Determination of Equilibrium Constants for the Reaction between Acetone and HO_2 Using Infrared Kinetic Spectroscopy

The reaction between the hydroperoxy radical, HO_2, and acetone may play an important role in acetone removal and the budget of HO_x radicals in the upper troposphere. We measured the equilibrium constants of this reaction over the temperature range of 215–272 K at an overall pressure of 100 Torr using a flow tube apparatus and laser flash photolysis to produce HO_2. The HO_2 concentration was monitored as a function of time by near-IR diode laser wavelength modulation spectroscopy. The resulting [HO_2] decay curves in the presence of acetone are characterized by an immediate decrease in initial [HO_2] followed by subsequent decay. These curves are interpreted as a rapid (<100 μs) equilibrium reaction between acetone and the HO_2 radical that occurs on time scales faster than the time resolution of the apparatus, followed by subsequent reactions. This separation of time scales between the initial equilibrium and ensuing reactions enabled the determination of the equilibrium constant with values ranging from 4.0 × 10^(–16) to 7.7 × 10^(–1)8 cm^3 molecule^(–1) for T = 215–272 K. Thermodynamic parameters for the reaction determined from a second-law fit of our van’t Hoff plot were Δ_(r)H°_(245) = −35.4 ± 2.0 kJ mol^(–1) and Δ_(r)S°_(245) = −88.2 ± 8.5 J mol^(–1) K^(–1). Recent ab initio calculations predict that the reaction proceeds through a prereactive hydrogen-bonded molecular complex (HO_2–acetone) with subsequent isomerization to a hydroxy–peroxy radical, 2-hydroxyisopropylperoxy (2-HIPP). The calculations differ greatly in the energetics of the complex and the peroxy radical, as well as the transition state for isomerization, leading to significant differences in their predictions of the extent of this reaction at tropospheric temperatures. The current results are consistent with equilibrium formation of the hydrogen-bonded molecular complex on a short time scale (100 μs). Formation of the hydrogen-bonded complex will have a negligible impact on the atmosphere. However, the complex could subsequently isomerize to form the 2-HIPP radical on longer time scales. Further experimental studies are needed to assess the ultimate impact of the reaction of HO_2 and acetone on the atmosphere

Acetonyl Peroxy and Hydro Peroxy Self- and Cross-Reactions: Kinetics, Mechanism, and Chaperone Enhancement from the Perspective of the Hydroxyl Radical Product

Pulsed laser photolysis coupled with infrared (IR) wavelength modulation spectroscopy and ultraviolet (UV) absorption spectroscopy was used to study the kinetics and branching fractions for the acetonyl peroxy (CH₃C(O)CH₂O₂) self-reaction and its reaction with hydro peroxy (HO₂) at a temperature of 298 K and pressure of 100 Torr. Near-IR and mid-IR lasers simultaneously monitored HO₂ and hydroxyl, OH, respectively, while UV absorption measurements monitored the CH₃C(O)CH₂O₂ concentrations. The overall rate constant for the reaction between CH₃C(O)CH₂O₂ and HO₂ was found to be (5.5 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, and the branching fraction for OH yield from this reaction was directly measured as 0.30 ± 0.04. The CH₃C(O)CH₂O₂ self-reaction rate constant was measured to be (4.8 ± 0.8) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, and the branching fraction for alkoxy formation was inferred from secondary chemistry as 0.33 ± 0.13. An increase in the rate of the HO₂ self-reaction was also observed as a function of acetone (CH₃C(O)CH₃) concentration which is interpreted as a chaperone effect, resulting from hydrogen-bond complexation between HO₂ and CH₃C(O)CH₃. The chaperone enhancement coefficient for CH₃C(O)CH₃ was determined to be k_A″ = (4.0 ± 0.2) × 10⁻²⁹ cm⁶ molecule⁻² s⁻¹, and the equilibrium constant for HO₂·CH₃C(O)CH₃ complex formation was found to be K_c(R14) = (2.0 ± 0.89) × 10⁻¹⁸ cm³ molecule⁻¹; from these values, the rate constant for the HO₂ + HO₂·CH₃C(O)CH₃ reaction was estimated to be (2 ± 1) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. Results from UV absorption cross-section measurements of CH₃C(O)CH₂O₂ and prompt OH radical yields arising from possible oxidation of the CH₃C(O)CH₃-derived alkyl radical are also discussed. Using theoretical methods, no likely pathways for the observed prompt OH radical formation have been found and the prompt OH radical yields thus remain unexplained

Acetonyl Peroxy and Hydro Peroxy Self- and Cross- Reactions: Kinetics, Mechanism, and Chaperone Enhancement from the Perspective of the Hydroxyl Radical Product

Pulsed laser photolysis coupled with infrared (IR) wavelength modulation spectroscopy and ultraviolet (UV) absorption spectroscopy was used to study the kinetics and branching fractions for the acetonyl peroxy (CH₃C(O)CH₂O₂) self-reaction and its reaction with hydro peroxy (HO₂) at a temperature of 298 K and pressure of 100 Torr. Near-IR and mid-IR lasers simultaneously monitored HO₂ and hydroxyl, OH, respectively, while UV absorption measurements monitored the CH₃C(O)CH₂O₂ concentrations. The overall rate constant for the reaction between CH₃C(O)CH₂O₂ and HO₂ was found to be (5.5 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and the branching fraction for OH yield from this reaction was directly measured as 0.30 ± 0.04. The CH₃C(O)CH₂O₂ self-reaction rate constant was measured to be (4.8 ± 0.8) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and the branching fraction for alkoxy formation was inferred from secondary chemistry as 0.33 ± 0.13. An increase in the rate of the HO₂ self-reaction was also observed as a function of acetone (CH₃C(O)CH₃) concentration which is interpreted as a chaperone effect resulting from hydrogen-bond complexation between HO₂ and CH₃C(O)CH₃. The chaperone enhancement coefficient for CH₃C(O)CH₃ was determined to be k”A = (4.0 ± 0.2) x 10⁻²⁹ cm⁶ molecule⁻² s⁻¹ and the equilibrium constant for HO₂•CH₃C(O)CH₃ complex formation was found to be K_c(R15) = (2.0 ± 0.89) × 10⁻¹⁸ cm³ molecule⁻¹; from these values the rate constant for the HO₂ + HO₂•CH₃C(O)CH₃ reaction was estimated to be (2 ± 1) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. Results from UV absorption cross-section measurements of CH₃C(O)CH₂O₂ and prompt OH radical yields arising from possible oxidation of the CH₃C(O)CH₃-derived alkyl radical are also discussed. Using theoretical methods, no likely pathways for the observed prompt OH radical formation have been found and thus remains unexplained

THE LASER INDUCED FLUORESCENCE SPECTRUM OF TRAPPED BrCN CATIONS

Laser induced fluorescence spectra of mass selected BrCN{sup +} confined in a three dimensional quadrupole trap are presented. Using this technique, the {tilde B}{sup 2}{Pi}-{tilde X}{sup 2}{Pi} system of BrCN{sup +} is investigated. Approximate values for the C-Br stretching frequencies ({nu}'{sub 1} = 441 {+-} 18 cm{sup -1}, {nu}''{sub 1} = 509 {+-} 16 cm{sup -1}) and the difference of the spin-orbit splitting constants (A''-A' = 224 {+-} 43 cm{sup -1}) are determined. However, the spectrum is complicated by irregular vibrational and spin-orbit splitting spacing yielding a large uncertainty in these values. The existence of sequence bands and a Fermi resonance interaction provide possible explanations for these irregularities

Rotational analysis of bands in the 460 nm system of nickel dichloride produced in a free-jet expansion: Determination of the structure and electronic ground state of nickel dichloride

By use of a free-jet expansion which incorporates a heated nozzle, we have recorded the laser excitation spectrum of the 460 nm band system of NiCl2 at rotational resolution. The rotational temperature in these recordings was about 12 K. Several bands have been recorded and analyzed for three isotopomers, 58Ni35Cl2, 60Ni35Cl2, and 58Ni35Cl37Cl in natural abundance. Spin components with O values of 0 and 1 have been identified in both the upper and lower states of the transition. Accurate values for all three vibrational intervals ?1, ?2, and ?3 have been determined for nickel dichloride in the upper state and for the bending wave number ?2 in the lower state. The results show that the molecule is linear in both states involved in the transition and that the lower (ground) state is 3S-g in character. Evidence is presented from the nickel isotope shifts to show that the transition is vibronically induced through the bending vibration and that the upper state is vibronically 3?u in character; it probably derives from an electronic 3?g state. The zero-point averaged bond lengths are determined for both states as r0'=0.209?435(13) nm and r0?=0.205?317(14) nm. The fine structure parameters for the math?3S-g state are interpreted in terms of low lying 1S+g and 3?g states, which are shown to lie a few thousand reciprocal centimeters above the ground state