Ion Mobility and Fourier Transform Ion Cyclotron Resonance Collision Cross Section Techniques Yield Long-Range and Hard-Sphere Results, Respectively.

We determined collision cross section (CCS) values for singly and doubly charged cucurbit[]uril ( = 5-7), decamethylcucurbit[5]uril, and cyclohexanocucurbit[5]uril complexes of alkali metal cations (Li-Cs). These hosts are relatively rigid. CCS values calculated using the projection approximation (PA) for computationally modeled structures of a given host are nearly identical for +1 and +2 complexes, with weak metal ion dependence, whereas trajectory method (TM) calculations of CCS for the same structures consistently yield values 7-10% larger for the +2 complexes than for the corresponding +1 complexes and little metal ion dependence.

Halide Size-Selective Binding by Cucurbit[5]uril-Alkali Cation Complexes in the Gas Phase.

We report data that suggest complexes with alkali cations capping the portals of cucurbit[5]uril (CB[5]) bind halide anions size-selectively as observed in the gas phase: Cl binds inside the CB[5] cavity, Br is observed both inside and outside, and I binds weakly outside. This is reflected in sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments: all detected Cl complexes dissociate at higher energies, and Br complexes exhibit unusual bimodal dissociation behavior, with part of the ion population dissociating at very low energies and the remainder dissociating at significantly higher energies comparable to those observed for Cl. Decoherence cross sections measured in SF using cross-sectional areas by Fourier transform ion cyclotron resonance techniques for [CB[5] + MX] (M = Na, X = Cl or Br) are comparable to or less than that of [CB[5] + Na] over a wide energy range, suggesting that Cl or Br in these complexes are bound inside the CB[5] cavity.