Structures of dibenzo-24-crown-8 complex with an NH4+ ion studied by cryogenic ion mobility-mass spectrometry

Conformation and Photodissociation Process of Benzo-15-Crown-5 and Benzo-18-Crown-6 Complexes with Ammonium Ions Investigated by Cold UV and IR Spectroscopy in the Gas Phase

We examined the conformation of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with ammonium ions, NH4+, CH3NH3+ (MeNH3+), CH3CH2NH3+ (EtNH3+), and CH3CH2CH2NH3+ (PrNH3+), using cold UV and IR spectroscopy in the gas phase. We measured the UV photodissociation (UVPD) spectra of the ammonium complexes and compared them with those of the K+(B15C5) and K+(B18C6) complexes in order to identify the conformation on the basis of the band position. The number of possible conformations for the ammonium complexes of B15C5 is limited compared with alkali metal ions with similar ionic radii. The NH4+(B15C5), MeNH3+(B15C5), and EtNH3+(B15C5) complexes show two conformers, whereas the K+(B15C5) complex has three stable conformers. In the case of the PrNH3+(B15C5) complex, one conformer was found predominantly in the UVPD spectrum. The ammonium complexes of B15C5 prefer to adopt crown conformations with large dihedral angles on the C-O-C-C atoms around the benzene moiety. In the case of the ammonium complexes of B18C6, two or three conformers were found in the UVPD spectra. One conformation of the B18C6 complexes is similar to that of the K+(B18C6) complex, which has a planar form on the C-O-C-C atoms around the benzene moiety. The other but dominant conformations of the ammonium complexes could be attributed to those with large C-O-C-C dihedral angles. These conformational findings for the ammonium complexes suggest that the benzo-crown ethers tend to adopt nonplanar conformations around the benzene moiety to encapsulate the ammonium ions. The IR-UV double-resonance (DR) spectra of the B15C5 and B18C6 complexes were compared to those of benzo-12-crown-4 (B12C4) and dibenzo-18-crown-6 (DB18C6) complexes. The N-H···O hydrogen bond (H-bond) is weaker with increasing ring size from B12C4 to B18C6, although the calculated binding energy is smaller for B12C4 than for B18C6. This result indicates that cooperative H-bonds with three N-H groups can strengthen the intermolecular bond between the ammonium ions and B18C6. The difference in the conformational preference between the ammonium and K+ complexes is attributed to directed N-H···O H-bonds in the ammonium complexes. Proton transfer and dissociation of the crown ring were also observed for the photoexcitation of the NH4+(B15C5) and NH4+(B18C6) complexes.

Structural studies of supramolecular complexes and assemblies by ion mobility mass spectrometry

Recent advances in instrumentation and development of computational strategies for ion mobility mass spectrometry (IM-MS) studies have contributed to an extensive growth in the application of this analytical technique to comprehensive structural description of supramolecular systems. Apart from the benefits of IM-MS for interrogation of intrinsic properties of noncovalent aggregates in the experimental gas-phase environment, its merits for the description of native structural aspects, under the premises of having maintained the noncovalent interactions innate upon the ionization process, have attracted even more attention and gained increasing interest in the scientific community. Thus, various types of supramolecular complexes and assemblies relevant for biological, medical, material, and environmental sciences have been characterized so far by IM-MS supported by computational chemistry. This review covers the state-of-the-art in this field and discusses experimental methods and accompanying computational approaches for assessing the reliable three-dimensional structural elucidation of supramolecular complexes and assemblies by IM-MS.

Intra-host π-π interactions in crown ether complexes revealed by cryogenic ion mobility-mass spectrometry

Cryogenic ion mobility-mass spectrometry was performed to investigate the relative abundance of conformers of dinaphtho-24-crown-8 (DN24C8) complexes with alkali metal cations M+ (M = Li, Na, K, Rb, and Cs). The "closed" conformers of M+(DN24C8) with short distances between two naphthalene rings in the crown ethers were predominantly observed for all complexes at 86 K. The two noncovalent interactions, host-guest and intra-host interactions, were analyzed separately by density functional theory calculations to reveal the origin of the stability of the closed conformers. As a result, it was revealed that the intra-host π-π interactions have a more critical role in determining the stability of the conformers than the host-guest interactions. The closed conformers of M+(DN24C8) also have wider regions of the π-π interactions than those of the M+(dibenzo-24-crown-8) complexes.

Large Conformational Change in the Isomerization of Flexible Crown Ether Observed at Low Temperature

The dynamic processes of conformational changes of supramolecules are important to understand the motion in synthetic supramolecules. Although a host-guest complex is the most basic supramolecule, a detailed mechanism of its conformational changes has rarely been studied. Here, we observed the large conformational change of a dibenzo-24-crown-8 complex with four guest ions (Ag⁺, Na⁺, K⁺, and NH₄⁺) at low temperature in the gas phase. The isomerization between the two types of conformers, which have different distances between the two benzene rings, proceeds even at 86 K. Using variable-temperature ion mobility-mass spectrometry (IM-MS) at 100-210 K, the activation energy for the isomerization is determined to be rather small (4.8-9.0 kJ mol^-1). Reaction pathway calculations revealed that the isomerization is caused by the sequential rotation of two single bonds in the crown ether ring. The present cryogenic IM-MS study of the host-guest complexes at the molecular level opens an approach to detailed understanding of the motion in supramolecules.