Awadhesh Kumar

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

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 2...

Dynamics of acetic acid dissociation at 193.3 nm: selectivity in OH reaction channel

Abstract The photodissociation dynamics of acetic acid at 193.3 nm is investigated by probing the nascent photoproduct OH using LIF spectroscopy. The photoproduct OH is formed mainly in the ground vibrational state with a rotational energy of 1.6±0.2 kcal/mol . The energy partitioned as photofragment translational energy is 10.0 −2.8 +5.0 kcal/mol . The present study in combination with the available dynamic information implies that the C–O single bond fission is the dominant OH-producing reaction. The modeling of the observed energy partitioning indicates that about 80% of the excess energy above the barrier is retained in the CO moiety.

Kinetics of OH radical reaction with allyl alcohol (H2CCHCH2OH) and propargyl alcohol (HCCCH2OH) studied by LIF

Abstract The rate constants for the reactions of hydroxyl radical (OH) with two unsaturated alcohols (ROH) namely, allyl alcohol (H 2 CCHCH 2 OH) and propargyl alcohol (HCCCH 2 OH) in the gas phase have been measured. The kinetic measurements were carried out using laser photolysis (LP) combined with laser induced fluorescence (LIF) technique at room temperature over a pressure range of 10–20 Torr. The bimolecular rate constants for the reactions OH+ROH→products, are determined at room temperature to be (3.7±0.5)×10 −11 , (9.2±1.4)×10 −12 cm 3 molecule −1 s −1 , respectively, for allyl alcohol and propargyl alcohol. The measured rate constants in combination with ab initio molecular orbital calculation provide a better understanding of the structure-reactivity rules.

Photodissociation Dynamics of Phosphorus Trichloride (PCl3) at 235 nm Using Resonance Enhanced Multiphoton Ionization (REMPI) with Time-of-Flight (TOF) Mass Spectrometry

The photodissociation dynamics of phosphorus trichloride (PCl(3)) has been studied in a supersonic beam by resonance enhanced multiphoton ionization (REMPI), using time-of-flight (TOF) mass spectrometry. The study is focused on the nascent state of the primary chlorine atom, formed on excitation of the (n, sigma*) transition of the molecule around 235 nm. Dissociation of PCl(3) and the REMPI detection of chlorine atoms are performed, using the same laser around 235 nm. The photofragments, namely, Cl((2)P(3/2)) and Cl*((2)P(1/2)), are probed, using the 2+1 REMPI scheme in the 234-236 nm region. We have determined the photofragment speed distribution, the recoil anisotropy parameter beta, and the spin-orbit branching ratio for chlorine atom elimination channels. Polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment anisotropies. The anisotropy parameters for Cl and Cl* are characterized by values of 0.0 +/- 0.05 and 0.20 +/- 0.05, respectively. Two components, namely, the fast and the slow, are observed in the speed distribution (P(v)) of Cl and Cl* atoms, formed from different potential energy surfaces. The average translational energies for the Cl and Cl* channels for the fast component are 29.7 and 30.6 kcal/mol, respectively. Similarly, for the slow component, the average translational energies for the Cl and Cl* channels are 9.5 and 9.1 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of an impulsive model, for the fast component, and a statistical model, for the slow component. Apart from the chlorine atom elimination channel, molecular chlorine (Cl(2)) elimination is also observed in the photodissociation of PCl(3). The observation of the molecular chlorine in the dissociation process and the bimodal translational energy distribution of the chlorine atom clearly indicate the existence of a crossover mechanism from the initially prepared state to the ground state.

Dynamics of OH formation in photodissociation of pyruvic acid at 193 nm

The dynamics of the formation of OH radical upon 193 nm excitation of pyruvic acid has been investigated by the laser-photolysis laser-induced-fluorescence technique. OH radicals were generated in the ground electronic state, with no vibrational excitation. The estimated rotational temperature is 720±90 K, and the translational energy is 18.7±6.5 kcal mol−1. Ab initio calculations on excited electronic states were performed at the configuration interaction with single electronic excitation level with 6-31+G(d,p) basis function. All low-lying electronic excited states (S1–S3 and T1–T6) were characterized and the transitions were identified. A transition state for the C–OH dissociation channel has been obtained from the T1 state with a late exit barrier. A mechanism for the formation of OH radicals involving internal conversion and intersystem crossing from the initially populated S3 state to T1 state and the dissociation from the T1 potential energy surface with the calculated barrier is proposed, which re...

Dynamics of OH formation in the dissociation of acrylic acid in its (n, π*) and (π, π*) transitions excited at 248 and 193 nm

The (n,π*) and (π,π*) transitions in acrylic acid (H2C=CHCOOH) are excited by KrF (248 nm) and ArF (193 nm) laser pulses, respectively, and the dynamics of its photodissociation to give OH fragments is studied using laser induced fluorescence technique. At both the photolysis wavelengths, the OH fragments produced are vibrationally cold, but have different rotational state distributions. To get an insight into the potential energy surface involved in the dissociation process, spin–orbit and Λ-doublets ratios are also measured. Average relative translational energy partitioned into the photofragments is determined using linewidth of the Doppler profiles to be 13.2±3.1 and 10.2±2.8 kcal/mol at 193 and 248 nm excitations, respectively. High percentage of translational energy released into the photofragments suggests the presence of an exit barrier for the dissociation. On 248 nm excitation, the OH radicals are formed instantaneously during the laser pulse, while on 193 nm excitation, a risetime of ∼2 μs is s...

Photodissociation of carboxylic acids: dynamics of OH formation

Abstract In this review article, recent studies on the photodissociation dynamics of carboxylic acids carried out in our laboratory are presented. The dynamics are investigated by mapping the energy partitioning in the nascent photoproduct OH using laser-induced fluorescence spectroscopy. To understand the effect of the nature of the CC bond on the dissociation dynamics, both saturated (acetic) as well as unsaturated (acrylic and propiolic) carboxylic acids are investigated. In all of the carboxylic acids studied, a high percentage of the available energy is partitioned into the product translational state, indicating the presence of an exit barrier in the dissociative potential energy surface. Based on the energy partitioning, the quantum yield and the OH formation rate, the photoexcitation dynamics of carboxylic acids are revealed.

Time-resolved study of the transients produced in the CO2 and ArF laser flash photolysis of gaseous silacyclobutane and 1,3-disilacyclobutane

A time-resolved study of the transients produced in the TEA CO2 or ArF laser-induced decomposition of gaseous silacyclobutane (SCB) and 1,3-disilacyclobutane (DSCB) is reported. Both compounds produce transient H2CSiH2 as the major primary product, which has been identified by its optical absorption spectrum, with λmax≈ 260 nm. Under conditions of low laser fluence, this species has two decay channels: a unimolecular process (k= 2.3 ± 0.7 × 104 s–1) and a reaction with the parent compound (kSCB= 2.0 ± 0.3 × 10–13 cm3 molecule–1 s–1 and kDSCB= 3.0 ± 0.5 × 10–13 cm3 molecule–1 s–1). At high fluence (7.2 J cm–2 for the CO2 laser and 6 J cm–2 for the ArF laser), the transient absorption signals become very complex owing to the onset of a number of other reactions and the formation of several additional transient species which appear to have strong absorption in the 250–650 nm region, with peaks/shoulders at ca. 260, 320 and 435 nm but these could not be identified unambiguously.

Dynamics of Cl (2Pj) atom formation in the photodissociation of fumaryl chloride (ClCO - CH = CH - COCl) at 235 nm: a resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) study.

The photodissociation dynamics of fumaryl chloride (ClCO-CH═CH-COCl) has been studied in a supersonic molecular beam around 235 nm using resonance enhanced multiphoton ionization (REMPI) time-of-flight (TOF) technique by detecting the nascent state of the primary chlorine atom. A single laser has been used for excitation of fumaryl chloride and the REMPI detection of chlorine atoms in their spin-orbit states, Cl ((2)P(3/2)) and Cl* ((2)P(1/2)). We have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin-orbit branching ratio for chlorine atom elimination channels. To obtain these, measured polarization-dependent and state-specific TOF profiles are converted into kinetic energy distributions, using a least-squares fitting method, taking into account the fragment recoil anisotropies, β(i). The TOF profiles for both Cl and Cl* are found to be independent of laser polarization; i.e., β is well characterized by a value of 0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distribution, P(E(T)), of Cl and Cl* atoms, and assigned to be formed from different potential energy surfaces. The average translational energies for the fast components of the Cl and Cl* channels are 14.9 ± 1.6 and 16.8 ± 1.6 kcal/mol, respectively. Similarly, for the slow components, the average translational energies of the Cl and Cl* channels are 3.4 ± 0.8 and 3.1 ± 0.8 kcal/mol, respectively. The energy partitioning into the translational modes is interpreted with the help of various models, such as impulsive and statistical models. Apart from the chlorine atom elimination channel, molecular hydrogen chloride (HCl) elimination is also observed in the photodissociation process. The HCl product has been detected, using a REMPI scheme in the region of 236-237 nm. The observation of the molecular HCl in the dissociation process highlights the importance of the relaxation process, in which the initially excited parent molecule relaxes to the ground state from where the molecular (HCl) elimination takes place.

Kinetics of Gas-Phase Reaction of OH with Morpholine: An Experimental and Theoretical Study

Kinetics of reaction of OH radical with morpholine, a heterocyclic molecule with both oxygen and nitrogen atoms, has been investigated using laser photolysis-laser-induced fluorescence technique, in the temperature range of 298-363 K. The rate constant at room temperature (k(298)) is (8.0 +/- 0.1) x 10(-11) molecule(-1) cm(3) s(-1). The rate constant decreases with temperature in the range studied, with the approximate dependence given by k(T) = (1.1 +/- 0.1) x 10(-11) exp[(590 +/- 20)/T] cm(3) molecule(-1) s(-1). The rate constants are high compared with those of similar heterocyclic molecules with oxygen atom but comparable to those reported for aliphatic amines. Ab initio molecular orbital calculations show that prereactive complexes, 5-7 kcal mol(-1) lower in energy as compared with the reactants, are formed because of hydrogen bond interaction between OH and the N/O atom of morpholine. The stability of the complex involving the nitrogen atom is found to be more than that involving the oxygen atom. The optimized transition-state structures and energies for the different pathways of hydrogen abstraction from these prereactive complexes explain the observation of negative activation energy.