Investigating the Phase-Dependent Reactivity of Chlorine Dioxide Using Resonance Raman Spectroscopy

Femtosecond pump-probe studies of actinic-wavelength dependence in aqueous chlorine dioxide photochemistry

The actinic or photolysis-wavelength dependence of aqueous chlorine dioxide (OClO) photochemistry is investigated using femtosecond pump-probe spectroscopy. Following photoexcitation at 310, 335, and 410 nm the photoinduced evolution in optical density is measured from the UV to the near IR. Analysis of the optical-density evolution illustrates that the quantum yield for atomic chlorine production (Phi(Cl)) increases with actinic energy, with Phi(Cl)=0.16+/-0.02 for 410 nm excitation and increasing to 0.25+/-0.01 and 0.54+/-0.10 for 335 and 310 nm excitations, respectively. Consistent with previous studies, the production of Cl occurs through two channels, with one channel corresponding to prompt (<5 ps) Cl formation and the other corresponding to the thermal decomposition of ClOO formed by OClO photoisomerization. The partitioning between Cl production channels is dependent on actinic energy, with prompt Cl production enhanced with an increase in actinic energy. Limited evidence is found for enhanced ClO production with an increase in actinic energy. Stimulated emission and excited-state absorption features associated with OClO populating the optically prepared (2)A(2) surface decrease with an increase in actinic energy suggesting that the excited-state decay dynamics are also actinic energy dependent. The studies presented here provide detailed information on the actinic-wavelength dependence of OClO photochemistry in aqueous solution.

Femtosecond Pump−Probe Studies of Nitrosyl Chloride Photochemistry in Solution

We present a femtosecond pump-probe study of the primary events of nitrosyl chloride (ClNO) photochemistry in solution. Following 266 nm photolysis, the resulting evolution in optical density is measured for ClNO dissolved in acetonitrile, chloroform, and dichloromethane. The results demonstrate that photolysis results in the production of a photoproduct that has an absorption band maximum at 295 nm in acetonitrile and 330 nm in chloroform and dichloromethane. To determine the extent of Cl production, comparative photochemical studies of methyl hypochlorite (MeOCl) and ClNO are performed. Photolysis of MeOCl in solution results in the production of the Cl:solvent charge-transfer complex; therefore, a comparison of the spectral evolution observed following MeOCl and ClNO photolysis under identical photolysis conditions is performed to determine the extent of Cl production following ClNO photolysis. We find that similar to the gas-phase photochemistry, Cl and NO formation is the dominant photochemical channel in acetonitrile. However, the photochemistry in chloroform and dichloromethane is more complex, with a second product formed in addition to Cl and NO. It is proposed that in these solvents photoisomerization also occurs, resulting in the production of ClON. The results presented here represent the first detailed examination of the solution phase photochemistry of ClNO.

A 2A2←X 2B1 absorption and Raman spectra of the OClO molecule: A three-dimensional time-dependent wave packet study

Time-dependent wave packet calculations of the (A (2)A(2)<--X (2)B(1)) absorption and Raman spectra of the OClO molecule are reported. The Fourier grid Hamiltonian method in three dimensions is employed. The X (2)B(1) ground state ab initio potential energy surface reported by Peterson is used together with his corresponding A (2)A(2) state surface or the revised surface of the A (2)A(2) state by Xie and Guo. Radau coordinates are used to describe the vibrations of a nonrotating OClO molecule. The split-operator method combined with fast Fourier transform is applied to propagate the wave function. We find that the ab initio A (2)A(2) potential energy surface better reproduces the detailed structures of the absorption spectrum at long wavelength, while the revised surface of the A (2)A(2) state, consistent with the work of Xie and Guo, better reproduces the overall shape and the energies of the vibrational levels. Both surfaces of the A (2)A(2) state can reasonably reproduce the experimental Raman spectra but neither does so in detail for the numerical model employed in the present work.