Conformation and\nPhotodissociation Process of Benzo-15-Crown‑5\nand Benzo-18-Crown‑6 Complexes with Ammonium Ions Investigated\nby Cold UV and IR Spectroscopy in the Gas Phase

We examined the conformation of benzo-15-crown-5 (B15C5)\nand benzo-18-crown-6\n(B18C6) complexes with ammonium ions, NH<sub>4</sub><sup>+</sup>,\nCH<sub>3</sub>NH<sub>3</sub><sup>+</sup> (MeNH<sub>3</sub><sup>+</sup>), CH<sub>3</sub>CH<sub>2</sub>NH<sub>3</sub><sup>+</sup> (EtNH<sub>3</sub><sup>+</sup>), and CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>NH<sub>3</sub><sup>+</sup> (PrNH<sub>3</sub><sup>+</sup>), using\ncold UV and IR spectroscopy in the gas phase. We measured the UV photodissociation\n(UVPD) spectra of the ammonium complexes and compared them with those\nof the K<sup>+</sup>(B15C5) and K<sup>+</sup>(B18C6) complexes in\norder to identify the conformation on the basis of the band position.\nThe number of possible conformations for the ammonium complexes of\nB15C5 is limited compared with alkali metal ions with similar ionic\nradii. The NH<sub>4</sub><sup>+</sup>(B15C5), MeNH<sub>3</sub><sup>+</sup>(B15C5), and EtNH<sub>3</sub><sup>+</sup>(B15C5) complexes\nshow two conformers, whereas the K<sup>+</sup>(B15C5) complex has\nthree stable conformers. In the case of the PrNH<sub>3</sub><sup>+</sup>(B15C5) complex, one conformer was found predominantly in the UVPD\nspectrum. The ammonium complexes of B15C5 prefer to adopt crown conformations\nwith large dihedral angles on the C–O–C–C atoms\naround the benzene moiety. In the case of the ammonium complexes of\nB18C6, two or three conformers were found in the UVPD spectra. One\nconformation of the B18C6 complexes is similar to that of the K<sup>+</sup>(B18C6) complex, which has a planar form on the C–O–C–C\natoms around the benzene moiety. The other but dominant conformations\nof the ammonium complexes could be attributed to those with large\nC–O–C–C dihedral angles. These conformational\nfindings for the ammonium complexes suggest that the benzo-crown ethers\ntend to adopt nonplanar conformations around the benzene moiety to\nencapsulate the ammonium ions. The IR-UV double-resonance (DR) spectra\nof the B15C5 and B18C6 complexes were compared to those of benzo-12-crown-4\n(B12C4) and dibenzo-18-crown-6 (DB18C6) complexes. The N–H···O\nhydrogen bond (H-bond) is weaker with increasing ring size from B12C4\nto B18C6, although the calculated binding energy is smaller for B12C4\nthan for B18C6. This result indicates that cooperative H-bonds with\nthree N–H groups can strengthen the intermolecular bond between\nthe ammonium ions and B18C6. The difference in the conformational\npreference between the ammonium and K<sup>+</sup> complexes is attributed\nto directed N–H···O H-bonds in the ammonium\ncomplexes. Proton transfer and dissociation of the crown ring were\nalso observed for the photoexcitation of the NH<sub>4</sub><sup>+</sup>(B15C5) and NH<sub>4</sub><sup>+</sup>(B18C6) complexes.