Heterogeneous reactions of SO2 with HOCl and HOBr on ice surfaces

The heterogeneous reactions of SO2 + HOX (X = Cl or Br) --> products on ice surfaces at low temperature have been investigated in a flow reactor coupled with a differentially pumped quadrupole mass spectrometer. Pseudo-first-order loss of SO2 over the ice surfaces has been measured under the conditions of concurrent HOX flow. The initial uptake coefficient of SO2 reaction with HOX has been determined as a function of HOX surface coverage, theta(HOX), on the ice. The initial uptake coefficients increase as the HOX coverage increases. The uptake coefficient can be expressed as gamma(t) = k(h)theta(HOX), where k(h) is an overall rate constant of SO2 + HOCl, which was determined to be (2.3 +/- 0.6) x 10(-19) and (1.7 +/- 0.5) x 10(-19) molecules(-1) x cm2 at 190 and 210 K, and k(h) of SO2 + HOBr is (6.1 +/- 2.0) x 10(-18) molecules(-1) x cm2 at 190 K. theta( HOX) is in the range 8.1 x 10(13)-9.1 x 10(14) molecules x cm(-2). The kinetic results of the heterogeneous reaction of SO2 + HOX on ice surface are interpreted using the Eley-Rideal mechanism. The activation energy of the heterogeneous reaction of SO2 with HOCl on ice surface was determined to be about -37 +/- 10 kJ/mol in the 190-238 K range.     

Uptake of SO2 on HOBr-ice surfaces

The uptake of SO2 on HOBr-treated ice surfaces has been studied using a flow reactor coupled with a differentially pumped quadrupole mass spectrometer at 190-240 K. The initial uptake coefficient was determined as a function of HOBr surface coverage, theta(HOBr), on the ice. The uptake coefficients increase as the HOBr coverage increases. The uptake coefficient can be expressed as gamma(t) = k(h)theta(HOBr), where k(h) = 1.5 x 10(-19) molecules(-1) cm(-2) at 191 K and k(h) = 6.4 x 10(-21) molecules(-1) cm(-2) at 210 K and theta(HOBr) is in the range of 8 x 10(13) to 1.2 x 10(15) molecules cm(-2). The effects of temperature and film thickness on the uptake coefficients of SO2 by the HOBr-treated ice films were also studied. The activation energy E(a) of SO(2) on HOBr-ice surfaces is approximately -81 +/- 8 kJ/mol in the 190-215 K range. Kinetic results were interpreted in terms of the Eley-Rideal mechanism. This study suggests that the uptake of SO2 on ice/snow surfaces is enhanced by the presence of HOBr near the ice surface. The implication for atmospheric chemistry is that HOBr-ice surfaces may not provide a significant pathway to oxide S(IV) in the boundary layer due to both lower uptake coefficient and smaller HOBr surface coverage at T > 220 K.     

Temperature dependence of the heterogeneous reaction of carbonyl sulfide on magnesium oxide

The experimental determination of rate constants for atmospheric reactions and how these rate constants vary with temperature remain a crucially important part of atmosphere science. In this study, the temperature dependence of the heterogeneous reaction of carbonyl sulfide (COS) on magnesium oxide (MgO) has been investigated using a Knudsen cell reactor and a temperature-programmed reaction apparatus. We found that the adsorption and the heterogeneous reaction are sensitive to temperature. The initial uptake coefficients (gammat(Ini)) of COS on MgO decrease from 1.07 +/- 0.71 x 10-6 to 4.84 +/- 0.60 x 10-7 with the increasing of temperature from 228 to 300 K, and the steady state uptake coefficients (gammat(SS)) increase from 5.31 +/- 0.06 x 10-8 to 1.68 +/- 0.41 x 10-7 with the increasing of temperature from 240 to 300 K. The desorption rate constants (kdes) were also found to increase slightly with the enhancement of temperature. The empirical formula between the uptake coefficients, desorption rate constants and temperature described in the form of Arrhenius expression were obtained. The activation energies for the heterogeneous reaction and desorption of COS on MgO were measured to be 11.02 +/- 0.34 kJ.mol-1 and 6.30 +/- 0.81 kJ.mol-1, respectively. The results demonstrate that the initial uptake of COS on MgO is mainly contributed by an adsorption process and the steady state uptake is due to a catalytic reaction. The environmental implication was also discussed.     

Heterogeneous uptake of gaseous nitric acid on dolomite (CaMg(CO3)2) and calcite (CaCO3) particles: a Knudsen cell study using multiple, single, and fractional particle layers

In this study, the heterogeneous uptake of gaseous nitric acid on dolomite, CaMg(CO3)2, and calcite, CaCO3, particles under dry conditions at 296 K was investigated. A Knudsen cell reactor was used to measure heterogeneous uptake coefficients for these reactions. Several different experiments were performed including those on many, single, and fractional layers of particles. For experiments using multiple particle layers, the Knudsen cell data were modeled to take into account gas diffusion into the underlying layers of the sample. From this analysis, initial heterogeneous uptake coefficients, gamma(o,t), were determined to be (5 +/- 2) x 10(-4) and (2 +/- 1) x 10(-3), for dolomite and calcite, respectively, at a nitric acid concentration of 6.5 x 10(10) molecules cm(-3). For experiments that employed single or fractional particle layers, the initial heterogeneous uptake coefficient was analyzed using a recent method described in the literature. Values of the initial heterogeneous uptake coefficient using this analysis were in agreement with the above analysis and determined to be (7 +/- 4) x 10(-4) and (2 +/- 0.4) x 10(-3) for CaMg(CO3)2 and CaCO3, respectively. In addition, these results are compared to previous literature values.     

Low temperature NH(X 3sigma-) radical reactions with NO, saturated, and unsaturated hydrocarbons studied in a pulsed supersonic laval nozzle flow reactor between 53 and 188 K

The reactions of ground-state imidogen radicals (NH(X 3sigma-)) with NO and select saturated and unsaturated hydrocarbons have been measured in a pulsed supersonic expansion Laval nozzle flow reactor in the temperature range 53-188 K. The rate coefficients for the NH + NO system display negative temperature dependence in the temperature regime currently investigated and a global temperature-dependent fit is best represented in a modified power law functional form, with k1(NH + NO) = (4.11 +/- 0.31) x 10(-11) x (T/300)(-0.30+/-0.17) x exp(77+/-21/T) cm3/s. The reactions of NH with ethylene, acetylene, propene, and diacetylene were measured over the temperature range 53-135 K. In addition, the reactions of NH with methane and ethane were also measured at 53 K, for reasons discussed later. The temperature dependence of the reactions of NH with the unsaturated hydrocarbons are fit using power law expressions, k(T) = A(T/300)(-n), and are as follows: k4 = (2.3 +/- 1.2) x 10(-12) x (T/300)(-1.09+/-0.33) cm3/s, k5 = (4.5 +/- 0.3) x 10(-12) x (T/300)(-1.07+/-0.04) cm3/s, k6 = (5.6 +/- 1.9) x 10(-12) x (T/300)(-1.23+/-0.21) cm3/s, and k7 = (7.4 +/- 1.8) x 10(-12) x (T/300)(-1.23+/-0.15) cm3/s for ethylene, acetylene, propene, and diacetylene, respectively. The rate for NH + ethane at 53 K is measured to be k3 = (6.8 +/- 1.7) x 10(-12) cm3/s, while that for methane at the same temperature represents an upper bound of k2 < or = (1.1 +/- 4.3) x 10(-12) cm3/s, as this is at the limits of measurement with our current technique. The behavior of these systems throughout the temperature range explored indicates that these reactions occur over a potential energy surface without an appreciable barrier through a complex formation mechanism. Implications for chemistry in low temperature environments where these species are found are briefly discussed.     

Experimental and theoretical investigations of the reactions NH(X 3Sigma-) + D(2S)-->ND(X 3Sigma-) + H(2S) and NH(X 3Sigma-) + D(2S)-->N(4S) + HD(X 1Sigmag+)

The rate coefficient of the reaction NH(X (3)Sigma(-))+D((2)S)-->(k(1) )products (1) is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures. The NH(X) radicals are produced by quenching of NH(a (1)Delta) (obtained in the photolysis of HN(3)) with Xe and the D atoms are generated in a D(2)/He microwave discharge. The NH(X) concentration profile is measured in the presence of a large excess of D atoms. The room-temperature rate coefficient is determined to be k(1)=(3.9+/-1.5) x 10(13) cm(3) mol(-1) s(-1). The rate coefficient k(1) is the sum of the two rate coefficients, k(1a) and k(1b), which correspond to the reactions NH(X (3)Sigma(-))+D((2)S)-->(k(1a) )ND(X (3)Sigma(-))+H((2)S) (1a) and NH(X (3)Sigma(-))+D((2)S)-->(k(1b) )N((4)S)+HD(X (1)Sigma(g) (+)) (1b), respectively. The first reaction proceeds via the (2)A(") ground state of NH(2) whereas the second one proceeds in the (4)A(") state. A global potential energy surface is constructed for the (2)A(") state using the internally contracted multireference configuration interaction method and the augmented correlation consistent polarized valence quadrupte zeta atomic basis. This potential energy surface is used in classical trajectory calculations to determine k(1a). Similar trajectory calculations are performed for reaction (1b) employing a previously calculated potential for the (4)A(") state. The calculated room-temperature rate coefficient is k(1)=4.1 x 10(13) cm(3) mol(-1) s(-1) with k(1a)=4.0 x 10(13) cm(3) mol(-1) s(-1) and k(1b)=9.1 x 10(11) cm(3) mol(-1) s(-1). The theoretically determined k(1) shows a very weak positive temperature dependence in the range 250< or =TK< or =1000. Despite the deep potential well, the exchange reaction on the (2)A(") ground-state potential energy surface is not statistical.     

Kinetic and equilibria studies of the aquation of the trinuclear platinum phase II anticancer agent [(trans-PtCl(NH(3))(2))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2))](4+) (BBR3464)

The hydrolysis profile of the bifunctional trinuclear phase II clinical agent [(trans-PtCl(NH(3))(2))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)NH(2))(2))](4+) (BBR3464, 1) has been examined using [(1)H,(15)N] heteronuclear single quantum coherence (HSQC) 2D NMR spectroscopy. Reported are estimates of the rate and equilibrium constants for the first and second aquation steps, together with the acid dissociation constant (pK(a1) approximately equal to pK(a2) approximately equal to pK(a3)). The equilibrium constants for the aquation determined by NMR at 298 and 310 K (I = 0.1 M, pH 5.3) are similar, pK(1) = pK(2) = 3.35 +/- 0.04 and 3.42 +/- 0.04, respectively. At lower ionic strength (I = 0.015 M, pH 5.3) the values at 288, 293, and 298 K are pK(1) = pK(2) = 3.63 +/- 0.05. This indicates that the equilibrium is not strongly ionic strength or temperature dependent. The aquation and anation rate constants for the two-step aquation model at 298 K in 0.1 M NaClO(4) (pH 5.3) are k(1) = (7.1 +/- 0.2) x 10(-5) s(-1), k(-1) = 0.158 +/- 0.013 M(-1) s(-1), k(2) = (7.1 +/- 1.5) x 10(-5) s(-1), and k(-2) = 0.16 +/- 0.05 M(-1) s(-1). The rate constants in both directions increase 2-fold with an increase in temperature of 5 K, and rate constants increase with a decrease in solution ionic strength. A pK(a) value of 5.62 plus minus 0.04 was determined for the diaqua species [(trans-Pt(NH(3))(2)(OH(2)))(2)(mu-trans-Pt(NH(3))(2)(NH(2)(CH(2))(6)-NH(2))(2))](6+) (3). The speciation profile of 1 under physiological conditions is explored and suggests that the dichloro form predominates. The aquation of 1 in 15 mM phosphate was also examined. No slowing of the initial aquation was observed, but reversible reaction between aquated species and phosphate does occur.     

Kinetics and mechanisms of CF3CHFOCH3, CF3CHFOC(O)H, and FC(O)OCH3 reactions with OH radicals

The kinetics and mechanism of oxidation of CF3CHFOCH3 was studied using an 11.5-dm3 environmental reaction chamber. OH radicals were produced by UV photolysis of an O3-H2O-He mixture at an initial pressure of 200 Torr in the chamber. The rate constant of the reaction of CF3CHFOCH3 with OH radicals (k1) was determined to be (1.77 +/- 0.69) x 10(-12) exp[(-720 +/- 110)/T] cm3 molecule(-1)(s-1) by means of a relative rate method at 253-328 K. The mechanism of the reaction was investigated by FT-IR spectroscopy at 298 K. CF3CHFOC(O)H, FC(O)OCH3, and COF2 were determined to be the major products. The branching ratio (k1a/k1b) for the reactions CF3CHFOCH3 + OH --> CF3CHFOCH2* + H2O (k1a) and CF3CHFOCH3 + OH --> CF3CF*OCH3 + H2O (k1b) was estimated to be 4.2:1 at 298 K from the yields of CF3CHFOC(O)H, FC(O)OCH3, and COF2. The rate constants of the reactions of CF3CHFOC(O)H (k2) and FC(O)OCH3 (k3) with OH radicals were determined to be (9.14 +/- 2.78) x 10(-13) exp[(-1190 +/- 90)/T] and (2.10 +/- 0.65) x 10(-13) exp[(-630 +/- 90)/T] cm3 molecule(-1)(s-1), respectively, by means of a relative rate method at 253-328 K. The rate constants at 298 K were as follows: k1 = (1.56 +/- 0.06) x 10-13, k2 = (1.67 +/- 0.05) x 10-14, and k3 = (2.53 +/- 0.07) x 10-14 cm3 molecule(-1)(s-1). The tropospheric lifetimes of CF3CHFOCH3, CF3CHFOC(O)H, and FC(O)OCH3 with respect to reaction with OH radicals were estimated to be 0.29, 3.2, and 1.8 years, respectively.     

Determination of the rate constants for the NCO(X2Pi) + Cl(2P) and Cl(2P) + ClNCO(X1A') reactions at 293 and 345 K

The rate constant for the reaction of the isocyanato radical, NCO(X2Pi) with chlorine atoms, Cl(2P), has been measured at 293 +/- 2 and 345 +/- 3 K to be (6.9 +/- 3.8) x 10(-11) and (4.0 +/- 2.2) x 10(-11) cm3 molecules(-1) s,(-1) respectively, where the uncertainties include both random and systematic errors. The measurements were carried out at pressures of 1.3-6.2 Torr with either Ar or CF4 as the bath gas and were independent of both pressure and nature of the third body. Equal concentrations of NCO and Cl atoms were created by 248 nm photolysis of ClNCO. The reaction was monitored by following the temporal dependence of NCO(X2Pi) using time-resolved infrared absorption spectroscopy on rotational transitions of the NCO(10(1)1) <-- (00(1)0) combination band. The reaction rate constant was determined by using a simple chemical model and minimizing the sum of the residuals between the experimental and computer generated temporal NCO concentration profiles. The reaction Cl + ClNCO --> Cl2 + NCO was found to contribute to the observed NCO. The rate constant for this reaction was found to be (2.4 +/- 1.6) x 10(-13) and (1.9 +/- 1.2) x 10(-13) cm3 molecules(-1) s,(-1) at 293 and 345 K, respectively, where the uncertainties include both random and systematic error.     

Kinetic and product study of the gas-phase reactions of OH radicals, NO(3) radicals, and O(3) with (C(2)H(5)O)(2)P(S)CH(3) and (C(2)H(5)O)(3)PS

Rate constants for the reactions of OH radicals and NO3 radicals with O,O-diethyl methylphosphonothioate [(C(2)H(5)O)(2)P(S)CH(3); DEMPT] and O,O,O-triethyl phosphorothioate [(C(2)H(5)O)(3)PS; TEPT] have been measured using relative rate methods at atmospheric pressure of air over the temperature range 296-348 K for the OH radical reactions and at 296 +/- 2 K for the NO(3) radical reactions. At 296 +/- 2 K, the rate constants obtained for the OH radical reactions (in units of 10(-11) cm(3) molecule(-1) s(-1)) were 20.4 +/- 0.8 and 7.92 +/- 0.27 for DEMPT and TEPT, respectively, and those for the NO(3) radical reactions (in units of 10(-15) cm(3) molecule(-1) s(-1)) were 2.01 +/- 0.20 and 1.03 +/- 0.10, respectively. Upper limits to the rate constants for the reactions of O(3) with DEMPT and TEPT of <6 x 10(-20) cm(3) molecule(-1) s(-1) were determined in each case. Rate constants for the OH radical reactions, measured relative to k(OH + alpha-pinene) = 1.21 x 10(-11) e(436/T) cm(3) molecule(-1) s(-1), resulted in the Arrhenius expressions k(OH + DEMPT) = 1.08 x 10(-11) e(871+/-25)/T cm(3) molecule(-1) s(-1) and k(OH + TEPT) = 8.21 x 10(-13) e(1353+/-49)/T cm(3) molecule(-1) s(-1) over the temperature range 296-348 K, where the indicated errors are two least-squares standard deviations and do not include the uncertainties in the reference rate constant. Diethyl methylphosphonate was identified and quantified from the OH radical and NO(3) radical reactions with DEMPT, with formation yields of 21 +/- 4%, independent of temperature, from the OH radical reaction and 62 +/- 11% from the NO(3) radical reaction at 296 +/- 2 K. Similarly, triethyl phosphate was identified and quantified from the OH radical and NO(3) radical reactions with TEPT, with formation yields of 56 +/- 9%, independent of temperature, from the OH radical reaction and 78 +/- 15% from the NO(3) radical reaction at 296 +/- 2 K.     

Quantum chemical study of the mechanism of reaction between NH (X 3sigma-) and H2, H2O, and CO2 under combustion conditions

Reactions of ground-state NH (3sigma-) radicals with H2, H2O, and CO2 have been investigated quantum chemically, whereby the stationary points of the appropriate reaction potential energy surfaces, that is, reactants, products, intermediates, and transition states, have been identified at the G3//B3LYP level of theory. Reaction between NH and H2 takes place via a simple abstraction transition state, and the rate coefficient for this reaction as derived from the quantum chemical calculations, k(NH + H2) = (1.1 x 10(14)) exp(-20.9 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K, is found to be in good agreement with experiment. For reaction between triplet NH and H2O, no stable intermediates were located on the triplet reaction surface although several stable species were found on the singlet surface. No intersystem crossing seam between triplet NH + H2O and singlet HNO + H2 (the products of lowest energy) was found; hence there is no evidence to support the existence of a low-energy pathway to these products. A rate coefficient of k(NH + H2O) = (6.1 x 10(13)) exp(-32.8 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K for the reaction NH (3sigma-) + H2O --> NH2 (2B) + OH (2pi) was derived from the quantum chemical results. The reverse rate coefficient, calculated via the equilibrium constant, is in agreement with values used in modeling the thermal de-NO(x) process. For the reaction between triplet NH and CO2, several stable intermediates on both triplet and singlet reaction surfaces were located. Although a pathway from triplet NH + CO2 to singlet HNO + CO involving intersystem crossing in an HN-CO2 adduct was discovered, no pathway of sufficiently low activation energy was discovered to compare with that found in an earlier experiment [Rohrig, M.; Wagner, H. G. Proc. Combust. Inst. 1994, 25, 993.].     

Reaction of HO with hydroxyacetone (HOCH2C(O)CH3): rate coefficients (233-363 K) and mechanism

Absolute rate coefficients for the title reaction, HO + HOCH(2)C(O)CH(3)--> products (R1) were measured over the temperature range 233-363 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. The rate coefficient displays a slight negative temperature dependence, which is described by: k(1)(233-363 K) = (2.15 +/- 0.30) x 10(-12) exp{(305 +/- 10)/T} cm(3) molecule(-1) s(-1), with a value of (5.95 +/- 0.50) x 10(-12) cm(3) molecule(-1) s(-1) at room temperature. The effects of the hydroxy-substituent and hydrogen bonding on the rate coefficient are discussed based on theoretical calculations. The present results, which extend the database on the title reaction to a range of temperatures, indicate that R1 is the dominant loss process for hydroxyacetone throughout the troposphere, resulting in formation of methylglyoxal at all atmospheric temperatures. As part of this work, the rate coefficient for reaction of O((3)P) with HOCH(2)C(O)CH(3) (R4) was measured at 358 K: k(4)(358 K) = (6.4 +/- 1.0) x 10(-14) cm(3) molecule(-1) s(-1) and the absorption cross section of HOCH(2)C(O)CH(3) at 184.9 nm was determined to be (5.4 +/- 0.1) x 10(-18) cm(2) molecule(-1).     

Experimental and theoretical investigation of the reaction NH(X3Sigma-) + H(2S)-->N(4S) + H2(X1)Sigmag +)

The rate coefficient of the reaction NH(X (3)Sigma(-)) + H((2)S)-->(k(1a) )N((4)S) + H(2)(X (1)Sigma(g) (+)) is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures (2 mbar< or =p< or =10 mbar). The NH(X) radicals are produced via the quenching of NH(a(1)Delta) (obtained by photolyzing HN(3)) with Xe whereas the H atoms are generated in a H(2)He microwave discharge. The NH(X) concentration profile is measured under pseudo-first-order condition, i.e., in the presence of a large excess of H atoms. The room temperature rate coefficient is determined to be k(1a) = (1.9 +/- 0.5) x 10(12) cm(3) mol(-1) s(-1). It is found to be independent of the pressure in the range considered in the present experiment. A global potential energy surface for the (4)A(") state is calculated with the internally contracted multireference configuration interaction method and the augmented correlation consistent polarized valence quadruple zeta atomic basis. The title reaction is investigated by classical trajectory calculations on this surface. The theoretical room temperature rate coefficient is k(1a) = 0.92 x 10(12)cm(3) mol(-1) s(-1). Using the thermodynamical data for the atoms and molecules involved, the rate coefficient for the reverse reaction, k(-1a), is also calculated. At high temperatures it agrees well with the measured k(-1a).     

Oxidation of [Ru(NH3)5isn](BF4)2 by hyochlorous acid and chlorine in aqueous acidic media

UV-vis stopped-flow studies of the reaction of [Ru(NH3)5isn](2+) (isn = isonicotinamide) with excess HOCl at 25 degrees C demonstrate that it proceeds in two time-resolved steps. In the first step [Ru(NH3)5isn](3+) is produced with the rate law -d[Ru(II)]/dt = 2(aK(h)[H(+)] + b[H(+)][Cl(-)] + c[Cl(-)])[HOCl](tot)[Ru(II)]/(K(h) + [H(+)][Cl(-)]). Here, K(h) is 1.3 x 10(-3) M(2) and corresponds to the equilibrium hydrolysis of Cl2, a is (8.34 +/- 0.19) x 10(3) M(-2) s(-1) and represents the acid-assisted reduction of HOCl, b is (4.04 +/- 0.13) x 10(4) M(-1) s(-1) and represents the reduction of Cl2, and c is (6.25 +/- 0.59) x 10(2) s(-1) and represents the Cl(-)-assisted reduction of HOCl. In the second step [Ru(NH3)5isn](3+) undergoes further oxidation to a mixture of products with the rate law -d[Ru(III)]/dt = e[Ru(III)][HOCl]/[H(+)] where e is (1.18 +/- 0.01) x 10(-2) s(-1). This step is assigned a mechanism with Cl(+) transfer from HOCl to [Ru(III)(NH3)4(NH2)isn](2+) occurring in the rate-limiting step. These results underline the resistance of HOCl to act as a simple outer-sphere one-electron oxidant.     

Determination of the rate constant for the radical-radical reaction NCO(X2Pi) + CH3(X2A2 ') at 293 K and an estimate of possible product channels

The rate constant for the reaction of the cyanato radical, NCO(X2Pi), with the methyl radical, CH3(X2A2' '), has been measured to be (2.1 +/- 1.3(-0.80)) x 10(-10) cm3 molecule(-1) s(-1), where the uncertainty includes both random and systematic errors at the 68% confidence level. The measurements were conducted over a pressure range of 2.8-4.3 Torr of CH4 and at a temperature of 293 +/- 2 K. The radicals were generated by the 248-nm photolysis of ClNCO in a large excess of CH4. The subsequent rapid reaction, Cl + CH4, generated the CH3 radical. The rate constant for the Cl + CH4 reaction was measured to be (9.2 +/- 0.2) x 10(-14) cm3 molecule(-1) s(-1), where the uncertainty is the scatter of one standard deviation in the data. The progress of the reaction was followed by time-resolved infrared absorption spectroscopy on single rovibrational transitions from the ground vibrational level. Multiple species were detected in these experiments, including NCO, CH3, HCl, C2H6, HCN, HNC, NH, and HNCO. Temporal concentration profiles of the observed species were simulated using a kinetic model, and rate constants were determined by minimizing the sum of the squares of the residuals between experimental observations and model calculations. Both HCN and HNC seem to be minor products (<0.3% each) of the NCO + CH3 reaction. The peak concentrations of NH and HNCO were small, accounting for <1% of the initial NCO concentration; however, their temporal profiles could not be fit by the model kinetics. The observed C2H6 temporal profile always peaked at significantly higher concentrations than the model predictions, and several reaction models were constructed to help explain these observations. The most likely product channel seems to be the recombination channels, producing CH3NCO and CH3OCN.     

Electron attachment to MoF6, ReF6, and WF6; reaction of MoF6(-) with ReF6 and reaction of Ar+ with MoF6

Rate constants were measured for electron attachment to MoF(6), ReF(6), and WF(6) in 133 Pa of helium gas using a flowing-afterglow Langmuir-probe apparatus. The experiment is a thorny one because the molecules tend to form oxide impurities on feedline surfaces and because of thermal decomposition of MoF(6) on surfaces as the gas temperature is increased. The electron attachment rate constant for MoF(6) is (2.3+/-0.8)x10(-9) cm(3) s(-1) at 297 K; only MoF(6) (-) is formed in the temperature range of 297-385 K. The rate constant increases with temperature up to the point where decomposition becomes apparent. Electron attachment to ReF(6) occurs with a rate constant of (2.4+/-0.8)x10(-9) cm(3) s(-1) at 297 K; only ReF(6) (-) is produced. MoF(6) (-) reacts with ReF(6) to form ReF(6) (-) on essentially every collision, showing definitively that the electron affinity of ReF(6) is greater than that of MoF(6). A rate constant of (5.0+/-1.3)x10(-10) cm(3) s(-1) was measured for this ion-molecule reaction at 304 K. The reverse reaction is not observed. The reaction of Ar(+) with MoF(6) was found to produce MoF(5) (+)+F, with a rate constant of (1.8+/-0.5)x10(-9) cm(3) s(-1). WF(6) attaches electrons so slowly at room temperature that the attachment rate was below detection level (< or =10(-12) cm(3) s(-1)). By 552 K, the attachment rate constant reaches a value of (2+/-1)x10(-10) cm(3) s(-1).     

Heterogeneous reactions of gaseous HNO3 and NO2 on the clay minerals kaolinite and pyrophyllite

Airborne clay mineral particles have long atmospheric lifetimes due to their relatively small size. To assess their impact on trace atmospheric gases, we investigated heterogeneous reactions on prototype clay minerals. Diffuse reflectance infrared spectroscopy identified surface-adsorbed products formed from the uptake of gaseous nitric acid and nitrogen dioxide on kaolinite and pyrophyllite. For kaolinite, a 1:1 phyllosilicate, HNO3 molecularly adsorbed onto the octahedral aluminum hydroxide and tetrahedral silicon oxide surfaces. Also detected on the aluminum hydroxide surface were irreversibly adsorbed monodentate, bidentate, bridged, and water-coordinated nitrate species as well as surface-adsorbed water. Similar adsorbed products formed during the uptake of NO2 on kaolinite at relative humidity (RH) of 0%, and the reaction was second order with respect to reactive surface sites and 1.5 +/- 0.1 for NO2. Reactive uptake coefficients, calculated using Brunauer, Emmett, and Teller surface areas, increased from (8.0 +/- 0.2) x 10(-8) to (2.3 +/- 0.4) x 10(-7) for NO2 concentrations ranging from 0.56 x 10(13) to 8.8 x 10(13) molecules cm(-3). UV-visible spectroscopy detected gaseous HONO as a product for the reaction of NO2 on wet kaolinite. The uptake of HNO3 on pyrophyllite, a 2:1 phyllosilicate, resulted in stronger signal for nitric acid molecularly adsorbed on the silicon oxide surface compared to kaolinite. Monodentate, bridged, and water-coordinated nitrate species bound to aluminum sites also formed during this reaction indicating that reactive sites on edge facets are important for this system. The uptake of NO2 on pyrophyllite, gammaBET = (7 +/- 1) x 10(-9), was significantly lower than kaolinite because NO2 did not react with the dominant tetrahedral silicon oxide surface. These results highlight general trends regarding the reactivity of tetrahedral silicon oxide and octahedral aluminum hydroxide clay surfaces and indicate that the heterogeneous chemistry of clay aerosols varies with mineralogy and cannot be predicted by elemental analysis.     

Kinetic study of IO radical with RO2 (R = CH(3), C2H5, and CF3) using cavity ring-down spectroscopy

The reactions of iodine monoxide radical, IO, with alkyl peroxide radicals, RO(2) (R = CH(3), C(2)H(5), and CF(3)), have been studied using cavity ring-down spectroscopy. The rate constant of the reaction of IO with CH(3)O(2) was determined to be (7.0 +/- 3.0) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 100 Torr of N(2) diluent. The quoted uncertainty is two standard deviations. No significant pressure dependence of the rate constant was observed at 30-130 Torr total pressure of N(2) diluent. The temperature dependence of the rate constants was also studied at 213-298 K. The upper limit of the branching ratio of OIO radical formation from IO + CH(3)O(2) was estimated to be <0.1. The reaction rate constants of IO + C(2)H(5)O(2) and IO + CF(3)O(2) were determined to be (14 +/- 6) x 10(-11) and (6.3 +/- 2.7) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, 100 Torr of N(2) diluent, respectively. The upper limit of the reaction rate constant of IO with CH(3)I was <4 x 10(-14) cm(3) molecule(-1) s(-1).     

Competitive Substitution and Electron Transfer in Reactions between Haloamminegold(III) and Halocyanoaurate(III) Complexes and Thiocyanate

Kinetics for reactions between thiocyanate and trans-Au(CN)(2)Cl(2)(-), trans-Au(CN)(2)Br(2)(-), and trans-Au(NH(3))(2)Cl(2)(+) in an acidic, 1.00 M perchlorate aqueous medium have been studied by use of conventional and diode-array UV/vis spectroscopy and high-pressure and sequential-mixing stopped-flow spectrophotometry. Initial, rapid formation of mixed halide-thiocyanate complexes of gold(III) is followed by slower reduction to Au(CN)(2)(-) and Au(NH(3))(2)(+), respectively. This is an intermolecular process, involving attack on the complex by outer-sphere thiocyanate. Second-order rate constants at 25.0 degrees C for reduction of trans-Au(CN)(2)XSCN(-) are (6.9 +/- 1.1) x 10(4) M(-)(1) s(-)(1) for X = Cl and (3.1 +/- 0.7) x 10(3) M(-)(1) s(-)(1) for X = Br. For reduction of trans-Au(CN)(2)(SCN)(2)(-) the second-order rate constant at 25.0 degrees C is (3.1 +/- 0.1) x 10(2) M(-)(1) s(-)(1) and the activation parameters are DeltaH() = (55 +/- 3) x 10(2) kJ mol(-)(1), DeltaS() = (-17.8 +/- 0.8) J K(-)(1) mol(-)(1), and DeltaV() = (-4.6 +/- 0.5) cm(3) mol(-)(1). The activation volume for substitution of one chloride on trans-Au(NH(3))(2)Cl(2)(+) is (-4.5 +/- 0.5) cm(3) mol(-)(1), and that for reduction of trans-Au(NH(3))(2)(SCN)(2)(+) (4.6 +/- 0.9) cm(3) mol(-)(1). The presence of pi-back-bonding cyanide ligands stabilizes the transition states for both substitution and reductive elimination reactions compared to ammine. In particular, complexes trans-Au(CN)(2)XSCN(-) with an unsymmetric electron distribution along the X-Au-SCN axis are reduced rapidly. The observed entropies and volumes of activation reflect large differences in the transition states for the reductive elimination and substitution processes, respectively, the former being more loosely bound, more sensitive to solvational changes, and probably not involving any large changes in the inner coordination sphere. A transition state with an S-S interaction between attacking and coordinated thiocyanate is suggested for the reduction. The stability constants for formation of the very short-lived complex trans-Au(CN)(2)(SCN)(2)(-) from trans-Au(CN)(2)X(SCN)(-) (X = Cl, Br) by replacement of halide by thiocyanate prior to reduction can be calculated from the redox kinetics data to be K(Cl,2) = (3.8 +/- 0.8) x 10(4) and K(Br,2) = (1.1 +/- 0.4) x 10(2).     

Kinetics and mechanism of (CF3)2CHOCH3 reaction with OH radicals in an environmental reaction chamber

The atmospheric chemistry of (CF3)2CHOCH3, a possible HCFC/HFC alternative, was studied using a smog chamber/FT-IR technique. OH radicals were prepared by the photolysis of ozone in a 200-Torr H2O/O3/O2 gas mixture held in an 11.5-dm3 temperature-controlled chamber. The rate constant, k1, for the reaction of (CF3)2CHOCH3 with OH radicals was determined to be (1.40 +/- 0.28) x 10(-12) exp[(-550 +/- 60)/T] cm3 molecule(-1) s(-1) by means of a relative rate method at 253-328 K. The value of k1 at 298 K was (2.25 +/- 0.04) x 10(-13) cm3 molecule(-1) s(-1). The random errors are reported with +/-2 standard deviations, and potential systematic errors of 15% could increase k(1). In considering OH-radical reactions, we estimated the tropospheric lifetime of (CF3)2CHOCH3 to be 2.0 months using the rate constant at 288 K. The degradation mechanism of (CF3)2CHOCH3 initiated by OH radicals was also investigated using FT-IR spectroscopy at 298 K. Products (CF3)2CHOC(O)H, CF3C(OH)2CF3, CF3C(O)OCH3, and COF(2) were identified and quantified. The branching ratio, k1a/k1b, was estimated to be 2.1:1 for reactions (CF3)2CHOCH3 + OH --> (CF3)2CHOCH2*+ H2O (k1a) and (CF3)2CHOCH3 + OH --> (CF3)2C*OCH3 + H2O (k1b).