Microhydration of Dibenzo-18-Crown-6 Complexes with K+, Rb+, and Cs+ Investigated by Cold UV and IR Spectroscopy in the Gas Phase

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.

Molecular Cage Reports on Its Contents: Spectroscopic Signatures of Cryo-Cooled K+- and Ba2+-Benzocryptand Complexes

UV photofragment spectroscopy and IR-UV double resonance methods are used to determine the structure and spectroscopic responses of a three-dimensional [2.2.2]-benzocryptand cage to the incorporation of a single K⁺ or Ba²⁺ imbedded inside it (labeled as K⁺-BzCrypt, Ba²⁺-BzCrypt). We studied the isolated ion-cryptand complex under cryo-cooled conditions, brought into the gas phase by nano-electrospray ionization. Incorporation of a phenyl ring in place of the central ethyl group in one of the three N-CH₂-CH₂-O-CH₂-CH₂-O-CH₂-CH₂-N chains provides a UV chromophore whose S₀-S₁ transition we probe. K⁺-BzCrypt and Ba²⁺-BzCrypt have their S₀-S₁ origin transitions at 35,925 and 36,446 cm^-1, respectively, blue-shifted by 174 and 695 cm^-1 from that of 1,2-dimethoxybenzene. These origins are used to excite a single conformation of each complex selectively and record their IR spectra using IR-UV dip spectroscopy. The alkyl CH stretch region (2800-3000 cm^-1) is surprisingly sensitive to the presence and nature of the encapsulated ion. We carried out an exhaustive conformational search of cage conformations for K⁺-BzCrypt and Ba²⁺-BzCrypt, identifying two conformations (A and B) that lie below all others in energy. We extend our local mode anharmonic model of the CH stretch region to these strongly bound ion-cage complexes to predict conformation-specific alkyl CH stretch spectra, obtaining quantitative agreement with experiment for conformer A, the gas-phase global minimum. The large electrostatic effect of the charge on the O- and N-lone pairs affects the local mode frequencies of the CH₂ groups adjacent to these atoms. The localized CH₂ scissors modes are pushed up in frequency by the adjacent O/N-atoms so that their overtones have little effect on the alkyl CH stretch region. However, the localized CH₂ wags are nearly degenerate and strongly coupled to one another, producing an array of delocalized wag normal modes, whose highest frequency members reach up above 1400 cm^-1. As such, their overtones mix significantly with the CH stretch modes, most notably involving the CH₂ symmetric stretch fundamentals of the central ethyl groups in the all-alkyl chains and the CH stretches adjacent to the N-atoms and antiperiplanar to the nitrogen lone pair.