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.