J. M. Nicovich

Temperature-dependent kinetics studies of aqueous phase reactions of SO4− radicals with dimethylsulfoxide, dimethylsulfone, and methanesulfonate

A laser flash photolysis – long path UV-visible absorption – competitive kinetics technique has been employed to investigate the kinetics of the aqueous phase reactions between the hydroxyl radical (OH) and three organic sulfur species of atmospheric interest, dimethylsulfoxide (DMSO; CH3S(O)CH3), dimethylsulfone (DMSO2; CH3(O)S(O)CH3), and methanesulfonate (MS; CH3(O)S(O)O–). Thiocyanate anion (SCN–) has been employed as the competitor. Temperature dependent rate coefficients for the reactions of interest are reported for the first time. The following Arrhenius expressions adequately summarize the kinetic data obtained over the temperature range 275 – 310 K (units are M–1 s–1): ln kDMSO = (26.88 ± 0.14) – {(1270 ± 40)/T}; ln kDMSO2 ≤ (22.36 ± 0.17) – {(1690 ± 50)/T}; ln kMS = (25.20 ± 0.13) – {(2630 ± 40)/T}. Uncertainties in the Arrhenius parameters are 2σ and represent precision only. The accuracies of individual measured rate coefficient ratios, kX(T)/kSCN–(T) (X = DMSO, DMSO2, or MS), are estimated to be ±5% independent of T and the identity of X. Taking uncertainties in kSCN–(T) into account, we estimate the absolute accuracy of reported rate coefficients to be 16% at 275 K, 9% at 295 K, and 12% at 310 K. The implications of the new kinetics results for understanding the atmospheric sulfur cycle are discussed.

Kinetics, mechanism, and thermochemistry of the gas-phase reaction of atomic chlorine with pyridine

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction of atomic chlorine with pyridine (C(5)H(5)N) as a function of temperature (215-435 K) and pressure (25-250 Torr) in nitrogen bath gas. At T> or = 299 K, measured rate coefficients are pressure independent and a significant H/D kinetic isotope effect is observed, suggesting that hydrogen abstraction is the dominant reaction pathway. The following Arrhenius expression adequately describes all kinetic data at 299-435 K for C(5)H(5)N: k(1a) = (2.08 +/- 0.47) x 10(-11) exp[-(1410 +/- 80)/T] cm(3) molecule(-1) s(-1) (uncertainties are 2sigma, precision only). At 216 K < or =T< or = 270 K, measured rate coefficients are pressure dependent and are much faster than computed from the above Arrhenius expression for the H-abstraction pathway, suggesting that the dominant reaction pathway at low temperature is formation of a stable adduct. Over the ranges of temperature, pressure, and pyridine concentration investigated, the adduct undergoes dissociation on the time scale of our experiments (10(-5)-10(-2) s) and establishes an equilibrium with Cl and pyridine. Equilibrium constants for adduct formation and dissociation are determined from the forward and reverse rate coefficients. Second- and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the addition reaction: Delta(r)H = -47.2 +/- 2.8 kJ mol(-1), Delta(r)H = -46.7 +/- 3.2 kJ mol(-1), and Delta(r)S = -98.7 +/- 6.5 J mol(-1) K(-1). The enthalpy changes derived from our data are in good agreement with ab initio calculations reported in the literature (which suggest that the adduct structure is planar and involves formation of an N-Cl sigma-bond). In conjunction with the well-known heats of formation of atomic chlorine and pyridine, the above Delta(r)H values lead to the following heats of formation for C(5)H(5)N-Cl at 298 K and 0 K: Delta(f)H = 216.0 +/- 4.1 kJ mol(-1), Delta(f)H = 233.4 +/- 4.6 kJ mol(-1). Addition of Cl to pyridine could be an important atmospheric loss process for pyridine if the C(5)H(5)N-Cl product is chemically degraded by processes that do not regenerate pyridine with high yield.

Spectroscopic and Kinetic Study of the Gas-Phase CH3I-Cl and C2H5I-Cl Adducts †

Time-resolved UV-visible absorption spectroscopy has been coupled with UV laser flash photolysis of Cl2/RI/N2/X mixtures (R = CH3 or C2H5; X = O2, NO, or NO2) to generate the RI-Cl radical adducts in the gas phase and study the spectroscopy and reaction kinetics of these species. Both adducts were found to absorb strongly over the wavelength range 310-500 nm. The spectra were very similar in wavelength dependence with lambda(max) approximately 315 nm for both adducts and sigma(max) = (3.5 +/- 1.2) x 10(-17) and (2.7 +/- 1.0) x 10(-17) cm(2) molecule(-1) (base e) for CH3I-Cl and C2H5I-Cl, respectively (uncertainties are estimates of accuracy at the 95% confidence level). Two weaker bands with lambda max approximately 350 and 420 nm were also observed. Over the wavelength range 405-500 nm, where adduct spectra are reported both in the literature and in this study, the absorption cross sections obtained in this study are a factor of approximately 4 lower than those reported previously [Enami et al. J. Phys. Chem. A 2005, 109, 1587 and 6066]. Reactions of RI-Cl with O2 were not observed, and our data suggest that upper limit rate coefficients for these reactions at 250 K are 1.0 x 10(-17) cm(3) molecule(-1) s(-1) for R = CH3 and 2.5 x 10(-17) cm(3) molecule(-1) s(-1) for R = C2H5. Their lack of reactivity with O2 suggests that RI-Cl adducts are unlikely to play a significant role in atmospheric chemistry. Possible reactions of RI-Cl with RI could not be confirmed or ruled out, although our data suggest that upper limit rate coefficients for these reactions at 250 K are 3 x 10(-13) cm(3) molecule(-1) s(-1) for R = CH3 and 5 x 10(-13) cm(3) molecule(-1) s(-1) for R = C2H5. Rate coefficients for CH3I-Cl reactions with CH3I-Cl (k9), NO (k22), and NO2 (k24), and C2H5I-Cl reactions with C2H5I-Cl (k14), NO (k23), and NO2 (k25) were measured at 250 K. In units of 10(-11) cm(3) molecule(-1) s(-1), the rate coefficients were found to be 2k9 = 35 +/- 12, k22 = 1.8 +/- 0.4, k24 = 3.3 +/- 0.6, 2k14 = 40 +/- 16, k23 = 1.8 +/- 0.3, and k25 = 4.0 +/- 0.9, where the uncertainties are estimates of accuracy at the 95% confidence level.

Temperature-dependent kinetics study of the reactions of O(1D2) with N2 and O2

A laser flash photolysis–resonance fluorescence technique has been employed to investigate the kinetics of the reactions of electronically excited oxygen atoms, O(1D2), with N2 (k1) and O2 (k2) as a function of temperature (197–427 K) in helium buffer gas at pressures of 11–40 Torr. The results are well-described by the following Arrhenius expressions (units are 10−11 cm3 molecule−1 s−1): k1(T) = (1.99 ± 0.06) exp{(145 ± 9)/T} and k2(T) = (3.39 ± 0.03) exp{(63 ± 3)/T}. Uncertainties in the Arrhenius parameters are 2σ and represent precision only; estimated accuracies of reported k1(T) and k2(T) values at the 95% confidence level are ±10% around room temperature and ±15% at the temperature extremes of the study. The O(1D2) + O2 kinetic data reported in this study are in very good agreement with available literature values. However, the kinetic data reported in this study (and two other new studies reported in this issue) suggest that the O(1D2) + N2 reaction is significantly faster than previously thought, a finding that has important implications regarding production rates of tropospheric HOx radicals as well as stratospheric HOx and NOx radicals calculated in atmospheric models.

Quenching of by Cl2CO: kinetics and yield

Abstract Absolute rate coefficients ( k 1 ) for the deactivation of electronically excited oxygen atoms, O ( 1 D 2 ) , by phosgene (Cl 2 CO) have been measured as a function of temperature over the range 194– 425 K. The results are well described by the Arrhenius expression k 1 (T)=2.04×10 −10 exp (+27/T) cm 3 molecule −1 s −1 . The reported values for k 1 ( T ) are considerably lower than the values currently recommended for use in modeling stratospheric chemistry. The yield of ground state oxygen atoms, O ( 3 P J ) , is found to be 0.20±0.04.