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Vibrational relaxation of H

The bending force constant associated with formation water that is selected for the potential energy surface describing the OH/OD + GeH4 reaction is shown to be important for the degree of bending excitation of H2O and HOD.

The gas-phase unimolecular reactions of C

The five unimolecular HX and DX (X = F, Cl) elimination pathways of CD

The recombination of CF

The room-temperature gas-phase recombination of CH2F and CD2Cl radicals was used to prepare CH2FCD2Cl molecules with 91 kcal mol(-1) of vibrational energy. Three unimolecular processes are in competition with collisional deactivation of CH2FCD2Cl; HCl and DF elimination to give CHF═CD2 and CH2═CDCl plus isomerization to give CH2ClCD2F by the interchange of F and Cl atoms. The Cl/F interchange reaction was observed, and the rate constant was assigned from measurement of CHCl═CD2 as a product, which is formed by HF elimination from CH2ClCD2F. These experiments plus previously published results from chemically activated CH2ClCH2F and electronic structure and RRKM calculations for the kinetic-isotope effects permit assignment of the three rate constants for CH2FCD2Cl (and for CH2ClCD2F). The product branching ratio for the interchange reaction versus elimination is 0.24 ± 0.04. Comparison of the experimental rate constant with the RRKM calculated rate constant permitted the assignment of a threshold energy of 62 ± 3 kcal mol(-1) for this type-1 dyotropic rearrangement. On the basis of electronic structure calculations, the nature of the transition state for the rearrangement reaction is discussed. The radical recombination reactions in the chemical system also generate vibrationally excited CD2ClCD2Cl and CH2FCH2F molecules, and the rate constants for DCl and HF elimination were measured in order to confirm that the photolysis of CD2ClI and (CH2F)2CO mixtures was giving reliable data for CH2FCD2Cl.

The recombination of ·CHF

Chemically activated C

The recombination of chloromethyl and t-butyl radicals at room temperature was used to generate neopentyl chloride molecules with 89 kcal mol(-1) of internal energy. The observed unimolecular reactions, which give 2-methyl-2-butene and 2-methyl-1-butene plus HCl, as products, are explained by a mechanism that involves the interchange of a methyl group and the chlorine atom to yield 2-chloro-2-methylbutane, which subsequently eliminates hydrogen chloride by the usual four-centered mechanism to give the observed products. The interchange isomerization process is the rate-limiting step. Similar experiments were done with CD(2)Cl and C(CH(3))(3) radicals to measure the kinetic-isotope effect to help corroborate the proposed mechanism. Density functional theory was employed at the B3PW91/6-31G(d',p') level to verify the Cl/CH(3) interchange mechanism and to characterize the interchange transition state. These calculations, which provide vibrational frequencies and moments of inertia of the molecule and transition state, were used to evaluate the statistical unimolecular rate constants. Matching the calculated and experimental rate constants, gave 62 ± 2 kcal mol(-1) as the threshold energy for interchange of the Cl atom and a methyl group. The calculated models also were used to reinterpret the thermal unimolecular reactions of neopentyl chloride and neopentyl bromide. The previously assumed Wagner-Meerwein rearrangement mechanism for these reactions can be replaced by a mechanism that involves the interchange of the halogen atom and a methyl group followed by HCl or HBr elimination from 2-chloro-2-methylbutane and 2-bromo-2-methylbutane. Electronic structure calculations also were done to find threshold energies for several related molecules, including 2-chloro-3,3-dimethylbutane, 1-chloro-2-methyl-2-phenylpropane, and 1-chloro-2-methyl-2-vinylpropane, to demonstrate the generality of the interchange reaction involving a methyl, or other hydrocarbon groups, and a chlorine atom. The interchange of a halogen atom and a methyl group located on adjacent carbon atoms can be viewed as an extension of the halogen atom interchange mechanisms that is common in 1,2-dihaloalkanes.

The recombination of CH(2)Cl and CH(2)F radicals generates vibrationally excited CH(2)ClCH(2)Cl, CH(2)FCH(2)F, and CH(2)ClCH(2)F molecules with about 90 kcal mol(-1) of energy in a room temperature bath gas. New experimental data for CH(2)ClCH(2)F have been obtained that are combined with previously published studies for C(2)H(4)Cl(2) and C(2)H(4)F(2) to define reliable rate constants of 3.0 x 10(8) (C(2)H(4)F(2)), 2.4 x 10(8) (C(2)H(4)Cl(2)), and 1.9 x 10(8) (CH(2)ClCH(2)F) s(-1) for HCl and HF elimination. The product branching ratio for CH(2)ClCH(2)F is approximately 1. These experimental rate constants are compared to calculated statistical rate constants (RRKM) to assign threshold energies for HF and HCl elimination. The calculated rate constants are based on transition-state models obtained from calculations of electronic structures; the energy levels of the asymmetric, hindered, internal rotation were directly included in the state counting to obtain a more realistic measure for the density of internal states for the molecules. The assigned threshold energies for C(2)H(4)F(2) and C(2)H(4)Cl(2) are both 63 +/- 2 kcal mol(-1). The threshold energies for CH(2)ClCH(2)F are 65 +/- 2 (HCl) and 63 +/- 2 (HF) kcal mol(-1). These threshold energies are 5-7 kcal mol(-1) higher than the corresponding values for C(2)H(5)Cl or C(2)H(5)F, and beta-substitution of F or Cl atoms raises threshold energies for HF or HCl elimination reactions. The treatment presented here for obtaining the densities of states and the entropy of activation from models with asymmetric internal rotations with high barriers can be used to judge the validity of using a symmetric internal-rotor approximation for other cases. Finally, threshold energies for the 1,2-fluorochloroethanes are compared to those of the 1,1-fluorochloroethanes to illustrate substituent effects on the relative energies of the isomeric transition states.

Vibrationally excited CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) molecules were prepared in the gas phase at 300 K with approximately 93 kcal mol(-1) of energy by recombination of CF(2)ClCHF and C(2)H(5) or C(2)D(5) radicals. Three unimolecular reactions were observed. 1,2-ClF interchange converts CF(2)ClCHFC(2)H(5)(CF(2)ClCHFC(2)D(5)) into CF(3)CHClC(2)H(5)(CF(3)CHClC(2)D(5)), and subsequent 2,3-ClH (ClD) elimination gives CF(3)CH=CHCH(3) (CF(3)CH=CDCD(3)). 2,3-FH(FD) elimination gives cis- and trans-CF(2)ClCH=CHCH(3) (CF(2)ClCH=CDCD(3)), and 1,2-ClH elimination gives CF(2)=CFCH(2)CH(3) (CF(2)=CFCD(2)CD(3)). The experimental rate constants for CF(2)ClCHFC(2)H(5) (CF(2)ClCHFC(2)D(5)) were 1.3 x 10(4) (0.63 x 10(4)) s(-1) for 1,2-FCl interchange and 2.1 x 10(4) (0.61 x 10(4)) s(-1) with a trans/cis ratio of 3.7 for 2,3-FH(FD) elimination. The 1,2-ClH process was the least important with a branching fraction of only 0.08 +/- 0.04. The rate constants for 2,3-ClH (ClD) elimination from CF(3)CHClC(2)H(5) (CF(3)CHClC(2)D(5)) were 1.8 x 10(6) (0.49 x 10(6)) s(-1) with a trans/cis ratio of 2.4. Density functional theory was used to compute vibrational frequencies and structures needed to obtain rate constants from RRKM theory. Matching theoretical and experimental rate constants provides estimates of the threshold energies, E0, for the three reaction pathways; 1,2-FCl interchange has the lowest E0. The unimolecular reactions of CF(2)ClCHFC(2)H(5) are compared to those of CF(2)ClCHFCH(3). Both of these systems are compared to CH(3)CHFC(2)H(5) to illustrate the influence of a CF(2)Cl group on the E0 for FH elimination.

Chemically activated CF(3)CFClCH(3), CF(3)CFClCD(3), CF(3)CFClCH(2)D, and CF(3)CFClCHD(2) molecules with 94 kcal mol(-1) of internal energy were formed by the combination of CF(3)CFCl radicals with CH(3), CD(3), CH(2)D, and CHD(2) radicals, which were generated from UV photolysis of CF(3)CFClI and CH(3)I, CD(3)I, CH(2)DI, or CHD(2)I. The total (HF + HCl) elimination rate constants for CF(3)CFClCH(3) and CF(3)CFClCD(3) were 5.3 x 10(6) and 1.7 x 10(6) s(-1) with product branching ratios of 8.7 +/- 0.6 in favor of HCl (or DCl). The intermolecular kinetic isotope effects were 3.22 and 3.18 for the HCl and HF channels, respectively. The product branching ratios were 10.3 +/- 1.9 and 11.8 +/- 1.8 (10.8 +/- 3.8 and 11.6 +/- 1.7) for HCl/HF and DCl/DF, respectively, from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)). The intramolecular kinetic-isotope effects (without correction for reaction path degeneracy) for HCl/DCl and HF/DF elimination from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)) were 2.78 +/- 0.16 and 2.98 +/- 0.12 (0.82 +/- 0.04 and 0.91 +/- 0.03), respectively. Density function theory at the B3PW91/6-311+G(2d,p) and B3PW91/6-31G(d',p') levels was investigated, and the latter was chosen to calculate frequencies and moments of inertia for the molecules and transition states. Rate constants, branching ratios and kinetic-isotope effects then were calculated using RRKM theory with torsional motions treated as hindered internal rotations. Threshold energies for HF and HCl elimination from CF(3)CFClCH(3) were assigned as 61.3 +/- 1.5 and 58.5 +/- 1.5 kcal mol(-1), respectively. The threshold energy for Cl-F interchange was estimated as 67 kcal mol(-1). The difference between the transition states for HCl and HF elimination is discussed.

Vibrationally activated CF3CH2CH2Cl molecules were prepared with 94 kcal mol-1 of vibrational energy by the combination of CF3CH2 and CH2Cl radicals and with 101 kcal mol-1 of energy by the combination of CF3 and CH2CH2Cl radicals at room temperature. The unimolecular rate constants for elimination of HCl from CF3CH2CH2Cl were 1.2 × 107 and 0.24 × 107 s-1 with 101 and 94 kcal mol-1, respectively. The product branching ratio, kHCl/kHF, was 80 ± 25. Activated CH3CH2CH2Cl and CD3CD2CH2Cl molecules with 90 kcal mol-1 of energy were prepared by recombination of C2H5 (or C2D5) radicals with CH2Cl radicals. The unimolecular rate constant for HCl elimination was 8.7 × 107 s-1, and the kinetic isotope effect was 4.0. Unified transition-state models obtained from density-functional theory calculations, with treatment of torsions as hindered internal rotors for the molecules and the transition states, were employed in the calculation of the RRKM rate constants for CF3CH2CH2Cl and CH3CH2CH2Cl. Fitting the calculated ...

The effect of hydrogen on the concentration of singlet oxygen in the a1Δg and b1Σ states, generated from a microwave discharge in O2 and in an O2/Ar mixture, was studied in flow reactors. The addition of hydrogen, in a range of 0.01–1 of concentration of the O2, increased the yields of singlet oxygen by factor of 5–20. In addition to the higher O2 (a and b) concentrations, the addition of hydrogen removed the usual NO2 fluorescence, making observation of the O2(b X) transition at 762 nm much easier in the flow reactor. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 12–17, 2006

The formation of water molecules via hydrogen atom abstraction by hydroxyl radicals and their formation via unimolecular elimination from vibrationally excited alcohols and organic acids are important processes in a variety of gas-phase chemical environments. The nascent vibrational distributions of the water molecules produced by such reactions have been obtained by analysis of the infrared chemiluminescence from H 2 O, HOD and D 2 O. The analysis required computer simulation of the spectra obtained by recording emission from a fast-flow reactor at 298 K with a low-resolution Fourier transform spectrometer. By combining the information deduced from simulation of the H 2 O and HOD emission spectra from the reactions of OH and OD, the total vibrational energy released to the water molecule and the distribution between the stretch and bending modes have been assigned. The present report provides a summary of results from hydroxyl radicals reacting with inorganic hydride molecules, with the primary, secondar...

The reactions of Ar(4s 3 P 2 ) and Ar(4s' 3 P 0 ) metastable atoms with H 2 and D 2 have been studied at 300 and 80 K by the stationary and flowing-afterglow techniques. Total quenching constants were measured for both metastable atoms for H 2 and D 2 at 300 K and for H 2 at 80 K. Optical pumping in the stationary-afterglow experiment was used to select one or the other metastable atoms, and chemiluminescence spectra were obtained for state-selected metastable atoms reacting with H 2 and D 2 . The branching fractions for chemiluminescence, which range from 0.07 to 0.30, are higher for D 2 than H 2 and higher for Ar(4s' 3 P 0 ) atoms than for Ar(4s 3 P 2 ) atoms. The chemiluminescence is assigned to the ArH(B 2 Π-X 2 Σ + ) and ArD(B 2 Π-X 2 Σ + ) transitions. The total quenching rate constants for H 2 decline by factors of 80 and 30 at 80 K. relative to the room-temperature values, for the reactions of Ar(4s 3 P 2 ) and Ar(4s' 3 P 0 ) atoms, respectively. Both the chemiluminescence and dark product channels of these reactions probably proceed by chemical reaction rather than by electronic energy transfer.

Stimulated emission (SE) induced by pulsed two-photon excitation of the 6p′[1/2]0, 6p′[3/2]2, 7p[1/2]0, 7p[3/2]2, and 7p[5/2]2 states of the Xe atom has been studied. Spectra of SE were recorded in the 800–3500 nm region, which covers the 6p′, 7p → 7s (2500–3500 nm), 6p′, 7p → 5d (1000–2000 nm), 7s → 6p (1200–1900 nm) transitions, as well as the near-IR 6p → 6s (800–1000 nm) transitions. It is shown that excitation of the 7p states results in radiative cascade via the 7s states, 7p → 7s → 6p, although at least one case of cascading via the 5d states is also observed. Spectra of SE induced by excitation of the 6p′ states are dominated by the 6p′ → 6s′ transitions in the near IR and the 6p′ → 5d[1/2]1 → 6p[1/2]1 → 6s[3/2]1 cascade; the 6p′ → 7s → 6p cascade is also observed, although the secondary 7s → 6p emission is rather weak in comparison with excitation of the 7p states.

Infrared chemiluminescence from a flow reactor has been used to study the H + CH2XI and N + CH2X(X = Cl, F, I, H) reactions at 300 K. Both the HI + CH2Cl and HCl + CH2I channels were identified for the H + CH2ClI reaction. The HCl channel involves adduct, HICH2Cl, formation as confirmed by the D + CH2ClI reaction, which gave both HCl and DCl products. The nascent HCl(upsilon) distribution from the H + CH2ClI reaction was P-1-P-5 = 25 : 29 : 26 : 13 : 7. The rate constant for the HCl(upsilon) formation channel is estimated to be 4 times smaller than that for the H + Cl-2 reaction. The highest HCl(upsilon) level observed from the H + CH2ClI reaction implies that the C-Cl bond energy is 50.2 kJ mol(-1) lower than that of the Cl-CH3 bond, which is in modest agreement with recent theoretical estimates. The H + CH2FI reaction gave a HF(upsilon) distribution of P-1-P-3 = 77 : 15 : 8. The C-F bond energy in CH2FI is estimated to be less than or equal to 460.2 kJ mol(-1), based on the highest HF(upsilon) level observed, the upper bound being the same as that of F-CH3. When N atoms are added to the flow reactor, the HCl(upsilon) emission intensities from H + CH2ClI increased by up to 2-fold, which is attributed to the N + CH2Cl --> HCl + HCN reaction. Concomitant weak emission from HCN and HNC could also be observed; however, the main product channel is thought to be NCH2 + Cl. Strong visible CN(A-X) emission was also observed when H/N/CH2XI were present in the reactor. If the CH2X radicals were produced by the F + CH3X reaction in the presence of N atoms, similar results were obtained. The N + CH2N reaction is proposed as the first step that leads to CN(A) formation with NCN as an intermediate.

The total quenching rate constants of argon atoms in the 4p′[1/2]0, 4p[1/2]0, 4p[3/2]2, and 4p[5/2]2 states (2p1, 2p5, 2p6, and 2p8, respectively, in the Paschen numbering system) by rare gases, H2, D2, N2, CO, NO, O2, F2, Cl2, CO2, NO2, CH4, C2H2, C2H4, C2H6, CF4, CHF3, and SF6 have been determined at room temperature. These four excited states of argon (energy 13.09–13.48 eV) were selectively prepared by two-photon excitation from the ground state using VUV (184–190 nm range) laser pulses. The total quenching rates were deduced from the pressure dependence of the decay times of the excited-state atoms, measured by observing their fluorescence emission intensities in the presence of added reagents. The quenching constants increase from values of ≅0.01×10−10 cm3 atom−1 s−1 for Ne, to ≅0.1×10−10 cm3 atom−1 s−1 for He and Ar, and to very large values, (5–15)×10−10 cm3 atom−1 s−1, for most polyatomic molecules, F2, Cl2, and O2. The quenching mechanisms of the Ar(4p,4p′) atoms are briefly discussed and compar...

The total quenching rate constants for NCl(a1Δ) molecules were measured at room temperature for 40 molecular reagent species and for F, H, N and O atoms. The Cl + N3 reaction was used to provide the NCl(a1Δ) molecules in a room temperature flow reactor; the azide radicals were obtained from the F + HN3 reaction. In most cases, rate constants were obtained for both NF(a1Δ) and NCl(a1Δ) so that the data could be confirmed by comparison with earlier studies of NF(a1Δ). The quenching rate constants for NCl(a1Δ) range from ∼1 × 10-15 to 4 × 10-11 cm3 molecule-1 s-1. Except for O2, F, H, CH3Cl, HCl, HBr, and HI, rate constants for the NCl(a1Δ) molecule are generally smaller than or comparable to those for NF(a1Δ), and NCl(a1Δ) is not especially reactive at 300 K. Small rate constants were obtained for common gases, such as H2, CO2, and CH4; the rate constants are larger for unsaturated hydrocarbons and they increase with the number of carbon atoms in the molecule. A correlation was found between the basicity of...

Product branching fractions and kinetic isotope effects for the reactions of OH and OD radicals with CH3SH and CH3SD were determined from the infrared chemiluminescent spectra of the H2O, HDO and D2O products. The spectra were acquired by a Fourier transform spectrometer that viewed the emission from a discharge flow reactor. Abstraction of H atom from the methyl group accounts for (11 ± 4)% of the total reaction rate from the OD + CH3SD (or OD + CH3SH) reaction and for (24 ± 8)% from the OH + CH3SH (or OH + CH3SD) reaction. The major product channel involves interaction with the sulfur end of the molecule. The difference in branching fractions for OD and OH reactions is due to the large inverse secondary kinetic-isotope effect for the addition−elimination channel, which occurs by transfer of the hydrogen atom from the sulfur atom to the oxygen atom in the adduct. Transition state models for the elimination channel are discussed to show that the experimentally determined secondary kinetic isotope effect f...

The D' (2 (3)P2) left and right arrow A'(2 (3)Pi) transition in ClF is studied in emission, in fluorescence, and through OODR excitation experiments. The emission spectrum is excited in a flowing Tesla discharge of ClF in half an atmosphere of Ar and is photographed at high resolution. The excitation and fluorescence spectra are obtained by pumping through perturbed levels of the B (0(+) 3Pi) valence state. The emission data yield a detailed characterization of v' levels 0 and 1 and v" levels 3-8. The excitation spectra extend the v' range to v' = 16 and put both states on an absolute energy scale. The low-resolution fluorescence spectra span v" = 1-17, the upper limit of which lies within approximately 20 cm-1 of dissociation. Key spectroscopic parameters (cm-1) from the study, valid for 35Cl19F, are: T'e = 55 253, omega'e = 365.22, omegaex'e = 2.005, T"e = 18 257, omega"e = 363.53, omegaex"e = 8.30, B'e = 0.21790, a'e = 0.00175, B"e = 0.33412, a"e = 0.00631, R'e = 2.5069 A, R"e = 2.024 A. A near-dissociation fit of the D' --> A' emission and fluorescence data locates the first dissociation limit at 21 495(5) cm-1, which is lower than the accepted value, but by just one standard error. Copyright 1999 Academic Press.

The optical–optical double resonance method has been used to excite selected vibrational levels of the E(0+) and f(0+) ion-pair states of ClF. Subsequent bound–free transitions terminating on three repulsive valence states, Y(3Σ0+−), B1(3Π0+), and C(1Σ0++), have been analyzed. The bound–free continua have been modeled to obtain analytical expressions for the three 0+ repulsive potentials and to improve the description for the E(0+) and f(0+) state potentials. One of the repulsive states is Y(3Σ0+−), which is responsible for the predissociation of B(3Π0+). The transition dipole functions also have been assigned for the bound–free transitions. The molecular orbital configurations of the lower states are discussed. Finally, the influence of homogeneous interaction between the E(0+) and f(0+) states on the bound–free spectra from certain vibrational levels is discussed.

The room-temperature reactions of OH(OD) radicals with cyclo-C6H12, n-C4H10, and neo-C5H12 have been investigated by observing the infrared chemiluminescence from the H2O(HOD) molecules generated in a fast-flow reactor. These hydrocarbon molecules are representative for abstraction from secondary and primary C–H bonds. The total vibrational energy released to H2O(HOD) was in the range of 〈fv〉=0.55–0.65. The majority (80%–85%) of the vibrational energy is in the stretching modes and the main energy release is to the local mode associated with the new OH bond. The dynamics associated with the energy disposal to H2O(HOD) resemble the H+L−H dynamics for the analogous reactions of F atoms. The data from H2O and HOD are complementary because of the different collisional coupling between the energy levels of the ν1, ν2, and ν3 modes; however, no specific isotope effect was found for the energy disposal to H2O versus HOD for reactions with the hydrocarbon molecules. In contrast, a very unusual isotope effect was ...

Four of the six ion-pair states of ClF that correlate to Cl+(3PJ) and F−(1S0), the E(0+,3P2), f(0+,3P0), β(1,3P2), and G(1,3P1) states, have been identified by sequential, two-photon excitation via the ClF(B 3Π0+) valence state. The Ω=1 states, β and G, were studied by selection of rotational levels of the B 3Π0+ state that are perturbed by ClF(A 3Π1). Spectroscopic data from laser excitation and fluorescence spectra permit the assignment of vibrational energies and rotational constants to 30 levels. These four ion-pair states exhibit extensive homogeneous and heterogeneous interactions, and neither the vibrational energy nor the rotational constants are regular with increasing vibrational quantum number. The vibrational and rotational constants of the A 3Π1 state were identified from the low resolution ClF[β(1)−A 3Π1] emission spectra, and the dissociation limits of the A 3Π1 and B 3Π0+ states are compared. The dissociation energy of ClF(X) is confirmed to be 21 110 cm−1. Some qualitative information als...

The infrared chemiluminescence of vibrationally excited H2O and HDO from the highly exothermic reactions of OH and OD radicals with HI and GeH4 was observed in the 2200–5500 cm−1 range. The experiments utilized a fast-flow reactor with 0.3–1 Torr of Ar carrier gas at 300 K; the OH(OD) radicals were produced via the H(D)+NO2 reaction and the H or D atoms were generated by a discharge in a H2(D2)/Ar mixture. The H2O and HOD vibrational distributions were determined by computer simulation of the emission spectra in the 2200–3900 cm−1 range. The total vibrational energy released to H2O and HOD molecules is, respectively, 〈fv〉=0.36 and 0.41 from HI and 〈fv〉=0.46 and 0.51 from GeH4. These values are significantly smaller than for the reactions of OH and OD with HBr, 〈fv〉=0.61 and 0.65. The populations of the O–H stretching vibration of HOD and the collisionally coupled ν1 and ν3 stretching modes of H2O decrease with increasing vibrational energy. In contrast, the vibrational distribution from the HBr reaction i...

The quasiclassical trajectory (QCT) method was used to study the dynamics of the OH(X 2Π) and OD(X 2Π)+HBr chemical reactions on an empirical potential energy surface (PES). The main emphasis in the calculation was the vibrational energy distributions of H2O (and HDO) and the magnitude and temperature dependence of the rate constant. However, this PES also serves as a generic model for the dynamics of direct H atom abstraction by OH radicals. Since this PES has no formal potential energy barrier, variational transition‐state theory was used to obtain rate constants for comparison with the QCT calculations and experimental results. The parameters of the potential energy surface were adjusted to obtain better agreement with the experimentally measured fraction of H2O vibrational energy, 〈fV(H2O)〉=0.6, without significantly changing the entrance channel. No isotope effect for the partition of energy to H2O vs HOD was found. Analysis of the trajectories indicates that the reactant OH(OD) bond is a spectator, ...

Vibrationally excited H2O, HOD, and D2O molecules formed by unimolecular elimination from deutero‐substituted ethanol molecules C2H5OH*, C2H5OD*, CH2DCH2OH*, and CH2DCH2OD* with an excitation energy of about 100 kcal mol−1 were observed by infrared chemiluminescence in the 2400–3900 cm−1 range. The activated ethanol molecules were produced via the successive reactions H+CH2ICH2OH→HI+CH2CH2OH and H+CH2CH2OH→CH3CH2OH* in a fast flow reactor that was observed with a Fourier transform spectrometer. The vibrational distributions of the H2O, HOD, and D2O molecules were determined by computer simulation of the experimental spectra; the distributions decline with increasing vibrational energy giving 〈fv〉=0.15 and 〈fv〉=0.14 for H2O and HOD from the decomposition of C2H5OH* and C2H5OD*, respectively. The vibrational energy in the bending mode of H2O is comparable to the energy in the stretching modes. Comparison with the statistical vibrational distributions shows a substantial overpopulation of the bending levels ...

A new, simple method for the generation of Xe(6s[3/2]1) and Xe(6s′[1/2]1) atoms is described. The method involves resonant two‐photon excitation of Xe(6p[1/2]0 and 6p′[3/2]2) states followed by amplified spontaneous emission (ASE) to the Xe(6s[3/2]1 and 6s′[1/2]1) states. The vacuum ultraviolet transitions, Xe(6s[3/2]1→5p6(1S0)) at 147 nm and Xe(6s′[1/2]1→5p6(1S0)) at 129.6 nm, were used to monitor the time dependence of the resonance state atom concentrations. The quenching rate constants of these resonance atoms with ten molecules were measured at 300 K. The quenching cross‐sections of the Xe(6s and 6s′) resonance atoms are compared to the cross‐sections of the metastable Xe(6s[3/2]2) atoms and Xe(6p[3/2]2) atoms. The correlation between quenching cross‐sections and photoabsorption cross‐section of the molecules predicted by the resonance dipole–dipole energy transfer model is discussed. The applicability of the two‐photon driven ASE method for the generation of other resonance state atoms is considered.

One‐photon laser excitation of Xe(6s[3/2]2) and Kr(5s[3/2]2) atoms that were generated in a discharge‐flow reactor was used to study the collisional relaxation of the Kr(5p′[3/2]1, [3/2]2, and [1/2]1), the Xe(7p[3/2]2, [3/2]1, [5/2]2, and [5/2]3), and the Xe(6p′[3/2]1, [3/2]2, and [1/2]1) states in He and Ar. Both cw and pulsed laser excitation techniques were utilized to obtain the total deactivation rate constants and product formation rate constants at 300 K. Collisions with He mainly produce Xe* and Kr* product states with small energy defects, but the rate constants can be as large as 20×10−10 cm3 atom−1 s−1, which correspond to thermally averaged cross sections of 150 A2. Because of the rapid collisional coupling of populations in nearly isoenergetic levels, multicomponent exponential decay of the initially produced state is frequently observed. The deactivation rate constants for Ar are smaller than for He, but the product distributions tend to be more diverse than for He, and arguments based only ...

Two-photon excitation of Xe atoms in a static gas cell was used to prepare the Xe(7p[3/2]2, 7p[5/2]2, and 6p‘[3/2]2) states in mixtures of Xe with rare gases, H2(D2), CO, Cl2, HCl, CCl2F2, CCl4, SF6, NF3, CF4, CH4, CH3F, and N2O at 300 K. The total quenching rate constants were measured, and product fluorescence spectra were used to assign reaction pathways. The total quenching constants for these Xe* Rydberg states by molecular reagents are very large, corresponding to cross sections of 200−1500 A2. Qualitative models are introduced to discuss the quenching processes, which are mainly reactive quenching and excitation transfer. For reagents with very large electron affinities, ion-pair formation may be important. A correlation of the quenching cross section with the dipole moment of the reagent was observed. These three Xe* states have nearly the same energy (∼11 eV), but the Xe(7p) states nominally have the Xe+(2P3/2) ion-core, whereas the Xe(6p‘) states have the Xe+(2P1/2) core. The XeCl(D) and XeF(D) ...

Emission spectra have been used to characterize the excitation‐transfer reactions from Kr(5s’,3P0) and Kr(5s,3P2) metastable atoms to 12CO and 13CO at 300 K. The most important products from the Kr(3P0) reactions are 12CO and 13CO(b 3Σ+,v’=0 and 1) and 12CO(a’ 3Σ+,v’=34 and 35) and 13CO(a’ 3Σ+,v’=35 and 36). The rotational distributions of the CO(a’ 3Σ+) and CO(b 3Σ+,v’=1) levels are cold, but the CO(b 3Σ+,v’=0) distribution is rotationally excited. The populations in the 12CO(a’,v’=34 and 35) levels are transferred to CO(b,v’=0) by collisions with He and the rate constants are 0.4–1.0×10−10 cm3 s−1. Emission spectra from the Kr(3P2) reaction identified 12CO(a’,v’=23–26) and 13CO(a’,v’=24–27) and CO(d 3Δ,v’=20 and 21) for both 12CO and 13CO as important products; the CO(d 3Δ,v’=20 and 21) states previously were identified by Tsuji and co‐workers. The vacuum ultraviolet spectra from the Kr(3P2) reaction with 12CO and 13CO showed that CO(A 1Π) is a primary product and that it also is formed from CO(d 3Δ) an...

NCl(b 1Σ+), PCl(b 1Σ+) and PBr(b 1Σ+) molecules have been generated in a flow reactor by passing dilute flows of NFCl2, PCl3 and PBr3 in He through a dc discharge, and the total rate constants for quenching of NCl(b 1Σ+) and PCl(b 1Σ+) by several diatomic and small polyatomic molecules have been measured at 300 K. Except for O2, NO2 and molecular halogens, the rate constants are in the 10–13– 10–14 cm3 molecule–1 s–1 range. The dependence of the rate constants on the properties of the reagent suggests that the dominant quenching mechanism for PCl(b) and NCl(b) is electronic-to-vibrational (E–V) energy transfer with the a 1Δ state as the product, just as for the reactions of NF(b 1Σ+) and PF(b 1Σ+). The rate constants for quenching of NCl(b 1Σ+) and PCl(b 1Σ+) by O2 are 1.0 × 10–12 and ⩽ 10–15 cm3 molecule–1 s–1, respectively, and are in accord with an electronic-to-electronic (E–E) energy transfer with the final states being O2(b 1Σ+)+ NCl(X 3Σ–) or PCl(X 3Σ–). Formation of PBr(b 1Σ+) from the PCl(b 1Σ+)+ Br2 reaction was observed, and quenching of PCl(b 1Σ+) by F2, Cl2 and Br2 proceeds by a chemical mechanism with rate constants of 2.3 × 10–12, 0.79 × 10–12 and 16 × 10–12 cm3 molecule–1 s–1, respectively. The radiative lifetimes for NCl(b 1Σ+), PCl(b 1Σ+) and PBr(b 1Σ+) are 2.0 ± 0.4, 4.9 ± 0.8 and 0.8 ± 0.2 ms, respectively, as determined from the decay of the respective b 1Σ+→ X 1Σ– emission intensities along the flow reactor.

A dc discharge in a dilute flow of PF[sub 3] in He was used as a source of PF(b[sup 1][Sigma][sup +]) molecules in a flow reactor to measure the radiative lifetime and quenching rate constants for 38 reagent molecules at 300 K. The concentration of PF(b[sup 1][Sigma][sup +]) in the reactor was about 2 x 10[sup 8] molecule cm[sup [minus]3]. The decay rate of the PF(b[sup 1][Sigma][sup +]) concentration was monitored from the intensity of the PF(b-X) transition, and the radiative lifetime was measured to be 9.7 [+-] 1.2 ms. The quenching rate constants are small, except for O[sub 2] and Cl[sub 2], and in the range 10[sup [minus]12]-10[sup [minus]14] cm[sup 3] molecule[sup [minus]1] s[sup [minus]1]. The dependence of the other rate constants on properties of the reagent is consistent with an electronic-to-vibrational (E-V) quenching mechanism, and this was confirmed by studying several isotopic pairs of molecules, such as H[sub 2]/D[sub 2], H[sub 2]O/D[sub 2]O, and CH[sub 3]CN/CD[sub 3]CN. The quenching rate constants are compared to predictions of a model for E-V energy transfer. The reaction with O[sub 2] proceeds by electronic energy transfer to give O[sub 2](b[sup 1][Sigma][sup +]). 39 refs., 9 figs., 3 tabs.

The primary and secondary reactions in the H+NFCl 2 system have been studied by infrared chemiluminescence in a fast flow reactor at 300 K. The primary reaction is exclusively Cl atom abstraction to give HCl(υ=0-4) +NFCl with a total rate constant of (1.9±0.4)×10 -11 cm 3 molecule -1 s -1 and an inverted vibrational distribution of P 0 -P 4 =9:20:32:27:12. The rate constant for HF formation from H+NFCl was estimated as (0.9±0.4)×10 -11 cm 3 molecule -1 s -1 , and the HF vibrational distribution, P 0 -P 3 =42:34:18:6, is characteristics of unimolecular HF elimination reactions. These data for the HF+NCl(a) product channel from the H+ NFCl reaction are compared to earlier studies, which provided information about the HCl+NF(a) product channel

The reaction of HNCO with F atoms has been investigated as a source for NCO(X~ 2 Π) and NF(X 3 Σ - ) in a room temperature flow reactor by monitoring the NCO concentration using laser-induced fluorescence. The first step gives HF+NCO with a rate constant of (3.4±0.7)×10 -11 cm 3 s -1 ; the second step gives NF(X 3 Σ - )+CO with a rate constant of(9.2±1.8)×10 -12 cm 3 s -1 . The overall reaction seems to be stoichiometric for NF(X 3 Σ - ) formation and for properly adjusted reaction conditions the F/HNCO system can be used to provide a selectEd [NCO] or [NF(X)]

A flow reactor has been used to measure the room temperature rate constants for quenching of NF(a 1 Δ) by molecules containing the NF bond and by SiF 4 , HNCO, and NCO. The quenching rate constants decrease in the series NF(X), N 2 F 4 , NF 2 , and NF 3 . The rate constant for NF(X) is (3±1)×10 -13 cm 3 molecule -1 s -1 , which is smaller than the bimolecular self-destruction rate constant of NF(a). The quenching rate was so slow for NF 3 that only an upper limit to the rate constant, 1.6×10 -17 cm 3 molecule -1 s -1 , could be measured

The excitation spectra and the transition rates (absorption cross sections) for the two-photon, laser-assisted reactions between Xe/Cl 2 , Xe/ClF, Xe/F 2 , and Kr/F 2 pairs have been measured for modest pressures and 300 K. For these conditions the photoassisted reactions occur with approximately equal concentrations of bound van der Waals molecules and free pairs. The shapes of the Xe/Cl 2 , Xe/ClF, and Kr/F 2 excitation spectra are similar, and the maximum in the spectrum is associated with the transition originating from pairs located near the minimum of the van der Waals potential and terminating on the reactive ionic potential surface

Optical-optical double resonance experiments have been used to identify and characterize five ion-pair states and several of the bound and repulsive valence states of ClF. This report provides a description of these experiments for and , and a summary of the current knowledge of the valence and ion-pair states. The important role of perturbations among the rovibronic levels of the bound valence states and their utilization in the double resonance technique is discussed. The ion-pair states of the same symmetry, = (E and f) and 1( and G) interact very strongly and the spectroscopy of these states is anomalous and, hence, interesting. Comparison is made to some recent ab initio calculations for ClF. One possible explanation of the irregular vibrational energy levels and rotational constants of the ion-pair states of and 1 symmetry is a crossing of the diabatic potentials of these states. Some currently unresolved questions about ClF spectroscopy are posed for future work. Where appropriate, analogy is made between the electronic states of ClF and the corresponding valence and ion-pair states of

The recombination reactions of CH(2)Br and CH(2)Cl radicals have been used to generate vibrationally excited CH(2)BrCH(2)Br and CH(2)BrCH(2)Cl molecules with 91 kcal mol(-1) of energy in a room-temperature bath gas. The experimental unimolecular rate constants for elimination of HBr and HCl were compared to calculated statistical rate constants to assign threshold energies of 58 kcal mol(-1) for HBr elimination from C(2)H(4)Br(2) and 58 and 60 kcal mol(-1), respectively, for HBr and HCl elimination from C(2)H(4)BrCl. The Br-Cl interchange reaction was demonstrated and characterized by studying the CH(2)BrCD(2)Cl system generated by the recombination of CH(2)Br and CD(2)Cl radicals. The interchange reaction was identified from the elimination of HBr and DCl from CH(2)ClCD(2)Br. The interchange reaction rate is much faster than the rates of either DBr or HCl elimination from CH(2)BrCD(2)Cl, and a threshold energy of congruent with43 kcal mol(-1) was assigned to the interchange reaction. The statistical rate constants were calculated from models of the transition states that were obtained from density functional theory using the B3PW91 method with the 6-31G(d',p') basis set. The model for HBr elimination was tested versus published thermal and chemical activation data for C(2)H(5)Br. A comparison of Br-Cl interchange with the Cl-F and Br-F interchange reactions in 1,2-haloalkanes is presented.

The two‐photon, laser‐assisted reaction between Xe and Cl2 has been used to generate XeCl(B,C) molecules for subsequent kinetic studies at 300 K. Two‐body quenching rate constants for the collisionally coupled XeCl(B,C) molecules were determined from the pressure dependence of their decay rates on added molecular gases. The quenching constants for XeCl(B,C) by most molecules are large, even though acceptor electronic states are not available for quenching by an excitation‐transfer mechanism. At higher pressure (>100 Torr) of Xe, Xe2Cl(4 2Γ) molecules were formed via three‐body quenching of XeCl(B,C), and the quenching rate constants for Xe2Cl(4 2Γ) by added gases were determined and compared with those for XeCl(B,C). In all cases, the quenching rate constants for XeCl(B,C) are larger. The two‐body quenching mechanisms for XeCl(B,C) and Xe2Cl(4 2Γ), which are ionic molecules, are discussed. Three‐body quenching of XeCl(B,C) with formation of Xe2Cl(4 2Γ) was observed for Xe+N2, Xe+CF4, and Xe+SF6.

The reactions of NF(a 1 Δ) with the molecular halogens (X 2 ) and inter-halogens (XY) have been studied in a flow reactorat 300 K. Observation of the rate of removal of NF(a) by X 2 gives the total quenching rate constants, which increase in the order listed on the title, to a value of 15×10 -11 cm 3 s -1 for I 2

The unimolecular reactions of 1-propanol, 3,3,3-propan-1-ol-d3, 3,3,3-trifluoropropan-1-ol, and 3-chloropropan-1-ol have been studied by the chemical activation technique. The recombination of CH3, CD3, CF3, and CH2Cl radicals with CH2CH2OH radicals at room temperature was used to generate vibrationally excited CH3CH2CH2OH, CD3CH2CH2OH, CF3CH2CH2OH, and CH2ClCH2CH2OH molecules. The principal unimolecular reaction for propanol and propanol-d3 with 90 kcal mol(-1) of vibrational energy is 1,2-H2O elimination with rate constants of 3.4 x 10(5) and 1.4 x 10(5) s(-1), respectively. For CH2ClCH2CH2OH also with 90 kcal mol(-1) of energy, 2,3-HCl elimination with a rate constant of 3.0 x 10(7) s(-1) is more important than 1,2-H2O elimination; the branching fractions are 0.95 and 0.05. For CF3CH2CH2OH with an energy of 102 kcal mol(-1), 1,2-H2O elimination has a rate constant of 7.9 x 10(5) and 2,3-HF elimination has a rate constant of 2.6 x 10(5) s(-1). Density functional theory was used to obtain models for the molecules and their transition states. The frequencies and moments of inertia from these models were used to calculate RRKM rate constants, which were used to assign threshold energies by comparing calculated and experimental rate constants. This comparison gives the threshold energy for H2O elimination from 1-propanol as 64 kcal mol(-1). The threshold energies for 1,2-H2O and 2,3-HCl elimination from CH2ClCH2CH2OH were 59 and 54 kcal mol(-1), respectively. The threshold energies for H2O and HF elimination from CF3CH2CH2OH are 62 and 70 kcal mol(-1), respectively. The structures of the transition states for elimination of HF, HCl, and H2O are compared.