59th Annual Report on Research 2014

The Criegee intermediates were produced by photolyzing gem-diiodo precursors entrained in a 20% O₂/Ar gas mixture within a quartz capillary tube affixed to a pulsed valve, with subsequent cooling in a supersonic expansion.  The (CH₃)₂CI₂ and CH₃CH₂CHI₂ precursors were synthesized in our laboratory.  The photoproducts were then ionized with VUV excitation at 118 nm (10.5 eV) for mass- and isomer-selective detection of the parent R₁R₂COO⁺ ion in a time-of-flight (TOF) mass spectrometer. 

UV spectroscopic studies of the Criegee intermediates were carried out by electronic excitation on the B-X transition in the 280-380 nm region.  UV excitation resonant with the B-X transition depletes the ground state population of the Criegee intermediate, which is detected as a reduced VUV photoionization signal for R₁R₂COO⁺.  As an example, a time-of-flight (TOF) mass spectrum for the dimethyl-substituted Criegee intermediate (CH₃)₂COO at m/z=74 obtained by VUV photoionization is shown in Fig. 1, along with the UV-induced depletion (at 323 nm) of the VUV photoionization signal (UV+VUV) and the difference signal determined by subtracting the UV-on from UV-off signal.  The absorption cross section is then obtained from the magnitude of the depletion of the ground state and the photon flux. 

The resultant UV spectra obtained for the simplest Criegee intermediate CH₂OO and several alkyl-substituted Criegee intermediates are shown in Fig. 2.  In each case, the data has been fit to a simple Gaussian form with uncertainty indicated by the shaded region.  The peak cross sections are ~5 x 10^-17 cm² molec^-1, indicating very strong UV absorptions by the Criegee intermediates over a broad spectral region.  The UV absorption spectra of the alkyl-substituted Criegee intermediates are similar to one another, yet shifted ~15 nm to shorter wavelength than CH₂OO, indicating the presence of an attractive intramolecular interaction between an alpha-H of the alkyl group and terminal O of the COO group. 

Since hydroxyl (OH) radicals are important decay products of energized Criegee intermediates, we have looked for OH radicals produced concurrently with the generation of the Criegee intermediates in the quartz capillary tube.  The OH radicals are then detected in their lowest quantum state following cooling in the supersonic expansion using 1+1ʹ REMPI with OH A-X (1,0) R₁(1) excitation and VUV ionization at 118 nm.  The OH⁺ signals shown in Fig. 3 are scaled relative to the corresponding Criegee intermediate ion signals obtained by VUV photoionization in the same mass spectrum.  We observe OH signals increasing in a ratio of 1: 4: 6: 10 for CH₂OO: CH₃CHOO: (CH₃)₂COO: CH₃CH₂CHOO.  Our results are supported by the IUPAC preferred OH yields from alkene ozonolysis.  The trend in OH generation for the alkyl substituted Criegee intermediates may reflect their respective barriers for 1,4 hydrogen shift from the Criegee intermediates to vinyl hydroperoxides along the unimolecular dissociation pathway to OH products.  Examination of the mechanism for hydrogen transfer and unimolecular decay of the alkyl-substituted Criegee intermediates is an important topic of ongoing research.

Fig. 1  TOF mass spectra for (CH₃)₂COO upon VUV photoionization showing depletion induced by UV excitation.

Fig. 2  Overview of UV absorption spectra (Gaussian fits shown) for the strong B-X transition characteristic of the carbonyl oxide group for CH₂OO and several alkyl substituted Criegee intermediates obtained in this laboratory.

Fig. 3  OH photoionization signals scaled relative to the corresponding Criegee intermediate ion signal.