Multi-state and Multi-mode Vibronic Coupling Effects in the Photoionization Spectroscopy of Acetaldehyde

Multi-state and multi-mode vibronic dynamics in the seven energetically low-lying (X~2A', A~2A″, B~2A', C~2A', D~2A″, E~2A', and F~2A') electronic states of the acetaldehyde radical cation is theoretically studied in this article. Adiabatic energies of these electronic states are calculated by ab initio quantum chemistry methods. A vibronic coupling model of seven electronic states is constructed in a diabatic electronic basis to carry out the first-principles nuclear dynamics study. The vibronic spectrum is calculated and compared with the experimental findings reported in the literature. The progressions of vibrational modes found in the spectrum are assigned. The findings reveal that the X~2A' and F~2A' electronic states are energetically well-separated from the other electronic states and the remaining states (A~2A″ to E~2A') are energetically very close or even quasi-degenerate at the equilibrium geometry of the reference electronic ground state of acetaldehyde. The energetic proximity of A~2A″ to E~2A' electronic states results in multiple multi-state conical intersections. The impact of electronic nonadiabatic interactions due to conical intersections on the vibronic structure of the photoionization band and nonradiative internal conversion dynamics is 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.

On the Formation of the Popcorn Flavorant 2,3-Butanedione (CH3 COCOCH3 ) in Acetaldehyde-Containing Interstellar Ices

Acetaldehyde (CH₃ CHO) is ubiquitous throughout the interstellar medium and has been observed in cold molecular clouds, star forming regions, and in meteorites such as Murchison. As the simplest methyl-bearing aldehyde, acetaldehyde constitutes a critical precursor to prebiotic molecules such as the sugar deoxyribose and amino acids via the Strecker synthesis. In this study, we reveal the first laboratory detection of 2,3-butanedione (diacetyl, CH₃ COCOCH₃ ) - a butter and popcorn flavorant - synthesized within acetaldehyde-based interstellar analog ices exposed to ionizing radiation at 5 K. Detailed isotopic substitution experiments combined with tunable vacuum ultraviolet (VUV) photoionization of the subliming molecules demonstrate that 2,3-butanedione is formed predominantly via the barrier-less radical-radical reaction of two acetyl radicals (CH₃ ĊO). These processes are of fundamental importance for a detailed understanding of how complex organic molecules (COMs) are synthesized in deep space thus constraining the molecular structures and complexity of molecules forming in extraterrestrial ices containing acetaldehyde through a vigorous galactic cosmic ray driven non-equilibrium chemistry.