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[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.

Collision Cross-Section Measurements of Collision-Induced Dissociation Precursor and Product Ions in an FTICR-MS and an IM-MS: A Comparative Study.

Sustained off-resonance irradiation-cross-sectional areas by Fourier transform ion cyclotron resonance mass spectrometry (SORI-CRAFTI) is an FTICR-MS strategy to collisionally activate precursor ions and then measure their ion-neutral collision cross sections, as well as those of selected products, at the same time. We benchmarked SORI-CRAFTI using protonated leucine-enkephalin, to excellent agreement (typically within 1-2%) with previous studies performed via collision-induced dissociation-ion mobility (CID-IMS). SORI-CRAFTI was then applied to alkali metal-cationized leucine-enkephalin and compared with CID-IMS via precursor/product cross-section ratios. Qualitative agreement between SORI-CRAFTI and CID-IMS was excellent (again, usually within 1-2%); however, neither SORI-CRAFTI nor CID-IMS could determine if metalated leucine-enkephalin was present in its canonical or zwitterionic form. When SORI-CRAFTI was used on [2.2.2]-cryptand+Cs+, SORI activation resulted in a 5% decrease in collision cross section, consistent with migration of the externally bound Cs+ into the cryptand's cavity and similar to the cross section observed when electrospraying from an isopropanol-rich solvent. Thus, SORI-CRAFTI is useful for studying gas-phase ion chemistry of small- to medium-sized molecules and host-guest systems.

Prototypical Allosterism in a Simple Ditopic Ligand: Gas-Phase Topologies of Cucurbit[n]uril·n-Alkylammonium Complexes Controlled by Binding in the Second Site.

We have employed mass spectrometry, ion mobility, and computational techniques to characterize complexes of n-alkylammonium ions with cucurbit[5]uril (CB[5]) and cucurbit[6]uril (CB[6]) ligands in the gas phase. Nonrotaxane structures are energetically preferred and experimentally observed for all CB[5] complexes. Pseudorotaxane structures are computationally favored and experimentally observed for [CB[6]·n-alkylammonium]+ complexes, but the addition of a second cation (proton, alkali metal ion, another alkylammonium ion, or guanidinium) on the opposite rim of CB[6] causes sufficiently unfavorable steric interactions that n-pentylammonium and longer chains no longer remain threaded through the CB[6] cavity; nonrotaxane topologies are then favored. This provides a very simple example of negative allosteric interactions and molecular structure switching in these complexes.

Multi-CRAFTI: Relative Collision Cross Sections from Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Line Width Measurements.

Determination of collision cross sections (CCS) using the cross-sectional areas by the Fourier transform ion cyclotron resonance (CRAFTI) technique is limited by the requirement that accurate pressures in the trapping cell of the mass spectrometer must be known. Experiments must also be performed in the energetic hard-sphere regime such that ions decohere after single collisions with neutrals; this limits application to ions that are not much more massive than the neutrals. To mitigate these problems, we have resonantly excited two (or more) ions of different m/z to the same center-of-mass kinetic energy in a single experiment, subjecting them to identical neutral pressures. We term this approach "multi-CRAFTI". This facilitates measurement of relative CCS without requiring knowledge of the pressure and enables determination of absolute CCS using internal standards. Experiments with tetraalkylammonium ions yield CCS in reasonable agreement with the one-ion-at-a-time CRAFTI approach and with ion mobility spectrometry (IMS) when differences in collision energetics are taken into account (multi-CRAFTI generally yields smaller CCS than does IMS due to the higher collision energies employed in multi-CRAFTI). Comparison of multi-CRAFTI and IMS results with CCS calculated from structures computed at the M06-2X/6-31+G* level of theory using projection approximation or trajectory method values, respectively, indicates that the computed structures have CCS increasingly smaller than the experimental CCS as m/z increases, implying the computational model overestimates interactions between the alkyl arms. For ions that undergo similar collisional decoherence processes, relative CCS reach constant values at lower collision energies than do absolute CCS values, suggesting a means of increasing the accessible upper m/z limit by employing multi-CRAFTI.