Acetonyl Peroxy and Hydroperoxy Self- and Cross-Reactions: Temperature-Dependent Kinetic Parameters, Branching Fractions, and Chaperone Effects.

The temperature-dependent kinetic parameters, branching fractions, and chaperone effects of the self- and cross-reactions between acetonyl peroxy (CH3C(O)CH2O2) and hydro peroxy (HO2) have been studied using pulsed laser photolysis coupled with infrared (IR) wavelength-modulation spectroscopy and ultraviolet absorption (UVA) spectroscopy. Two IR lasers simultaneously monitored HO2 and hydroxyl (OH), while UVA measurements monitored CH3C(O)CH2O2. For the CH3C(O)CH2O2 self-reaction (T = 270-330 K), the rate parameters were determined to be A = (1.5-0.3+0.4) × 10-13 and Ea/R = -996 ± 334 K and the branching fraction to the alkoxy channel, k2b/k2, showed an inverse temperature dependence following the expression, k2b/k2 = (2.27 ± 0.62) - [(6.35 ± 2.06) × 10-3] T(K). For the reaction between CH3C(O)CH2O2 and HO2 (T = 270-330 K), the rate parameters were determined to be A = (3.4-1.5+2.5) × 10-13 and Ea/R = -547 ± 415 K for the hydroperoxide product channel and A = (6.23-4.4+15.3) × 10-17 and Ea/R = -3100 ± 870 K for the OH product channel. The branching fraction for the OH channel, k1b /k1, follows the temperature-dependent expression, k1b/k1 = (3.27 ± 0.51) - [(9.6 ± 1.7) × 10-3] T(K). Determination of these parameters required an extensive reaction kinetics model which included a re-evaluation of the temperature dependence of the HO2 self-reaction chaperone enhancement parameters due to the methanol-hydroperoxy complex. The second-law thermodynamic parameters for KP,M for the formation of the complex were found to be ΔrH250K° = -38.6 ± 3.3 kJ mol-1 and ΔrS250K° = -110.5 ± 13.2 J mol-1 K-1, with the third-law analysis yielding ΔrH250K° = -37.5 ± 0.25 kJ mol-1. The HO2 self-reaction rate coefficient was determined to be k4 = (3.34-0.80+1.04) × 10-13 exp [(507 ± 76)/T]cm3 molecule-1 s-1 with the enhancement term k4,M″ = (2.7-1.7+4.7) × 10-36 exp [(4700 ± 255)/T]cm6 molecule-2 s-1, proportional to [CH3OH], over T = 220-280 K. The equivalent chaperone enhancement parameter for the acetone-hydroperoxy complex was also required and determined to be k4,A″ = (5.0 × 10-38 - 1.4 × 10-41) exp[(7396 ± 1172)/T] cm6 molecule-2 s-1, proportional to [CH3C(O)CH3], over T = 270-296 K. From these parameters, the rate coefficients for the reactions between HO2 and the respective complexes over the given temperature ranges can be estimated: for HO2·CH3OH, k12 = [(1.72 ± 0.050) × 10-11] exp [(314 ± 7.2)/T] cm3 molecule-1 s-1 and for HO2·CH3C(O)CH3, k15 = [(7.9 ± 0.72) × 10-17] exp [(3881 ± 25)/T] cm3 molecule-1 s-1. Lastly, an estimate of the rate coefficient for the HO2·CH3OH self-reaction was also determined to be k13 = (1.3 ± 0.45) × 10-10 cm3 molecule-1 s-1.

Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate.

Isoprene has the highest emission into Earth's atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO2 and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO3 and identifying organic hydroperoxide formation from reaction with SO2 and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

Functionalized Hydroperoxide Formation from the Reaction of Methacrolein-Oxide, an Isoprene-Derived Criegee Intermediate, with Formic Acid: Experiment and Theory.

Methacrolein oxide (MACR-oxide) is a four-carbon, resonance-stabilized Criegee intermediate produced from isoprene ozonolysis, yet its reactivity is not well understood. This study identifies the functionalized hydroperoxide species, 1-hydroperoxy-2-methylallyl formate (HPMAF), generated from the reaction of MACR-oxide with formic acid using multiplexed photoionization mass spectrometry (MPIMS, 298 K = 25 °C, 10 torr = 13.3 hPa). Electronic structure calculations indicate the reaction proceeds via an energetically favorable 1,4-addition mechanism. The formation of HPMAF is observed by the rapid appearance of a fragment ion at m/z 99, consistent with the proposed mechanism and characteristic loss of HO2 upon photoionization of functional hydroperoxides. The identification of HPMAF is confirmed by comparison of the appearance energy of the fragment ion with theoretical predictions of its photoionization threshold. The results are compared to analogous studies on the reaction of formic acid with methyl vinyl ketone oxide (MVK-oxide), the other four-carbon Criegee intermediate in isoprene ozonolysis.

Formic acid catalyzed isomerization and adduct formation of an isoprene-derived Criegee intermediate: experiment and theory.

Isoprene is the most abundant non-methane hydrocarbon emitted into the Earth's atmosphere. Ozonolysis is an important atmospheric sink for isoprene, which generates reactive carbonyl oxide species (R1R2C[double bond, length as m-dash]O+O-) known as Criegee intermediates. This study focuses on characterizing the catalyzed isomerization and adduct formation pathways for the reaction between formic acid and methyl vinyl ketone oxide (MVK-oxide), a four-carbon unsaturated Criegee intermediate generated from isoprene ozonolysis. syn-MVK-oxide undergoes intramolecular 1,4 H-atom transfer to form a substituted vinyl hydroperoxide intermediate, 2-hydroperoxybuta-1,3-diene (HPBD), which subsequently decomposes to hydroxyl and vinoxylic radical products. Here, we report direct observation of HPBD generated by formic acid catalyzed isomerization of MVK-oxide under thermal conditions (298 K, 10 torr) using multiplexed photoionization mass spectrometry. The acid catalyzed isomerization of MVK-oxide proceeds by a double hydrogen-bonded interaction followed by a concerted H-atom transfer via submerged barriers to produce HPBD and regenerate formic acid. The analogous isomerization pathway catalyzed with deuterated formic acid (D2-formic acid) enables migration of a D atom to yield partially deuterated HPBD (DPBD), which is identified by its distinct mass (m/z 87) and photoionization threshold. In addition, bimolecular reaction of MVK-oxide with D2-formic acid forms a functionalized hydroperoxide adduct, which is the dominant product channel, and is compared to a previous bimolecular reaction study with normal formic acid. Complementary high-level theoretical calculations are performed to further investigate the reaction pathways and kinetics.

Pressure and Temperature Dependencies of Rate Coefficients for the Reaction OH + NO2 + M → Products.

The OH + NO2 reaction is a critically important process for radical chain termination in the atmosphere with a major impact on the ozone budgets of the troposphere and stratosphere. Rate constants for the reaction of OH + NO2 + M → products have been measured under conditions relevant to the upper troposphere/lower stratosphere with a laser photolysis-laser-induced fluorescence (LP-LIF) technique augmented by in situ optical spectroscopy for quantification of [NO2]. The experiments are carried out over the temperature range of 230-293 K and the pressure range 50-750 Torr of N2 and air and as a function of [O2]. The observed rate coefficients in N2 agree with the newest experimental literature data sets and are within experimental uncertainty of current recommended literature values at 293 K but are systematically higher by up to 22% at 700 Torr and 230 K. The efficacy of different falloff parametrizations has been examined and compared to those in literature sources. The collisional quenching efficiency of O2 was found to be in excellent agreement with current literature sources, and rate coefficients determined in air at 293 and 245 K were observed to be within uncertainty of the rate coefficients measured in N2 bath gas. This work has improved confidence in the literature rate coefficients under conditions of the lower troposphere (∼760 Torr, 280-310 K) toward the stratosphere (10-100 Torr, 220-250 K).

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 kA″ = (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 Kc(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.

Experimental Evidence of Dioxole Unimolecular Decay Pathway for Isoprene-Derived Criegee Intermediates.

Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth's atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide (anti) and MACR-oxide (syn) with the vinyl substituent adjacent to the terminal O atom. The resulting dioxoles are predicted to undergo rapid unimolecular decay to oxygenated hydrocarbon radical products, including acetyl, vinoxy, formyl, and 2-methylvinoxy radicals. In the presence of O2, these radicals rapidly react to form peroxy radicals (ROO), which quickly decay via carbon-centered radical intermediates (QOOH) to stable carbonyl products that were identified in this work. The carbonyl products were detected under thermal conditions (298 K, 10 Torr He) using multiplexed photoionization mass spectrometry (MPIMS). The main products (and associated relative abundances) originating from unimolecular decay of anti-MVK-oxide and subsequent reaction with O2 are formaldehyde (88 ± 5%), ketene (9 ± 1%), and glyoxal (3 ± 1%). Those identified from the unimolecular decay of syn-MACR-oxide and subsequent reaction with O2 are acetaldehyde (37 ± 7%), vinyl alcohol (9 ± 1%), methylketene (2 ± 1%), and acrolein (52 ± 5%). In addition to the stable carbonyl products, the secondary peroxy chemistry also generates OH or HO2 radical coproducts.

Predissociation dynamics of the HCl-(H2O)3 tetramer: An experimental and theoretical investigation.

The cyclic HCl-(H2O)3 tetramer is the largest observed neutral HCl-(H2O)n cluster. The vibrational predissociation of HCl-(H2O)3 is investigated by theory, quasiclassical trajectory (QCT) calculations, and experiment, following the infrared excitation of the hydrogen-bonded OH-stretch fundamental. The energetically possible dissociation pathways are HCl + (H2O)3 (Pathway 1) and H2O + HCl-(H2O)2 (Pathway 2). The HCl and H2O monomer fragments are observed by 2 + 1 resonance enhanced multiphoton ionization combined with time-of-flight mass spectrometry, and their rotational energy distributions are inferred and compared to the theoretical results. Velocity map images of the monomer fragments in selected rotational levels are used for each pathway to obtain pair-correlated speed distributions. The fragment speed distributions obtained by experiment and QCT calculations are broad and structureless, encompassing the entire range of allowed speeds for each pathway. Bond dissociation energies, D0, are estimated experimentally from the endpoints of the speed distributions: 2100 ± 300 cm-1 and 2400 ± 100 cm-1 for Pathway 1 and Pathway 2, respectively. These values are lower but in the same order as the corresponding calculated D0: 2426 ± 23 cm-1 and 2826 ± 19 cm-1. The differences are attributed to contributions from vibrational hot bands of the clusters that appear in the high-speed tails of the experimental pair-correlated distributions. Satisfactory agreement between theory and experiment is achieved when comparing the monomer fragments' rotational energies, the shapes of the speed distributions, and the average fragment speeds and center-of-mass translational energies. Insights into the dissociation mechanism and lifetime are gained from QCT calculations performed on a previously reported many-body potential energy surface. It is concluded that the dissociation lifetime is on the order of 10 ps and that the final trimer products are in their lowest energy cyclic forms.

Vibrational Predissociation of the HCl-(H2O)3 Tetramer.

The vibrational predissociation of the HCl-(H2O)3 tetramer, the largest HCl-(H2O)n cluster for which HCl is not predicted to be ionized, is reported. This work focuses on the predissociation pathway giving rise to H2O + HCl-(H2O)2 following IR laser excitation of the H-bonded OH stretch fundamental. H2O fragments are monitored state selectively by 2 + 1 resonance-enhanced multiphoton ionization (REMPI) combined with time-of-flight mass spectrometry (TOF-MS). Velocity map images of H2O in selected rotational levels are used to determine translational energy distributions from which the internal energy distributions in the pair-correlated cofragments are derived. From the maximum translational energy release, the bond dissociation energy, D0 = 2400 ± 100 cm-1, is determined for the investigated channel. The energy distributions in the fragments are broad, encompassing the entire range of allowed states. The importance of cooperative (nonpairwise) interactions is discussed.

Pulse duration and energy dependence of photodamage and lethality induced by femtosecond near infrared laser pulses in Drosophila melanogaster.

The potential application of nonlinear optical imaging diagnosis and treatment using femtosecond laser pulses in humans accentuates the need for studies carried out in whole organisms instead of single cells or cell cultures. While there is a general consensus that in order to minimize the level of photodamage the excitation power has to be kept as low as possible, it has yet to be determined if shorter pulses have greater benefit than longer pulses. Here we evaluate the rate of death in Drosophila melanogaster as the integral parameter related to photodamage resulting from femtosecond near infrared (NIR) laser irradiation under conditions comparable to those used in two-photon excited fluorescence (TPEF) microscopy. We found that the lethality (resulting from photodamage) as a function of laser energy fluence fits a 3-region dose-response curve. The lethality was accompanied with development of necrosis and apoptosis in irradiated tissues. Quantitative analysis showed that the damage has a mostly linear character on energy fluence per pulse, and for a given TPEF signal, shorter (37 fs) pulse duration results in lower lethality than longer (100 fs) pulse duration. These results have important implications for the use of femtosecond NIR laser pulses in microscopy as well as in vivo medical imaging.