Near-Infrared Laser-Induced Generation of Three Rare Conformers of Glycolic Acid

Abstract

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.

High vibrational overtone excitation‐induced conformational isomerization of glycolic acid in solid argon matrix

“…Structures of the glycolic acid conformers. The conformer abbreviations are according to Halasa et al In the SSC, GAC, AAT, and SST symbolic names, S, G, A, C, and T stand for syn, gauche, anti, cis, and trans, respectively. The first letter describes the orientation of the HO alcohol group (syn, anti, or gauche, around the HOCC torsional angle); the second letter describes the syn or anti orientation of the alcohol and carboxyl groups, around the OCCO torsional angle); the third letter describes the orientation of the HO carboxyl group (cis or trans, around the OCOH torsional angle…”

“…The experimental characterization and the photo‐induced isomerization of glycolic acid has been mostly studied with a combination of Fourier‐transform infrared and matrix isolation technique . The solid rare gas (or in general inert) matrices at low temperatures provide a suitable environment to trap and investigate molecules in a low‐temperature solid environment .…”

“…The solid rare gas matrices are optically transparent from vacuum ultraviolet to far infrared, so photo‐induced processes as well as optical detection of trapped species can be done in wide spectral regions . The vibrational spectroscopy of SSC conformer is characterized in solid argon and nitrogen matrices . Due to their low population at room temperature, only the signatures of the weak vibrational bands of the GAC and AAT conformers are observed after the deposition of gaseous mixture onto low‐temperature substrate.…”

“…The broadband IR irradiation‐induced isomerization of the SSC conformer has revealed two isomerization products in an argon matrix. The primary photoisomerization product is the GAC conformer, and the secondary product (AAT) forms in a two‐step process of SSC → GAC → AAT . The excitation energy of the broadband IR irradiation in near‐infrared exceeds the height of the potential energy barriers of the isomerization reactions; thus, the excess vibrational energy redistributes to the CC bond and promotes an internal rotation around the C‐C bond.…”

A robust and accurate formulation of molecular and colloidal electrostatics

“…Spectroscopy and photochemistry on glycolic acid and its Hbonded complexes with water have been extensively studied by Vaida's group 7,8 and others. 9 The direct study of ionic glycolic acid and its complexes is difficult owing to its lack of strong electronic absorption bands, and its ionization potential (IP) lies in the vacuum ultraviolet region, ∼120 nm. Its aromatic analog, 9-hydroxy-9-fluorene carboxylic acid (9HFCA) in Figure 1, is ideal for H-bonding network studies for neutral and cation.…”