Abstract
The mechanisms of the photodissociation of single isolated methanol (CH3OH) molecules in the lowest singlet-excited (S1) state were systematically studied using the complete active-space second-order perturbation theory (CASPT2) and transition state theory (TST). This theoretical study focused on the nonradiative relaxation processes that transform the S0 → S1 vertically excited molecule to the products in their respective electronic ground states. The results confirmed that O–H dissociation is the predominant exothermic process and that the formation of formaldehyde (CH2O), in which the O–H dissociated species are the precursors for the reaction in the S0 state, is the second most favorable process. For C–O dissociation, the theoretical results suggested a thermally excited precursor in a different Franck–Condon region in the S0 state, from which vertical excitation leads to a transition structure in the S1 state and spontaneously to the [CH3]· and [OH]· products in their electronic ground states. The CASPT2 and TST results also revealed the possibility of [CH3OH] → [CH2OH2] isomerization dissociation, in which another thermally excited precursor is vertically excited, and C–O dissociation and intermolecular proton transfer lead to the singlet and triplet [CH2]–[H2O] H-bond complexes in their electronic ground states. Although sufficient thermal energy to generate the precursors in the S0 state is available and the reactions are kinetically feasible at high temperatures, the strongly kinetically controlled O–H dissociation predominates the C–O and [CH3OH] → [CH2OH2] isomerization dissociations. The present results verified and confirmed the reported theoretical and experimental findings and provided insights into the thermal selectivity and interplay between thermal excitation and photoexcitation.
Highlights
The photochemistry of molecules in the gas and condensed phases has been extensively studied in the past decades, with the most investigated topic being the photodissociation of small molecules in the Earth’s atmosphere, which leads to serious environmental problems.1 Solar radiation induces photochemical reactions through the formation of radicals, anions, and cations
S1 relax-scan potential energy curves and transition state theory (TST) calculations confirmed that O–H dissociation is the predominant exothermic process, whereas the formation of formaldehyde (CH2O) and hydrogen (H2) molecules, in which the O–H dissociated species becomes the precursor in the S0 state, is the second most favorable process
For C–O dissociation, the relax-scan potential energy curves suggested a thermally excited precursor (ΔH = 45 kJ/mol) in a different Franck–Condon region in the S0 state, whose S0 → S1 vertical excitation leads to a transition structure and spontaneous formation of the [CH3]⋅ and [OH]⋅ products in their respective electronic ground states via an exothermic process (ΔHRel = −165 kJ/mol); S0 → S1 vertical excitation at the same excitation wavelength (196 nm) could lead spontaneously to direct C– O dissociation in the S1 state on a “barrierless” potential energy curve
Summary
The photochemistry of molecules in the gas and condensed phases has been extensively studied in the past decades, with the most investigated topic being the photodissociation of small molecules in the Earth’s atmosphere, which leads to serious environmental problems. Solar radiation induces photochemical reactions through the formation of radicals, anions, and cations. Solar radiation induces photochemical reactions through the formation of radicals, anions, and cations. Modern spectroscopic techniques, such as fluorescence, resonance-enhanced multiphoton ionization (REMPI), and time-resolved vibrational spectroscopy, in combination with quantum chemical methods, have been proven to be powerful tools for studying photodissociation reactions.. The photodissociation of methanol (CH3OH) has received particular interest because the methoxy ([CH3O]⋅) and hydroxymethyl ([CH2OH]⋅) radical products are reactive intermediates in atmospheric, combustion, and industrial processes.. In the Earth’s atmosphere, [CH3O]⋅ can be generated via the oxidation of CH4.4 The photolysis of CH3OH in the gas phase has been studied extensively using various theoretical and experimental techniques.. Radical intermediates and products, electronic states, and associated absorption spectra have attracted general interest. At least five unimolecular dissociation channels depicted, exist for CH3OH in the along with their standard gas phase. enthalpies of
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