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

Journal Article · Fri Jul 21 00:00:00 EDT 2023 · Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory

DOI:https://doi.org/10.1021/acs.jpca.3c03457· OSTI ID:2317742

Foley, Casey D.  ^[1]; Allen, Cole D. ^[2]; Au, Kendrew ^[1]; Lee, Chin  ^[1]; Rempe, Susan B.  ^[3]; Ren, Pengyu ^[2]; Sibert III, Edwin L. ^[4]; Zwier, Timothy S.  ^[1]

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1. Sandia National Laboratories (SNL-CA), Livermore, CA (United States)
2. Univ. of Texas, Austin, TX (United States)
3. Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States)
4. Univ. of Wisconsin, Madison, WI (United States)

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. Here, 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.

• Research Organization: Sandia National Laboratories (SNL), Albuquerque, NM, and Livermore, CA (United States); Sandia National Lab. (SNL-CA), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA); USDOE Laboratory Directed Research and Development (LDRD) Program; USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); National Science Foundation (NSF)
• Grant/Contract Number: NA0003525; 21-0149; 1885207; CHE-1900095
• OSTI ID: 2317742
• Alternate ID(s): OSTI ID: 2320346
• Report Number(s): SAND-2024-02428J; SAND-2024-02813J
• Journal Information: Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory, Vol. 127, Issue 30; ISSN 1089-5639
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

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