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

UV photofragment spectroscopy and IR-UV doubleresonancemethods are used to determine the structure and spectroscopic responsesof a three-dimensional [2.2.2]-benzocryptand cage to the incorporationof a single K+ or Ba2+ imbedded inside it (labeledas K+-BzCrypt, Ba2+-BzCrypt). We studied theisolated ion-cryptand complex under cryo-cooled conditions, broughtinto the gas phase by nano-electrospray ionization. Incorporationof a phenyl ring in place of the central ethyl group in one of thethree N-CH2-CH2-O-CH2-CH2-O-CH2-CH2-N chains provides a UV chromophorewhose S-0-S-1 transition we probe. K+-BzCrypt and Ba2+-BzCrypt have their S-0-S-1 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 originsare used to excite a single conformation of each complex selectivelyand record their IR spectra using IR-UV dip spectroscopy. Thealkyl CH stretch region (2800-3000 cm(-1))is surprisingly sensitive to the presence and nature of the encapsulatedion. We carried out an exhaustive conformational search of cage conformationsfor K+-BzCrypt and Ba2+-BzCrypt, identifyingtwo conformations (A and B) that lie below all others in energy. Weextend our local mode anharmonic model of the CH stretch region tothese strongly bound ion-cage complexes to predict conformation-specificalkyl CH stretch spectra, obtaining quantitative agreement with experimentfor conformer A, the gas-phase global minimum. The large electrostaticeffect of the charge on the O- and N-lone pairs affects the localmode frequencies of the CH2 groups adjacent to these atoms.The localized CH2 scissors modes are pushed up in frequencyby the adjacent O/N-atoms so that their overtones have little effecton 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 highestfrequency members reach up above 1400 cm(-1). As such,their overtones mix significantly with the CH stretch modes, mostnotably involving the CH2 symmetric stretch fundamentalsof the central ethyl groups in the all-alkyl chains and the CH stretchesadjacent to the N-atoms and antiperiplanar to the nitrogen lone pair.