Gas-Phase UV Spectroscopy of Chemical Intermediates Produced in Solution: Oxidation Reactions of Phenylhydrazines by DDQ

In this study, we demonstrated cold gas-phase spectroscopy of chemical intermediates produced in solution. Herein, we combined an electrospray ion source with a T-shaped solution mixer for introducing chemical intermediates in solution into the gas phase. Specifically, the oxidation reaction of 2-(4-nitrophenyl)­hydrazinecarboxaldehyde (NHCA) by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) was initiated by mixing the methanol solutions of NHCA and DDQ in the T-shaped mixer, and the chemical species were injected into the vacuum apparatus for ultraviolet photodissociation (UVPD) spectroscopy. A cationic intermediate was strongly observed at m/z 150 in the mass spectrum, and the UVPD spectrum was observed under cold (∼10 K) gas-phase conditions. The UVPD spectrum showed a strong, broad absorption at ∼38,000 cm–1, accompanied by a relatively weak component at ∼34,000 cm–1. These spectral patterns can be ascribed to a diazonium cation intermediate, whose existence has been predicted in a previous study. This report indicates that cold gas-phase UV spectroscopy can be a useful method for identifying the structure of chemical intermediates produced in solution.