Femtosecond Pump−Probe Studies of Nitrosyl Chloride Photochemistry in Solution

Comparing molecular photofragmentation dynamics in the gas and liquid phases

This article explores the extent to which insights gleaned from detailed studies of molecular photodissociations in the gas phase (i.e. under isolated molecule conditions) can inform our understanding of the corresponding photofragmentation processes in solution. Systems selected for comparison include a thiophenol (p-methylthiophenol), a thioanisole (p-methylthioanisole) and phenol, in vacuum and in cyclohexane solution. UV excitation in the gas phase results in RX-Y (X = O, S; Y = H, CH3) bond fission in all cases, but over timescales that vary by ~4 orders of magnitude - all of which behaviours can be rationalised on the basis of the relevant bound and dissociative excited state potential energy surfaces (PESs) accessed by UV photoexcitation, and of the conical intersections that facilitate radiationless transfer between these PESs. Time-resolved UV pump-broadband UV/visible probe and/or UV pump-broadband IR probe studies of the corresponding systems in cyclohexane solution reveal additional processes that are unique to the condensed phase. Thus, for example, the data clearly reveal evidence of (i) vibrational relaxation of the photoexcited molecules prior to their dissociation and of the radical fragments formed upon X-Y bond fission, and (ii) geminate recombination of the RX and Y products (leading to reformation of the ground state parent and/or isomeric adducts). Nonetheless, the data also show that, in each case, the characteristics (and the timescale) of the initial bond fission process that occurs under isolated molecule conditions are barely changed by the presence of a weakly interacting solvent like cyclohexane. These condensed phase studies are then extended to an ether analogue of phenol (allyl phenyl ether), wherein UV photo-induced RO-allyl bond fission constitutes the first step of a photo-Claisen rearrangement.

On the photolysis of simple anions and neutral molecules as sources of O−/OH, SOx−and Cl in aqueous solution

This contribution examines the aqueous phase photolysis processes of simple anions such as nitrate, nitrite, peroxodisulfate and neutral molecules such as H2O2. The review includes new results on absolute effective quantum yields for the photodissociation processes of NO3(-), NO2(-), S2O8(2-), HSO5(-), S2O6(2-), HOCl, and chloroacetone in an aqueous solution. The quantum yields for the photolysis of nitrate and nitrite have also been determined as a function of temperature. Models to interpret the wavelength and the temperature dependencies of the quantum yields for the different systems are discussed and a simple model treatment is developed to quantify the effects of (i) impulse conservation, (ii) electrostatic interaction (e.g., ion-dipole, dipole-dipole and coulomb interaction between the photofragments directly after photolytic fragmentation), and (iii) diffusion and recombination. The combined impulse-interaction-diffusion (IID) model is compared to the experimentally observed effective radical formation quantum yields and reasonable agreement is found for a number of systems. It is shown that the temperature dependencies for effective quantum yields of photolysis processes in aqueous solution are not only governed by the temperature dependence of the viscosity of water but also determined by the temperature dependence of the rate constants of the photofragment recombination reactions.