Mechanism of UV-Induced Conformational Changes among Enol-Type Isomers of (Trifluoroacetyl)acetone Studied by Low-Temperature Matrix-Isolation Infrared Spectroscopy and Density Functional Theory Calculation

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2004-03, Vol.108 (12), p.2353-2362
• Main Authors: Minoura, Yukiko, Nagashima, Naoko, Kudoh, Satoshi, Nakata, Munetaka
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp031192y

UV-induced conformational changes among chelated and nonchelated enol isomers of (trifluoroacetyl)acetone in a low-temperature argon matrix were investigated by Fourier transform infrared spectroscopy. An infrared spectrum measured after deposition before UV irradiation was assigned to a chelated enol isomer, 1,1,1-trifluoro-4-hydroxy-3-penten-2-one, where no bands for the other chelated enol isomer, 5,5,5-trifluoro-4-hydroxy-3-penten-2-one, were found, the latter being formed from the former by moving of the hydrogen atom in the O−H group to the CO group. Conformations of 3 nonchelated enol isomers among 14 were determined by a comparison of the infrared spectra measured after UV irradiation with the spectral patterns obtained by the density functional theory calculations at the DFT/B3LYP/6-31G* level. This selective isomerization was explained in terms of the inhibition of internal rotation in vibrational relaxation of photoexcited species, which was proposed in our previous studies of acetylacetone and hexafluoroacetylacetone. An infrared spectrum of a transient photoproduct was measured while the matrix sample was continuously exposed to the UV light. It was identified as another nonchelated enol isomer.