The question of a pressure effect in the reaction OH + HNO3. A laser flash-photolysis resonance-absorption study

Absolute rate constants, k, of the reaction OH + HNO₃ were determined using a pulsed laser photolysis-resonance absorption technique. The measured values, in cm³ mol^-1 s^-1 at ±3σ, 10^-10k(1–16 Torr HNO₃) = 7.57 ± 0.64, k(500 Torr N₂) = 7.20 ± 0.66 and k(600 Torr SF₆) = 8.37 ± 0.45, indicate that any pressure effect on k at 297 K is less than the experimental uncertainty of 10%.     

The kinetic isotope effect for carbon and oxygen in the reaction CO + OH

The kinetic isotope effect (KIE) for carbon and oxygen in the reaction CO + OH has been measured over a range of pressures of air and at 0.2 and 1.0 atm of oxygen, argon, and helium. The reaction was carried out with 21–86% conversion under static conditions, utilizing the photolysis of H₂O₂ as a source of OH radicals. The value of the KIE for carbon varies with pressure and the kind of ambient gas; for air the ratio of the reaction rates ¹²k/¹³k has the value 1.007 at 1.00 atm and decreases to 0.997 at 0.2 atm; for oxygen and argon over the same pressure range the values are 1.002–0.994 and 1.000–0.991, respectively. The value of the KIE for the CO oxygen atom is ¹⁶k/¹⁸k = 0.990 over the pressure range 0.2–1.0 atm and is independent of the kind of ambient gas. No exchange of the oxygen atoms in the activated complex, followed by decomposition to the starting molecules, was observed. From the mechanistic standpoint the normal KIE observed for carbon at the high pressure is attributed to the initial formation of the activated HOCO radical, whereas the inverse KIE observed at low pressures is a result of the KIE for the reverse reaction HOCO? → CO + OH being greater than that for the forward reaction HOCO^? → CO₂ + H. The derived isotopic equilibrium constant for HOCO ?CO favors the enrichment of ¹³C in the more strongly bound HOCO.     

Rate constants for the reaction OH + CO as functions of temperature and water concentration

Rate constants for the reaction OH + CO have been measured as functions of temperature (340–1220 K) and water concentration in the presence of 1 atm of argon. Results at zero water concentration yield the expression, log k_? (cm³ molecule^?1 S^?1) = ?12.96 + 4.7 × 10^?4 T, for the reaction rate constant as a function of temperature. These results are in very good agreement with previous direct measurements and in reasonable agreement with flame and shock tube measurements. Explanations are offered for the involvement of the water molecule in the present experiments and earlier measurements from this laboratory throughout the entire temperature range. Results are consistent with previous results showing little, if any, pressure effect of Ar on the reaction up to 1 atm of Ar.     

The reaction of OH with CO

Hydroxyl radicals were prepared from the photolysis of N₂O at 213.9 nm in the presence of excess H₂. The O(¹D) produced in the primary photolytic act reacts with H₂ to produce OH radicals. If CO is also present, then OH can react either with H₂ or CO: The competition between reactions (1) and (2) was measured by measuring the CO₂ yield at various values of the ratio [CO]/[H₂] at 217–298°K. At 298°K the ratio of the rate coefficients k₁/k₂ increased with pressure from a low-pressure limiting value of 14 to a high-pressure limiting value of 50. The low-pressure limiting value agrees well with the low-pressure values found by others. At lower temperatures our high-pressure values of k₁/k₂ were larger than deduced from the accepted low-pressure Arrhenius expression and could be fitted to the expression The mechanism which seems to fit the results best is with k₁° = k_ak_b/k_-a and k₁^∞ = k_a.     

The ultraviolet absorption spectra of the acetyl radical and the kinetics of the CH3 + CO reaction at room temperature

A continuum-absorption spectrum between 200 and 240 nm is assigned to the acetyl radical. Kinetic measurements using molecular modulation spectroscopy show for the reaction CH₃ + CO (+M) → CH₃CO + M the rate constants are (1.8 ± 0.2) × 10^?18 cm³ molecule^?1 s^?1 at 100 Torr and (6 ± 1) × 10^?18 at 750 Torr. The rate constant for acetyl combination 2CH₃CO → (CH₃CO)₂ is (3.0 ± 10) × 10^?11 at 25°C.     

Water effect on the OH + HCl reaction

Hydrochloric acid is a major reservoir for chlorine radicals in the atmosphere. Chlorine radicals are chemically reactivated by the relatively slow attack of OH radical on HCl. Through the formation of hydrogen-bonded complexes, water has a dramatic effect on the rate of this reaction. The introduction of water opens several new reaction pathways with rate coefficients that are faster than the "bare" reaction. Accounting for the low fraction of hydrogen bonded water complexes in the atmosphere, the present results suggest that these new mechanisms involving water can contribute, although modestly, to the total chemical reactivation of chlorine from HCl in the lower troposphere. The first reported value for the equilibrium constant for the formation of H(2)O·HCl complex, which is important in understanding the removal of HCl from the atmosphere by deposition, is presented.     

Rate constant for the OH + CO reaction: Pressure dependence and the effect of oxygen

The effect of pressure on the rate constant of the OH + CO reaction has been measured for Ar, N₂, and SF₆ over the pressure range 200–730 torr. All experiments were at room temperature. The method involved laser-induced fluorescence to measure steady-state OH concentrations in the 184.9 nm photolysis of H₂O-CO mixtures in the three carrier gases, combined with supplementary measurements of the CO depletion in these same carrier gases in the presence and absence of competing reference reactants. The effect of O₂ on the pressure effect was determined. A pressure enhancement of the rate constant was observed for N₂ and SF₆, but not for Ar, within an experimental error of about 10%. The pressure effect for N₂ was somewhat lower than previous literature reports, being about 40% at 730 torr. For SF₆ a factor of two enhancement was seen at 730 torr. In each case it was found that O₂ had no effect on the pressure enhancement. The roles of the radical species HCO and HOCO were evaluated.     

The mechanism of the OH + CO reaction and the stability of the HOCO radical

The mechanism of the reaction between OH radicals and CO is discussed in relation to recent experiments which indicate that the rate constant, k = ?(dln[OH]/dt)/[CO], depends on total pressure. It is shown that this observation is quite consistent with the known spectroscopic and thermodynamic properties of the HOCO radical, as long as the dissociation of HOCO to H + CO₂ is no faster than that to OH + CO.     

Reactions in a non-uniform flow tube temperature profile: effect on the rate coefficient for the reaction C2H5OH+OH

The influence of non-linear temperature profiles in a flow reactor on the determination of rate constants is discussed for the case of pseudo-first-order reactions. The effects are investigated for the reaction C₂H₅OH+OH and taken into account in the evaluation of experimental data.     

The rate constant of the reaction HO2 + COCO2 + OH

The high-temperature oxidation of formaldehyde in the presence of carbon monoxide was investigated to determine the rate constant of the reaction HO₂ + CO ? CO₂ + OH (10). In the temperature range of 878–952°K from the initial parts of the kinetic curves of the HO₂ radicals and CO₂ accumulation at small extents of the reaction, when the quantity of the reacted formaldehyde does not exceed 10%, it was determined that the rate constant k₁₀ is A computer program was used to solve the system of differential equations which correspond to the high-temperature oxidation of formaldehyde in the presence of carbon monoxide. The computation confirmed the experimental results. Also discussed are existing experimental data related to the reaction of HO₂ with CO.     

Development of a room temperature Hirao reaction

Arylphosphonates were prepared at 25 °C through the palladium-catalyzed coupling of aryl iodides with a silver phosphonate. A wide range of aryl iodides were successfully employed including phenolic substrates as well as those containing an ortho substituent.     

The role of dimers in complex forming reactions at low temperature: full dimension potential and dynamics of (H2CO)2+OH reaction

The (H CO) +OH and H CO-OH+H CO reaction dynamics are studied theoretically for temperatures below 300 K. For this purpose, a full dimension potential energy surface is built, which reproduces well accurate ab initio calculations. The potential presents a submerged reaction barrier, as an example of the catalytic effect induced by the presence of the third molecule. However, quasi-classical and ring polymer molecular dynamics calculations show that the dominant channel is the dimer-exchange mechanism below 200 K, and that the reactive rate constant tends to stabilize at low temperatures, because the effective dipole of either dimer is reduced with respect to that of formaldehyde alone. The reaction complex formed at low temperatures does not live long enough to produce complete energy relaxation, as assumed in statistical theories. These results show that the reactivity of the dimers cannot explain the large rate constants measured at temperatures below 100 K.     

Steric acceleration of an uncatalysed ene reaction at room temperature

The bulky trityl steric buttress is used to effect an intramolecular, uncatalysed ene reaction that operates at room temperature, whilst smaller buttresses require heat.     

Kinetics of the OH + HCNO reaction

The kinetics of the OH + HCNO reaction was studied. The total rate constant was measured by LIF detection of OH using two different OH precursors, both of which gave identical results. We obtain k = (2.69 +/- 0.41) x 10(-12) exp[(750.2 +/- 49.8)/T] cm(3) molecule(-1) s(-1) over the temperature range 298-386 K, with a value of k = (3.39 +/- 0.3) x 10(-11) cm(3) molecule(-1) s(-1) at 296 K. CO, H(2)CO, NO, and HNO products were detected using infrared laser absorption spectroscopy. On the basis of these measurements, we conclude that CO + H(2)NO and HNO + HCO are the major product channels, with a minor contribution from H(2)CO + NO.     

Kinetics of the NCN + NO reaction over a broad temperature and pressure range

Rate coefficients for the reaction (3)NCN + NO → products (R3) were measured in the temperature range 251-487 K at pressures from 10 mbar up to 50 bar with helium as the bath gas. The experiments were carried out in slow-flow reactors by using pulsed excimer laser photolysis of NCN(3) at 193 or 248 nm for the production of NCN. Pseudo-first-order conditions ([NCN](0) ? NO) were applied, and NCN was detected time-resolved by resonant laser-induced fluorescence excited near 329 nm. The measurements at the highest pressures yielded values of k(3) ～ 8 × 10(-12) cm(3) s(-1) virtually independent of temperature and pressure, which indicates a substantially smaller high-pressure limiting value of k(3) than predicted in earlier works. Our experiments at pressures below 1 bar confirm the negative temperature and positive pressure dependence of the rate coefficient k(3) found in previous investigations. The falloff behavior of k(3) was rationalized by a master equation analysis based on a barrierless association step (3)NCN + NO ? NCNNO((2)A″) followed by a fast internal conversion NCNNO((2)A″) ? NCNNO((2)A'). From 251-487 K and above 30 mbar, the rate coefficient k(3) is well represented by a Troe parametrization for a recombination/dissociation reaction, k(3)(T,P) = k(4)(∞)k(4)(0)[M]F(k(4)(0)[M] + k(4)(∞))(-1), where k(4) represents the rate coefficient for the recombination reaction (3)NCN + NO. The following parameters were determined (30% estimated error of the absolute value of k(3)): k(4)(0)[M=He] = 1.91 × 10(-30)(T/300 K)(-3.3) cm(6) s(-1)[He], k(4)(∞) = 1.12 × 10(-11) exp(-23 K/T) cm(3) s(-1), and F(C) = 0.28 exp(173 K/T).     

Experimental and modeling studies of the pressure and temperature dependences of the kinetics and the OH yields in the acetyl + O2 reaction

The acetyl + O(2) reaction has been studied by observing the time dependence of OH by laser-induced fluorescence (LIF) and by electronic structure/master equation analysis. The experimental OH time profiles were analyzed to obtain the kinetics of the acetyl + O(2) reaction and the relative OH yields over the temperature range of 213-500 K in helium at pressures in the range of 5-600 Torr. More limited measurements were made in N(2) and for CD(3)CO + O(2). The relative OH yields were converted into absolute yields by assuming that the OH yield at zero pressure is unity. Electronic structure calculations of the stationary points of the potential energy surface were used with a master equation analysis to fit the experimental data in He using the high-pressure limiting rate coefficient for the reaction, k(∞)(T), and the energy transfer parameter, (ΔE(d)), as variable parameters. The best-fit parameters obtained are k(∞) = 6.2 × 10(-12) cm(-3) molecule(-1) s(-1), independent of temperature over the experimental range, and (ΔE(d))(He) = 160(T/298?K) cm(-1). The fits in N(2), using the same k(∞)(T), gave (ΔE(d))(N(2)) = 270(T/298?K) cm(-1). The rate coefficients for formation of OH and CH(3)C(O)O(2) are provided in parametrized form, based on modified Troe expressions, from the best-fit master equation calculations, over the pressure and temperature ranges of 1 ≤ p/Torr ≤ 1.5 × 10(5) and 200 ≤ T/K ≤ 1000 for He and N(2) as the bath gas. The minor channels, leading to HO(2) + CH(2)CO and CH(2)C(O)OOH, generally have yields <1% over this range.     

Proton transfer reaction of a new orthohydroxy Schiff base in protic solvents at room temperature

Ground and excited state inter- and intramolecular proton transfer reactions of a new o-hydroxy Schiff base, 7-ethylsalicylidenebenzylamine (ESBA) have been investigated by means of absorption, emission and nanosecond spectroscopy in different protic solvents at room temperature and 77 K. The excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission (approximately 11000 cm(-1)) at a selected excited energy in alcoholic solvents. Spectral characteristics obtained reveal that ESBA exists in more than one structural form in most of the protic solvents, both in the ground and excited states. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rate constants, which are mainly represented by nonradiative decay rates. At 77 K the fluorescence spectra are found to be contaminated with phosphorescence spectra in glycerol and ethylene glycol. It is shown that the fluorescence intensity and nature of the species present are dependent upon the excitation energy.