Insights into the mechanistic photodissociation of methyl formate

Photodissociation Dynamics of CO-Forming Channels on the Ground-State Surface of Methyl Formate at 248 nm: Direct Dynamics Study and Assessment of Generalized Multicenter Impulsive Models

The photodissociation dynamics of methyl formate in the electronic ground state S₀, initiated by a 248 nm-wavelength laser, is studied by direct dynamics simulations. We analyze five channels, where four of them have as products CH₃OH + CO, one leading to the formation of three fragments, H₂CO + H₂ + CO, and a channel characterized by a roaming transition state. The analysis of energy distribution among the degrees of freedom of the product and the comparison with experimental results previously published by other groups provide the ingredients to distinguish the examined dissociation pathways. The interpretation of the results proves that the characterization of dissociation mechanisms must rely on a dynamics approach involving multiple electronic states, including considerations on the features of the S₁/S₀ conical intersection. Here, we also assess the generalized multicenter impulsive model, GMCIM, that has been designed for dissociation processes with exit barriers, and the energy distribution in the products is predicted on the basis of information from the saddle points and the intrinsic reaction coordinates. Main features, advantages, limits, and future perspectives of the method are reported and discussed.

Electron-induced vibrational excitation and dissociative electron attachment in methyl formate

We probe the low-energy electron collisions with methyl formate HCOOCH3, focusing on its resonant states. Experimentally, we (i) use two-dimensional electron energy loss spectroscopy to gain information about the vibrational excitation and (ii) report the absolute dissociative electron attachment cross sections. The electron scattering spectra reveal both the threshold effects due to the long-range electron-molecule interaction and a pronounced π* resonance centered around 2.1 eV. This resonance gives rise to dissociative electron attachment into three different anionic channels, the strongest one being the production of the formate anion. Theoretically, we characterize this resonant state using the complex absorbing potential approach combined with multistate multireference perturbation theory, which predicts its position and width in excellent agreement with the experiment.

Quantum Chemical Investigation on Photochemical Reactions of Nonanoic Acids at Air-Water Interface

Photoinduced chemical reactions of organic compounds at the marine boundary layer have recently attracted significant experimental attention because this kind of photoreactions has been proposed to have substantial impact on local new particle formation and their photoproducts could be a source of secondary organic aerosols. In this work, we have employed first-principles density functional theory method combined with cluster models to systematically explore photochemical reaction pathways of nonanoic acids (NAs) to form volatile saturated and unsaturated C₉ and C₈ aldehydes at air-water interfaces. On the basis of the results, we have found that the formation of C₉ aldehydes is not initiated by intermolecular Norrish type II reaction between two NAs but by intramolecular T₁ C-O bond fission of NA generating acyl and hydroxyl radicals. Subsequently, saturated C₉ aldehydes are formed through hydrogenation reaction of acyl radical by another intact NA. Following two dehydrogenation reactions, unsaturated C₉ aldehydes are generated. In parallel, the pathway to C₈ aldehydes is initiated by T₁ C-C bond fission of NA, which generates octyl and carboxyl radicals; then, an octanol is formed through recombination reaction of octyl with hydroxyl radical. In the following, two dehydrogenation reactions result into an enol intermediate from which saturated C₈ aldehydes are produced via NA-assisted intermolecular hydrogen transfer. Finally, two dehydrogenation reactions generate unsaturated C₈ aldehydes. In these reactions, water and NA molecules are found to play important roles. They significantly reduce relevant reaction barriers. Our work has also explored oxygenation reactions of NA with molecular oxygen and radical-radical dimerization reactions.

Regulation of nonadiabatic processes in the photolysis of some carbonyl compounds

An energy scheme involving S_o → S₁ excitation, followed by dissociation channels via diabatic coupling, internal conversion, transition state and roaming mechanisms.

The involvement of triplet receiver states in the ultrafast excited state processes of small esters

The photoinduced processes of methyl formate and methyl acetate have been probed by femtosecond time-resolved mass spectrometry and photoelectron spectroscopy experiments supported by quantum chemical calculations.

Dynamical, spectroscopic and computational imaging of bond breaking in photodissociation: roaming and role of conical intersections

Recent experimental and theoretical advances in the study of the dissociation of excited molecules are revealing unexpected mechanisms, when their outcomes are tackled by combining (i) space-time ion imaging of translational features, with (ii) spectroscopic probing of rotational and vibrational distributions; crucial is the assistance of (iii) the quantum chemistry of structural investigations of rearrangements of chemical bonds, and of (iv) the simulations of molecular dynamics to follow the evolution of selective bond stretching and breaking. Here we present results of such an integrated approach to methyl formate, HCOOCH3, the simplest of esters; the main focus is on the rotovibrationally excited CO (v=1) product and in general on the energy distribution in the fragments. Previous laser studies of dissociation into CO and CH3OH at a sequence of various wavelengths discovered signatures of a roaming mechanism by the late arrival of CO (v=0) products in time-of-flight ion imaging. Subsequent detailed investigations as a function of excitation energy provided the assessment of the threshold, which opens for triple breakdown into CO and further fragments H and CH3O, as spectroscopically characterized by ion imaging and FTIR respectively. Accompanying quantum mechanical electronic structure calculations and classical molecular dynamics simulations clarify the origin of these fragments through "roaming" pathways involving incipient radical intermediates at energies below the triple fragmentation threshold: a specific role is played by nonadiabatic transitions at a conical intersection between ground and excited states; alternative pathways focalize our attention to regions of the potential energy surfaces other than those in the neighbourhoods of saddle points along minimum energy paths: eventually this leads us to look for avenues in reaction kinetics beyond those of venerable transition state theories.

Photocatalytic homolysis of methyl formate to dry formaldehyde on PdO/TiO2: photocatalytic reverse Tishchenko reaction of methyl formate

Photocatalytic homolysis of dry methyl formate (MF) to dry formaldehyde readily takes place in high selectivity (≥80%) upon irradiation of MF vapour on PdO/TiO₂ in the 385–1050 nm region.

Sequential photo-oxidation of methanol to methyl formate on TiO2(110)

Methyl formate is produced from the photo-oxidation of methanol on preoxidized TiO(2)(110). We demonstrate that two consecutive photo-oxidation steps lead to methyl formate using mass spectrometry and scanning tunneling microscopy. The first step in methanol oxidation is formation of methoxy by the thermal dissociation of the O-H bond to yield adsorbed CH(3)O and water. Formaldehyde is produced via hole-mediated oxidation of adsorbed methoxy in the first photochemical step. Next, transient HCO is made photochemically from formaldehyde. The HCO couples with residual methoxy on the surface to yield methyl formate. Exposure of the titania surface to O(2) is required for these photo-oxidation steps in order to heal surface and near-surface defects that can serve as hole traps. Notably, residual O adatoms are not required for photochemical production of methyl formate or formaldehyde. All O adatoms react thermally with methanol to form methoxy and gaseous water at rt, leaving a surface devoid of O adatoms. The mechanism provides insight into the photochemistry of TiO(2) and suggests general synthetic pathways that are the result of the ability to activate both alkoxides and aldehydes using photons.