Infrared Spectrum and UV-Induced Photochemistry of Matrix-Isolated 5-Hydroxyquinoline

Abstract

The structure, infrared spectrum, and photochemistry of 5-hydroxyquinoline (5HQ) were studied by matrix isolation infrared spectroscopy, complemented by theoretical calculations performed at the DFT(B3LYP)/6-311++G(d,p) level of approximation. According to the calculations, the trans conformer of 5HQ (with the OH group pointing to the opposite direction of the pyridine ring of the molecule) is more stable than the cis form (by ∼8.8 kJ mol(-1)). The main factors determining the relative stability of the two conformers were rationalized through natural bond orbital (NBO) and charge density analyses. The compound was trapped in solid nitrogen at 10 K, and its infrared spectra registered and interpreted, showing the sole presence in the matrix of the more stable trans conformer. Broadband in situ UV irradiations (λ ≥ 288 nm and λ ≥ 235 nm) allowed for the observation of different chemical transformations, which started by excitation to the S1 state of 5HQ, followed by homolytic cleavage of the O-H bond, and subsequent reattachment of the H atom to the 5HQ radical to form quinolin-5(6H)-one and quinolin-5(8H)-one. The first of these two quinolinones was found to convert to open-ring isomeric ketenes, especially when irradiation was performed at higher energy, whereas the second is rather stable under the used experimental conditions. As a whole, the observed photochemistry of matrix-isolated 5HQ closely matches those previously reported for phenol and thiophenol. A detailed mechanistic interpretation for the observed photochemical processes is here proposed, which received support from time-dependent DFT calculations.

Structural Relevance of Intramolecular H-Bonding in Ortho-Hydroxyaryl Schiff Bases: The Case of 3-(5-bromo-2-hydroxybenzylideneamino) Phenol

“…In addition, moderate size barriers (up to ca. 20–25 kJ mol −1 ) associated with transformations involving only movement of a hydrogen atom may also allow conformational decay by quantum mechanical tunneling through the barrier, this phenomenon being common for molecules bearing the hydroxyl moiety, in particular for phenols and naphtols and for carboxylic acids [ 57 , 58 , 59 , 60 , 61 , 62 ]. In order to estimate the energy barriers separating the low-energy conformers of BHAP, relaxed potential energy scans were performed (at the B3LYP/6-311++G(d,p) level) along the relevant coordinates.…”

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

“…Finally, the UV-induced photochemistry of the matrixisolated monomeric gallic acid was investigated and found to follow the general patterns exhibited by phenol-type compounds. [50][51][52] It was concluded that the 2,4-cyclohexadienone derivative, resulting from photoinduced cleavage of the free OH bond of gallic acid followed by recombination of the hydrogen atom with the formed phenoxyl radical analogue at the ortho-position, is the major photorearrangement product, while photofragmentation products (such as acetylene, CO 2 , ketene, and ethynediol) were also found to be present in the irradiated matrices.…”

“…From results previously reported on the photochemistry of matrix-isolated phenols, [49][50][51] it could be expected that, upon UV irradiation, GA undergoes photoinduced cleavage of the free OH bond and forms phenoxyl radical analogue. This is followed by recombination of the detached hydrogen atom with the phenoxyl radical analogue at the ortho or/and para position, leading to formation of the 2,4or 2,5-cyclohexadienone derivatives.…”

“…molecule isolated in solid xenon at 2334 and 667 cm −1 , 52 and the last to ketene (the presence of CO or of ketene species resulting from ring-opening reactions of the 2,4-CHD forms cannot be excluded since they give also rise to bands in this region). [49][50][51]53,54 The band observed at 586 cm −1 in the spectrum of the photolysed matrix can also be assigned to the ketene species (γCH 2 mode, predicted at 587 cm −1 with a large intensity; 107.3 km mol −1 ). In turn, the band observed at ∼767 cm −1 (partially overlapped by a reactant band) and possibly also the small bands at 3303 and 3298 cm −1 , may be ascribed to acetylene (reference values: 728, 3281, and 3267 cm −1 ), 55 while that observed at 1206 cm −1 probably originates in ethynediol, whose most intense IR band is predicted at 1200 cm −1 (see Table S4 in the supplementary material).…”

Recent advances in ketene chemistry

“…Photolysis of 5-hydroxyquinoline, forming ketene 50, was studied by matrix isolation spectroscopy and computations (Scheme 24). 53 The ketene was identified by its characteristic IR absorption in the region 2112-2129 cm …”