Photo-transformations of 2-aminothiazole-4-carboxylic acid in low-temperature matrices

The structure, spectroscopic properties and photo-induced conformational changes of 2-aminothiazole-4-carboxylic acid (ACA) isolated in argon and nitrogen matrices were studied using matrix-isolation FTIR spectroscopy and DFT calculations undertaken at the B3LYPD3/6-311++G(3df,3pd) level of theory. Out of 18 stable isomers, the three most stable structures with the five-membered ring stabilized by two C=C and C=N double bonds and with different arrangement of the carboxyl group were identified in both matrices after deposition. Conformer ACA1 has trans and conformers ACA2 and ACA3 have cis conformation of the COOH group and the calculated abundance of 69.5%, 18.2% and 12.2%, respectively. Following irradiation at 285 nm, a rotation of the OH group around the C-O bond in ACA1 was detected leading to increase of ACA2. The corresponding reverse photorotamerization was detected at 296 nm, resulting in the recovery of the initial isomer ACA1. Upon NIR irradiation a dominant process was rotamerization within the carboxylic group leading to changes in the population of the ACA1 trans and ACA2 cis forms.