Quantum chemical exploration of the binding motifs and binding energies of neutral molecules, radicals and ions with small water clusters: characterisation and astrochemical implications

Accurate binding energies of molecules to water clusters are relevant for understanding intermolecular interactions and various chemical applications. They enter models of interstellar chemical processes, as binding to icy grains influences surface reactions and thus affects calculated gas-phase abundances. Unfortunately, many astrochemical molecules (especially radicals and ions) are incompletely characterised in these models. To address this, we report computational searches for optimal structures and benchmark binding and condensation energies for sets of neutral, radical, cationic, and anionic molecules of astrochemical relevance with clusters of N=1−4 water molecules. These calculations utilised reliable density functionals for geometry optimisation, and coupled cluster (CCSD(T)) single point calculations with large basis sets. Four energetic binding motifs (weak, intermediate, strong or covalently bonded) were observed depending on the chemical nature of the guest molecule. Neutral closed and open-shell molecules with strong dipoles and a greater potential for hydrogen bonding are more tightly bound to water clusters compared to non-polar ones. For closed-shell cationic and anionic species, barrier-less reactions with water clusters occur, which reveals radical-free routes to molecular processing in the gas phase and on amorphous ice surfaces.</p

Formation and Stability of C₆H₃⁺ Isomers

The stability of the five main isomers of C₆H₃⁺ was investigated using quantum chemical calculations. The cyclic isomers are stabilized by two complementary aromatic effects, first 6-electron π aromaticity, and second a more unusual three-center two-electron σ aromaticity. Two cyclic isomers sit at the bottom of the potential energy surface with energies very close to each other, with a third cyclic isomer slightly higher. The reaction barriers for the interconversion of these isomers, as well as to convert to low-energy linear isomers, are found to be very high with transition states that break both the π and the σ aromaticities. Finally, possibilities for forming the cyclic isomers via association reactions are discussed

Photochemistry and Photophysics of <i>n</i>-Butanal, 3-Methylbutanal, and 3,3-Dimethylbutanal: Experimental and Theoretical Study

Dilute mixtures of <i>n</i>-butanal, 3-methylbutanal, and 3,3-dimethylbutanal in synthetic air, different N₂/O₂ mixtures, and pure nitrogen (up to 100 ppm) were photolyzed with fluorescent UV lamps (275–380 nm) at 298 K. The main photooxidation products were ethene (<i>n</i>-butanal), propene (3-methylbutanal) or <i>i</i>-butene (3,3-dimethylbutanal), CO, vinylalcohol, and ethanal. The photolysis rates and the absolute quantum yields were found to be dependent on the total pressure of synthetic air but not of nitrogen. At 100 Torr, the total quantum yield Φ₁₀₀ = 0.45 ± 0.01 and 0.49 ± 0.07, whereas at 700 Torr, Φ₇₀₀ = 0.31 ± 0.01 and 0.36 ± 0.03 for 3-methylbutanal and 3,3-dimethybutanal, respectively. Quantum yield values for <i>n</i>-butanal were reported earlier by Tadić et al. (<i>J. Photochem. Photobiol. A</i> <b>2001</b> <i>143</i>, 169–179) to be Φ₁₀₀ = 0.48 ± 0.02 and Φ₇₀₀ = 0.32 ± 0.01. Two decomposition channels were identified: the radical channel RCHO → R + HCO (Norrish type I) and the molecular channel CH₃CH(CH₃)CH₂CHO → CH₂CHCH₃ + CH₂CHOH or CH₃C(CH₃)₂CH₂CHO → CHC(CH₃)CH₃ + CH₂CHOH, (Norrish type II) having the absolute quantum yields of 0.123 and 0.119 for 3-methybutanal and 0.071 and 0.199 for 3,3-dimethylbutanal at 700 Torr of synthetic air. The product ethenol CH₂CHOH tautomerizes to ethanal. We have performed ab initio and density functional quantum (DFT) chemical computations of both type I and type II processes starting from the singlet and triplet excited states. We conclude that the Norrish type I dissociation produces radicals from both singlet and triplet excited states, while Norrish type II dissociation is a two-step process starting from the triplet excited state, but is a concerted process from the singlet state

Evidence for the Formation of Pyrimidine Cations from the Sequential Reactions of Hydrogen Cyanide with the Acetylene Radical Cation

Herein, we report the first direct evidence for the formation of pyrimidine ion isomers by sequential reactions of HCN with the acetylene radical cation in the gas phase at ambient temperature using the mass-selected variable temperature and pressure ion mobility technique. The formation and structures of the pyrimidine ion isomers are theoretically predicted via coupled cluster and density functional theory calculations. This ion–molecule synthesis may indicate that pyrimidine is produced in the gas phase in space environments before being incorporated into condensed-phase ices and transformed into nucleic acid bases such as uracil