Dynamics of OH formation in the photodissociation of enolic-acetonylacetone at 193 nm

• First results on dynamics of OH radical formation in the photodissociation of enolic conformer of acetonylacetone.
• Мechanism for OH radical formation is reported.
• Effect of hydrogen bonding in the photodissociation process is reported.

The photodissociation dynamics of enolic-acetonylacetone (H3CC(O)CH2CH C(OH)CH3), in gas phase, is studied using Laser Photolysis–Laser Induced Fluorescence (LP–LIF) technique at room temperature. The nascent state distribution of the OH radical is probed formed after photo-excitation of the parent molecule to its πC=C−πC=C* state using 193 nm laser light. The ro-vibrational distribution of the nascent OH photofragment is measured using LIF in collision free conditions. The rotational distribution is Boltzmann-like, and characterized by a single rotational temperature of 650 ± 50 K. The spin-orbit and Λ-doublet ratios of OH fragment formed in the dissociation process are also determined. For OH producing channel, the average translational energy partitioned into the photofragment pairs in the center-of-mass co-ordinate is found to be 25.0 ± 4.0 kcal/mol, corresponding to a fT value of 0.43. The experimental studies along with theoretical calculations employing time-dependent density functional theory (TD-DFT) along with ab initio   molecular orbital (MO) theory suggest that the initially preparedπC=C−πC=C* state crosses over to a nearby σ* repulsive state along the CO bond, from where the dissociation takes place. The dynamics of OH formation is compared with that of acetylacetone photodissociation and effect of H-bonding is discussed.

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