Resonance Raman intensity analysis of chlorine dioxide dissolved in chloroform: The role of nonpolar solvation

Resonance Raman intensity analysis of ClNO(2) dissolved in methanol

Halogens such as chlorine are converted from halides, including ClNO(2), to reactive radicals by UV solar radiation. These radicals can affect ozone production and destruction in the stratosphere. Recently, it became clear that halogen radicals can also play a significant role in the chemistry of the troposphere. The photochemistry of ClNO(2) has been the subject of several studies in the gas and solid state that demonstrated a clear phase-dependent reactivity. Here, we report our initial studies of nitryl chloride in solution. Resonance Raman (RR) spectra of ClNO(2) dissolved in methanol after excitation within the 1(1)A(1)-2(1)A(1) absorption band (D band) in the region 200-240 nm are presented. RR intensity along the NO symmetric stretch coordinate (v(1)) at 1291 cm(-1) is observed at all excitation wavelengths, whereas limited intensity corresponding to the transition of the N-Cl symmetric stretch (v(3)) was only observed at 199.8 nm, whereas no intensity corresponding to the O-N-O symmetric bend (v(2)) was observed. Depolarization ratios and absolute resonance Raman cross sections for v(1) were obtained at several excitation wavelengths spanning the D band. Depolarization ratios were found to deviate significantly from 1/3, consistent with more than a single dipole-allowed electronic transition contributing to the scattering. RR intensity analysis (RRIA) reveals that two closely spaced excited electronic states contribute to the scattering, which are dissociative along the Cl-N coordinate. In this study the role the solvent environment plays in ClNO(2) state energetics and excited structural evolution along fundamental coordinates is discussed.

Enhanced and broadened SRS spectra of toluene mixed with chloroform in liquid-core fiber

Stimulated Raman scattering of toluene and its mixture with chloroform is studied in liquid-core optical fiber. The results show a remarkably broadened Raman line of the mixture from about 630 nm to 650 nm by a pumping wavelength at 532 nm, which is assigned to the interaction of CH vibrations of the two liquids. The results suggest that interactions between adjacent vibrations can produce strong and wide Raman spectra in liquid-core fiber which may prove a new simple way for supercontinuum generation.

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.

Solvent Effects on the Resonance Raman and Hyper-Raman Spectra and First Hyperpolarizability of N,N-Dipropyl-p-nitroaniline

Linear absorption spectra, resonance Raman spectra and excitation profiles, and two-photon-resonant hyper-Rayleigh and hyper-Raman scattering hyperpolarizability profiles are reported for the push-pull chromophore N,N-dipropyl-p-nitroaniline in seven solvents spanning a wide range of polarities. The absorption spectral maximum red shifts by about 2700 cm(-1), and the symmetric -NO2 stretch shifts to lower frequencies by about 11 cm(-1) from hexane to acetonitrile, indicative of significant solvent effects on both the ground and excited electronic states. The intensity patterns in the resonance Raman and hyper-Raman spectra are similar and show only a small solvent dependence except in acetonitrile, where both the Raman and hyper-Raman intensities are considerably reduced. Quantitative modeling of all four spectroscopic observables in all seven solvents reveals that the origin of this effect is an increased solvent-induced homogeneous broadening in acetonitrile. The linear absorption oscillator strength is nearly solvent-independent, and the peak resonant hyperpolarizability, beta(-2omega;omega,omega), varies by only about 15% across the wide range of solvents examined. These results suggest that the resonant two-photon absorption cross sections in this chromophore should exhibit only a weak solvent dependence.

Structural Changes upon Photoexcitation into the Metal-to-Ligand Charge-Transfer State of [Cu(pqx)(PPh3)2]+ Probed by Resonance Raman Spectroscopy and Density Functional Theory

The structural changes that occur when [Cu(pqx)(PPh(3))(2)](+) (pqx is 2-(2'-pyridyl)quinoxaline) undergoes excitation through a metal-to-ligand charge-transfer (MLCT) transition are investigated using resonance Raman excitation profiles coupled with density functional theory (DFT). The DFT calculations predict bond lengths to within 3 pm and absolute deviations of 7 cm(-1) for the vibrational frequencies of [Cu(pqx)(PPh(3))(2)](+). TD-DFT calculations of oscillator strengths (f = 0.089) and band positions (419 nm) showed close agreement with experiment (f = 0.07, 431 nm). Resonance Raman spectra show the 527 cm(-1) (nu(29)) and 1476 cm(-1) (nu(75)) modes undergo the largest dimensionless displacement (Delta = 1.5 and 1.1, respectively) following photoexcitation into the MLCT Franck-Condon region. The solvent couples strongly to the MLCT transition and resonance Raman intensity analysis (RRIA) gives a solvent reorganization energy of 3400 cm(-1) for dichloromethane and 2800 cm(-1) for chloroform solutions. A large inner-sphere reorganization of 3430 cm(-1) in dichloromethane solution (3520 cm(-1) in chloroform solution) was found for [Cu(pqx)(PPh(3))(2)](+), indicating that the molecule as a whole undergoes significant distortion following MLCT excitation.

Time resolved infrared absorption studies of geminate recombination and vibrational relaxation in OClO photochemistry

Ultrafast time-resolved infrared absorption studies of aqueous chlorine dioxide (OClO) photochemistry are reported. Following photoexcitation at 401 nm, the evolution in optical density at frequencies between 1000 to 1100 cm(-1) is monitored to investigate vibrational energy deposition and relaxation along the asymmetric-stretch coordinate following the reformation of ground-state OClO via geminate recombination of the primary photofragments. The measured kinetics are compared to two proposed models for the vibrational-relaxation dynamics along the asymmetric-stretch coordinate. This comparison demonstrates that the perturbation model derived from molecular dynamics studies is capable of qualitatively reproducing the observed kinetics, where the collisional model employed in previous UV-pump, visible probe experiments demonstrates poor agreement with experiment. The ability of the perturbation model to reproduce the optical-density evolution observed in these studies demonstrates that for aqueous OClO, frequency dependence of the solvent-solute coupling is important in defining the level-dependent vibrational relaxation rates along the asymmetric-stretch coordinate. The absence of optical-density evolution corresponding to the population of higher vibrational levels (n>8) along the asymmetric-stretch coordinate suggests that following geminate recombination, energy is initially deposited into a local Cl-O stretch, with the relaxation of vibrational energy from this coordinate providing for delayed vibrational excitation of the asymmetric- and symmetric-stretch coordinates relative to geminate recombination, as previously observed.

Understanding the phase-dependent reactivity of chlorine dioxide using resonance Raman spectroscopy

Progress in understanding the phase-dependent reactivity of chlorine dioxide (OClO) is outlined. Resonance Raman intensity analysis studies of gaseous and solution-phase OClO are presented which demonstrate that the optically prepared excited state undergoes significant modification in solution. In addition, time-resolved resonance Raman studies are presented which demonstrate that geminate recombination of the primary photoproducts, resulting in the re-formation of ground-state OClO, dominates the photochemical reaction dynamics in solution. The current picture of aqueous OClO photochemistry derived from these studies is discussed, and future directions of investigation are outlined.