Protonation Isomer Specific Ion–Molecule Radical Reactions

Abstract

Through a combination of ion-mobility filtering and laser-equipped quadrupole ion–trap mass spectrometry, the gas-phase reaction kinetics of two protonation isomers of the distonic-radical quinazoline cation are independently measured with ethylene. For these radical addition reactions, protonation site variations drive significant changes in nearby radical reactivity, and this is primarily due to through–space electrostatic effects. Furthermore, quantum chemical methods specifically designed for calculating long-range interactions, such as double-hybrid density functional theory, are required to rationalize the experimentally measured difference in reactivity.

Observation of Solvent-Dependence in the Mechanism of Neutral-Catalyzed Isomerization of para-Aminobenzoic Acid Protomers

“…This would be the next step in upgrading the model for these systems. As we have shown in previous distonic radical ion–molecule studies, these reaction rates are particularly sensitive to these barrier height energies. ,, …”

“…The double-hybrid DSD-PBEP86-D3­(BJ) method has been benchmarked to calculate energies accurately for minima and transition state geometries found in long-range noncovalent complexes, such as those encountered in the vehicle mechanisms of this work. These double-hybrid methods perform exceptionally well for water-catalyzed proton transfer reactions with reported mean absolute deviations scores below the threshold of typical chemical accuracy (<4.2 kJ mol –1 ), and some comparison of methods for ion–molecule complexes is provided in our recent study . Each minimum energy structure was optimized and verified to contain no imaginary frequencies.…”

“…Intramolecular proton transfera subset of proton transfer reactionsinvolves the transport of a proton between protonation sites of a molecule. For positive mode mass spectrometry, this process has been implicated in the formation of protonation isomers − (often called protomers) and thus are influential to the kinetic and thermodynamic stability of ions under different ionization and activation conditions. − Exploring these internal proton transfer reactions for small molecules provides a foundation to interpret the proton transfer reactions in larger biologically relevant molecules and ensure the efficient detection of analytes under mass spectrometry analysis.…”

Gas-Phase Phenyl Radical + O2 Reacts via a Submerged Transition State

“…Previous studies using this modeling approach indicate that the energy of the submerged transition state (TS inner ) primarily dominates the bimolecular kinetics of these reactions, which competes with the redissociation of the PRC. ,, Therefore, to examine the sensitivity of our model, rate coefficients were also determined assuming a ±1 kJ mol –1 uncertainty in TS inner . Figure shows a schematic of the four-point RRKM-ME model used for the phenyl + O 2 system, including the energies of TS inner determined using the aug-cc-pVTZ (−1.6 kJ mol –1 ) and aug-cc-pVQZ (−4.6 kJ mol –1 ) basis sets.…”

Vibrational and electronic spectra of protonated vanillin: exploring protonation sites and isomerisation

“…Depending on the nature of each group and their relative positions in the aromatic ring (or rings), several (one-or multiple-) competing protonation sites can be observed. [1][2][3][4] The photophysics of different possible protonated isomers (protomers) in aromatic derivatives and similar compounds may exhibit substantial differences. [5][6][7][8][9][10] In such a case, the reactivity of a protonated molecule in this family of compounds or its photodegradation mechanisms may depend on the favoured protomer.…”