R.D. Saini

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.

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.

UV spectrum and decay kinetics of transient methylsilene produced in the 193 nm photolysis of gaseous 1-methyl-1-silacyclobutane

Abstract 193 nm photolysis of gaseous 1-methyl-1-silacyclobutane with a laser fluence of 50 mJ cm−2 yields a H2CSi(H)CH3 transient, which decays exclusively by dimerisation to form 1,3-dimethyl-1,3-disilacyclobutane as the stable product with an estimated rate constant of ( 2.4±0.5)∗x10 −11 cm 3 molecule−1 s−1.

Formation and isolation of uranium(V) in the photochemical reduction of uranyl ion in aqueous carbonate medium

Abstract Photochemical reduction of UO 2 (CO 3 ) 3 4- was investigated in deoxygenated aqueous carbonate solutions of pH 11.2 or greater containing ethanol or sodium formate. The photolytic product is a dark-coloured, sparingly soluble U V complex which is sensitive to aerial oxidation. The evidence for the presence of pentavalent uranium in this complex was obtained by optical absorption spectroscopy, cyclic voltammetry, chemical synthesis and chemical analysis. On the basis of its physical and chemical properties, the molecular formula of the uranium(V) complex is proposed as [(UO 2 ) 4 (OH) 6 (H 2 O 9 ] 2- .

Photoluminescence studies of the uranyl carbonate system

Abstract An investigation was carried out on the effects of hydrolysis and complex formation on the optical absorption spectra, photoluminescence and excited state lifetimes of various uranyl species formed under different pH conditions and various carbonate to uranyl concentration ratios in aqueous solutions. The hydrolysis increases both the quantum efficiency of luminescence Φ L and the lifetimes τ of the excited states up to a pH of about 6.2; the exact pH depends on the total uranyl concentration in the solution. The effect is attributed to the formation of large polynuclear species with hydroxo bridges, such as (UO 2 ) 3 (OH) 7 − , which introduce a rigidity into the molecular structure of the emitting species and inhibit the collisional quenching due to their large size and high molecular weight. At a pH of greater than 6.2, at a total uranyl concentration of 5 × 10 −4 M, both Φ L and τ decline continuously. This is explained as being due to the transformation of polynuclear species with hydroxo bridges to species with oxo bridges, with a concomitant reduction in the molecular rigidity. Complex formation between uranyl(VI) species and carbonate ion always leads to the quenching of uranyl luminescence. The Φ L value decreases with incresing carbonate to uranyl concentration ratio, so much so that the UO 2 (CO 3 ) 3 4− complex is non-luminescent both in solution and in the solid state. Experimental evidence suggests that the quenching of uranyl luminescence by carbonate takes place by an intramolecular mechanism. At a pH close to 4.8, where the formation of very large polynuclear species such as (UO 2 ) 11 (OH) 12 (CO 3 ) 6 2− has been reported in the literature, kinetic such as (UO 2 ) 11 (OH) 12 (CO 3 ) 6 2– has been reported in the literature, kinetic evidence for the excited state dissociation of higher polynuclear species into smaller fragments is obtained. On dilution, uranyl carbonato complexes show enhanced luminescence which is interpreted as being due to the hydrolytic reactions.

Triplet state characteristics of 2,2′- and 4,4′-biphenyldiols studied by 248 nm nanosecond laser flash photolysis

Triplet state characteristics of 2,2′- and 4,4′-biphenyldiols have been investigated in different organic solvents using 248 nm nanosecond laser flash photolysis technique. While for 2,2′-biphenyldiol, the triplet state of the molecule is produced as the only transient, for 4,4′-biphenyldiol, some phenoxyl radicals are also formed along with the triplet state following the laser excitation. Triplet quantum yields of 2,2′-biphenyldiol in different solvents are seen to be much lower than those of 4,4′-biphenyldiol. The differences in the laser flash photolysis results of the two diols have been explained on the basis of the presence and the absence of intramolecular hydrogen bonding in the two molecules.

On the reaction of carbonate radical with uranyl ion in aqueous medium: flash photolytic and pulse radiolytic studies

Abstract During steady state and flash photolysis (λ > 248 nm), the uranyl ion in aqueous carbonate medium was observed to exhibit photochromic behaviour. Formation of a radical complex, UO2(CO3)2(C 3)4−, with a lifetime of approximately 4.2 ms or greater is postulated to account for this observation. Flash excitation with UV light produced a carbonate radical and a permanent product, in addition to photochromism. The carbonate radical is shown to result exclusively from the photoionization of free bicarbonate or carbonate ions in the solution. The permanent product was identified as a peroxy complex, {UO2(CO3)2OOH}3−; a reaction mechanism involving CO3∓− and UO2(CO3)2(C 3)4− is proposed to explain its formation. Pulse radiolytic studies do not support the occurrence of the reaction, CO3∓− + UVVI→products, which has been reported in the literature.

On the decay of the ozonide radical in aqueous alkaline solutions

Abstract In flash photolysis of an oxygenated aqueous potassium persulphate solution at pH 12.5 the decay of the ozonide radical has been found to follow 3 2 order kinetics which has been explained by reactions O − 3 + O − ⇌ 2 O − 2 and O − 3 + HO 2 → 2 O 2 + OH −

Photodissociation and photoionization mechanisms of 2,2’- and 4,4’-biphenyldiols: a laser flash photolysis study

Nanosecond and picosecond laser flash photolysis (LFP) of 2,2’- and 4,4’-biphenyldiols has been investigated in aqueous acidic and alkaline solutions to understand the details of the photodissociation (PD) and photoionization (PI) mechanisms of the two diols. For 2,2’-biphenyldiol, the photodissociation and photoionization following the excitation using UV light (248 or 266 nm) is seen to be a bi-photonic process, whereas that of the 4,4’-biphenyldiol is seen to follow a monophotonic mechanism. A detailed analysis of the nanosecond and picosecond LFP results indicate that the PD/PI of 2,2’-biphenyldiol involves the participation of the triplet (T1) state of the molecule. For 4,4’-biphenyldiol, however, the PD/PI occurs from the excited singlet (S1) state. It is inferred that the difference in the PD/PI behavior of 2,2’- and 4,4’-biphenyldiols arises due to the presence and absence of intramolecular hydrogen bonding in the two respective molecules. Qualitative potential energy diagrams have been presented to rationalize how the presence and absence of intramolecular hydrogen bonding causes a difference in the stabilization of the electronic states in the two diols and that ultimately determines the possibility of the bi-photonic mono-photonic PD/PI mechanisms for 2,2’- and 4,4’-biphenyldiols, respectively.