S. Dhanya

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...

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.

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.

Laser flash photolysis of 2,2′-bipyridine solution in cyclohexane

Abstract The lowest excited triplet 3 bpy * has been found to be the principal transient species with a quantum yield of 0.83 in the 248 nm laser flash photolysis of 2,2′-bipyridine (bpy) in cyclohexane. The transient absorbs in the UV and blue region with λ max = 355 nm. The molar absorbance ϵ of the transient 3 bpy * at 355 nm has been determined to be 53 600 ± 2100 M −1 cm −1 . In deaerated solutions 3 bpy * has been found to decay by mixed first-order and second-order kinetics with rate constants of 1.33 X 10 4 s −1 and 3.34 X 10 10 M −1 s −1 respectively. It has also been found that the transient is scavenged by oxygen with a rate constant of 5.19 X 10 8 M −1 s −1 . Some of the end-products of photolysis have been identified and a reaction mechanism is proposed.

Fluorescence behaviour of 2,2′-bipyridine in aqueous solution

Abstract The fluorescence behaviour of an aqueous solution of 2,2′-bipyridine (Bpy) (concentration, greater than 10 −3 M) was investigated under various conditions. When complexed with Zn 2+ or Cd 2+ the excitation maximum changed from 365 to 370 nm and the fluorescence maximum was red shifted from 430 to 455 nm. The enhancement of the fluorescence intensity on complexation is at a maximum for the 1 : 1 complex. The fluorescence intensity is affected by the presence of additives such as urea and LiCl. These observations suggest that the fluorescence originates from the excited state of the Bpy dimer or higher aggregates.

Is H Atom Abstraction Important in the Reaction of Cl with 1-Alkenes?

The relative yields of products of the reaction of Cl atoms with 1-alkenes (C4-C9) were determined to see whether H atom abstraction is an important channel and if it is to identify the preferred position of abstraction. The presence of all the possible positional isomers of long chain alkenones and alkenols among the products, along with chloroketones and chloroalcohols, confirms the occurrence of H atom abstraction. A consistent pattern of distribution of abstraction products is observed with oxidation at C4 (next to allyl) being the lowest and that at CH2 groups away from the double bond being the highest. This contradicts with the higher stability of allyl (C3) radical. For a better understanding of the relative reactivity, ab initio calculations at MP2/6-311+G (d,p) level of theory are carried out in the case of 1-heptene. The total rate coefficient, calculated using conventional transition state theory, was found to be in good agreement with the experimental value at room temperature. The preferred position of Cl atom addition is predicted to be the terminal carbon atom, which matches with the experimental observation, whereas the rate coefficients calculated for individual channels of H atom abstraction do not explain the observed pattern of products. The distribution of abstraction products except at C4 is found to be better explained by reported structure activity relationship, developed from experimental rate coefficient data. This implies the reactions to be kinetically dictated and emphasizes the importance of secondary reactions.

Formation of Furan along with HO2 during the OH-Initiated Oxidation of 2,5-DHF and 2,3-DHF: An Experimental and Computational Study

Experimental characterization of products during OH-initiated oxidation of dihydrofurans (DHF) confirms the formation of furan accompanied by the formation of HO2 to be a significant channel in 2,5-DHF (21 ± 3%), whereas it is absent in 2,3-DHF. Theoretical investigations on the reaction of OH with these molecules are carried out to understand this difference. All possible channels of reaction are studied at M06-2X level with 6-311G* basis set, and the stationary points on the potential energy surface are optimized. The overall rate coefficients calculated using conventional TST with Wigner tunneling correction for 2,5-DHF and 2,3-DHF are 2.25 × 10(-11) and 4.13 × 10(-10) cm(3) molecule(-1) s(-1), respectively, in the same range as the previously determined experimental values. The branching ratios of different channels were estimated using the computed rate coefficients. The abstraction of H atom, leading to dihydrofuranyl radical, is found to be a significant probability, equally important as the addition of OH to the double bond in the case of 2,5-DHF. However, this probability is very small in the case of 2,3-DHF because the rate coefficient of the addition reaction is more than 10 times that of the abstraction reaction. This explains the conspicuous absence of furan among the products of the reaction of OH with 2,3-DHF. The calculations also indicate that the abstraction reaction, and hence furan formation, may become significant for OH-initiated oxidation of 2,3-DHF at temperatures relevant to combustion.

Reactivity of Cl Atom with Triple-Bonded Molecules. An Experimental and Theoretical Study with Alcohols

The reactivities of the Cl atom with triple-bonded molecules were examined by determining the rate coefficients of reactions of four triple-bonded alcohols (TA), namely, 2-propyn-1-ol, 3-butyn-1-ol, 3-butyn-2-ol, and 2-methyl-3-butyn-2-ol, using the relative rate method, at 298 K. The rate coefficients (k) of reaction of the four alcohols with Cl vary in the range (3.5-4.3) × 10(-10) cm(3) molecule(-1) s(-1). These values imply significant contribution of the Cl reaction in the tropospheric degradation of TAs in the conditions of the marine boundary layer. A striking difference is observed in the reactivity trend of Cl from that of OH/O3. Although the reactivity of OH/O3 is lower with triple-bonded molecules, as compared to the double-bonded analogues, the reactivity of the Cl atom is similar for both. For a deeper insight, the reactions of Cl and OH with the simplest TA, 2-propyn-1-ol, are investigated theoretically. Conventional transition state theory is applied to compute the values of k, using the calculated energies at QCISD and QCISD(T) levels of theory of the optimized geometries of the reactants, transition states (TS), and the product radicals of all the possible reaction pathways at the MP2/6-311++G(d,p) level. The k values calculated at the QCISD level for Cl and the QCISD(T) level for OH reactions are found to be very close to the experimental values at 298 K. In the case of the Cl reaction, the abstraction of α-H atoms as well as the addition at the terminal and middle carbon atoms have submerged TS and the contribution of the abstraction reaction is found to be significant at room temperature, at all levels of calculations. Addition at the terminal carbon atom is prominent compared to that at the middle carbon. In contrast to the Cl reaction, only addition at the middle carbon is associated with such low lying TS in the case of OH. The individual rate coefficients of addition and abstraction of OH are lower than that of Cl. The negative temperature dependence of the computed rate coefficients in the temperature range 200-400 K shows that the difference in the TS energy of Cl and OH affects the pre-exponential factor more than the activation energy.

Laser flash photolysis of 2,2′-bipyridine in aqueous solution

Abstract The transient species produced on laser flash photolysis (248 nm) of an aqueous solution of 2,2′-bipyridine (Bpy) have been identified as the excited triplet (3Bpy*), radical (BpyH and radical cation (Bpy+ ). BpyH and Bpy+ are formed via a biphotonic ionization reaction; 3Bpy* is produced predominantly via monophotonic absorption. The transient absorption maximum at 340 nm is assigned to Bpy+ ; since it is also observed on pulse radiolysis of Bpy in 1,2-dichloroethane. The molar absorbance of Bpy+ at 340 nm in aqueous solution is 2.33 × 104 M−1 cm−1. The decay of Bpy+ follows first-order kinetics independent of pH with a rate constant k of 1.6 × 105 s−1. 3Bpy* and BpyH have absorption maxima in the region 350–370 nm, and their decay kinetics are complex.

Reaction of electronically excited benzene molecule with nitrate ion

Abstract From the present experiments on the UV photolysis of aqueous nitric acid solutions containing benzene, it is inferred that nitrous acid is formed through a chemical reaction between electronically excited benzene and ground state nitrate ion.