Photodissociation dynamics of acetylacetone: The OH product state distribution

Acetylacetone Photolysis at 280 nm Studied by Velocity-Map Ion Imaging

The photolysis of acetylacetone (AcAc) has been studied using velocity-map ion imaging with pulsed nanosecond lasers. The enolone tautomer of AcAc (CH3C(O)CH═C(OH)CH3) was excited in the strong UV absorption band by UV pulses at 280 nm, preparing the S2(ππ*) state, and products were probed after a short time delay by single-photon VUV ionization at 118.2 nm. Two-color UV + VUV time-of-flight mass spectra show enhancement of fragments at m/z = 15, 42, 43, 58, and 85 at the lowest UV pulse energies and depletion of the parent ion at m/z = 100. Ion images of the five major fragments are all isotropic, indicating dissociation lifetimes that are long on the timescale of molecular rotation but shorter than the laser pulse duration (<6 ns). The m/z = 15 and 85 fragments have identical momentum distributions with moderate translational energy release, suggesting that they are formed as a neutral product pair and likely via a Norrish type I dissociation of the enolone to form CH3 + C(O)CH═C(OH)CH3 over a barrier on a triplet surface. The m/z = 43 fragment may be tentatively assigned to the alternative Norrish type I pathway that produces CH3CO + CH2C(O)CH3 on S0 following phototautomerization to the diketone, although alternative mechanisms involving dissociative ionization of a larger primary photoproduct cannot be conclusively ruled out. The m/z = 42 and 58 fragments are not momentum-matched and consequently are not formed as a neutral pair via a unimolecular dissociation pathway on S0. They also likely originate from the dissociative ionization of primary photofragments. RRKM calculations suggest that unimolecular dissociation pathways that lead to molecular products on S0 are generally slow, implying an upper-limit lifetime of <46 ns after excitation at 280 nm. Time-dependent measurements suggest that the observed photofragments likely do not arise from dissociative ionization of energized AcAc S0*.

UV photoreaction pathways of acetylacetaldehyde trapped in cryogenic matrices

The broadband UV photochemistry kinetics of acetylacetaldehyde, the hybrid form between malonaldehyde and acetylacetone (the two other most simple molecules exhibiting an intramolecular proton transfer), trapped in four cryogenic matrices, neon, nitrogen, argon, and xenon, has been followed by FTIR and UV spectroscopy. After deposition, only the two chelated forms are observed while they isomerize upon UV irradiation toward nonchelated species. From previous UV irradiation effects, we have already identified several nonchelated isomers, capable, in turn, of isomerizing and fragmenting; even fragmentation seems to be most unlikely due to cryogenic cages confinement. Based on these findings, we have attempted an approach to understand the reaction path of electronic relaxation. Indeed, we have demonstrated, in previous studies, that in the case of malonaldehyde, this electronic relaxation pathway proceeds through singlet states while it proceeds through triplet ones in the case of acetylacetone. We observed CO and CO₂ formations when photochemistry is almost observed among nonchelated forms, i.e., when the parent molecule is almost totally consumed. In order to identify a triplet state transition, we have tried to observe a "heavy atom effect" by increasing the weight of the matrix gas, from Ne to Xe, and to quench the T₁ state by doping the matrices with O₂. It appears that, as in the case of acetylacetone, it is the nonchelated forms that fragment. It also appears that these fragmentations certainly take place in the T₁ triplet state and originate in an Π* ← n transition.

Sulfonyl Nitrene and Amidyl Radical: Structure and Reactivity

Photocatalytic generation of nitrenes and radicals can be used to tune or even control their reactivity. Photocatalytic activation of sulfonyl azides leads to the elimination of N₂ and the resulting reactive species initiate C−H activations and amide formation reactions. Here, we present reactive radicals that are generated from sulfonyl azides: sulfonyl nitrene radical anion, sulfonyl nitrene and sulfonyl amidyl radical, and test their gas phase reactivity in C−H activation reactions. The sulfonyl nitrene radical anion is the least reactive and its reactivity is governed by the proton coupled electron transfer mechanism. In contrast, sulfonyl nitrene and sulfonyl amidyl radicals react via hydrogen atom transfer pathways. These reactivities and detailed characterization of the radicals with vibrational spectroscopy and with DFT calculations provide information necessary for taking control over the reactivity of these intermediates.

UV Photochemistry of Acetylacetaldehyde Trapped in Cryogenic Matrices

The broad band UV photochemistry of acetylacetaldehyde, the hybrid form between malonaldehyde and acetylacetone (the two other most simple molecules exhibiting an intramolecular proton transfer), trapped in four cryogenic matrices, neon, nitrogen, argon, and xenon, has been studied by IRTF spectroscopy. These experimental results have been supported by B3LYP/6-311G++(2d,2p) calculations in order to get S₀ minima together with their harmonic frequencies. On those minima, we have also calculated their vibrationally resolved UV absorption spectra at the time-dependent DFT ωB97XD/6-311++G(2d,2p) level. After deposition, only the two chelated forms are observed while they isomerize upon UV irradiation toward nonchelated species. From UV irradiation effects we have identified several nonchelated isomers, capable, in turn, of isomerizing and fragmenting, even if this last phenomenon seems to be most unlikely due to cryogenic cages confinement. On the basis of these findings, we have attempted a first approach to the reaction path of electronic relaxation. It appeared that, as with acetylacetone, the path of electronic relaxation seems to involve triplet states.

Unraveling Conformer-Specific Sources of Hydroxyl Radical Production from an Isoprene-Derived Criegee Intermediate by Deuteration

Ozonolysis of isoprene, the most abundant volatile organic compounds emitted into the Earth's troposphere after methane, yields three distinct Criegee intermediates. Among these, methyl vinyl ketone oxide (MVK-oxide) is predicted to be the major source of atmospheric hydroxyl radicals (OH) from isoprene ozonolysis. Previously, Barber et al. [ J. Am. Chem. Soc., 2018, 140, pp 10866-10880] demonstrated that syn-MVK-oxide conformers undergo unimolecular decay via 1,4-hydrogen (H) transfer from the methyl group to the adjacent terminal oxygen atom, followed by the prompt release of OH radical products. Here, we selectively deuterate the methyl group of MVK-oxide (d₃-MVK-oxide) and record its IR action spectrum in the vinyl CH stretch overtone (2ν_CH) region. The resultant time-dependent appearance of OD radical products, detected by laser-induced fluorescence, demonstrates that a unimolecular decay of d₃-MVK-oxide proceeds by an analogous 1,4-deuterium (D) atom transfer mechanism anticipated for syn conformers. The experimental spectral and temporal results are compared with the calculated IR absorption spectrum and unimolecular decay rates predicted by the Rice-Ramsperger-Kassel-Marcus (RRKM) theory for syn-d₃-MVK-oxide, as well as the prior study on syn-MVK-oxide. The d₃-MVK-oxide IR action spectrum is similar to that for MVK-oxide, yet exhibits notable changes as the overtone and combination transitions involving CD stretch shift to a lower frequency. The unimolecular decay rate for d₃-MVK-oxide is predicted to be a factor of 40 times slower than that for MVK-oxide in the 2ν_CH region. Experimentally, the temporal profile of the OD products reflects the slower unimolecular decay of d₃-MVK-oxide compared to that for MVK-oxide to OH products as well as experimental factors. Both experiment and theory demonstrate that quantum mechanical tunneling plays a very important role in the 1,4-H/D-transfer processes at energies in the vicinity of the transition-state barrier. The similarities of the IR action spectra and changes in the unimolecular decay dynamics upon deuteration indicate that syn conformers make the main contribution to the IR action spectra of MVK-oxide and d₃-MVK-oxide.

Short-wavelength probes in time-resolved photoelectron spectroscopy: an extended view of the excited state dynamics in acetylacetone

Time-resolved photoelectron spectroscopy using a vacuum ultraviolet probe brings new insight to the excited state dynamics operating in acetylacetone.

Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production

The reaction of ozone with isoprene, one of the most abundant volatile organic compounds in the atmosphere, produces three distinct carbonyl oxide species (RR'COO) known as Criegee intermediates: formaldehyde oxide (CH₂OO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R' = H, CH₃, CH═CH₂) and conformations of the Criegee intermediates control their subsequent chemistry in the atmosphere. In particular, unimolecular decay of MVK-OO is predicted to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis. This study reports the initial laboratory synthesis and direct detection of MVK-OO through reaction of a photolytically generated, resonance-stabilized monoiodoalkene radical with O₂. MVK-OO is characterized utilizing infrared (IR) action spectroscopy, in which IR activation of MVK-OO with two quanta of CH stretch at ca. 6000 cm^-1 is coupled with ultraviolet detection of the resultant OH products. MVK-OO is identified by comparison of the experimentally observed IR spectral features with theoretically predicted IR absorption spectra. For syn-MVK-OO, the rate of appearance of OH products agrees with the unimolecular decay rate predicted using statistical theory with tunneling. This validates the hydrogen atom transfer mechanism and computed transition-state barrier (18.0 kcal mol^-1) leading to OH products. Theoretical calculations reveal an additional roaming pathway between the separating radical fragments, which results in other products. Master equation modeling yields a thermal unimolecular decay rate for syn-MVK-OO of 33 s^-1 (298 K, 1 atm). For anti-MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products.

Photochemistry of UV-excited trifluoroacetylacetone and hexafluoroacetylacetone I: infrared spectra of fluorinated methylfuranones formed by HF photoelimination

The photochemistry of gas-phase 1,1,1-trifluoroacetylacetone (TFAA) excited with ultraviolet (UV) light involves a significant photoelimination channel that produces hydrogen fluoride and a fluorinated methylfuranone, 2,2-difluoro-5-methyl-3(2H)-furanone (2FMF). This pathway is remarkable because it is a gas-phase unimolecular reaction that forms a five-membered ring product. This report is the first of such a TFAA photoelimination channel, which is similar to one observed with 1,1,1,5,5,5-hexafluoroacetylacetone (HFAA), resulting in 2,2-difluoro-5-trifluoromethyl-3(2H)-furanone. We present infrared spectral observations of 2FMF produced by pulsed, UV-laser excitation of TFAA, along with analogous results from HFAA, supported by density functional theory (DFT) computational studies. DFT results for the infrared spectrum of 5-methyl-3(2H)-furanone, the expected comparable acetylacetone photoelimination product, help suggest that UV excitation of acetylacetone fails to follow a similar type of photoelimination. We use a weighted RMS approach as a figure of merit for comparing calculated infrared frequencies with experimental data. Results from the three acetylacetones reveal how the presence of fluorine atoms in acetylacetone influences the gas-phase molecular photochemistry.

Atmospheric chemistry of acetylacetone

A kinetic study on the reactions of the OH radical and ozone with acetylacetone (AcAc) has been performed in a 1080 L quartz glass reaction chamber using in situ FTIR spectroscopy analysis. Temperature dependent rate coefficients for the reaction of AcAc with the OH radical were determined over the temperature range 285-310 K using the relative kinetic method. The following Arrhenius expression was derived: k = 3.35 x 10(-12) exp((983 +/- 130)/T) cm3 molecule(-1) s(-1), where the indicated error is the two least-squares deviation. A rate coefficient (in units of cm3 molecule(-1) s(-1)) of (1.03 +/- 0.31) x 10(-18) has been obtained at (298 +/- 3) K for the reaction of ozone with AcAc. A product investigation on the gas-phase reaction of OH radical with AcAc was conducted in a 405 L borosilicate glass chamber using in situ FTIR spectroscopy to monitor reactants and products. Methylglyoxal, acetic acid, peroxy acetic nitrate (PAN) were positively identified as products with molar yields of (20.8 +/- 4.5)%, (16.9 +/- 3.4)%, and (2.0 +/- 0.5)%, respectively. From the residual infrared spectrum the main products are attributed to 2,3,4-pentantrione (CH3-CO-CO-CO-CH3) and its hydrated analogue pentan-2,3-dione-4-diol (CH3-CO-CO-C(OH)2-CH3). Based on the observed products, a simplified mechanism for the reaction of the OH radical with AcAc is proposed.

UV and IR Photoisomerization of Acetylacetone Trapped in a Nitrogen Matrix

UV- and IR-induced photoisomerization of acetylacetone trapped in a nitrogen matrix at 4.3 K have been carried out using a tunable optical parametric oscillator type laser, or a mercury vapor lamp, coupled with Fourier Transform IR and UV spectroscopies. After deposition, the main form present in the cryogenic matrix is that chelated (enol). Upon UV irradiation, the intramolecular H bond is broken leading to nonchelated isomers among seven possible open forms. These forms have then been irradiated by resonant pi* <-- pi UV irradiation, or by resonant nuOH irradiation. The selective UV irradiation allows us to suggest a first vibrational assignment while the nuOH irradiation leads us to observe interconversions between the nonchelated isomers. In order to support our vibrational assignment, we have carried out theoretical calculations at the B3LYP/6-311++G(2d,2p) level of theory. This study shows that only five isomers are observed among eight postulated.

Kinetics and mechanism of the reactions of CH3CO and CH3C(O)CH2 radicals with O2. Low-pressure discharge flow experiments and quantum chemical computations

The reactions CH(3)CO + O(2)--> products (1), CH(3)CO + O(2)--> OH +other products (1b) and CH(3)C(O)CH(2) + O(2)--> products (2) have been studied in isothermal discharge flow reactors with laser induced fluorescence monitoring of OH and CH(3)C(O)CH(2) radicals. The experiments have been performed at overall pressures between 1.33 and 10.91 mbar of helium and 298 +/- 1 K reaction temperature. OH formation has been found to be the dominant reaction channel for CH(3)CO + O(2): the branching ratio, Gamma(1b) = k(1b)/k(1), is close to unity at around 1 mbar, but decreases rapidly with increasing pressure. The rate constant of the overall reaction, k(2), has been found to be pressure dependent: the fall-off behaviour has been analysed in comparison with reported data. Electronic structure calculations have confirmed that at room temperature the reaction of CH(3)C(O)CH(2) with O(2) is essentially a recombination-type process. At high temperatures, the further reactions of the acetonyl-peroxyl adduct may yield OH radicals, but the most probable channel seems to be the O(2)-catalysed keto-enol transformation of acetonyl. Implications of the results for atmospheric modelling studies have been discussed.

Experimental and Theoretical UV Characterizations of Acetylacetone and Its Isomers

Cryogenic matrix isolation experiments have allowed the measurement of the UV absorption spectra of the high-energy non-chelated isomers of acetylacetone, these isomers being produced by UV irradiation of the stable chelated form. Their identification has been done by coupling selective UV-induced isomerization, infrared spectroscopy, and harmonic vibrational frequency calculations using density functional theory. The relative energies of the chelated and non-chelated forms of acetylacetone in the S0 state have been obtained using density functional theory and coupled-cluster methods. For each isomer of acetylacetone, we have calculated the UV transition energies and dipole oscillator strengths using the excited-state coupled-cluster methods, including EOMCCSD (equation-of-motion coupled-cluster method with singles and doubles) and CR-EOMCCSD(T) (the completely renormalized EOMCC approach with singles, doubles, and non-iterative triples). For dipole-allowed transition energies, there is a very good agreement between experiment and theory. In particular, the CR-EOMCCSD(T) approach explains the blue shift in the electronic spectrum due to the formation of the non-chelated species after the UV irradiation of the chelated form of acetylacetone. Both experiment and CR-EOMCCSD(T) theory identify two among the seven non-chelated forms to be characterized by red-shifted UV transitions relative to the remaining five non-chelated isomers.

Photodissociation dynamics of acetoxime in gas phase

The dynamics of photodissociation of acetoxime at 193 nm, leading to the formation of (CH3)2C=N and OH fragments, has been investigated. The nascent OH radicals, which are both rotationally and vibrationally excited, were probed by laser photolysis-laser induced fluorescence technique. OH fragments in both v" = 1 and v" = 0 vibrational states were detected with a ratio of population in the higher to lower level of 0.07+/-0.01. The rotational temperatures of v" = 0 and 1 levels of OH radicals are 2650+/-150 K and 1290+/-20 K, respectively. More than 30% of the available energy, i.e., 115+/-21 kJ mol(-1) is partitioned into the relative translational energy of the fragments. The results of excited electronic state and transition state calculations at the configuration interaction with single electronic excitation level suggest that the dissociation takes place with an exit barrier of approximately 126 kJ mol(-1) at the triplet state (T2) potential energy surface, formed by internal conversions/intersystem crossing from the initially populated S2 state. Using the calculated transition state geometry and its energy, the observed energy distribution pattern can be reproduced by the hybrid model within experimental uncertainties. The presence of an exit barrier is further supported by the observation of N-OH dissociation upon 248 nm excitation, where the relative translational energy of the fragments is found to be approximately 96 kJ mol(-1). The photodissociation dynamics of acetoxime is compared with C-OH dissociation in enols and carboxylic acid and N-OH dissociation in nitrous acid. The observed emission (lambda(max)=430 nm) and the N-OH dissociation dynamics indicate crossing of the initially populated state to an emissive state of acetoxime, which is different from the dissociative state.