Walter Hack

Experimental and theoretical investigation of the reaction NH(X3∑-)+H(2S) → N(4S)+H2(X1∑+g)

The rate coefficient of the reaction NH(XΣ−3)+H(S2)→k1aN(S4)+H2(XΣg+1) is determined in a quasistatic laser-flash photolysis, laser-induced fluorescence system at low pressures (2mbar⩽p⩽10mbar). The NH(X) radicals are produced via the quenching of NH(aΔ1) (obtained by photolyzing HN3) with Xe whereas the H atoms are generated in a H2∕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 k1a=(1.9±0.5)×1012cm3mol−1s−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 A″4 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...

Experimental and theoretical investigations of the reactions NH(XΣ−3)+D(S2)→ND(XΣ−3)+H(S2) and NH(XΣ−3)+D(S2)→N(S4)+HD(XΣg+1)

The rate coefficient of the reaction NH(XΣ−3)+D(S2)→k1products (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(aΔ1) (obtained in the photolysis of HN3) with Xe and the D atoms are generated in a D2/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 k1=(3.9±1.5)×1013cm3mol−1s−1. The rate coefficient k1 is the sum of the two rate coefficients, k1a and k1b, which correspond to the reactions NH(XΣ−3)+D(S2)→k1aND(XΣ−3)+H(S2) (1a) and NH(XΣ−3)+D(S2)→k1bN(S4)+HD(XΣg+1) (1b), respectively. The first reaction proceeds via the A″2 ground state of NH2 whereas the second one proceeds in the A″4 state. A global potential energy surface is constructed for the A″2 state using the internally contracted multireference configuration interaction method and the augmented correlation consistent ...

Mechanism and rate of the reaction CH3+ O—revisited

The primary products and the rate of the reaction of methyl radicals with oxygen atoms in the gas phase at room temperature have been studied using three different experimental arrangements: (A) laser flash photolysis to produce CH3 and O from the precursors CH3I and SO2 (the educts and the products were detected by quantitative FTIR spectroscopy); (B) the coupling of a conventional discharge flow reactor via a molecular sampling system to a mass spectrometer with electron impact ionization, which allowed the determination of labile and stable species; (C) laser induced multiphoton ionization combined with a TOF mass spectrometer-molecular beam sampling-flow reactor, which was used for the specific and sensitive detection of the CH3, CD3, C2H5 and C2D5 radicals and the determination of rate coefficients. The branching ratio of the reaction channels was determined by the experimental arrangements (A) and (B) leading to CH3 + O --> HCHO + H (55 +/- 5)% --> CO + H2 + H (45 +/- 5)%. The rate coefficients of the normal and deuterated methyl and ethyl radicals with atomic oxygen showed no isotope effect: k(CD3 + O)/k(CH3 + O) = 0.99 +/- 0.12, k(C2D5 + O)/k(C2H5 + O) = 1.01 +/- 0.07 (statistical error, 95% confidence level). The absolute rate coefficient of the reaction CH3 + O was derived with reference to the reaction C2H5 + O (k = 1.04 x 10(14) cm3 mol(-1) s(-1)) leading to k(CH3 + O) = (7.6 +/- 1.4) x 10(13) cm3 mol(-1) s(-1).

Mechanisms and rates of the reactions C2H5+O and 1-C3H7+O

The mechanisms and rates of the reactions of the primary alkyl radicals ethyl and l-propyl with oxygen atoms at room temperature and low pressure (around 5 mbar) have been studied using two independent experimental arrangements. The reactants were generated by UV-laser flash photolysis with different precursors (C 2 H 5 COC 2 H 5 , C 2 H 6 +CFCl 3 , C 2 H 5 I, C 3 H 7 COC 3 H 7 , SO 2 ). Stable species concentrations were measured quantitatively by Fourier transform IR and OH radical concentrations of the ground ( v =0) and first vibrational ( v =1) state by time-resolved laser-induced fluorescence. For both reaction 1 and reaction 2, the mechanism is explained in terms of the formation and subsequent decomposition of a chemically activated alkoxy radical and a competing abstraction channel leading directly to OH and the alkene: C 2 H 5 +O→C 2 H 5 (reaction Ia)/C 2 H 5 O→HCHO+CH 3 (reaction la 1 )/CH 3 CHO+H (reaction 1a 2 )//C 2 H 5 +O→C 2 H 4 +OH (reaction 1b). The absolute branching ratio was determined preferentially using diethyl ketone as the C 2 H 5 radical source leading to (1a 2 )/(1a 2 )/(1b), 32/44/24. Relative branching ratios for the C 2 H 5 radical sources C 2 H 6 +Cl and C 2 H 5 I were derived as (1a 1 /(1a 2 )=1/1.5 and 1/1.55, respectively. The overall rate coefficient of the reaction C 2 H 5 +O was measured as k 1 =(1.04±0.1)×10 14 cm 3 mol −1 s −1 and in addition k (C 2 H 5 +OH)=(7.0±1)×10 13 cm 3 mol −1 s −1 . The mechanism and the rate of reaction 2 were found as 1-C 3 H 7 +O→1-C 3 H 7 O (reaction 2a)/1-C 3 H 7 O→HCHO+C 2 H 5 (reaction 2a 1 )/C 2 H 5 CHO+H (reaction 2a 2 )//1.C 3 H 7 +O→C 3 H 6 +OH (reaction 2b) (branching ratio (2a 1 )/(2b), 44/32/24 and k 2 =(8.2±1)×10 3 mo −1 s −1 The results are discussed in terms of statistical rate theory.

Mechanism of the 1-C4H9+ O reaction and the kinetics of the intermediate 1-C4H9O radical

The 1-C4H9 + O reaction has been investigated in two quasi-static reactors with different detection systems. From a time-resolved measurement of OH formation by laser induced fluorescence (T = 295 K, p = 21 mbar, bath gas: He) an inverted vibrational state distribution for OH X2Π ( = 0, 1, 2) was observed. By using Fourier transform infrared spectroscopy, relative product yields of 0.55 ± 0.08 for 1-C4H8, 0.397 ± 0.05 for HCHO and 0.053 ± 0.02 for C3H7CHO were determined (T = 298 K, p = 2 mbar, bath gas: He). The results are explained in terms of the formation and subsequent decomposition of an intermediate chemically activated 1-C4H9O radical and a competing abstraction channel leading directly to OH + 1-C4H8. A modeling by statistical rate theory based on ab initio results for the stationary points of the potential energy surface of C4H9O allows the quantitative description of the product branching ratios. From this modeling, threshold energies of E06 = 55 ± 6 and E07 = 88 ± 6 kJ mol−1 for the β-C–C and the β-C–H bond dissociation, respectively, in 1-C4H9O are obtained. For the 1,5 H atom shift, a most probable value of E05 = 40 ± 5 kJ mol−1 follows from a comparison of our quantum chemical results with data from the literature.

Methanol and ethanol decomposition in supercritical water

Abstract The decomposition of CH3OH and C2H5OH in supercritical water was studied in a flow reactor tube (Ni/Mo/Cr/Fe alloy) in the temperature range 597 ≤ T/K ≤ 797 at a pressure of p = 315 bar for technical application of scH2O for hazardous chemical waste destruction. The CH3OH and C2H5OH concentrations in the liquid as a function of the residence times were determined by a Raman spectrometer. The [CH3OH] and [C2H5OH] resp. decay followed first order kinetics and a rate constant k1(653 K) = 1.3 × 10−2 s−1 for CH3OH and k2(653 K) = 5.5 × 10−2 s−1 for C2H5OH was determined. The rate constant k1 was found to be independent of the initial CH3OH concentration in the mass fraction range 0.002 ≤ xm ≤ 0.04. The rate depended on the history of the reactor. Treatment with NH4OH, C2H5OH or with H2O2 at T = 653 K, did not change the rate. Treatment with HNO3/H2O2, however, at T = 838 K reduced the rate by about a factor of 1000. The Arrhenius-activation energy over the above temperature range was determined to be EA = 164 kJ/mol for methanol and EA = 145 kJ/mol for ethanol. The major products from methanol decomposition were CH4, H2, and CO2 as observed by gas chromatography and CH4 and CO2 by FTIR-spectrometry. No other products were found. The products were not effected by the pretreatment of the reactor wall. A non-radical mechanism, which explains the formation of only these products, will be discussed.

Relaxation of NH(a1Δ, v = 1) in collisions with H(2S): An experimental and theoretical study.

Collisions of electronically and vibrationally excited NH(a(1)Delta, v = 1) with H atoms were investigated by experimental, quantum mechanical (QM) wavepacket, and quasiclassical trajectory (QCT) methods. The NH(a(1)Delta, v = 1) total loss rate constant, corresponding to the sum of the NH vibrational relaxation, N((2)D)+H(2) formation, and electronic quenching to NH(X(3)Sigma(-)), was measured at room temperature. Most of the calculations were performed within the Born-Oppenheimer approximation, neglecting electronic quenching due to Renner-Teller coupling because QCT calculations showed that for the loss of NH(a(1)Delta, v = 1) the contribution of quenching is negligible. The QM study included Coriolis couplings, and the QCT study counted only trajectories ending close to a vibrational quantum level of the product diatom. The collisions are dominated by long-lived intermediate complexes, and QM probabilities and cross sections thus exhibit pronounced resonances. QM and QCT cross sections and rate coefficients of the various processes are in very good agreement. The measured rate constant is (9.1 +/- 3.3) x 10(-11) cm(3) s(-1), compared with (14.4 +/- 0.5) x 10(-11) and (15.6 +/- 1.6) x 10(-11) cm(3) s(-1), as obtained from QM and QCT calculations, respectively. The reason for the theoretical overestimation is unknown.

The Elementary Reaction of CHF(X˜1A´) with Ozone

The reaction CHF(X˜1A´) + O3 → products (1) has been studied in an isothermal flow reactor in the temperature range from 235 K to 443 K and at a pressure of about p = 2 mbar under pseudo first order conditions, [O3]0 >> [CHF]0. He was the main carrier gas. The CHF(X˜) radical was produced in the reaction sequence CH3F + F → CH2F + HF; CH2F + F → CHF(X˜) + HF and detected by laser induced fluorescence. For reaction (1) a value of the rate constant: k1(T) = (6.1±0.6) · 1012(T/298 K)-0.6±0.4[cm3/mol s] was obtained. No change in the rate constant was observed, substituting He by the quenchers N2 or SF6. The reaction mechanism is discussed.

The t-C4H9OCH2 radical in the gas phase: Detection by multiphoton ionization and the reactions with O, O2, O3, H, and NO

The detection of the t -C 4 H 9 OCH 2 and the CH 3 OCH 2 radical by the multiphoton ionization/mass spectrometry/discharge fast flow reactor technique has been investigated in the wavelength region λ=420–530 nm. For both radicals, unstructured bands were observed. The rate of the five reactions of the t -C 4 H 9 OCH 2 radical with O, O 2 , O 3 , H, and NO have been determined with reference to the corresponding CH 3 OCH 2 radical reactions in the temperature range of 249–684 K at low pressure (around 2 mbar) t -C 4 H 9 OCH 2 +O→products k 1 =(1.80±0.5)·10 14 ( T /300 K) 0.46±0.15 cm 3 mol −1 s −1 (253≤ T/K ≤684) (1) t -C 4 H 9 OCH 2 +O 2 →products k 2 =(6.7±0.04)·10 11 ( T /300K) −1.1±0.2 cm 3 mol −1 s −1 (2) t -C 4 H 9 OCH 2 +O 3 →products k 3 =(6.7±0.1)·10 12 ( T /300K) −0.73±0.3 cm 3 mol −1 s −1 (249≤ T/K ≤363) (3) t -C 4 H 9 OCH 2 +H products k 4 =(2.5±0.1)·10 13 ( T /300 K) −0.50±0.1 cm 3 mol −1 s −1 (4) t -C 4 H 9 OCH 2 +NO→products k 5 =(2.6±0.1)·10 10 ( T /300 K) 1.5±0.6 cm 3 mol −1 s −1 (5) The formation of the primary products is discussed on the basis of mass spectrometric detection using low-energy electron impact ionization.

The Rate constant of the Reaction of OH(A2Σ+) with H2O2(X˜1A)

The rate constant of the depletion of OH radicals in the first electronically excited state with hydrogenperoxid: OH(A2Σ+) + H2O2 → products (1) was determined at room temperature under pseudo first-order conditions [OH(A)]o << [H2O2]o. The OH(A) was produced by laser excitation and detected via its fluorescence in He as buffer gas (p = 20 mbar). The rate constant is: k 1 = (3.7±0.5)×1014 cm3/mol s, similar to the quenching rate constant of OH(A) by H2O.