Photodissociation Dynamics of Enolic 1,2-Cyclohexanedioneat 266, 248, and 193 nm: Mechanism and Nascent State Product Distributionof OH.

The photodissociation dynamics of1,2-cyclohexanedione (CHD), whichexists in enolic form in gas phase, is studied using pulsed laserphotolysis (LP)-laser induced fluorescence (LIF) “pump-and-probe”technique at room temperature. The nascent state distribution of theOH radical, formed after initial photoexcitation of the molecule toit is (π, π*) and Rydberg states, is determined. The initial(π, π*) and Rydberg states are prepared by excitationwith the fourth harmonic output of Nd:YAG (266 nm)/KrF (248 nm) andArF (193 nm) lasers, respectively. The ro-vibrational distributionof the nascent OH photofragment is measured in collision-free conditionsusing LIF. The OH fragments are formed in the vibrationally cold stateat all the above wavelengths of excitation but differ in rotationalstate distributions. At 266 nm photolysis, the rotational populationof OH shows a curvature in Boltzmann plot, which is fairly describedby two types of Boltzmann-like distributions characterized by rotationaltemperatures of 3100 ± 100 and 900 ± 80 K. However, at 248nm photolysis, the rotational distribution is described by a singlerotational temperature of 950 ± 80 K. The spin–orbit andΛ-doublets ratios of OH fragments formed in the dissociationprocess are also measured. The average translational energy in thecenter-of-mass coordinate, partitioned into the photofragment pairsof the OH formation channels, is determined to be 12.5 ± 3.0,12.7 ± 3.0, and 12.0 ± 3.0 kcal/mol at 266, 248, and 193nm excitation, respectively. The energy partitioning into variousdegrees of freedom of products is interpreted with the help of differentmodels, namely, statistical, impulsive, and hybrid models. To understandthe nature of the dissociative potential energy surface involved inthe OH formation channel, detailed ab initio calculations are performedusing configuration interaction-singles (CIS) method. It is proposedthat at 266 nm photolysis, the OH fragment is formed from two differentexcited state structures, one with a strong H bonding, similar tothat in the ground state, and another without effective H bonding,whereas, at 248 nm photodissociation, it seems that the OH formationoccurs mainly from the excited state, which lacks effective H-bonding.At 193 nm excitation, the initially prepared population in the Rydbergstate crosses over to a nearby σ* repulsive state along theC–O bond, from where the dissociation takes place. The exitbarrier for the OH dissociation channel is estimated to be 14 kcal/mol.The existence of dynamical constraint due to strong hydrogen bondin the ground state is effectively present in the dissociation processat 266 and somewhat deficient at 248 nm photolysis. [ABSTRACT FROM AUTHOR]