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).