Photochemistry of glycine in the interstellar media: a theoretical study Kritsana Sagarik, Pannipa Panajapo, Jirayu Nirasok Phorntep Promma and Parichart Suwannakham School of Chemistry, Suranaree University of Technology, Thailand
Photochemistry of amino acids has been of interest because previous studies have shown that they can be formed via UV irradiation. Being the smallest proteinogenic molecule that was found as one of the most abundant amines and amino acids in a cometary sample (comet 81P/Wild 2) in NASA Stardust spacecraft program, 1, 2 the mechanisms for glycine (NH 2 C a H 2 COOH) dissociation 3 and formation 1 in the cold interstellar media have received special attention in theoretical and experimental points of view. 4 In this work, photodissociation and formation of glycine were studied in the electronic ground (S 0 ) and excited (S 1 ) states using quantum chemical methods, transition state theory (TST) and microcanonical molecular dynamics simulations with surface hopping dynamics (NVE-MDSH).For glycine dissociation, 5 the potential energy surfaces obtained from quantum chemical methods revealed the intersections of the S 1 and S 0 states for distorted glycine structures, C a –N and N–H dissociations and N–H®O=C isomerization, and NVE-MDSH simulations suggested that the isomerization- mediated dissociation and exothermic S 1 ®S 0 relaxation energy could generate reactive radicals and stable molecular products in different temperature ranges (Figure 1). These thermally selective reactions favored the lowest energy glycine conformer (structure Ip ) as the precursor, from which the molecular products could be formed above threshold exothermic relaxation temperatures (Figure 2). The optimized reaction paths showed that among all the photochemical reactions considered, 6 formation of glycine was energetically more favorable through consecutive bimolecular reactions, compared with concerted termolecular reactions. The bimolecular reactions began with the S 0 ®S 1 vertically excited Van der Waals complex precursor (e.g., [H 2 O…CO]), nonradiative S 1 ®S 0 relaxation of the photoexcited complex at the S 1 /S 0 intersection and formation of the singlet dihydroxy carbene [HOCOH] intermediate (Step_1 in Figure 3a), followed by formation of glycine in the S 0 state through the H-bond complex intermediate (e.g., [HOCOH…CH 2 NH], Step_2 in Figure 3b). The TST and NVE-MDSH results revealed that although the bimolecular reactions via the nonradiative S 1 ®S 0 relaxation were energetically more favorable than the bimolecular reactions occurring only in the S 0 state, glycine formation on this reaction path was not thermodynamically favorable due to the entropic effect. These theoretical results showed in detail how UV irradiation and nonradiative S 1 ®S 0 relaxation help create molecules in cold interstellar media without heat transfer from the surroundings. References 1. J. E. Elsila, J. P. Dworkin, M. P. Bernstein, M. P. Martin and S. A. Sandford, The Astrophysical Journal , 2007, 660 . 2. J. E. Elsila, D. P. Glavin and J. P. Dworkin, Meteoritics &; Planetary Science , 2009, 44 , 1323-1330. 3. N. F. Xavier Jr, L. Baptista and G. F. Bauerfeldt, Monthly Notices of the Royal Astronomical Society , 2019, 486 , 2153-2164. 4. R. Maul, M. Preuss, F. Ortmann, K. Hannewald and F. Bechstedt, The Journal of Physical Chemistry A , 2007, 111 , 4370- 4377. 5. J. Nirasok, P. Panajapo, P. Promma, P. Suwannakham and K. Sagarik, Journal of Photochemistry and Photobiology A: Chemistry , 2023, 436 , 114354. 6. S. L. Miller and H. C. Urey, Science 1959 130 , 245-251.
Figure 2 The correlation between the exothermic relaxation temperature in the S 0 state and vertical excitation energy.
Figure 3 a)-b) An example of consecutive bimolecular reactions for formation of glycine via the nonradiative S 1 ®S 0 relaxation.
Figure 1 Reactive and stable molecular products generated from the photolysis of glycine.
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© The Author(s), 2023
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