The effect of middle range forces on the rate constant of a fast chemical reaction within adiabatic capture theory

Elementary reactions for glycine production in hot and dense interstellar media from CH 3 COOH , HCOOH, and NH 2 CH

CONTEXT

In this work, we investigate three elementary reactions involved in the production of glycine in the interstellar medium (ISM) employing trustworthy electronic structure and chemical kinetics methodologies. We considered three elementary reactions: HCOOH + NH 2 CH → NH 2 CH 2 COOH (R 1 ),CH 3 COOH + NH → CH 2 COOH + NH 2 (R 2 ) andCH 2 COOH + NH 2 → NH 2 CH 2 COOH ( R 3 ) under conditions consistent with hot molecular cores of massive star-forming regions. Our results indicate that the elementary reactions are feasible in these environments, with reaction barriers of 18.8 ( R 1 ) and 18.4 kcal · mol - 1 ( R 2 ). The rate coefficients for these reactions were calculated to be 1.4 × 10 - 17 and 9.3 × 10 - 16 cm 3 · molecule - 1 · s - 1 at 1000 K. Additionally, if the products of ( R 2 ) couple on a singlet surface, R 3 connects to the ground state of glycine via a barrierless path presenting a rate coefficient equal to 8.7 × 10 - 9 cm 3 · molecule - 1 · s - 1 at 298.15 K. Given that the molecules involved in these reactions have been detected in regions such as Sgr B2, our findings suggest that these elementary reactions should be included in mechanisms to study the production of glycine in such locations.

METHODS

The single-reference electronic structure calculations were carried out with the ORCA 4.1.2 package while the multi-reference calculations were performed with the COLUMBUS 7.0 package. The DFT functionals employed were M06-2X, ω B97X, and ω B97X-D3, with the 6-31+G* and def2-TZVP, and for the wave function-based calculations, the CCSD(T), DLPNO-CCSD(T), MRCI, and CASSF methods were employed using the aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets. The chemical kinetic calculations for the elementary reactions with well-defined saddle points were performed using the Pilgrim package employing the TST, CVT, and CVT/SCT approaches.

Proton leap: shuttling of protons onto benzonitrile

The detailed description of chemical transformations in the interstellar medium allows deciphering the origin of a number of small and medium - sized organic molecules. We present density functional theory analysis of proton transfer from the trihydrogen cation and the ethenium cation to benzonitrile, a recently discovered species in the Taurus Molecular Cloud 1. Detailed energy transformations along the reaction paths were analyzed using the interacting quantum atoms methodology, which elucidated how the proton carrier influences the lightness to deliver the proton to benzonitrile's nitrogen atom. The proton carriers' deformation energy represents the largest destabilizing effect, whereas a proton's promotion energy, the benzonitrile-proton Coulomb attraction, as well as non-classical benzonitrile-proton and carrier-proton interaction are the dominant stabilizing energy components. As two ion-molecule reactions proceed without energy barriers, rate constants were estimated using the classical capture theory and were found to be an order of magnitude larger for the reaction with the trihydrogen cation compared to that with the ethenium cation (∼10^-8 and 10^-9 cm³ s^-1, respectively). These results were obtained both with quantum chemical and ab initio molecular dynamics simulations (the latter performed at 10 K and 100 K), confirming that up to 100 K both systems choose energetically undemanding routes by tracking the corresponding minimum energy paths. A concept of a turning point is introduced, which is an equivalent to the transition state in barrierless reactions.