Vibrational spectra and UV photochemistry of (CH2)2O thin films and (CH2)2O in amorphous water ice

Neural canonical transformations for vibrational spectra of molecules

The behavior of polyatomic molecules around their equilibrium positions can be regarded as that of quantum-coupled anharmonic oscillators. Solving the corresponding Schrödinger equations enables the interpretation or prediction of the experimental spectra of molecules. In this study, we developed a novel approach for solving the excited states of anharmonic vibrational systems. The normal coordinates of the molecules are transformed into new coordinates through a normalizing flow parameterized by a neural network. This facilitates the construction of a set of orthogonal many-body variational wavefunctions. This methodology has been validated on an exactly solvable 64-dimensional coupled harmonic oscillator, yielding numerical results with a relative error of 10-6. The neural canonical transformations are also applied to calculate the energy levels of two specific molecules, acetonitrile (CH3CN) and ethylene oxide (C2H4O). These molecules involve 12 and 15 vibrational modes, respectively. A key advantage of this approach is its flexibility concerning the potential energy surface, as it requires no specific form. Furthermore, this method can be readily implemented on large-scale distributed computing platforms, making it easy to extend to investigating complex vibrational structures.

Radiation-induced transformations of matrix-isolated ethanol molecules at cryogenic temperatures: an FTIR study

Ethanol (C2H5OH) is one of the most common alcohol molecules observed in various space media (molecular clouds, star formation regions, and, highly likely, interstellar ices), where it is exposed to light and ionizing radiation, leading to more complex organic molecules and eventually to the biologically important species. To better understand the radiation-induced evolution of ethanol molecules in icy media, we have examined the transformations of isolated C2H5OH and C2D5OH under the action of X-rays and vacuum ultraviolet (VUV) radiation in solid inert matrices (Ne, Ar, Kr, and Xe) at 4.4 K using Fourier transform infrared (FTIR) spectroscopy. The results obtained with X-ray irradiation demonstrate the formation of a variety of radiolysis products corresponding to dehydrogenation (CH3CHOH˙, CH3CHO, CH2CHOH, CH3CO˙, H2CCO-H2, H2CCO, HCCO˙, CCO) and C-C bond rupture (H2CO, HCO˙, CO, CH4, and CH3˙). The absorptions of the CH3CHOH˙ radical related to the CCO stretching (the bands at 1249.1, 1247.0, 1246.2, and 1245.1 cm-1, in Ne, Ar, Kr, and Xe, respectively) were first tentatively characterized on the basis of comparison with available computational data. In addition, the C2H2⋯H2O complex, which corresponds to dehydrogenation, was found followed by C-O bond cleavage. The results were confirmed by experiments with isotopic substitution. It was found that dehydrogenation strongly predominated in a xenon matrix, while skeleton bond rupture is more feasible in neon and argon. The matrix effect was attributed to a significant role of "hot" reaction channels in neon and argon, which are efficiently quenched due to relaxation in more polarizable xenon. The VUV photolysis (185 nm) in Ar and Xe matrices yields a similar set of products, except for CH3CHOH˙ and CH2CHOH, which were not found (the nonobservation of the former species may be explained by its efficient secondary photolysis). The plausible mechanisms of product formation and astrochemical implications of the results are discussed.

Radiation-induced transformations of acetaldehyde molecules at cryogenic temperatures: a matrix isolation study

Acetaldehyde is one of the key small organic molecules involved in astrochemical and atmospheric processes occurring under the action of ionizing and UV radiation. While the UV photochemistry of acetaldehyde is well studied, little is known about the mechanism of processes induced by high-energy radiation. This paper reports the first systematic study on the chemical transformations of CH₃CHO molecules resulting from X-ray irradiation under the conditions of matrix isolation in different solid noble gases (Ne, Ar, Kr, and Xe) at 5 K. CO, CH₄, H₂CCO, H₂CCO-H₂, C₂H₂⋯H₂O, CH₂CHOH, CH₃CO˙, CH₃˙, HCCO˙, and CCO were identified as the main radiolysis products. The dominant pathway of acetaldehyde degradation involves C-C bond cleavage leading to the formation of carbon monoxide and methane. The second important channel is dehydrogenation resulting in the formation of ketene, a potentially highly reactive species. It was found that the matrix significantly affected both the decomposition efficiency and distribution of the reaction channels. Based on these observations, it was suggested that the formation of the methyl radical as well as vinyl alcohol and the C₂H₂⋯H₂O complex presumably included a significant contribution of ionic pathways. The decomposition of acetyl radicals under photolysis with visible light leading to the CH₃˙-CO radical-molecule pair was observed in all matrices, while the recovery of CH₃CO˙ in the dark at 5 K was found only in Xe. This finding represents a prominent example of matrix-dependent chemical dynamics (probably, involving tunnelling), which deserves further theoretical studies. Probable mechanisms of acetaldehyde radiolysis and their implications for astrochemistry, atmospheric chemistry and low-temperature chemistry are discussed.

Hydrogen abstraction in astrochemistry: formation of ˙CH2CONH2 in the reaction of H atom with acetamide (CH3CONH2) and photolysis of ˙CH2CONH2 to form ketene (CH2CO) in solid para-hydrogen

The amide bond of acetamide is unaffected by hydrogen exposure, but the hydrogen abstraction on its methyl site activates this molecule to react with other species to extend its size as a first step to form interstellar complex organic molecules.

A comparison of two methods for selecting vibrational configuration interaction spaces on a heptatomic system: ethylene oxide

Two recently developed methods for solving the molecular vibrational Schrodinger equation, namely, the parallel vibrational multiple window configuration interaction and the vibrational mean field configuration interaction, are presented and compared on the same potential energy surface of ethylene oxide, c-C(2)H(4)O. It is demonstrated on this heptatomic system with strong resonances that both approaches converge towards the same fundamental frequencies. This confirms their ability to tackle the vibrational problem of large molecules for which full configuration interaction calculations are not tractable.