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Structural transformations were induced in conformers of glycolic acid by selective excitation with monochromatic tunable near-infrared laser light. For the compound isolated in Ar matrixes, near-IR excitation led to generation of two higher-energy conformers (GAC; AAT) differing from the most stable SSC form by 180° rotation around the C-C bond. A detailed investigation of this transformation revealed that one conformer (GAC) is produced directly from the near-IR-excited most stable conformer. The other higher-energy conformer (AAT) was effectively generated only upon excitation of the primary photoproduct (GAC) with another near-IR photon. Once these higher-energy conformers of glycolic acid were generated in an Ar matrix, they could be subsequently transformed into one another upon selective near-IR excitations. Interestingly, no repopulation of the initial most stable SSC conformer occurred upon near-IR excitation of the higher-energy forms of the compound isolated in solid Ar. A dramatically different picture of near-IR-induced conformational transformations was observed for glycolic acid isolated in N2 matrixes. In this case, upon near-IR excitation, the most stable SSC form converted solely into a new conformer (SST), where the acid OH group is rotated by 180°. This conformational transformation was found to be photoreversible. Moreover, SST conformer, photoproduced in the N2 matrix, spontaneously converted to the most stable SSC form of glycolic acid, when the matrix was kept at cryogenic temperature and in the dark.

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The relative populations of two amino-hydroxy conformers of cytosine, differing in rotation of the OH group by approximately 180 degrees , were selectively and repeatedly manipulated with narrowband, near-infrared laser light. For cytosine monomers isolated in a low-temperature Ar matrix, laser irradiations at 7013 cm(-1) and at 7034 cm(-1) were found to induce effective transformations of the two conformers into each other.

Thiourea monomers isolated in a low-temperature argon matrix adopt exclusively the thione tautomeric form. Upon UV (λ > 300 nm) irradiation a photoreaction converting the initial isomer of the compound into its thiol tautomer occurred. Two structures of the photoproduct (with syn and anti orientation of the imino CN−H group with respect to the C−S bond) were identified in the matrix. Subsequently, a back reaction transforming the thiol form into the initial thione tautomer was observed for the matrix kept at 10 K and in darkness. The molecules with the anti conformation underwent this process, whereas those in the syn form remained unchanged. The only possible mechanism of the ground-state thiol → thione transformation at low temperature is proton-tunneling through the very high energy barrier of 9030 cm-1 (108 kJ mol-1) (calculated at the MP2/6-31++G(d,p) level). The experimentally obtained time constant of this process was 52 h. The structure of the photoproduced species was identified by comparing the experimental IR spectra with the theoretically calculated spectra obtained at the DFT(B3LYP)/6-31++G(d,p) level. UV-induced thione → thiol reaction of thiourea as well as the ground-state thiol → thione proton tunneling in the absence of light are reported for the first time.

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