Infrared characterization of formation and resonance stabilization of the Criegee intermediate methyl vinyl ketone oxide

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 O‒O 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 O‒O 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 C‒I bond rather than the central vinylic C‒I bond of the precursor upon photolysis. At high pressure, the adduct C2H3C(CH3)IOO was also observed. The reaction mechanism is characterized.


Supplementary Note 2. IR spectra of precursor (Z)-(CH2I)HC=C(CH3)I (1)
The IR spectrum of gaseous precursor (Z)-(CH2I)HC=C(CH3)I (1) in region 1450-850 cm 1 is presented in Supplementary Figure 5 (a). This spectrum is compared with the spectrum of (1) in a solid p-H2 matrix 1 and stick spectra of (1) predicted with the B2PLYP-D3 and B3LYP methods in Supplementary Figure 5. Four intense bands near 1434, 1294, 1152, and 1063 cm 1 and two weaker ones near 1384 and 1169 cm 1 (the latter appears as a shoulder of the band near 1152 cm 1 ) were observed, in agreement with those observed for the same conformer in a p-H2 matrix at 3.2 K. 1 According to the plot of experimental wavenumbers versus harmonic vibrational wavenumbers of (1) predicted with the B3LYP/aug-cc-pVTZ-pp method, we derived a linear scaling equation y = (0.9708 ± 0.0159) x + (9.3 ± 20.7), in which y and x are experimental and harmonic vibrational wavenumbers, respectively. All observed wavenumbers and intensities are compared with those observed in solid p-H2 and scaled harmonic vibrational wavenumbers and IR intensities in Supplementary Tables 5. We employed this equation to scale the predicted harmonic vibrational wavenumbers of other species considered in this work. On comparison of the spectrum of precursor reported by Barber et al. (in Supporting Information), 2 the samples that these authors used were clearly a mixture of both (Z)-and (E)-conformers, with the former dominant, as they stated. In this work, a nearly pure (Z)-conformer (1) was used.

Supplementary Note 3. IR spectra of the iodoalkenyl radical (Z)-C2H3C(CH3)I (2)
When the diiodoalkene precursor (1) in N2 was irradiated with light at 248 nm, the intensity of its lines decreased significantly, as shown in Supplementary Figure 6(b) as a difference spectrum obtained from the ac-channel recorded 0-3 µs after irradiation; intense negative bands indicate the destruction of the precursor, whereas the formation of products is indicated by some extremely weak positive features. The expanded spectra of products recorded 0-3 and 10-15 µs after irradiation are shown in Supplementary Figures S6(c) and S6(d), respectively, with the negative bands truncated.
The features corresponding to the primary photolysis product decreased with time, but a broad feature near 915 cm 1 and two sharp lines at 919 and 892 cm 1 increased continuously. We termed these six features near 1406, 1261, 1109, 1019, 925, and 873 cm 1 that are associated with the primary photolysis product as group A and marked them A1-A6 in Supplementary Figure 6 with grey. Some regions of the parent absorption could not be compensated completely because some precursors might become internally excited upon irradiation, so that their absorption spectrum is differed from that before irradiation.
The assignments of these new features in group A to the iodoalkenyl radical (Z)-C2H3C(CH3)I (2) is discussed in the main text. Comparison of IR spectra of features in group with the IR stick spectra of two possible photolysis products, (Z)-C2H3C(CH3)I (2) and (Z)-(CH2I)CHC(CH3), according to the scaled harmonic vibrational wavenumbers predicted with the B3LYP method are shown in Figure 2. The observed new features agree satisfactorily with lines predicted near 1418, 1261, 1108, 1018, 930, and 887 cm 1 for (2), as compared in Supplementary Table 11. Comparison of IR spectra of lines in group A with the IR stick spectra of (Z)-C2H3C(CH3)I (2) and (E)-C2H3C(CH3)I is presented in Supplementary Figure 7; the agreement of experiments with the latter is poor, indicating that the conversion from (Z)-to (E)-conformation did not occur.

Supplementary Note 4. Photolysis of (Z)-(CH2I)HC=C(CH3)I (1) in O2 at 35 Torr
The top trace in Supplementary Figure 8  (3) showed an initial rise from zero, followed by a slow decay; its rate of rise correlates well with the rate of decay of (2), supporting that (3) was produced from the reaction of (2) with O2 and that our spectral assignments are reasonable.
Supplementary Figure 14(b) presents the temporal profiles of iodoperoxy adduct (4) (black triangles and red inverted triangles) and MVKO (3) (blue circles) in an experiment of (1) in O2 at 347 Torr. Parts of bands C3 (1100-1120 cm 1 ), C6 (862-907 cm 1 ), and B6 (920-960 cm 1 ) were integrated to yield the temporal evolution of (4) and (3), respectively. All profiles were normalized for ease of comparison; species (3) and (4) showed a similarly sharp rise due to the rapid formation reaction of (2) with O2 at high pressure, followed by different slow decays. The similar rate of rise is consistent with the expectation from a parallel reaction, supporting that MVKO (3) and the iodoperoxy adduct (4) were produced from the same reaction, that of iodoalkenyl radical (2) with O2.
In contrast, the temporal profile of MVK (pink diamonds, integrated over 1250-1270 cm 1 ) had a slow rise, indicating the nature of secondary formation. The fraction of loss of MVKO (3) appeared to be smaller than that of (4). This might be because band B6 overlaps with an intense band of MVK in this region, and MVK is a major product when O2 pressure is high. The profile of (3) after 0.2 ms is thus unreliable and might reflect mostly the behavior of MVK. Further experiments with higher spectral resolution are needed to clarify this problem.
The spectra obtained at 35 Torr showed predominant production of (3) with little contribution of (4) (Supplementary Figure 8). In contrast, at 236 and 347 Torr, the yield of (3) decreased and that of (4) increased significantly ( Supplementary Figures 10 and 11). The analysis is listed in Supplementary Table 15 and discussed in the following section. This observation is also consistent with the expectation that the iodoperoxy adduct (4) is stabilized at higher pressure instead of decomposition to MVKO + I, similarly to what was observed in experiments of CH2I + O2. 4  in the photolysis region was calculated according to equation (1) Because band C6 of C2H3C(CH3)IOO (4) in region 862907 cm −1 overlaps with the weak band 9 B7 of MVKO (3), we performed spectrum subtraction to remove the contribution of (3) and estimated the error associated with this interference to be less than 11 %. Even including this possible errors, the observed intensity of band C6 relative to those of other bands in group C appeared, however, to be much greater than theoretically predicted; the absolute concentration of (4) according to band C6 might be overestimated. As shown in rows 10 and 11 of Supplementary Table S15, the estimated concentration of (4) according to bands C3 and C6 varied by factor 1.5-2.3.
Nevertheless, the percentage variation of concentrations estimated from each band as the total pressure increased, after taking into account the difference in photolysis yield of the precursor (1) in each experiment, is expected to be reliable. The derived relative concentrations of (3) and (