Both linear and branched isomers of propyl cyanide (PrCN; C3H7CN) are ubiquitous in interstellar space. To date, PrCN is one of the most complex molecules found in the interstellar medium. Furthermore, it is the only one observed species to share the branched atomic backbone of amino acids, some of the building blocks of life. Radical-radical chemical reactions are examined in detail using density functional theory, ab initio methods, and the energy resolved master equation formalism to compute rate constants in the low pressure limit prevalent in the ISM. Quantum chemical studies are reported for both isomers considering two possibilities: the gas phase association and the surface reactions of radicals on a 34-water amorphous ice model. The reaction mechanism involves the following radicals association: CH3CHCH3+CN, CH3+CH3CHCN and CH3CH2+CH2CN, CH3+CH2CH2CN, CN+CH3CH2CH2 for iso-PrCN and n-PrCN formation respectively. Two DFT methods: M062X and ωB97XD with the 6-311++G(d,p) basis set were tested for reactions in gas phase and on the ice mantle. In the gas phase, MP2/aug-cc-pVTZ level of theory is also used, and the energetics of the five reactions are calculated using explicitly correlated coupled cluster (CCSD(T)-F12) method. All reaction paths are exoergic and barrierless in the gas phase and on the ice-model, suggesting that the formation of iso-PrCN and n-PrCN is efficient on the ice model adopted in this paper. The gas phase rate constants of formation of both isomers can be eventually used as a high limit for the solid state reactions.

Propyl cyanide (PrCN) (C 3 H 7 CN) with both linear and branched isomers is ubiquitous in interstellar space and is important for astrochemistry as it is one of the most complex molecules found to date in the interstellar medium. Furthermore, it is the only one observed species to share the branched atomic backbone of amino acids, some of the building blocks of life. Radical-radical chemical reactions are examined in detail using density functional theory, second order Mφller Plesset perturbation theory, coupled cluster methods, and the energy resolved master equation formalism to compute the rate constants in the low pressure limit prevalent in the ISM. Quantum chemical studies are reported for the formation of propyl-cyanide (n-PrCN) and its branched isomer (iso-PrCN) from the gas phase association and surface reactions of radicals on a 34-water model ice cluster. We identify two and three paths for the formation of iso-PrCN, and n-PrCN respectively. The reaction mechanism involves the following radicals association: CH 3 CHCH 3 +CN, CH 3 +CH 3 CHCN for iso-PrCN formation and CH 3 CH 2 +CH 2 CN, CH 3 +CH 2 CH 2 CN, CN+CH 3 CH 2 CH 2 leading to n-PrCN formation. We employ the M062X/6-311++G(d,p) DFT functional and MP2/aug-cc-pVTZ for reactions on the ice model, and gas phase respectively to optimize the structures, compute minimum energy paths and zero-point vibrational energies of all reaction mechanisms. In gas phase, the energetics of the five reactions are also calculated using the explicitly correlated cluster ab initio methods (CCSD(T)-F12). All reaction paths are exoergic and barrier-less in gas phase and on the ice-model suggesting that the formation of iso-PrCN and n-PrCN is efficient on the water-ice model adopted in this paper. The gas phase formation of iso-PrCN and n-PrCN however requires a third body or spontaneous emission of a photon in order to stabilize the molecules.

Acetaldehyde (CH3CHO) is ubiquitous in interstellar space and is important for astrochemistry as it can contribute to the formation of amino acids. The reaction mechanism for its formation on a methanol ice grain may involve intersystem spin crossing.