UV-light induced conformational changes of 3-chlorosalicylic acid in low-temperature argon matrices

• Conformation of 3-chlorosalicylic acid in an argon matrix was investigated.
• Stabilization due to intramolecular hydrogen bonding was examined.
• Conformational changes to less stable isomers were caused by UV irradiation.
• Isomerization between less stable isomers occurred upon higher-energy irradiation.

Conformations and photoreaction pathways of 3-chlorosalicylic acid have been investigated by low-temperature matrix-isolation infrared spectroscopy with an aid of the density-functional-theory calculation. It was found that only one conformer having an intramolecular hydrogen bond of PhOH⋯OC in COOH existed in argon matrices after deposition before UV irradiation. Two less stable conformers were produced upon UV irradiation (λ > 330 nm); one has an intramolecular hydrogen bond of PhOH⋯OH in COOH, and the other has two intramolecular hydrogen bonds of COOH⋯OHPh and PhOH⋯Cl. Another conformational change between the less stable conformers was observed when the matrix was exposed to the shorter-wavelength UV light (λ > 290 nm) for 1 min following the 300-min longer-wavelength UV irradiation (λ > 330 nm). The photoreaction pathways due to the conformational changes were supported by a kinetic analysis using the IR absorbance changes of the photoproducts against the irradiation time.

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