UV-Induced Photoisomerization of Acetylacetone and Identification of Less-Stable Isomers by Low-Temperature Matrix-Isolation Infrared Spectroscopy and Density Functional Theory Calculation

Abstract

UV-induced photoisomerization of acetylacetone in low-temperature argon matrixes has been studied by Fourier transform infrared spectroscopy. Identifications of the species produced by UV irradiation (λ > 280 nm) were carried out with the aid of the density functional theory (DFT) calculation, in which the 6-31G* basis set was used to optimize the geometrical structures. By comparison of the observed infrared bands with the calculated spectral patterns, it was found that cis−trans isomerization around the C−C, CC, and C−O bonds occurs to produce less-stable enol isomers, 2-hydroxy-2-penten-4-one. Shorter-wavelength irradiation (λ < 280 nm) induced hydrogen-atom migration of the enol isomers to produce a keto isomer, 2,4-pentanedione.

Luminescence and Raman Spectra of Acetylacetone at Low Temperatures

“…A similar situation is found in acetylacetone, whose excited electronic states were studied by Chen et al, 8 Nakanishi et al, 27 Coussan et al, 28 Nagashima et al, 30 and Upadhyaya et al, 31 among others. The strong UV band appearing at 266 nm in the absorption spectra corresponds to the S 0 → S 2 transition.…”

“…Absorption selection rules for photons forbid singlet-to-triplet transitions, but it is known that, in the solid state, the excited states are not pure singlet or triplet states. 47 Figure 8 shows an energy level diagram depicting data from Matanović and Došlić, 10 Nagashima et al, 30 and Chen et al 8 In Table 2, we give the tentative assignment of the broad bands observed in polycrystalline acac shown in Figure 7. Three isomers have their energy approximately 4000−5000 cm -1 above the ground state of the CCC isomer.…”

“…There are more bands when the blue line is used, and their band centers shift a little. In the literature, [27][28][29][30][31][32][33][34] there are several studies reporting on the UV photoisomerization of acetylacetone and of experiments performed using infrared radiation for conversions of one matrix-trapped isomer into the other. Here, we report photoisomerization that is going on in the visible part of the spectrum when the sample is solid at low temperature.…”

Acetylacetone in hydrogen solids: IR signatures of the enol and keto tautomers and UV induced tautomerization

“…The temperature dependence of the keto/enol equilibrium was also investigated in the gas phase [4]. Studies in cryogenic matrices showed a large amount of enol form as in the gas phase at room temperature [5][6][7].…”

Comparative study of structure and photo-induced reactivity of malonaldehyde and acetylacetone isolated in nitrogen matrices

“…This cold solid-state environment prevents from both major fragmentation, and relaxation to the stable CCC form. A p ® p* UV excitation leads to the formation of several nonchelated rotamers [42][43][44] among the seven forms shown in Fig. 1, and subsequent resonant UV or IR irradiation of these created rotamers lead to other rotamer conversions [45,46].…”