Conformation of Benzo-12-Crown-4 Complexes with Ammonium Ions Investigated by Cold Gas-Phase Spectroscopy.

In this study, we examined the conformation and intermolecular interactions of benzo-12-crown-4 (B12C4) complexes with NH4+, CH3NH3+ (MeNH3+), CH3CH2NH3+ (EtNH3+), and CH3CH2CH2NH3+ (PrNH3+) using cold gas-phase spectroscopy. All of the B12C4 complexes showed sharp vibronic features in the UV photodissociation spectra, and the position of the 0-0 band was close to that of the B12C4 complex with an isotropic K+ guest. This result suggests that the conformation of B12C4 is maintained despite oriented interactions with ammonium guests via anisotropic N-H···O interactions. Further, we measured the IR-UV double-resonance spectra of these complexes in the NH stretching region. In the IR-UV spectra of the EtNH3+ and PrNH3+ complexes, two distinct IR fingerprints were observed depending on the UV probe wavelength selected, indicating the existence of another (second) conformer for these complexes. Quantum chemical calculations clarified that the second conformer of the EtNH3+ and PrNH3+ complexes was partially stabilized by the C-H···π hydrogen bond. The conformation of B12C4 complexes with ammonium ions is strongly affected by the interaction between the alkyl chain of the ion guest and the benzene ring of the B12C4 host, although the main intermolecular interaction occurs between the NH3+ group and crown cavity through the N-H···O hydrogen bonds.

Gas-Phase UV Spectroscopy of Chemical Intermediates Produced in Solution: Oxidation Reactions of Phenylhydrazines by DDQ.

In this study, we demonstrated cold gas-phase spectroscopy of chemical intermediates produced in solution. Herein, we combined an electrospray ion source with a T-shaped solution mixer for introducing chemical intermediates in solution into the gas phase. Specifically, the oxidation reaction of 2-(4-nitrophenyl)hydrazinecarboxaldehyde (NHCA) by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) was initiated by mixing the methanol solutions of NHCA and DDQ in the T-shaped mixer, and the chemical species were injected into the vacuum apparatus for ultraviolet photodissociation (UVPD) spectroscopy. A cationic intermediate was strongly observed at m/z 150 in the mass spectrum, and the UVPD spectrum was observed under cold (∼10 K) gas-phase conditions. The UVPD spectrum showed a strong, broad absorption at ∼38,000 cm-1, accompanied by a relatively weak component at ∼34,000 cm-1. These spectral patterns can be ascribed to a diazonium cation intermediate, whose existence has been predicted in a previous study. This report indicates that cold gas-phase UV spectroscopy can be a useful method for identifying the structure of chemical intermediates produced in solution.