Rabi Chhantyal-Pun

A kinetic study of the CH2OO Criegee intermediate self-reaction, reaction with SO2 and unimolecular reaction using cavity ring-down spectroscopy

Criegee intermediates are important species formed during the ozonolysis of alkenes. Reaction of stabilized Criegee intermediates with various species like SO2 and NO2 may contribute significantly to tropospheric chemistry. In the laboratory, self-reaction can be an important loss pathway for Criegee intermediates and thus needs to be characterized to obtain accurate bimolecular reaction rate coefficients. Cavity ring-down spectroscopy was used to perform kinetic measurements for various reactions of CH2OO at 293 K and under low pressure (7 to 30 Torr) conditions. For the reaction CH2OO + CH2OO (8), a rate coefficient k8 = (7.35 ± 0.63) × 10(-11) cm(3) molecule(-1) s(-1) was derived from the measured CH2OO decay rates, using an absorption cross section value reported previously. A rate coefficient of k4 = (3.80 ± 0.04) × 10(-11) cm(3) molecule(-1) s(-1) was obtained for the CH2OO + SO2 (4) reaction. An upper limit for the unimolecular CH2OO loss rate coefficient of 11.6 ± 8.0 s(-1) was deduced from studies of reaction (4). SO2 catalysed CH2OO isomerization or intersystem crossing is proposed to occur with a rate coefficient of (3.53 ± 0.32) × 10(-11) cm(3) molecule(-1) s(-1).

Direct Measurements of Unimolecular and Bimolecular Reaction Kinetics of the Criegee Intermediate (CH3)2COO

The Criegee intermediate acetone oxide, (CH3)2COO, is formed by laser photolysis of 2,2-diiodopropane in the presence of O2 and characterized by synchrotron photoionization mass spectrometry and by cavity ring-down ultraviolet absorption spectroscopy. The rate coefficient of the reaction of the Criegee intermediate with SO2 was measured using photoionization mass spectrometry and pseudo-first-order methods to be (7.3 ± 0.5) × 10-11 cm3 s-1 at 298 K and 4 Torr and (1.5 ± 0.5) × 10-10 cm3 s-1 at 298 K and 10 Torr (He buffer). These values are similar to directly measured rate coefficients of anti-CH3CHOO with SO2, and in good agreement with recent UV absorption measurements. The measurement of this reaction at 293 K and slightly higher pressures (between 10 and 100 Torr) in N2 from cavity ring-down decay of the ultraviolet absorption of (CH3)2COO yielded even larger rate coefficients, in the range (1.84 ± 0.12) × 10-10 to (2.29 ± 0.08) × 10-10 cm3 s-1. Photoionization mass spectrometry measurements with deuterated acetone oxide at 4 Torr show an inverse deuterium kinetic isotope effect, kH/kD = (0.53 ± 0.06), for reactions with SO2, which may be consistent with recent suggestions that the formation of an association complex affects the rate coefficient. The reaction of (CD3)2COO with NO2 has a rate coefficient at 298 K and 4 Torr of (2.1 ± 0.5) × 10-12 cm3 s-1 (measured with photoionization mass spectrometry), again similar to rate for the reaction of anti-CH3CHOO with NO2. Cavity ring-down measurements of the acetone oxide removal without added reagents display a combination of first- and second-order decay kinetics, which can be deconvolved to derive values for both the self-reaction of (CH3)2COO and its unimolecular thermal decay. The inferred unimolecular decay rate coefficient at 293 K, (305 ± 70) s-1, is similar to determinations from ozonolysis. The present measurements confirm the large rate coefficient for reaction of (CH3)2COO with SO2 and the small rate coefficient for its reaction with water. Product measurements of the reactions of (CH3)2COO with NO2 and with SO2 suggest that these reactions may facilitate isomerization to 2-hydroperoxypropene, possibly by subsequent reactions of association products.

Measurements of the Absolute Absorption Cross Sections of the Ã←X̃ Transition in Organic Peroxy Radicals by Dual-Wavelength Cavity Ring-Down Spectroscopy

We demonstrate an experimental method for the accurate measurement of the absorption cross section of transient species, such as organic peroxy radicals in which the concentration of the radicals is determined via the absorption of a stable coproduct that is produced stochiometrically. The requirements for the experimental apparatus, a dual-wavelength cavity ring-down spectrometer, and the chemical protocol for transient species generation are discussed. The capability of this approach is demonstrated by measuring the peak absorption cross section of the ethyl peroxy radical, C₂2H₅O₂, whose value for the Ã←X electronic transition at 7596 cm⁻¹ (λ = 1316.5 nm) is found to be σ(p)(EP) = 5.29(20) × 10⁻²¹ cm². These present results are compared to those obtained from other methods of measurement of σ(p)(EP). Possible random and systematic errors are discussed.

Detection and Characterization of Products from Photodissociation of XCH2CH2ONO (X = F, Cl, Br, OH)

Alkyl nitrites have been used previously to produce alkoxy radicals, which are important intermediates in the oxidation of alkanes in atmospheric and combustion processes. Substituted alkoxy radicals, particulary hydroxyalkoxy radicals, are also important intermediates in the atmospheric oxidation of alkenes and combustion of alcohols. In order to produce substituted alkoxy radicals we have photolyzed at 351 nm substituted alkyl nitrites, XCH(2)CH(2)ONO (X = F, Cl, Br, OH). Using laser-induced fluorescence only in the case of X = F do we observe the spectrum of substituted alkoxy radical, XCH(2)CH(2)O; but we always observe the electronic transitions of formaldehyde, HCHO, and vinoxy radical, CH(2)CHO. HCHO can be formed by the dissociation of XCH(2)CH(2)O in its ground state as the barrier to C-C bond dissociation is less than the photon energy remaining after O-NO bond breakage. However, the barrier along the reaction path directly leading from XCH(2)CH(2)O to CH(2)CHO + HX is much higher than the available energy remaining after O-NO bond breakage. A roaming mechanism, involving a frustrated dissociation of X followed by HX extraction, might explain the apparent paradox. Under the conditions of our observations vinoxy retains considerable vibrational excitation but the observed rotational temperatures of both HCHO and CH(2)CHO are ≲7 K.

Observation of the A-X electronic transitions of cyclopentyl and cyclohexyl peroxy radicals via cavity ringdown spectroscopy.

The A-X electronic absorption spectra of cyclopentyl, cyclohexyl, and cyclohexyl-d(11) peroxy radicals have been recorded at room temperature by cavity ringdown spectroscopy. By comparing the experimental spectra with predictions from ab initio and density functional calculations, we have assigned the band origins and vibrational structure of each of these species. The spectrum of cyclopentyl peroxy is interpreted primarily in terms of two overlapping gauche conformers, while that of cyclohexyl peroxy appears to be a superposition of axially and equatorially substituted gauche conformers, both based on the chair conformation of cyclohexane. Expectations from calculated Boltzmann factors indicate comparable populations for cis-conformers; however, no bands uniquely assignable to cis-conformers of either peroxy can be identified. Plausible assignments for cis-conformers are considered, and possible explanations for their absence are offered, including specifically lower oscillator strengths than for the gauche conformers. Mode mixing appears to be responsible for the appearance of multiple vibrations with COO bending character for both peroxies, particularly for cyclohexyl peroxy.

Temperature‐Dependence of the Rates of Reaction of Trifluoroacetic Acid with Criegee Intermediates

Abstract The rate coefficients for gas‐phase reaction of trifluoroacetic acid (TFA) with two Criegee intermediates, formaldehyde oxide and acetone oxide, decrease with increasing temperature in the range 240–340 K. The rate coefficients k(CH2OO + CF3COOH)=(3.4±0.3)×10−10 cm3 s−1 and k((CH3)2COO + CF3COOH)=(6.1±0.2)×10−10 cm3 s−1 at 294 K exceed estimates for collision‐limited values, suggesting rate enhancement by capture mechanisms because of the large permanent dipole moments of the two reactants. The observed temperature dependence is attributed to competitive stabilization of a pre‐reactive complex. Fits to a model incorporating this complex formation give k [cm3 s−1]=(3.8±2.6)×10−18 T2 exp((1620±180)/T) + 2.5×10−10 and k [cm3 s−1]=(4.9±4.1)×10−18 T2 exp((1620±230)/T) + 5.2×10−10 for the CH2OO + CF3COOH and (CH3)2COO + CF3COOH reactions, respectively. The consequences are explored for removal of TFA from the atmosphere by reaction with biogenic Criegee intermediates.

Imaging and scattering studies of the unimolecular dissociation of the BrCH2CH2O radical from BrCH2CH2ONO photolysis at 351 nm.

We report a study of the unimolecular dissociation of BrCH2CH2O radicals produced from the photodissociation of BrCH2CH2ONO at 351/355 nm. Using both a crossed laser-molecular beam scattering apparatus with electron bombardment detection and a velocity map imaging apparatus with tunable VUV photoionization detection, we investigate the initial photodissociation channels of the BrCH2CH2ONO precursor and the subsequent dissociation of the vibrationally excited BrCH2CH2O radicals. The only photodissociation channel of the precursor we detected upon photodissociation at 351 nm was O-NO bond fission. C-Br photofission and HBr photoelimination do not compete significantly with O-NO photofission at this excitation wavelength. The measured O-NO photofission recoil kinetic energy distribution peaks near 14 kcal/mol and extends from 5 to 24 kcal/mol. There is also a small signal from lower kinetic energy NO product (it would be 6% of the total if it were also from O-NO photofission). We use the O-NO photofission P(ET) peaking near 14 kcal/mol to help characterize the internal energy distribution in the nascent ground electronic state BrCH2CH2O radicals. At 351 nm, some but not all of the BrCH2CH2O radicals are formed with enough internal energy to unimolecularly dissociate to CH2Br + H2CO. Although the signal at m/e = 93 (CH2Br(+)) obtained with electron bombardment detection includes signal both from the CH2Br product and from dissociative ionization of the energetically stable BrCH2CH2O radicals, we were able to isolate the signal from CH2Br product alone using tunable VUV photoionization detection at 8.78 eV. We also sought to investigate the source of vinoxy radicals detected in spectroscopic experiments by Miller and co-workers ( J. Phys. Chem. A 2012 , 116 , 12032 ) from the photodissociation of BrCH2CH2ONO at 351 nm. Using velocity map imaging and photodissociating the precursor at 355 nm, we detected a tiny signal at m/e = 43 and a larger signal at m/e = 15 that we tentatively assign to vinoxy. An underlying signal in the time-of-flight spectra at m/e = 29 and m/e = 42, the two strongest peaks in the literature electron bombardment mass spectrum of vinoxy, is also apparent. Comparison of those signal strengths with the signal at HBr(+), however, shows that the vinoxy product does not have HBr as a cofragment, so the prior suggestion by Miller and co-workers that the vinoxy might result from a roaming mechanism is contraindicated.

Investigating the Tropospheric Chemistry of Acetic Acid Using the Global 3‐D Chemistry Transport Model, STOCHEM‐CRI

Acetic acid (CH3COOH) is one of the most abundant carboxylic acids in the troposphere. In the study, the tropospheric chemistry of CH3COOH is investigated using the 3-D global chemistry transport model, STOCHEM-CRI. The highest mixing ratios of surface CH3COOH are found in the tropics by as much as 1.6 ppb in South America. The model predicts the seasonality of CH3COOH reasonably well and correlates with some surface and flight measurement sites, but the model drastically underpredicts levels in urban and midlatitudinal regions. The possible reasons for the underprediction are discussed. The simulations show that the lifetime and global burden of CH3COOH are 1.6–1.8 days and 0.45–0.61 Tg, respectively. The reactions of the peroxyacetyl radical (CH3CO3) with the hydroperoxyl radical (HO2) and other organic peroxy radicals (RO2) are found to be the principal sources of tropospheric CH3COOH in the model, but the model-measurement discrepancies suggest the possible unknown or underestimated sources which can contribute large fractions of the CH3COOH burden. The major sinks of CH3COOH in the troposphere are wet deposition, dry deposition, and OH loss. However, the reaction of CH3COOH with Criegee intermediates is proposed to be a potentially significant chemical loss process of tropospheric CH3COOH that has not been previously accounted for in global modeling studies. Inclusion of this loss process reduces the tropospheric CH3COOH level significantly which can give even larger discrepancies between model and measurement data, suggesting that the emissions inventory and the chemical production sources of CH3COOH are underpredicted even more so in current global models.

Temperature Dependence of the Rates of Reaction of Trifluoracetic Acid with Criegee Intermediates

The rate coefficients for gas-phase reactions of trifluoroacetic acid (TFA) with two Criegee intermediates, formaldehyde oxide and acetone oxide, decrease with increasing temperature in the range 240 - 340 K. The rate coefficients k(CH2OO + TFA) = (3.4 ± 0.3) × 10-10 cm3 s-1 and k((CH3)2COO + TFA) = (6.1 ± 0.2) × 10-10 cm3 s-1 at 294 K exceed estimates for collision-limited values, suggesting rate enhancement by capture mechanisms because of the large permanent dipole moments of the two reactants. The observed temperature dependence is attributed to competitive stabilization of a pre-reactive complex. Fits to a model incorporating this complex formation give k [cm3 s-1] = (3.8±2.6)×10-18 T2 exp((1620±180)/T) + 2.5 × 10-10 and k [cm3 s-1] = (4.9±4.1)×10-18 T2 exp((1620±230)/T) + 5.2 × 10-10 for the CH2OO + CF3COOH and (CH3)2COO + CF3COOH reactions, respectively. The consequences are explored for removal of TFA from the atmosphere by reaction with biogenic Criegee intermediates.