Proton Transfer in Nitromethane–Ammonia Clusters under VUV Single-Photon Ionization Explored by Infrared Spectroscopy and Theoretical Calculations

Abstract

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 (CH 3 NO 2 -NH 3 ). It is found that in the neutral cluster, CH 3 NO 2 and NH 3 are mainly bonded by three intermolecular hydrogen bonds, in which electrostatic contribution plays a major role. After vacuum ultraviolet (VUV) single-photon ionization of CH 3 NO 2 -NH 3 , the positive charge redistributes from the ionized nitrogen atom of NH 3 to the CH 3 NO 2 molecule immediately. Then, the proton of CH 3 NO 2 transfers to NH 3 to form a proton-transferred type structure CH 2 NO 2 -NH 4 + , without any effective energy barrier, due to the positive hyperconjugation of cationic nitromethane. A closed loop of positive charge transfer takes place in the CH 3 NO 2 -NH 3 cluster after VUV ionization. The present work demonstrates that both the proton transfer reaction and charge transfer process have occurred in the ionized CH 3 NO 2 -NH 3 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.

Single-photon ionization induced C–C or C–N bond formation in pyrrole clusters

“…The IR laser beam was focused on the ionization region using a CaF 2 lens with a 200 mm focal length, in which the direction of the IR beam was counter-propagating with the VUV beam. 40,41…”

Proton Transfer Processes in 2-Butenenitrile Dimer Cation Studied by Mass-Selective Infrared Spectroscopy

“…Studies have demonstrated that the acidities of CH bonds in ionized alkanes, 33 alcohols, 34 and ethers 35 remarkably increase due to the hyperconjugation effect. Consequently, proton transfer reactions often proceed readily in ion−molecule reactions involving relevant clusters, like those between pentane and water, 36 nitromethane and ammonia, 37 trimethylamine dimer, 38 etc.…”