RESUMEN
Pure rotational transitions of methacrolein oxide (MACRO) were observed by Fourier-transform microwave spectroscopy. Among the four low-lying conformers existing within an energy window of 3 kcal/mol, only the lowest-energy conformer, the anti-trans conformer, was detected in a discharged jet of a 1,3-diiode-2-methylprop-1-ene and O2 mixture diluted in Ar. Nineteen pure rotational transitions, in the frequency range from 10 to 25 GHz, most of them showing A/E splitting due to the methyl-top internal rotation, were observed and analyzed by the XIAM program, yielding the internal rotation barrier of 559 cm-1, which very well agrees with a theoretically calculated value, 558 cm-1, at the CCSD(T)/cc-pVTZ level of theory.
RESUMEN
The reactions of Criegee intermediates with HNO3 are important in the polluted urban atmosphere because of their large rate coefficients and the significant concentration of HNO3. Employing a step-scan Fourier-transform spectrometer, we recorded infrared spectra of transient species and end products in the reaction CH2OO + HNO3 upon irradiation of a flowing mixture of CH2I2/HNO3/N2/O2 at 308 nm. Eight bands at 1686, 1426, 1348, 1294, 1052, 965, 891, and 825 cm-1 were assigned to the absorption of the adduct nitrooxymethyl hydroperoxide (NMHP, NO3CH2OOH). Additional products from two dissociation channels were observed. Four bands at 1709, 1325, 1276, and 886 cm-1 were assigned to H2C(O)ONO2 (with coproduct OH), produced from the fission of the O-O bond of internally hot NMHP (NMHP*). Simultaneous detection of H2CO (1746 cm-1), NO2 (1617 cm-1), and HO2 (1392 and 1098 cm-1) indicated a direct cleavage of the N-OC and C-OO bonds of NMHP*. The relative yields of these three channels in pressure range 10-150 Torr were estimated. At 10 Torr, the absorption of internally excited HNO3 near 885 and 1320 cm-1 was also detected at an early stage of the reaction. We investigated also the rate coefficient of the reaction CH2OO + HNO3 by probing the temporal profiles of the formation of NMHP and NO2 under total pressures of 40 and 70 Torr at 298 K. The rate coefficient kHNO3 = (2.4 ± 0.4) × 10-10 cm3 molecule-1 s-1 is less than half the only literature value, (5.4 ± 1.0) × 10-10 cm3 molecule-1 s-1, reported by Foreman et al. (Angew. Chem. Int. Ed. 2016, 55, 10419-10422).
RESUMEN
Methyl vinyl ketone oxide (MVKO) is an important Criegee intermediate in the ozonolysis of isoprene. MVKO is resonance stabilized by its allyl moiety, but no spectral characterization of this stabilization was reported to date. In this study, we photolyzed a mixture of 1,3-diiodo-but-2-ene and O2 to produce MVKO and characterized the syn-trans-MVKO, and tentatively syn-cis-MVKO, with transient infrared spectra recorded using a step-scan Fourier-transform spectrometer. The OâO stretching band at 948 cm-1 of syn-trans-MVKO is much greater than the corresponding bands of syn-CH3CHOO and (CH3)2COO Criegee intermediates at 871 and 887 cm-1, respectively, confirming a stronger OâO bond due to resonance stabilization. We observed also iodoalkenyl radical C2H3C(CH3)I upon photolysis of the precursor to confirm the fission of the terminal allylic CâI bond rather than the central vinylic CâI bond of the precursor upon photolysis. At high pressure, the adduct C2H3C(CH3)IOO was also observed. The reaction mechanism is characterized.
RESUMEN
Pure rotational transitions of methyl vinyl ketone oxide, or the so called methyl-vinyl Criegee intermediate, were observed by Fourier-transform microwave spectroscopy. Among the four possible conformers of this species, predicted by theory within an energy window of 3 kcal/mol, only the lowest-energy conformer, the syn-trans form, was detected in a discharged jet of a 1,3-diiode-but-2-ene (either in Z- or E-conformation) and O2 mixture diluted in Ar. Thirty pure rotational transitions with internal rotation splitting of the methyl top were observed. The observed frequencies were analyzed by the XIAM program, yielding an internal rotation barrier of 702.8(28) cm-1, which reasonably agrees with a theoretical value of 680 cm-1 determined with CCSD(T)/cc-pVTZ.
RESUMEN
The reaction of Criegee intermediate CH2OO with HCOOH is important in atmospheric chemistry, but its mechanism and kinetics are little investigated. We recorded time-resolved infrared absorption spectra of transient species produced upon irradiation at 308 nm of a flowing mixture of CH2I2/O2/N2/HCOOH at 298 K. Bands of CH2OO were observed initially upon irradiation; their decrease was accompanied with the appearance of several bands near 887, 925, 1052, 1115, 1170, 1342, 1391, and 1760 cm-1, assigned to the absorption of hydroperoxymethyl formate [HC(O)OCH2OOH, HPMF], that decreased in intensity at a later period with the appearance of absorption bands of the anti-conformer of formic acid anhydride [anti-(HCO)2O, FAN] near 998, 1101, 1767, and 1821 cm-1. The main contributions of the infrared absorption of HPMF are from an open-form conformer, but small contributions from the intramolecular hydrogen-bonded conformer that absorbs near 1070, 1170, and 1732 cm-1 were identified. Observed infrared spectra of both conformers of HPMF and anti-FAN agree satisfactorily with the anharmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. We derived a rate coefficient for CH2OO + HCOOH to be k = (1.4 ± 0.3) × 10-10 cm3 molecule-1 s-1 from formation of HPMF. We found also that anti-FAN was produced mainly from the open-form conformer with rate coefficient k = (1460 ± 30) s-1; the intramolecular hydrogen-bonded conformer of HPMF is stable.
RESUMEN
The reaction of the simplest Criegee intermediate CH2OO with NO2 is considered to be important in atmospheric chemistry because of its prospective contribution to the decay of CH2OO and the additional source of NO3, hence its effects on the NOx and HOx cycles. The reported rate coefficients of this reaction varied by a factor of 5. Employing a cw quantum-cascade laser with wavelength near 11 µm coupled with Herriott mirrors to probe CH2OO sensitively, we investigated in detail the reaction CH2OO + NO2 under total pressure 5.9-9.7 Torr at 298 K and reported a rate coefficient kNO2 = (1.0 ± 0.2) × 10-12 cm3 molecule-1 s-1, about one-seventh the value determined with direct measurements of CH2OO by Weltz et al. This smaller rate coefficient implies that the title reaction contributes to the atmospheric source of NO3 and the decay of CH2OO much less than previously conceived.