Mechanisms of the photodissociations of single isolated methanol

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.

Chemical Reactivity of Strongly Interacting, Hydrogen-Bond-Forming Molecules Following 193 nm Photon Irradiation: Methanol in Amorphous Solid Water at Low Temperatures

“…Interestingly we observe very little formaldehyde ( m / z = 30–32) which is often reported as a major product of photodissociation 31 , 33 , 63 and electron-induced processing 64 of methanol. We also observe relatively large amounts of CO, CO 2 , HD, and D 2 .…”

“…Mass m / z = 20 is assigned to both deuterated water (D 2 O) and deuterated methane (CD 4 ) (Figure c). In the literature, both of these products are proposed to form as a result of the direct photodissociation of methanol . The ΔP-TPD profile for m / z = 20 displays four peaks that increase with increasing photon irradiation: a “diffusion” peak, two “volcano” peaks (separated by 1–2 degrees), and a “trapped” peak.…”

“…Several dissociation pathways have been proposed for the absorption of 193 nm light by methanol in the gas phase. , Additionally, because the photon energy (6.4 eV) is larger than the work function of the clean Ru(0001) substrate (Φ = 5.52 eV), photoelectrons are emitted from the substrate when the CD 3 OD@(H 2 O) ASW films are irradiated by the 193 nm photons. Additionally, the adsorption of H 2 O and CD 3 OD on Ru(0001) lowers the work function even further at the layer thickness of 100 ML described here, as observed in measurements performed using a Kelvin probe (not shown here).…”

“…In the literature, both of these products are proposed to form as a result of the direct photodissociation of methanol. 33 The ΔP-TPD profile for m / z = 20 displays four peaks that increase with increasing photon irradiation: a “diffusion” peak, two “volcano” peaks (separated by 1–2 degrees), and a “trapped” peak. The two “volcano” peaks are probably observed because of homogeneous nucleation at slightly different crystallization temperatures of areas containing H 2 O–CD 3 OD as the dominant complex in addition to areas of pure H 2 O.…”

“…Öberg et al studied the UV photochemistry of methanol ices using a broad-band UV hydrogen microwave discharge lamp and found that a variety of complex molecules such as ethane, ethanol, dimethyl ether, formaldehyde, and acetaldehyde are formed upon sample heating in addition to smaller molecules such as methane, carbon monoxide, and carbon dioxide . The photochemistry of methanol at 193 nm has mostly been investigated (theoretically and experimentally) in the gas phase. − According to one study, in the solid phase, methanol does not absorb UV light at wavelengths longer than 185 nm . At shorter wavelengths, such as 157 nm, researchers have shown that the irradiation of amorphous solid methanol leads to the formation of methyl and hydroxyl radicals as well as hydrogen molecules .…”