We determined collision cross section (CCS) values for
singly and
doubly charged cucurbit[n]uril (n = 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. Experimentally, we measured relative
CCS values in SF6 for pairs of +1 and +2 complexes of the
cucurbituril hosts using the cross-sectional areas by Fourier transform
ion cyclotron resonance (“CRAFTI”) method. At center-of-mass
collision energies <∼30 eV, CRAFTI CCS values are sensitive
to the relative binding energies in the +1 and +2 complexes, but at
collision energies >∼40 eV (sufficient that ion decoherence
occurs on essentially every collision) that dependence is not evident.
Consistent with the PA calculations, these experiments found that
the +2 complex ions have CCS values ranging between 94 and 105% of
those of their +1 counterparts (increasing with metal ion size). In
contrast, but consistent with the TM CCS calculations, ion mobility
measurements of the same complexes at close to thermal energies in
much less polarizable N2 find the CCS of +2 complexes to
be in all cases 9–12% larger than those of the corresponding
+1 complexes, with little metal ion dependence.