posted on 2020-11-19, 12:34authored byAbdul Malik, Riccardo Spezia, William L. Hase
Thermometer ions
are widely used to calibrate the internal energy
of the ions produced by electrospray ionization in mass spectrometry.
Typically, benzylpyridinium ions with different substituents are used.
More recently, benzhydrylpyridinium ions were proposed for their lower
bond dissociation energies. Direct dynamics simulations using M06-2X/6-31G(d),
DFTB, and PM6-D3 are performed to characterize the activation energies
of two representative systems: para-methylbenzylpyridinium
ion (p-Me-BnPy+) and methyl,methylbenzhydrylpyridinium
ion (Me,Me-BhPy+). Simulation results are used to calculate
rate constants for the two systems. These rate constants and their
uncertainties are used to find the Arrhenius activation energies and
RRK fitted threshold energies which give reasonable agreement with
calculated bond dissociation energies at the same level of theory.
There is only one fragmentation mechanism observed for both systems,
which involves C–N bond dissociation via a loose transition
state, to generate either benzylium or benzhydrylium ion and a neutral
pyridine molecule. For p-Me-BnPy+ using
DFTB and PM6-D3 the formation of tropylium ion, from rearrangement
of benzylium ion, was observed but only at higher excitation energies
and for longer simulation times. These observations suggest that there
is no competition between reaction pathways that could affect the
reliability of internal energy calibrations. Finally, we suggest using
DFTB with a modified-Arrhenius model in future studies.