Structural Investigation of Photochemical Intermediates in Solution by Cold UV Spectroscopy in the Gas Phase: Photosubstitution of Dicyanobenzenes by Allylsilanes.

Electrospray ion sources with an in-line quartz cell were constructed to produce photochemical intermediates in solution. These ion sources can detect photochemical intermediates having lifetimes longer than a few seconds. Intermediates formed by photosubstitution of 1,4-dicyanobenzene (DCB) by allyltrimethylsilane (AMS) in acetonitrile using a Xe lamp were injected into the mass spectrometer. The cationic intermediate (C11H10N2·H+) was observed at m/z = 171, but no anionic intermediate was found, although C11H9N2- was expected based on prior studies. Theoretical studies suggested that C11H9N2- was simultaneously converted to neutral C11H10N2 and cationic C11H10N2·H+ species, which can be stable intermediates in the photosubstitution reaction. The UV photodissociation (UVPD) spectrum of C11H10N2·H+ under cold (∼10 K) gas-phase conditions determined the conformation of the C11H10N2 unit of the C11H10N2·H+ cation. This report demonstrates that cold gas-phase UV spectroscopy is a prospectively powerful tool for investigation of the electronic and geometric structures of photochemical intermediates produced in solution.