Further Investigation of Benzhydrylpyridinium Ions: Increasing the Size of Thermometer Ions for Characterization of Modern Mass Spectrometers

Benzhydrylpyridinium (R,R′-BhPy+) ions have emerged as a promising class of thermometer ions for characterizing the internal energy distributions generated in electrospray ionization (ESI) mass spectrometers. Compared to traditional benzylpyridinium (R-BnPy+) ions, R,R′-BhPy+ systems possess lower dissociation energies, making them more suitable for probing ions generated under relatively gentle ionization conditions. In this work, three new substituted benzhydrylpyridinium ions, H,Ph′-BhPy+, Me,Ph′-BhPy+, and Ph,Ph′-BhPy+, were synthesized and investigated using guided ion beam tandem mass spectrometry (GIBMS). Threshold collision-induced dissociation (TCID) experiments with Xe were performed to experimentally determine threshold energies for dissociation. All ions fragmented exclusively through cleavage of the C–N bond to yield substituted benzhydryl cations and neutral pyridine via a loose transition state. Experimental threshold energies for pyridine loss were measured for all three compounds, and the results were compared with those for several computational methods, including density functional theory (DFT), MP2, and DLPNO-CCSD(T) energies. The experimentally determined threshold energies closely match the DLPNO-CCSD(T), M06-2X, ωB97X-D, and counterpoise-corrected MP2 values, whereas lower-level DFT and MP2 without counterpoise approaches exhibit larger deviations. These results expand the library of experimentally characterized R,R′-BhPy+ thermometer ions and demonstrate that increasing ion size while maintaining relatively low dissociation energies provides useful probes for evaluating ion heating and activation processes in modern mass spectrometry platforms.