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Benzhydrylpyridinium Ions: A New Class of Thermometer Ions for the Characterization of Electrospray-Ionization Mass Spectrometers.

Previous attempts to characterize the internal energies of ions produced by electrospray ionization (ESI) have chiefly relied upon benzylpyridinium ions, R-BnPy+, as thermometer ions. However, these systems are not well suited for this purpose because of their relatively high dissociation energies. Here, we propose benzhydrylpyridinium ions, R,R'-BhPy+, as a new class of thermometer ions. DLPNO-CCSD(T)/CBS//PBE0-D3BJ calculations for R,R'-BhPy+ (R,R' = H,H'; Me,Me'; H,OMe'; Me,OMe'; OMe,OMe'; NPh2,NPh2') predict that these ions fragment by the loss of pyridine via loose transition states. The computed threshold energies of these fragmentations, 0.70 ≤ E0 ≤ 1.74 eV, are significantly lower than those of the dissociation of the benzylpyridinium ions. The theoretical predictions agree well with results from guided ion beam experiments, which find threshold energies of 1.79 ± 0.11, 1.55 ± 0.13, and 1.37 ± 0.14 eV for the fragmentation of R,R'-BhPy+, R,R' = H,H'; Me,Me'; H,OMe', respectively. The determined thermochemistry for these systems is then used to characterize the internal energies of ions produced by ESI from dichloromethane and methanol solutions under standard conditions. Correlating the measured survival yields of five of the R,R'-BhPy+ ions with the computed threshold energies including explicit consideration of their dissociation rates, we derive energy distributions with maxima at 2.06 ± 0.13/1.88 ± 0.11 eV and widths of 0.86 ± 0.07/0.86 ± 0.06 eV (dichloromethane/methanol). These energy distributions are comparable to ion temperatures between 620 ± 20/590 ± 20 and 710 ± 20/680 ± 20 K (dichloromethane/methanol).