Elementary reactions for glycine production in hot and dense interstellar media from \(\text {CH}_{3}\text {COOH}\), HCOOH, and \(\text {NH}_{2}\text {CH}\)

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: \(\text {HCOOH} + \text {NH}_{2}\text {CH} \rightarrow \text {NH}_{2}\text {CH}_{2}\text {COOH}\) $\text{HCOOH}+{\text{NH}}_2\text{CH}\to {\text{NH}}_2{\text{CH}}_2\text{COOH}$ ( \(\varvec{R}_{{\textbf {1}}}\) ${\mathit{R}}_{\mathbf{1}}$ ), \(\text {CH}_{3}\text {COOH} + \text {NH} \rightarrow \text {CH}_{2}\text {COOH} + \text {NH}_{2}\) ${\text{CH}}_3\text{COOH}+\text{NH}\to {\text{CH}}_2\text{COOH}+{\text{NH}}_2$ ( \(\varvec{R}_{\textbf {2}}\) ${\mathit{R}}_{\mathbf{2}}$ ) and \(\text {CH}_{2}\text {COOH} + \text {NH}_{2} \rightarrow \text {NH}_{2}\text {CH}_{2}\text {COOH}\) ${\text{CH}}_2\text{COOH}+{\text{NH}}_2\to {\text{NH}}_2{\text{CH}}_2\text{COOH}$ ( \(R_3\) $R_3$ ) 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 ( \(R_1\) $R_1$ ) and 18.4 \(\textbf{kcal}\cdot \textbf{mol}^{\varvec{-} {\textbf {1}}}\) $\mathbf{kcal}·{\mathbf{mol}}^{\mathbf{-}\mathbf{1}}$ ( \(R_2\) $R_2$ ). The rate coefficients for these reactions were calculated to be 1.4 \(\times 10^{-17}\) $\times {10}^{-17}$ and 9.3 \(\times 10^{-16}\) $\times {10}^{-16}$ \(\mathbf {cm^3}\cdot \textbf{molecule}^{-{\textbf {1}}}\cdot \textbf{s}^{-{\textbf {1}}}\) ${\mathbf{cm}}^{\mathbf{3}}·{\mathbf{molecule}}^{-\mathbf{1}}·{\mathbf{s}}^{-\mathbf{1}}$ at 1000 K. Additionally, if the products of ( \(R_2\) $R_2$ ) couple on a singlet surface, \(R_3\) $R_3$ connects to the ground state of glycine via a barrierless path presenting a rate coefficient equal to 8.7 \(\times 10^{-9}\) $\times {10}^{-9}$ \(\mathbf {cm^3}\cdot \textbf{molecule}^{-{\textbf {1}}}\cdot \textbf{s}^{-{\textbf {1}}}\) ${\mathbf{cm}}^{\mathbf{3}}·{\mathbf{molecule}}^{-\mathbf{1}}·{\mathbf{s}}^{-\mathbf{1}}$ 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.

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, \(\omega \) $\omega$ B97X, and \(\omega \) $\omega$ 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.