Propyl-cyanide isomer formation on interstellar ices from radical association: a quantum theoretical study

Abstract

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.

Theoretical Spectroscopic Study of Isopropyl Alcohol (CH3-CHOH-CH3)

“…It has contributed to increasing the catalog of nonrigid organic species discovered in the gas phase in extraterrestrial sources. In SrgB2 (N), isopropanol was found to be nearly as abundant as its isomer n-propanol, with an abundance ratio of 0.6, which is similar to the ratio of 0.4 for iso-and normal-propyl cyanide in Sgr B2(N2) (Belloche et al 2014;Garrod et al 2017;Kerkeni et al 2019Kerkeni et al , 2023.…”

Conformer Distributions of n-Propyl Cyanide in the Gas Phase and Following Ice Sublimation Measured by Broadband Rotational Spectroscopy

“…This causes the production of i -PrCN to become dominant. Recent work on the formation of PrCN isomers with DFT methods using M062X and ωB97XD functionals with the 6-311 ++ G­(d,p) basis set by Kerkeni and co-workers reports agreement with the formation pathways with cyanide addition and alkyl addition to alkyl cyanides on ice surfaces . However, given the range of assumptions made and outcomes found, it is clear that the models can benefit from additional detailed experimental guidance.…”