Fermi resonance interaction in hetero-dimer and trimer ions containing aniline+

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.

Induced Fit of Crown Cavity to Ammonium Ion Guests and Photoinduced Intracavity Reactions: Cold Gas-Phase Spectroscopy of Dibenzo-18-Crown-6 Complexes with NH4+, CH3NH3+, and CH3CH2NH3. J Phys Chem A 2020;124:3228-3241. [PMID: 32255649 DOI: 10.1021/acs.jpca.0c02341]

Ultraviolet photodissociation (UVPD) spectra of dibenzo-18-crown-6 (DB18C6) complexes with NH₄⁺, CH₃NH₃⁺ (MeNH₃⁺), and CH₃CH₂NH₃⁺ (EtNH₃⁺) [NH₄⁺(DB18C6), MeNH₃⁺(DB18C6), and EtNH₃⁺(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 NH₄⁺(DB18C6) complex provides various photofragment species, such as the C₈H₉O₂⁺ ion, resulting from cleavage of the DB18C6 component, whereas the dominant fragment ion for the MeNH₃⁺(DB18C6) and EtNH₃⁺(DB18C6) complexes is the ammonium ion itself. The UVPD investigation of deuterated systems suggests that after UV excitation of the NH₄⁺(DB18C6) complex, the dissociation process is initiated by proton transfer from NH₄⁺ to DB18C6, followed by the migration of hydrogen atoms in the crown cavity and the cleavage of the ether ring.

Structures of aniline(pyrrole)+, aniline(ethanol)+, and aniline-(benzene). SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2018;204:665-669. [PMID: 29982157 DOI: 10.1016/j.saa.2018.07.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Molecular structures of aniline(pyrrole)⁺, aniline(ethanol)⁺, and aniline(benzene)⁺ produced via resonance two-photon ionization at 266 nm were analyzed by infrared predissociation spectroscopy coupled with tandem mass spectrometry. Structural optimization and frequency calculation using density functional theory were carried out to suggest the most probable isomers which are in good agreement with the observed infrared absorption spectra. Intermolecular bonds in the cluster ions were formed such that the electronegative oxygen atom of the solvent molecule or the pi electron of the aromatic ring forms a hydrogen bonding to NH of aniline.

Anharmonic coupling between fundamental modes in tetramethylurea

In situ high pressure Raman spectra of tetramethylurea have been measured up to 25 GPa, liquid-solid and solid-solid phase transitions were detected at 0.2 GPa and 7.4 GPa, respectively. An unprecedented spectral phenomenon is the observation of a Fermi resonance between the fundamental modes. An exponential relationship between the intensity and the frequency difference was concluded. Pressure provides us a new way to study the correlation between Fermi resonance parameters.