Laser Spectroscopic Study of Cold Host-Guest Complexes of Crown Ethers in the Gas Phase

Laser spectroscopic study on the structure and dynamics of cold host-guest inclusion complexes of crown ethers (CEs) with various neutral and ionic species in the gas phase is presented. The complexes with neutral guest species are formed by using supersonic free jets, and those with ionic species are generated with an electrospray ionization (ESI) combined with a cold 22-pole ion trap. For CEs, various sizes of 3n-crown-n-ethers (n = 4, 5, 6 and 8) and their benzene-substituted species are used. For the guest species, we chose water, methanol, acetylene and phenol as neutral guest species, and for charged guest species, alkali metal cations are chosen. We measured electronic and vibrational spectra of the complexes by using various laser spectroscopic methods; electronic spectra for the neutral complexes are measured by laser-induced fluorescence (LIF). Discrimination of different species such as conformers is performed by ultraviolet-ultraviolet hole-burning (UV-UV HB) spectroscopy. The vibrational spectra of selected species are observed by infrared-ultraviolet double resonance (IR-UV DR) spectroscopy. For the ionic complexes, ultraviolet photodissociation (UVPD) and IR-UV DR spectroscopy are applied. The complex structures are determined by comparing the observed spectra with those of possible structures obtained by density functional theory calculations. We discuss how the host CEs change their conformation or which conformer prefers to form unique inclusion complexes. These results tell us the key interactions for forming special complex, leading to the molecular recognition