonics, South China Normal University, Guangzhou 510631, China.
J Phys Chem A. 2021 Apr 29;125(16):3279-3287. doi: 10.1021/acs.jpca.1c00255. Epub 2021 Apr 20.
It is known that the acidity and reactivity of the CH bond can be enhanced after ionization. Also, this property plays a pivotal role in proton transfer reaction and in the formation of new molecules. Herein, infrared spectroscopy and high-precision quantum chemical calculations are used to study the neutral and cationic clusters of nitromethane-ammonia (CHNO-NH). It is found that in the neutral cluster, CHNO and NH are mainly bonded by three intermolecular hydrogen bonds, in which electrostatic contribution plays a major role. After vacuum ultraviolet (VUV) single-photon ionization of CHNO-NH, the positive charge redistributes from the ionized nitrogen atom of NH to the CHNO molecule immediately. Then, the proton of CHNO transfers to NH to form a proton-transferred type structure CHNO-NH, without any effective energy barrier, due to the positive hyperconjugation of cationic nitromethane. A closed loop of positive charge transfer takes place in the CHNO-NH cluster after VUV ionization. The present work demonstrates that both the proton transfer reaction and charge transfer process have occurred in the ionized CHNO-NH cluster. Moreover, it is found that the proton transfer reaction is a result of the highly acidic CH bond caused by hyperconjugation between the σ (CH) bond and π orbital.
众所周知，碳氢键的酸度和反应活性在电离后会增强。此外，这一性质在质子转移反应和新分子形成过程中起着关键作用。在此，利用红外光谱和高精度量子化学计算研究了硝基甲烷-氨（CHNO-NH）的中性和阳离子团簇。研究发现，在中性团簇中，CHNO和NH主要通过三个分子间氢键结合，其中静电作用起主要作用。CHNO-NH经真空紫外（VUV）单光子电离后，正电荷立即从NH的电离氮原子重新分布到CHNO分子上。然后，由于阳离子硝基甲烷的正超共轭作用，CHNO的质子转移到NH上，形成质子转移型结构CHNO-NH⁺，且没有任何有效能垒。VUV电离后，CHNO-NH⁺团簇中发生了正电荷转移的闭环。本工作表明，在电离的CHNO-NH团簇中既发生了质子转移反应，也发生了电荷转移过程。此外，研究发现质子转移反应是由σ（CH）键与π轨道之间的超共轭作用导致的高酸性碳氢键的结果。
