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.

Geometric and Electronic Structures of Ag+(benzo-18-crown-6), Ag+(dibenzo-18-crown-6), and Ag+(dibenzo-15-crown-5) Complexes Investigated by Cold Gas-Phase Spectroscopy.

The UV photodissociation (UVPD) spectra of Ag+ complexes with benzo-18-crown-6 (B18C6), dibenzo-18-crown-6 (DB18C6), and dibenzo-15-crown-5 (DB15C5) [Ag+(B18C6), Ag+(DB18C6), and Ag+(DB15C5)] are observed under cold gas-phase conditions. Ag+(B18C6) and Ag+(DB18C6) show sharp vibronic bands in the 36000-37200 cm-1 region, while the UVPD spectrum of Ag+(DB15C5) is very broad. These UV bands are assigned to the π-π* transition, which is localized on the B18C6, DB18C6, and DB15C5 part of the complexes. Quantum chemical calculations suggest that the broad UV feature of Ag+(DB15C5) can be attributed to the short lifetimes of optically excited ππ* states due to internal conversion (IC) to low-lying excited states that are present only for this complex. The appearance of the π-π* transition in the same UV region as that of the neutral crown ethers and their complexes with alkali metal ions indicates that the positive charge is localized on the Ag atom in these complexes. However, the fragment ions produced after UV absorption are B18C6+, DB18C6+, and DB15C5+ radical ions, indicating that they are produced via charge transfer (CT) between the Ag+ ion and benzo-crown ethers. The CT during fragmentation is attributed to the higher ionization energy of Ag atom when compared to the benzo-crown ethers. In the complexes, the Ag+ ion is effectively encapsulated by the crown cavity of the benzo-crown ethers without transferring the positive charge from Ag+ to the crown. However, UV excitation of the Ag+(B18C6), Ag+(DB18C6), and Ag+(DB15C5) complexes can reduce the Ag+ ion and produce a Ag atom with high efficiency in the gas phase.