UV photodissociation spectroscopy of cryogenically cooled gas phase host–guest complex ions of crown ethers

Abstract

The geometric and electronic structures of cold host–guest complex ions of crown ethers (CEs) in the gas phase have been investigated by ultraviolet (UV) fragmentation spectroscopy. As host CEs, we chose 15-crown-5 (15C5), 18-crown-6 (18C6), 24-crown-8 (24C8), and dibenzo-24-crown-8 (DB24C8), and as guests protonated-aniline (aniline·H⁺) and protonated-dibenzylamine (dBAM·H⁺) were chosen. The ions generated by an electrospray ionization (ESI) source were cooled in a quadrupole ion-trap (QIT) using a cryogenic cooler, and UV spectra were obtained by UV photodissociation (UVPD) spectroscopy. UV spectroscopy was complemented by quantum chemical calculations of the most probable complex structures. The UV spectrum of aniline·H⁺·CEs is very sensitive to the symmetry of CEs; aniline·H⁺·18C6 shows a sharp electronic spectrum similar to aniline·H⁺, while aniline·H⁺·15C5 shows a very broad structure with poor Franck–Condon factors. In addition, a remarkable cage effect in the fragmentation process after UV excitation was observed in both complex ions. In aniline·H⁺·CE complexes, the cage effect completely removed the dissociation channels of the aniline·H⁺ moiety. A large difference in the fragmentation yield between dBAM·H⁺·18C6 and dBAM·H⁺·24C8 was observed due to a large barrier for releasing dBAM·H⁺ from the axis of rotaxane in the latter complex.