Yuan-Pern Lee

Infrared Absorption Spectrum of the Simplest Criegee Intermediate CH2OO

More Criegee Sightings The reaction of ozone with unsaturated hydrocarbons produces short-lived molecules termed Criegee intermediates. The simplest such molecule, H2CO2, was recently detected and monitored in the laboratory. Su et al. (p. 174; see the Perspective by Vereecken) have obtained its vibrational spectrum, which could ultimately enable direct measurements of its reactivity in the atmosphere. Taatjes et al. (p. 177; see the Perspective by Vereecken) report on the laboratory preparation and reactivity of the next heavier Criegee intermediate, which bears a methyl group in place of one of the hydrogen atoms. The vibrational spectrum of an intermediate involved in ozone chemistry may facilitate its direct study in the atmosphere. [Also see Perspective by Vereecken] The Criegee intermediates are carbonyl oxides postulated to play key roles in the reactions of ozone with unsaturated hydrocarbons; these reactions constitute an important mechanism for the removal of unsaturated hydrocarbons and for the production of OH in the atmosphere. Here, we report the transient infrared (IR) absorption spectrum of the simplest Criegee intermediate CH2OO, produced from CH2I + O2 in a flow reactor, using a step-scan Fourier-transform spectrometer. The five observed bands provide definitive identification of this intermediate. The observed vibrational frequencies are more consistent with a zwitterion rather than a diradical structure of CH2OO. The direct IR detection of CH2OO should prove useful for kinetic and mechanistic investigations of the Criegee mechanism.

Extremely rapid self-reaction of the simplest Criegee intermediate CH2OO and its implications in atmospheric chemistry

Criegee intermediates, which are carbonyl oxides produced when ozone reacts with unsaturated hydrocarbons, play an important role in the formation of OH and organic acids in the atmosphere, but they have eluded direct detection until recently. Reactions that involve Criegee intermediates are not understood fully because data based on their direct observation are limited. We used transient infrared absorption spectroscopy to probe directly the decay kinetics of formaldehyde oxide (CH2OO) and found that it reacts with itself extremely rapidly. This fast self-reaction is a result of its zwitterionic character. According to our quantum-chemical calculations, a cyclic dimeric intermediate that has the terminal O atom of one CH2OO bonded to the C atom of the other CH2OO is formed with large exothermicity before further decomposition to 2H2CO + O2((1)Δg). We suggest that the inclusion of this previously overlooked rapid reaction in models may affect the interpretation of previous laboratory experiments that involve Criegee intermediates.

Photo-induced fractionation of water isotopomers in the Martian atmosphere

The history and size of the water reservoirs on early Mars can be constrained using isotopic ratios of deuterium to hydrogen. We present new laboratory measurements of the ultraviolet cross-sections of H2O and its isotopomers, and modeling calculations in support of a photo-induced fractionation effect (PHIFE), that reconciles a discrepancy between past theoretical modeling and recent observations. This supports the hypothesis that Mars had an early warm atmosphere and has lost at least a 50-m global layer of water. Likely applications of PHIFE to other planetary atmospheres are sketched.

Perspective: Spectroscopy and kinetics of small gaseous Criegee intermediates.

The Criegee intermediates, carbonyl oxides proposed by Criegee in 1949 as key intermediates in the ozonolysis of alkenes, play important roles in many aspects of atmospheric chemistry. Because direct detection of these gaseous intermediates was unavailable until recently, previous understanding of their reactions, derived from indirect experimental evidence, had great uncertainties. Recent laboratory detection of the simplest Criegee intermediate CH2OO and some larger members, produced from ultraviolet irradiation of corresponding diiodoalkanes in O2, with various methods such as photoionization, ultraviolet absorption, infrared absorption, and microwave spectroscopy opens a new door to improved understanding of the roles of these Criegee intermediates. Their structures and spectral parameters have been characterized; their significant zwitterionic nature is hence confirmed. CH2OO, along with other products, has also been detected directly with microwave spectroscopy in gaseous ozonolysis reactions of ethene. The detailed kinetics of the source reaction, CH2I + O2, which is critical to laboratory studies of CH2OO, are now understood satisfactorily. The kinetic investigations using direct detection identified some important atmospheric reactions, including reactions with NO2, SO2, water dimer, carboxylic acids, and carbonyl compounds. Efforts toward the characterization of larger Criegee intermediates and the investigation of related reactions are in progress. Some reactions of CH3CHOO are found to depend on conformation. This perspective examines progress toward the direct spectral characterization of Criegee intermediates and investigations of the associated reaction kinetics, and indicates some unresolved problems and prospective challenges for this exciting field of research.

Synthesis and electron-transfer properties of benzimidazole-functionalized ruthenium complexes for highly efficient dye-sensitized solar cells

Novel heteroleptic ruthenium complexes--RD1, RD5, RD10 and RD11--with ligands based on benzimidazole were synthesized and characterized for application to dye-sensitized solar cells (DSSC); the remarkable performance of RD5-based DSSC is understood for its superior light-harvesting ability and slower charge-recombination kinetics.

Detailed mechanism of the CH2I + O2 reaction: Yield and self-reaction of the simplest Criegee intermediate CH2OO

The application of a new reaction scheme using CH2I + O2 to generate the simplest Criegee intermediate, CH2OO, has stimulated lively research; the Criegee intermediates are extremely important in atmospheric chemistry. The detailed mechanism of CH2I + O2 is hence important in understanding kinetics involving CH2OO. We employed ultraviolet absorption to probe simultaneously CH2I2, CH2OO, CH2I, and IO in the reaction system of CH2I + O2 upon photolysis at 248 nm of a flowing mixture of CH2I2, O2, and N2 (or SF6) in the pressure range 7.6-779 Torr to investigate the reaction kinetics. With a detailed mechanism to model the observed temporal profiles of CH2I, CH2OO, and IO, we found that various channels of the reaction CH2I + O2 and CH2OO + I play important roles; an additional decomposition channel of CH2I + O2 to form products other than CH2OO or ICH2OO becomes important at pressure less than 60 Torr. The pressure dependence of the derived rate coefficients of various channels of reactions of CH2I + O2 and CH2OO + I has been determined. We derived a rate coefficient also for the self-reaction of CH2OO as k = (8 ± 4) × 10(-11) cm(3) molecule(-1) s(-1) at 295 K. The yield of CH2OO from CH2I + O2 was found to have a pressure dependence on N2 and O2 smaller than in previous reports; for air under 1 atm, the yield of ~30% is about twice of previous estimates.

Infrared absorption of methanol clusters (CH3OH)n with n = 2−6 recorded with a time-of-flight mass spectrometer using infrared depletion and vacuum-ultraviolet ionization

We investigated IR spectra in the CH- and OH-stretching regions of size-selected methanol clusters, (CH(3)OH)(n) with n = 2-6, in a pulsed supersonic jet by using the IR-VUV (vacuum-ultraviolet) ionization technique. VUV emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer. The tunable IR laser emission served as a source of predissociation or excitation before ionization. The variations of intensity of protonated methanol cluster ions (CH(3)OH)(n)H(+) and CH(3)OH(+) and (CH(3)OH)(2)(+) were monitored as the IR laser light was tuned across the range 2650-3750 cm(-1). Careful processing of these action spectra based on photoionization efficiencies and the production and loss of each cluster due to photodissociation yielded IR spectra of the size-selected clusters. Spectra of methanol clusters in the OH region have been extensively investigated; our results are consistent with previous reports, except that the band near 3675 cm(-1) is identified as being associated with the proton acceptor of (CH(3)OH)(2). Spectra in the CH region are new. In the region 2800-3050 cm(-1), bands near 2845, 2956, and 3007 cm(-1) for CH(3)OH split into 2823, 2849, 2934, 2955, 2984, and 3006 cm(-1) for (CH(3)OH)(2) that correspond to proton donor and proton acceptor, indicating that the methanol dimer has a preferred open-chain structure. In contrast, for (CH(3)OH)(3), the splitting diminishes and the bands near 2837, 2954, and 2987 cm(-1) become narrower, indicating a preferred cyclic structure. Anharmonic vibrational wavenumbers predicted for the methanol open-chain dimer and the cyclic trimer with the B3LYP∕VPT2∕ANO1 level of theory are consistent with experimental results. For the tetramer and pentamer, the spectral pattern similar to that of the trimer but with greater widths was observed, indicating that the most stable structures are also cyclic.

Experimental and theoretical studies on vacuum ultraviolet absorption cross sections and photodissociation of CH3OH, CH3OD, CD3OH, and CD3OD

Absorption cross sections of CH3OH, CH3OD, CD3OH, and CD3OD are measured in a 107–220 nm spectral region using synchrotron radiation. Spectra of improved quality for four deuterated isotopomers, coupled with extensive calculations on low-lying excited states of methanol using time-dependent density functional theory with a large cc-pV5Z basis set, enable us to improve assignments of observed spectral features and to better understand the nature of these electronic transitions. Energies and oscillator strengths of all transitions predicted with calculations are consistent with experimental results. Observed isotopic shifts clearly indicate that absorption features in the 163–220 nm region (transition 1 1A″–X 1A′) are associated mainly with breaking of the O–H bond, consistent with theoretical predictions. In the 151–163 nm region (transition 2 1A″–X 1A′), observed small vibrational spacings (806 cm−1 for CH3OH) associated with the C–O stretching mode can be rationalized with a broad double-well-like potent...

I. Three-center versus four-center HCl-elimination in photolysis of vinyl chloride at 193 nm: Bimodal rotational distribution of HCl (v<=7) detected with time-resolved Fourier-transform spectroscopy

Following photodissociation of vinyl chloride at 193 nm, fully resolved vibration-rotational emission spectra of HCl in the spectral region 2000–3310 cm−1 are temporally resolved with a step-scan Fourier-transform spectrometer. Under improved resolution and sensitivity, emission from HCl up to v=7 is observed, with J>32 (limited by overlap at the band head) for v=1–3. All vibrational levels show bimodal rotational distribution with one component corresponding to ∼500 K and another corresponding to ∼9500 K for v⩽4. Vibrational distributions of HCl for both components are determined; the low-J component exhibits inverted vibrational population of HCl. Statistical models are suitable for three-center (α, α) elimination of HCl because of the loose transition state and a small exit barrier for this channel; predicted internal energy distributions of HCl are consistent but slightly less than those observed for the high-J component. Impulse models considering geometries and displacement vectors of transition sta...

Vibronic analysis of the Ã→X̃ laser‐induced fluorescence of jet‐cooled methoxy (CH3O) radical

The dispersed fluorescence of the A 2A1–X 2E system of CH3O and 13CH3O in a supersonic jet was recorded after excitation of various vibrational levels of the A state. Analysis of the spectra yielded ν2‘ = 1412, ν3‘ = 1047, ν5‘ = 1494, and ν6‘ = 653 cm−1 for the X state of 12CH3O, and ν2‘ = 1408, ν3‘ = 1027, ν5‘ = 1488, and ν6‘ = 649 cm−1 for 13CH3O. The least‐squares fitting of the observed 300–306 lines (with spin–orbit splitting removed) yielded ωe=1057±3 cm−1 and ωexe=7.0±0.7 cm−1 for ν3‘ of 12CH3O. Several tentative assignments were also made on the basis of the improved fluorescence spectra. The Fermi‐resonant levels 1289 and 1319 cm−1 above v=0 of the A state were also reassigned; the former is better represented as v2’ = 1, whereas the latter is represented as v3’ = 2.