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

Abstract

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′; NPh 2 ,NPh 2 ′) predict that these ions fragment by the loss of pyridine via loose transition states. The computed threshold energies of these fragmentations, 0.70 ≤ E 0 ≤ 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).

Unimolecular Fragmentation Properties of Thermometer Ions from Chemical Dynamics Simulations

“…6,51 For BnPy ions, this BDE is for the C-N bond between the substituted benzylium cation and the neutral pyridine whereas for BhPy ions it is the dissociation energy between substituted benzhydrilium ion and the neutral pyridine (see Scheme 1, in the present case R=Me). 52 It has been observed previously, 6,7,51 that this reaction proceeds through a loose transition state (TS) i.e. reverse activation barrier can be neglected and the BDE can be calculated as the zero-point energy difference between the reactants and the products.…”

“…Recently, a new set of thermometer ions was proposed, the benzhydrylpyridinium (BhPy) ions, which show lower activation energies than the original BnPy ions due to the addition of an extra phenyl ring that should weaken the C-N bond by stabilizing the resultant benzhydrylium ion. 7,8 In Scheme 1 we report the unimolecular fragmentations which characterize these two classes of thermometer ions.…”

Unimolecular Fragmentation Properties of Thermometer Ions from Chemical Dynamics Simulations

“…Note that the experimental values were obtained presuming that the reaction occurred via a loose TS. 7,56 Table S3).…”

“…3 and 4, respectively). Experimentally, rate constants were reported for the two systems, but only up to 6 and 4 eV for p-Me-BnPy + and Me,Me-BhPy + , respectively, 7,53 and thus a direct comparison cannot be done. However, since the values at the highest energies reported (about 4 •10 9 s -1 and 10 10 s -1 for p-Me-BnPy + and Me,Me-BhPy + , respectively) are on the order of magnitude of what is found in simulations at higher energies, we can expect that simulations at low energies will largely underestimate the reaction rates.…”

“…Recently, a new set of thermometer ions was proposed, the benzhydrylpyridinium (BhPy) ions, which show lower activation energies than the original BnPy ions due to the addition of an extra phenyl ring that weakens the C-N bond by stabilizing the resultant benzhydrylium ion. 7,8 In Scheme 1 we report the unimolecular fragmentations which characterize these two classes of thermometer ions.…”