Conformational Changes in Thiazole-2-carboxylic Acid Selectively Induced by Excitation with Narrowband Near-IR and UV Light

Abstract

Conformers and photoinduced conformational transformations were studied for monomers of thiazole-2-carboxylic acid (TCA). The matrix-isolation technique and excitations with narrowband near-IR and UV light, tuned in an optical parametric oscillator, were used for this purpose. Form I, with the carboxylic moiety in the trans orientation and with the hydrogen atom of the OH group directed toward the nitrogen atom of the ring, was the most abundant in low-temperature argon or nitrogen matrixes. Conformer II, differing from I by 180° rotation of the OH group around the C-O bond, was also trapped in the matrixes, but in much smaller amount. The abundance of form II was experimentally determined as ∼6% of the total amount of TCA molecules. Selective excitation of I with narrowband near-IR laser light resulted in I → II transformation. This near-IR-induced conformational change was photoreversible: form II converted back to I upon selective excitation of II with near-IR light of different wavelength. Conformational conversions of I into II, or vice versa, were also induced in TCA monomers by narrowband UV excitations at 300 nm (for I → II transformation) and at 305 nm (for II → I transformation). A spontaneous conversion of photogenerated II into the most stable form I was observed for the compound trapped in the matrix at 15 K and kept in the dark. This process was very slow; the estimated half-life time of conformer II was longer than 50 h. Finally, TCA was shown to thermally decompose at room temperature, yielding CO2 and thiazole.

Isocyano compounds newly recognized in photochemical reaction of thiazole: matrix-isolation FT-IR and theoretical studies

“…16 and those in the low-temperature inert-gas matrices were also reported in the ref. 12 The IR spectrum of thiazole (1) in an argon matrix at 10 K, shown in Fig. 2a, is consistent with the reported IR spectra.…”

“…11 Most recently, the conformational changes of a thiazole derivative (thiazole-2carboxylic acid) isolated in Ar and N 2 matrices have been reported by Halasa et al, using narrowband excitation with nearinfrared (IR) and UV light. 12 However, the photoreaction pathways of matrix-isolated thiazole, the parent molecule with no substitution groups, have never been reported yet because of their complication, even though thiazole is one of the most fundamental heterocyclic compounds having two hetero atoms.…”

Thermally and vibrationally induced conformational isomerizations, infrared spectra, and photochemistry of gallic acid in low-temperature matrices

“…Near-IR irradiation of one conformer at the frequency corresponding to a higher energy stretching O-H vibrational level can induce a conformational transformation. [41][42][43][44] This occurs because the molecule excited via OH stretching vibration can relax by transferring its energy excess into a lower energy torsional coordinate associated with an isomerization transformation. In order to investigate the possibility of formation of higher energy conformers of gallic acid which are not populated (VI, VII, VIII, and IX), or are scarcely populated (III, IV, and V) in the gas phase equilibrium, and the possibility of interconverting conformers I and II, a series of narrowband irradiation experiments has been undertaken, in which matrix-isolated gallic acid monomers in solid N 2 were irradiated at near-IR 2ν(O-H) frequencies.…”

Complexes of Glycolic Acid with Nitrogen Isolated in Argon Matrices. II. Vibrational Overtone Excitations

“…The concept of near-infrared photochemistry was suggested by Donaldson et al [1], for example, to describe OH-containing species such as HONO, HNO 3 , carboxylic acid, or alcohols absorbing NIR or visible radiation via vibrational overtones of the OH stretching mode that induced rotamerisation or even dissociation of the molecule. Up-to-now, the majority of overtone excitation studies has been reported for carboxylic acids [4,5,6,7,8,9,10,11,12,13]. As follows, glycolic acid (GA), the smallest α-hydroxycarboxylic acid, is also an advantageous object for such studies due to the presence of two hydroxyl groups with different surroundings in the same molecule.…”