TY - JOUR
T1 - Unimolecular Fragmentation Properties of Thermometer Ions from Chemical Dynamics Simulations
AU - Malik, Abdul
AU - Spezia, Riccardo
AU - Hase, William L.
N1 - Funding Information:
The research at Texas Tech University (TTU) was supported by the Robert A. Welch Foundation under Grant No. D-0005. The simulations were performed with the computer clusters Chemdynm of the Hase Research Group and Quanah of the TTU High Performance Computing Center. We thank Dr. A.F. Perez-Mellor for help in VENUS/DFTB+ software. We also thank the anonymous reviewers who helped improve the manuscript with their useful remarks.
Publisher Copyright:
©
PY - 2021/1/6
Y1 - 2021/1/6
N2 - 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.
AB - 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.
KW - benzhydrylpyridinium ions
KW - benzylpyridinium ions
KW - chemical dynamics simulations
KW - kinetic theories
KW - thermometer ions
KW - unimolecular dissociation
UR - http://www.scopus.com/inward/record.url?scp=85099429234&partnerID=8YFLogxK
U2 - 10.1021/jasms.0c00200
DO - 10.1021/jasms.0c00200
M3 - Article
C2 - 33210535
AN - SCOPUS:85099429234
VL - 32
SP - 169
EP - 179
JO - Journal of the American Society for Mass Spectrometry
JF - Journal of the American Society for Mass Spectrometry
SN - 1044-0305
IS - 1
ER -