Induced Fit of Crown Cavity to Ammonium Ion Guests and Photoinduced Intracavity Reactions: Cold Gas-Phase Spectroscopy of Dibenzo-18-Crown-6 Complexes with NH, CHNH, and CHCHNH.

Ultraviolet photodissociation (UVPD) spectra of dibenzo-18-crown-6 (DB18C6) complexes with NH, CHNH (MeNH), and CHCHNH (EtNH) [NH(DB18C6), MeNH(DB18C6), and EtNH(DB18C6), respectively] were observed under cold gas-phase conditions. We also measured the infrared (IR)-UV double-resonance spectra of these complexes in the NH stretching region to examine the encapsulation structure. The UVPD and IR-UV spectra were analyzed using quantum chemical calculations. All the ammonium complexes show sharp 0-0 bands at positions close to that of the K(DB18C6) complex; the conformation of the DB18C6 component in the ammonium complexes is similar to that in K(DB18C6). In addition, the ammonium complexes each have another type of isomer that the K(DB18C6) complex does not show in the gas phase. In these isomers, the conformation of the DB18C6 cavity changes, and the strength of the NH···O hydrogen bond increases. During the UVPD, the NH(DB18C6) complex provides various photofragment species, such as the CHO ion, resulting from cleavage of the DB18C6 component, whereas the dominant fragment ion for the MeNH(DB18C6) and EtNH(DB18C6) complexes is the ammonium ion itself. The UVPD investigation of deuterated systems suggests that after UV excitation of the NH(DB18C6) complex, the dissociation process is initiated by proton transfer from NH to DB18C6, followed by the migration of hydrogen atoms in the crown cavity and the cleavage of the ether ring.