Photodissociation dynamics of enolic-acetylacetone at 266, 248, and 193 nm: Mechanism and nascent state product distribution of OH

Abstract

The photodissociation dynamics of acetylacetone (H3C–CO–CH2–CO–CH3), which exists predominantly as an enolic form [H3C–COCH=C(OH)–CH3] in gas phase, is studied using pulsed laser photolysis laser induced fluorescence (LIF) “pump-and-probe” technique at room temperature. Although two pathways for OH formation have been observed, we have focused on the nascent state of the primary OH radical, formed after photo-excitation of the molecule to its (π,π*) and Rydberg states. The (π,π*) and Rydberg transitions are prepared by excitation with fourth harmonic of Nd:YAG (266 nm)/KrF (248 nm) and ArF (193 nm) lasers, respectively. The ro-vibrational distribution of the nascent OH photofragment is measured in collision-free conditions using LIF. The OH fragments are formed in vibrationally cold state at all the above wavelengths of excitation, but differ in rotational state distributions. The rotational distribution is Boltzmann-like, and characterized by rotational temperatures of 950±50, 1130±60, and 1010±80 K at 266, 248, and 193 nm photodissociation, respectively. The spin–orbit and Λ-doublets ratios of OH fragments formed in the dissociation process are also measured. The average translational energy partitioned into the photofragment pairs in the center-of-mass co-ordinate is found to be 16.0−4.0+1.0, 17.3±4.2, and 19.2±4.7 kcal/mol at 266, 248, and 193 nm excitation, respectively. The energy partitioning into various degrees of freedom of products is interpreted with the help of different models, namely, statistical, impulsive, and hybrid models. To understand the nature of the dissociative potential energy surface involved in the OH production channel, detailed ab initio calculations are performed using configuration interaction-singles method. Although acetylacetone is initially prepared in the (1ππ*) state at 266 and 248 nm excitation, it is concluded that the OH fragment is formed from the lowest (3ππ*) state. However, upon excitation at 193 nm, the initially prepared Rydberg state of acetylacetone crosses over fast to the nearby σ* repulsive state along the C–OH bond, and dissociates to give the OH radical.

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.…”

“…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.…”

“…First, if the sample is liquid, no absorption in the 2.2−2.5 eV interval can take place because the electronic states of molecules in liquid resemble those of free molecules, and in that energy interval, only the triplet T 2 state is predicted to exist (or a series of triple states for different isomers). 8,[29][30][31] We also recorded some UV−vis absorbance spectra of liquid acac, and there were no absorption bands in the 450−550 nm interval for liquid. Second, photons cannot be emitted if molecules make transitions from pure triplet states to the ground singlet state (spin zero); therefore, the electronic states to which acac molecules are excited and from which they decay into the ground state must have partial singlet character in the crystal.…”

“…25 After the discovery of Veierov et al 26 that acetylacetone displays isomerization upon UV illumination, a number of studies were devoted to isolation of different isomers and calculation of their relative energies. [27][28][29][30][31][32][33][34] Vibrational studies of acetylacetone gave a thorough assignment of bands for the enolic C s tautomer, and some, more intense, bands of the keto tautomer could be confidently attributed. induced UV and IR isomerizations of matrix-isolated acac.…”

Application of Infrared Spectra of UV Irradiated Acetylacetone Photoproducts to a Novel Actinometer for UV Laser Nanoprocessing

“…3. The IR spectra of HFA have been studied in detail (3) Although there are few reports on the photodissociation mechanism of HFA and the yield of the photoproducts, the photodissociation mechanisms of enolic acetylacetone, H 3 C-COCH=C(OH)-CH 3 , have been known (11) . As compared the photodissociation mechansisms of acetylacetone with the results of IR spectroscopy, the photodissociation mechanisms of HFA can be explained as follows: (1), (2), and (3).…”