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.