Simplified Multiple-Well Approach for the Master Equation Modeling of Blackbody Infrared Radiative Dissociation of Hydrated Carbonate Radical Anions

Blackbody infrared radiative dissociation (BIRD) in a
collision-free
environment is a powerful method for the experimental determination
of bond dissociation energies. In this work, we investigate temperature-dependent
BIRD of CO3·–(H2O)1,2 at 250–330 K to determine water binding energies
and assess the influence of multiple isomers on the dissociation kinetics.
The ions are trapped in a Fourier-transform ion cyclotron resonance
mass spectrometer, mass selected, and their BIRD kinetics are recorded
at varying temperatures. Experimental BIRD rates as a function of
temperature are fitted with rates obtained from master equation modeling
(MEM), using the water binding energy as a fit parameter. MEM accounts
for the absorption and emission of photons from black-body radiation,
described with harmonic frequencies and infrared intensities from
quantum chemical calculations. The dissociation rates as a function
of internal energy are calculated by Rice–Ramsperger–Kassel–Marcus
theory. Both single-well and multiple-well MEM approaches are used.
Dissociation energies derived in this way from the experimental data
are 56 ± 6 and 45 ± 3 kJ/mol for the first and second water
molecules, respectively. They agree within error limits with the ones
predicted by ab initio calculations done at the CCSD­(T)/aug-cc-pVQZ//CCSD/aug-cc-pVDZ
level of theory. We show that the multiple-well MEM approach described
here yields superior results in systems with several low-lying minima,
which is the typical situation for hydrated ions.