Studies via Near-Infrared Cavity Ringdown Spectroscopy and

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Studies via Near-IR Cavity Ringdown Spectroscopy and Electronic Structure Calculations of the Products of the Photolysis of Dihalomethane/N/O Mixtures 2

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Meng Huang, Terry A. Miller, Neal D. Kline, and Richard Dawes J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b10632 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

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The Journal of Physical Chemistry

Studies via Near-IR Cavity Ringdown Spectroscopy and Electronic Structure Calculations of the Products of the Photolysis of Dihalomethane/N2/O2 Mixtures Meng Huang,† Terry A. Miller,∗,† Neal Kline,†,¶ and Richard Dawes‡ Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH 43210, USA, and Department of Chemistry, Missouri University of Science and Technology, Rolla, MO 65409, USA E-mail: [email protected]

∗ To

whom correspondence should be addressed of Chemistry and Biochemistry, The Ohio State University, Columbus, OH 43210, USA ‡ Department of Chemistry, Missouri University of Science and Technology, Rolla, MO 65409, USA ¶ Research and Technology Directorate, Edgewood Chemical Biological Center, Aberdeen Proving Ground, Maryland 21010-5424, USA † Department

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Abstract Near-IR cavity ringdown spectra have been recorded following the photolysis of dihalomethanes in O2 /N2 mixtures. In particular photolysis of CH2 I2 under conditions previously reported to produce the simplest Criegee intermediate, CH2 O2 , gave a complex, structured spectrum bee of CH2 O2 might tween 6800 and 9000 cm−1 , where the lowest triplet-singlet transition(e a − X) be expected. To help identify the carrier of the spectrum, extensive electronic structure calculations have been preformed on the ae and Xe states of CH2 O2 and the lowest two doublet states of the iodomethylperoxy radical, CH2 IO2 , which also could be produced by the cheme − Xe transition likely lies in this spectral region. The conclusion of these istry and whose A calculations is that the ae − Xe transition of CH2 O2 clearly falls outside the observed spectral range and it would be extremely weak both because it is spin-forbidden and because of a large geometric change between the ae and Xe states. Moreover, only a shallow well (with a barrrier to dissociation of less than 1900 cm−1 ) is predicted on the ae state which likely precludes the e − Xe transition of CH2 IO2 are generally existence of long-lived states. Calculations for the A consistent with the observed spectrum, in terms of both the electronic origin and vibrational e state. To confirm the carrier assignment to CH2 IO2 , calculations beyond frequencies in the A the Franck-Condon approximation were carried out to explain the hot band structure of the large-amplitude, low-frequency OOCI torsion mode, ν12 . Photolysis of other dihalomethanes produced similar spectra which were analyzed and assigned to CH2 ClO2 and CH2 BrO2 . Exe state vibrations and perimental values for the electronic energies and frequencies for several A the ν12 vibration of the Xe state of each are reported. In addition the observed spectra were used to follow the self reaction of the CH2 IO2 species and its reaction with SO2 . The rates of these reactions are dramatically faster than those of unsubstituted alkyl peroxy radicals and approach those of the Criegee intermediate.

Introduction In the late 1940s Criegee 1,2 was investigating the mechanism of solution phase ozonolysis because of its wide-spread importance in synthetic chemistry. Based on these studies he proposed a mech2 ACS Paragon Plus Environment

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anism for the model ozonolysis reaction of alkenes by ozone which involves a 1,3-cycloaddition across the double bond to form an energy-rich, five membered ring called a primary ozonide. Due to its instability, the primary ozonide decomposes by cleaving at the C-C bond and one of the OO bonds. For example with the simplest alkene, ethene, the reaction produces a stable carbonyl molecule, CH2 O, and an unstable carbonyl oxide, CH2 O2 , known as a Criegee intermediate. Subsequent product studies of ozonolysis reactions have supported Criegee’s mechanism by identifying the main products as the carbonyl compounds predicted by Criegee while also identifying fragmentation products of Criegee intermediates (CO, CO2 ). 3–9 While Criegee’s original work pertained to solution chemistry, it has been broadly accepted that such ozonolysis reactions are a major means for the cleansing of unsaturated hydrocarbons from the troposphere. 10–13 Moreover, Criegee intermediates like CH2 O2 (which we shall also refer to as the methylene peroxy radical, for reasons that will become apparent in Sections 2 and 3) and its substituted analogs, R1 R2 CO2 , should be present in the troposphere. Nonetheless, despite widespread acceptance of the mechanism, no gas-phase physical detection of a Criegee intermediate had been reported until recently. The motivation for our studies via near-infrared (NIR) cavity ringdown spectroscopy (CRDS) of the products of the photolysis of dihalomethane/O2 mixtures began with a seminal article by Taatjes et al., 14 which reported the first physical detection of the simplest Criegee intermediate, CH2 O2 . This first detection was via photoionization mass spectroscopy and CH2 O2 was produced by a relatively complicated Cl-initiated oxidation of dimethyl sulfoxide. In 2012, the same detection technique was employed to identify 15 CH2 O2 produced via the much simpler reaction involving the photolysis of CH2 I2 in the presence of O2 . This second report sparked a flurry of activity to obtain other spectroscopic signatures of methylene peroxy that would characterize the geometric and electronic structure of the species which the mass spectrum could not, and also to provide alternative methods for following its chemistry. Later in 2012, an action spectrum showing a broad electronic absorption in the UV was reported by Beames et al., 16 and others. 17–19 In 2013 Nakajima and Endo 20 reported the first rotational spectrum using a Fourier-Transform microwave spectrometer and the first gas-phase vibrational spec-



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trum was reported 21,22 in 2013 also using a time-resolved Fourier transform infrared spectrometer. Since then, numerous other spectroscopic reports have followed which are nicely chronicled in the recent reviews of the gas-phase spectroscopy and reaction chemistry of Criegee intermediates by Osborn and Taatjes 12 and by Lee. 11 Our experiments began in 2012 just after the UV absorption had been reported with the aim of being able to obtain an electronic spectrum that showed vibrational and rotational structure which was absent in the UV band. Similar to most of the other investigations we employed the photolysis of CH2 I2 in the presence of O2 as a method of production for CH2 O2 . However our spectroscopic goal was not the Be − Xe electronic band in the UV. Rather we took our direction from the sparse theoretical work available at that time. Most of it was based upon generalized valence band (GVB) theory, which had concluded 23,24 that CH2 O2 should have an electronic structure resembling that of the isoelectronic ozone, O3 , and that the ground state of CH2 O2 should have a biradical character, hence its characterization as methylene peroxy. The work of Cremer, et al. 25 in 1993 suggested that the ground state had less biradical than Zwitterionic character, but did not question the analogy between the electronic structure of O3 and CH2 O2 . A recent study analyzes leading configurations in multireference wavefunctions and uses NBO analysis to conclude that the character is mixed between zwitterionic and biradical, neither dominant. 26 Another recent study describes the ground state as a zwitterion with very little biradical character, yet cites mixing with a biradical triplet state, to explain the observed reactivity. 27 Most importantly for our experiments the early GVB work predicted that the electronic absorption spectrum of CH2 O2 should have a strong similarity to that of O3 , which is well known and contains four absorption bands: the Hartley, Huggins, Chappuis, and Wulf bands. 28–31 All of these originate from the ground 1 A1 state with the Wulf band lowest in energy. Bouvier et al. 32–34 performed high-resolution, FTIR studies on the Wulf band determining an excited state geometry and rotational constants most consistent with a 3 A2 state. The singlet-triplet energy gap is expected to be related to the size of overlap between the two singly occupied p-orbitals in O3 or CH2 O2 . Wadt and Goddard 35 had earlier claimed that the overlap, and consequently the singlet-triplet energy


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gap, will be smaller in methylene peroxy than in ozone because the central lone pair of electrons will not delocalize as greatly across methylene peroxy as ozone. The origin of the ae3 A2 -Xe1 A1 transition in ozone is 33 9553.021(78) cm−1 . If the above overlap model is predictive then the analogous ae3 A0 -Xe1 A0 transition in methylene peroxy would be found at slightly lower energies in the near infrared (NIR). Armed with a cavity ringdown spectrometer (CRDS) routinely operating in the NIR and a convenient method of production established for CH2 O2 , we performed experiments designed to e spectrum, which we hoped would have well resolved vibrational and rotational observe its e a−X structure like the corresponding Wulf band of O3 . However while the new procedure for producing CH2 O2 may be experimentally simple, it is not necessarily mechanistically so. Indeed, the presently accepted 11,12 mechanism for the photolysis of a dihalomethane (CH2 XI, where X=Cl, Br, or I) in the presence of O2 with N2 as a third body is shown schematically in Fig. 1. For X=I, recent measurements indicate that yields for production of the iodomethyl peroxy, CH2 XO2 , and the methylene peroxy radicals, CH2 O2 , are broadly comparable 11,12 in the range from above 10 torr to over 1 atmosphere total pressure with a N2 /O2 mixture serving for a third body. Detailed measurements in the case of X=Cl or Br are not available but there appears to be no evidence in the literature for the channel in Fig. 1 producing the methylene peroxy intermediate from these precursors. However there have been a number of reports 36–38 of UV absorption spectra for halomethyl peroxy radicals (CH2 XO2 ), although for X=I, CH2 O2 may be responsible for some or all the absorption. 11 There have also been recent reports of vibrational spectra of CH2 IO2 in a p-H2 matrix 39 and an Ar matrix. 40 Fig. 2 shows the CRDS spectrum that we observed in the NIR when CH2 I2 is photolyzed by a KrF excimer laser at 248 nm. The spectrum contains features from more than one photoproduct, among which are a single line belonging to atomic I and a molecular spectrum with complex structure. The remainder of this paper is devoted to the identification of the carrier of the molecular spectrum, its spectral analysis, and its use to characterize the molecule and to gain insight into the accompanying reaction chemistry. To accomplish these tasks, we employ a couple of approaches.



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(i) Experimentally we examined the variation of the spectrum with various changes in the reaction chemistry including changes in N2 /O2 ratio and total pressure. We have observed the spectral region with several other dihalomethane precursors besides CH2 I2 including CH2 Cl2 , CH2 ClI, CH2 Br2 , and CH2 BrI. (ii) From a theoretical approach we have performed a number of electronic structure calculations for the ground and lowest excited states of possible spectral carriers. These e and Xe states of the halomethyl peroxy calculations include the ae and Xe states of CH2 O2 , and the A radicals, CH2 XO2 with X=Cl, Br, and I.

Experimental The apparatus used to observe the peroxy radicals in the NIR is shown in Fig. 3. The apparatus generally has been described previously, but we give a brief overview here to facilitate understanding of some observations described below. A PrecisionScan Sirah Dye Laser is pumped with the second harmonic of a Nd:YAG laser at 20 Hz and tuned over the range 575-670 nm using four different dyes: DCM, Rhodamine B, Rhodamine 101, and Pyrromethene 597. The output of the dye laser (60-110 mJ/pulse) is focused by a lens (f=50 cm) into a 70 cm single pass Raman cell purged with 300 psi H2 . This generates the required NIR radiation (1.51-1.10 µm) by stimulated Raman shifting (SRS). When the radiation exits the Raman cell the second Stokes component is isolated using 1000 nm long pass filters (Corion-LL-1000) and the resulting 1-3 mJ/pulse of NIR radiation is coupled into the ringdown cell. The cell is fabricated of stainless steel, 54 cm in length, and terminated by two 6 m radius-of-curvature, plano-concave mirrors (R≥99.995%, Los Gatos Research) which are housed in custom made flanges that allow for accurate alignment using finely threaded screws for adjustment. To cover the entire spectral region of interest, three sets of mirrors centered at 1.4, 1.3, and 1.2 µm were used, with the mirror sets having sufficient overlap to ensure continuous wavelength coverage. To prevent damage to the mirrors a constant purge of nitrogen was used to shield them. In addition to the mirror purge inlets, the ringdown cell was constructed with inlets for precursor


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gases, a Baratron pressure gauge, and an exhaust port leading to a mechanical pump. Once the NIR light exits the cell it is focused onto an amplified InGaAS photodiode detector (Thorlabs, PDA 400) whose output is recorded by a 12 bit 20 MHz digitizing card. During a typical scan approximately 20-40 consecutive laser shots are averaged at each dye laser frequency and, using the nonlinear Levenberg-Marquardt algorithm, the digitized signal of the decay of radiation is fit to a single exponential to acquire the ringdown time, initial amplitude, and baseline. The calculated ringdown time is converted to cavity absorption per pass and saved as a data point for a given frequency in the spectrum. The cell is fashioned with two rectangular apertures (2 x 16 cm) in the center of the cell separated by 15.6 cm which accept UV grade quartz windows that allow for photolysis of the precursor gases. An excimer laser is used to initiate the chemistry. The excimer is operated with a gas mixture of ArF or KrF to generate 193 or 248 nm light depending on the experiment being done. Once the radiation leaves the excimer a cylindrical and spherical lens shaped the beam to to a rectangular cross-section (13.0 cm along the cavity axis by 0.1 cm height) which passes through the photolysis windows. The excimer laser is fired 1-5 µs before each pulse of NIR light enters the cell, which allows enough time for the reactive intermediates to form, but not enough time for them to react significantly, be pumped out, or diffuse to the walls of the cell. At each dye laser frequency, a ringdown time is acquired with the photolysis excimer laser on and off. This will generate two traces: an on trace containing photolysis products and an off trace that contains background structures (i.e. precursor bands, background water lines, etc). These two traces are subtracted (on-off) to produce a trace dependent upon the photolysis products. To produce the chemical intermediates of interest, photolysis of a halogenated precursor in O2 was employed with N2 present as an additional third body. When CH2 I2 was photolyzed with 248 nm light the spectrum in Fig. 2 was produced. Photolysis of CH2 ClI (97%, Sigma Aldrich) and CH2 Cl2 (≥99.8%,Sigma Aldrich) with 248 and 193 nm light, respectively, produced a common spectrum different from the one observed from CH2 I2 . Photolysis of both CH2 Br2 (99%, Sigma Aldrich) and CH2 BrI (97%, Sigma Aldrich) both produced a common spectrum, but different



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from the previous two. In these cases the iodine atom will preferentially be photolyzed off the precursors that contain iodine, forming a CH2 X (X=Br or Cl) radical, while the CH2 X2 precursors have one X photolyzed off to create the aforementioned radicals. Traces of the spectra produced from precursors other than CH2 I2 are shown in the supplementary material. Liquid precursor samples were obtained commercially and were kept in glass sample bombs while performing experiments. The bombs were heated to 40◦ -50◦ C while N2 was bubbled through with a backing pressure of 1-5 psi maintaining ≈5.0-7.0 torr of this gas mixture in the ringdown cell. Typical partial pressures in the cell for this experiment were [N2 ]≈27.0-30.0 torr, [O2 ]≈30.0 torr, and halogenated hydrocarbon precursors ≈0.2-0.4 torr.

Theory and Computational Electronic Structure Calculations Xe and ae States of Criegee Intermediate, CH2 OO A series of high-level electronic structure calculations were performed to guide interpretation of the experimental spectrum in Fig. 2. As mentioned above, with CH2 I2 as a precursor the experimental conditions likely produce a mixture of CH2 IOO and CH2 OO, with pressure dependent branching. The initial goal of the calculations was to support or rule out a singlet-triplet transition in CH2 OO analogous to the Wulf band in ozone as the carrier of complex molecular spectra recorded near 7000 cm−1 . Reasoning based on orbital overlap concepts (mentioned in the introduction) had placed the ae3 A0 -Xe1 A0 gap significantly below the ozone Wulf band origin at 9553.021(78) cm−1 making such an assignment plausible. The ground Xe1 A0 state of the CH2 OO Criegee intermediate is a planar (CS symmetry) local minimum, significantly less stable than two other (dioxirane and methylenebis(oxy)) isomers. 22 The ground state near equilibrium is reasonably well described by single reference methods despite a T1 diagnostic 41 of about 0.044. A PES constructed at the CCSD(T)-F12a/AVTZ level yields


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generally good agreement with rovibrational spectra, although slightly overestimating the OOstretching frequency. 22 It is worth noting that single reference methods do break down trying to describe the singlet ground state for a small, high-energy range of torsional coordinate values φ = [70, 110] degrees (rotating the terminal O-atom above the plane of the formaldehyde fragment). In that range, divergent values of CCSD(T) energies are obtained accompanied by T1 diagnostic values greater than 0.20. 42 Photodissociation of CH2 OO (to formaldehyde + O-atom) via excitation to the Be1 A0 state has been the subject of several experimental and theoretical studies. 16–19,43 Dissociation along the OO bond is somewhat different from that of the isoelectronic ozone system. Ground state ozone is also a singlet and adiabatically dissociates to ground state O2 (X 3 Σ− ) + O(3 P) (both triplets). Diabatically it dissociates to the excited singlet delta state of O2 (a1 ∆) and excited O(1 D). Those states are involved in an avoided crossing causing a large barrier at the CASSCF level, which becomes a submerged reef or disappears completely for higher level calculations. 44–46 CH2 OO is spin-forbidden to dissociate to ground state products (singlet CH2 O(Xe1 A1 ) + O(3 P)), so adiabatically and diabatically it connects to CH2 O(Xe1 A1 ) + O(1 D). This means that there is no disruption caused by avoided crossings for the ground state, although the CASSCF method substantially underestimates the dissociation energy. 19 On the other hand the CCSD(T) method slightly overestimates the dissociation energy which contributes to pushing up the too-large OO-stretching frequency. 19 Perhaps the most reliable global description is obtained via multireference MRCI calculations. In a previous study, Davidson-corrected multistate calculations at the explicitly-correlated MRCIF12/VTZ-F12 level, were found to describe the low-lying singlet states quantitatively, matching e Xe band, as well as the dissociation closely both the experimental UV absorption spectrum of the B− energy of the ground state. However, the cost of calculations at this level is nearly prohibitive especially since a large active space was found to be required. 19 Here, tightly optimized structures and harmonic frequencies were obtained for the ground singlet Xe1 A0 state as well as the lowest lying triplet ae3 A0 state using the CCSD(T*)-F12b/VTZ-F12 and UCCSD(T*)-F12b/VTZ methods 47–49 (for the singlet and triplet states respectively), where



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(T*) refers to scaling of the triples contribution. All of the explicitly-correlated calculations reported here were performed using the MOLPRO code package. 50 The structures are shown with their geometric parameters in Fig. 4 and the harmonic frequencies are given in Table 1. The structure and harmonic frequencies recorded for the ground singlet state are very similar to those computed previously at the CCSD(T)-F12a/AVTZ level, and are in nearly exact agreement with the empirical structural parameters determined by McCarthy et al. 51 The equilibrium structure for the lowest triplet state is qualitatively different, with the terminal O-atom rotated out of plane above the formaldehyde fragment. Thus the lowest triplet state is 3 A0 for planar structures such as at the FC point above the Xe1 A0 state, but becomes 3 A00 with respect to a different plane of symmetry at its non-planar equilibrium geometry (see Fig. 4). The calculated value for the band origin is T00 = 10589 cm−1 at the (U)CCSD(T*)-F12b/VTZ level including ZPE-correction (far above the observed spectrum around 7000 cm−1 ). Since the T1 -diagnostic values for the singlet and triplet states at their equilibrium geometries are 0.044 and 0.038 respectively, single-point MRCI-F12 calculations 52 were performed to confirm the large gap obtained in the single reference calculations. Davidson-corrected MRCI-F12/VTZ-F12 calculations were performed using a (12e, 11o) active space in the CASSCF reference (the 2s orbitals on C and O-atoms were held doubly occupied starting with a full-valence active space). This produced a remarkably similar calculated value for the band origin of T00 = 10585 cm−1 , differing by only 4 cm−1 . As shown in Table 1, ZPE differences lower the gap by nearly 600 cm−1 (∆ZPE = -597 cm−1 ), so the adiabatic gap between minima is more than 11,000 cm−1 (Te = 11,186 cm−1 at the (U)CCSD(T*)-F12b/VTZ-F12 level, see Table 1. This is similar to a value of 11,292 cm−1 reported at the CASSCF/VTZ level. 27 The vertical gap is significantly larger due to significant differences in geometric parameters (e.g. rCO and φ ) shown in Fig. 4. Considering the torsional coordinate φ , we compute a barrier to planarity of 840 cm−1 at the (U)CCSD(T*)-F12b/VTZ-F12 level. Based on these calculations it was determined that the ae3 A00 -Xe1 A0 band of CH2 OO is not the carrier of the spectrum recorded around 7000 cm−1 . The error in gaps between electronic states obtained by these methods for small systems is usually less than 0.1 eV and often much better


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than that. 53 In addition, given the qualitatively different equilibrium structures, very small FranckCondon and overall intensity factors would be anticipated for this spin-forbidden transition. In the Supporting Information we follow a relaxed path dissociating the ae3 A00 state to CH2 O(Xe1 A1 ) + O(3 P) fragments. As has been mentioned previously, the electronic structure of ae3 A00 is found to be a simple biradical. 27 A dominant leading reference coefficient of about 0.94 is obtained throughout the rOO dissociation coordinate. Thus the single reference (U)CCSD(T*)-F12b/VTZ-F12 method is reliable and performs similarly to the much more costly MRCI-F12/VTZ-F12 method. More details and the behavior of the lowest six electronic states along this cut are provided by Figures S1-S3 of the Supporting Information. It is noteworthy that only a shallow well is predicted, with a barrier of less than 1,900 cm−1 to dissociation to the much more stable CH2 O(Xe1 A1 ) + O(3 P) products. The ZPE far exceeds the barrier which likely precludes the existence of long-lived states. e and 2 Xe states of the halomethyl In the next sub-section we will show that transitions between the 2 A peroxy radicals, CH2 XO2 with X=Cl, Br, and I are consistent with the observed spectra. e states of CH2 XOO(X = Cl, Br, I) Xe and A A series of ab initio calculations were performed with the Gaussian 09 54 package to obtain the equilibrium molecular geometry and vibrational frequencies of the CH2 XO2 molecules in their e For CH2 ClOO and CH2 BrOO radicals, B3LYP/ATVZ level of lowest two doublet states, Xe and A. theory and basis sets are used for the Xe state calculation while TDB3LYP/ATVZ level of theory and e state. To estimate the energy difference between the Xe and A e states, basis sets were used for the A e states, the G2 calculations 55 were performed on both states. In order to produce G2 results for the A electronic configuration was obtained by permuting the HOMO and SOMO orbitals of the radicals from the initial SCF reference. For the CH2 IOO radical, a basis set with a pseudo potential 56 was e states, respectively. combined with B3LYP and TDB3LYP calculations for the Xe and A For all the CH2 XO2 radicals, we also ran higher level electronic structure calculations with the UCCSD(T*)-F12b, MRCI-F12, and CASSCF levels of theory. Peterson’s new PP-based F12 basis sets 57 were used for Br and I-atoms, while all-electron descriptions of the other atoms including



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Cl were obtained using the VTZ-F12 and VDZ-F12 basis sets. 58 Relaxed scans of the Xe ground state along the torsional coordinate were generated using the UCCSD(T*)-F12b method. It was somewhat challenging (due to a tendency for orbital switching in/out of the active space) to obtain consistent yet affordable CASSCF and MRCI-F12 calculations for the full range of the torsional coordinate. Stable orbitals for the (13e, 12o) and more affordable (9e, 8o) active spaces were achieved by including the lowest three doublet states with relative weights of 1.0, 1.0, 0.2. A e state was thus obtained at the CASSCF relaxed scan of the torsional coordinate for the excited A level. A few single point MRCI-F12 calculations found that the energy variation along the torsional coordinate was not strongly dependent on dynamic correlation (the shape of the CASSCF curve was found to be quite accurate). The biggest limitation on the CASSCF calculations is the accuracy of the gap between states. Conveniently, at two points along the torsional coordinate (0 and 180 e state degrees) the system has CS symmetry, with the ground Xe state being A00 and the excited A being A0 . This means that the highly accurate and affordable UCCSD(T*)-F12b method can be used to describe both states at those two points (since the two states are each the lowest of their e state with the best respective symmetries). Thus to construct a torsional scan for the upper A estimate of the gap, the relaxed curve obtained at the CASSCF(9e, 8o) level was matched to the UCCSD(T*)-F12b gaps calculated at 0 and 180 degrees. The calculated geometric parameters for CH2 XO2 are listed in Table 2 and the calculated ene states are listed in Table 3. The normal mode frequencies for ergy differences between the Xe and A CH2 IO2 are listed in Table 4, and for CH2 BrO2 and CH2 ClO2 in Tables S1 and S2, respectively, of the supplementary materials. In Table 4 we compare our calculated frequencies with values obtained from the present work and previous matrix studies of the vibrational spectra. 39,40

Intensity Calculations In addition to the calculated vibrational frequencies, to aid in assigning the experimental spectrum it is useful to have theoretical predictions of the intensity of different spectral transitions and for this Franck-Condon factors typically are required. The eZspectrum package 59 provides 12 ACS Paragon Plus Environment

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Franck-Condon factors and, assuming a Boltzmann distribution of population in the ground state corresponding intensity simulations. The eigenvalues (normal mode frequencies) and eigenvectors (L-matrix elements) from the normal mode analysis, along with the molecular geometries from our quantum chemistry calculation are required as input for the eZspectrum package. All normal modes are represented in Cartesian coordinates and the Taylor expansions of the potential energy with respect to the Cartesian-coordinate displacements are truncated at second order in the eZspectrum package. The Franck-Condon factors are obtained as the square of the overlap between the e state normal mode eigenfunctions after the appropriate Duschinsky rotation e and upper, A, lower, X, is applied to the latter’s normal modes. While these approximations are reasonable for the other modes of the CH2 XO2 species, they may not be sufficient for the low-frequency torsional mode. To illustrate that more clearly, Fig. 5 e shows slices along the torsional coordinate of the potential energy surfaces (PES) for the Xe and A states based upon the electronic structure calculations for these two states of CH2 IO2 . In order to better understand the behavior of the torsional vibrations, we simulated the torsional spectrum using the results of a one-dimensional torsional calculation with internal coordinates as well as using eZspectrum. The vibrational Hamiltonian can be written as

Htor = −F

∂2 +V (τ) ∂ τ2

(1)

where the F parameter provides the effective rotational constant of the motion along the torsional coordinate. The F values (see Table 2) were calculated following the previous treatment of the methyl rotor system 60,61 using results from the electronic structure calculations for both the Xe and e states of each radical. Table 2 shows that as the mass of the halide increases, the F value deA creases because the effective moment of inertia along the C-O bond increases. The potential energy surfaces used here and shown in Fig. 5 are the ones calculated from the U-CCSD(T*) and CASSCF levels of electronic structure calculations. The eigenvalues, Em00 and Em0 , and eigenfunctions, Ψ00v e states are evaluated using discrete variable representations, and Ψ0v , of Htor on both the Xe and A where the τ values ranged from 0 to 360 degrees with an increment of 1.8 degree. 13 ACS Paragon Plus Environment


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Relative intensities of the vibronic transitions can then be calculated by

2

2 I ∝ Ψ0v Ψ0e |~µ| Ψ00e Ψ00v = Ψ0v |~µe (τ)| Ψ00v

(2)

e states, and the ~µ is the where Ψ00e and Ψ0e represent the electronic wavefunctions of the Xe and A dipole moment. The transition probability is thus directly related to the square of the electronic transition dipole,

~ e (τ) = Ψ0e |~µ| Ψ00e µ

(3)

Electronic transition dipole surfaces are calculated using the CASSCF level of electronic struce states for CH2 XO2 radicals. The value of the transition dipole ture calculation on both the Xe and A ~ e (τ) at each grid basis set point in the DVR calculation is obtained by a spline intermoment µ polation. If the electronic transition dipole moment is independent of the vibrational coordinate, specifically the torsional coordinate, τ, in this calculation, the intensity can be written as

2 ~ e |2 I ∝ Ψ0v |Ψ00v |µ

(4)

Therefore, the relative intensity of the transitions are only dependent on |hΨ0v |Ψ00v i|2 , which is the Franck-Condon factor, whose calculation is not now limited by the assumption in eZspectrum that Ψv is a harmonic oscillator eigenfunction.

Results Product Dependence on Chemistry Fig. 2 shows the CRDS spectrum obtained from the 248 nm photolysis of CH2 I2 . There are interferences from H2 O, the CH2 I2 precursor, and the iodine atom, which are marked in the trace but there is a substantial transient molecular spectrum generated via the photolysis. The spectrum 14 ACS Paragon Plus Environment

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displays distinct characteristics of a peroxy radical, presumably either methylene or iodomethyl. Most prominent in the spectrum is a defined origin region near 6800-6900 cm−1 and ∼900 cm−1 to the blue of that, what appears to be an OO stretch region. This should come as no surprise as both the CH2 IO2 and CH2 O2 molecules would have OO stretching modes. The spectrum also displays a high level of complexity as there are more than 25 bands clearly distinguishable. While the molecular spectrum is characteristic of a peroxy radical, this most obvious characteristic certainly does not distinguish between iodomethyl and methylene peroxy. At the time that we first obtained this spectrum, no reliable electronic structure calculations for both the ground and first excited states of either species were available to aid in the analysis of the spectrum. For this reason several experiments were performed to monitor the behavior of the spectrum as the chemistry was modified, in order to gain some insight as to the spectral carrier. The behavior of the molecular spectrum was monitored as a function of delay time between NIR probe beam and excimer photolysis, and also in the presence and absence of oxygen. The absorbance of the photoproduct was found to decrease with increasing delay between NIR probe beam and excimer photolysis clearly indicating its transient nature. When O2 was omitted from the cell, no absorption signals were observed confirming the necessity of O2 to produce the spectrum. Efforts to distinguish between CH2 IO2 and CH2 O2 by monitoring the signal intensity as a function of total O2 /N2 pressure were not definitive as the spectrum appeared to peak at ≈100 torr and decrease to both higher and lower pressure. Presumably a number of experimental factors contribute to the signal intensity as total pressure is varied and it is difficult to disentangle them. To gain further insight we photolyzed CH2 X2 and CH2 XI (X=Cl, Br). Fig. 6 shows a comparison between the three observed spectra. The comparison shows that there is a good deal of similarity among the spectra, possibly indicating that CH2 XO2 radicals are the carriers of all three spectra. This follows since, as previously argued, CH2 X is the expected initial photoproduct produced when CH2 X2 and CH2 XI (X=Cl, Br and I) are photolyzed. However, the spectrum resulting from the CH2 I + O2 reaction does contain some unique features. Particularly noticeable is the triplet structure in both the origin and OO stretch region, conceivably arising from spin splitting



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in the ae3 A0 state of CH2 O2 . Furthermore, the spectral origin shifts to the blue with respect to CH2 BrO2 while there is a consistent red shift from CH3 O2 to CH2 ClO2 to CH2 BrO2 . To gain further insight into the chemistry, more involved investigations were performed as described below.

Iodine Atom Absorption Studies have been done previously inferring the identity of the photoproduct that is being formed in the CH2 I + O2 reaction by monitoring the concentration of iodine atoms as a function of O2 pressure. 62,63 These works photolyzed CH2 I2 in the absence of O2 to obtain a baseline for photolytically produced iodine atoms and then added O2 incrementally. It was found that there is an initial linear increase in iodine atom concentration with added O2 , implying (see Fig. 1) that methylene peroxy is being formed, but the iodine atom concentration decreases slightly as large amounts of O2 are added to the cell. A limitation of these earlier experiments is that they selectively monitored iodine atoms, but not the organic photoproduct simultaneously. As our photoproduct signal occurs in the 6700-9100 cm−1 region which contains the 2 P1/2 - 2 P3/2 transition of I, simultaneous observation is possible. We similarly observe that with no O2 present there is a baseline absorption of iodine atom from photolysis of CH2 I2 and no molecular photoproduct is detected. After O2 is added there is an increase in the iodine absorption along with appearance of the photoproduct of the spectrum which is to be expected, if methylene peroxy is produced by the reaction of CH2 I radicals with O2 . As O2 pressure is increased further the iodine atom absorption decreases slightly while the photoproduct absorption does not change. The decrease in iodine atom signal previously has been interpreted as indicating CH2 IO2 is being collisionally stabilized by 3-body collisions involving O2 as well as N2 . In addition, iodine atoms could react with O2 to produce species like IO and IO2 which would contribute to the decrease in iodine atom absorption. The reason why the iodine atom signal decreases is not critical to the argument that the data indicate that methylene peroxy is being formed in our experiment. Nonetheless, how much methylene peroxy is being formed or whether it is the carrier of the observed spectrum is not directly established by these observations.


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Self reaction rate With our apparatus we are also able to determine approximate half-lives of reactive intermediates due to self-reaction by varying the delay between the NIR probe beam and the excimer photolysis beam. Of course, to obtain the self-reaction rate constant, self-reaction needs to be the only significant decay process and the concentration of the reactive species needs to be known. The initial concentration can be estimated by measuring the power output of the excimer laser behind our ringdown cell in the absence and presence of the CH2 I2 precursor. We measured the total photolysis beam flux, with and without CH2 I2 present, immediately before and after it passed through the ringdown cavity perpendicular to the probe beam. We found 2.0 mJ/pulse of 248 nm light (2.5 x 1015 photons) is absorbed by the CH2 I2 precursor. CH2 I2 has a quantum yield 64 of 1 yielding 2.5 x 1015 CH2 I radicals. Assuming all CH2 I radicals produce methylene peroxy (as Su et al. 65 did) we get 2.5 x 1015 methylene peroxy radicals formed. To determine the concentration of radicals we take the cross section of our excimer beam (12.0 cm x 0.1 cm) and multiply by the path length along which the beam is propagating (15.0 cm). Those dimensions give us a volume of 18.0 cm3 and a radical concentration of 1.4 x 1014 molecules cm−3 . We estimate the half-life of the reactive species from our time-delay measurements as 15 µs and use the second order equation, to produce an estimate of the self-reaction rate constant, kobs , of 5±3 x 10−10 cm3 molecule−1 s−1 , near the encounter limit. This value is consistent with the methylene peroxy value reported contemporaneously by Su et al. 65 of (4±2)×10−10 cm3 molecule−1 s−1 and even qualitatively consistent with the current, generally accepted value 11,12 of 6-8×10−11 cm3 molecule1 s−1 (based upon a more complete mechanism, including other loss processes than was used in our work or that of Su et al.). By way of comparison, methyl and ethyl peroxy radical have much smaller self-reaction rate constants 38 of 4.8 and 0.97×10−13 cm3 molecule−1 s−1 , respectively.

Reaction with SO2 A rate constant with less systematic uncertainty can be obtained by adding a foreign gas in excess to obtain pseudo-first-order kinetics. The reaction rate of Criegee intermediates with SO2 17 ACS Paragon Plus Environment


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has been found to be orders of magnitude faster than the reaction rate of alkyl peroxy radicals with it. Methylene peroxy’s reaction rate with SO2 is reported 11,12,15,63 as (3.4 − 4.1) x 10−11 cm3 molecule−1 s−1 while the reaction rate of methyl peroxy 66,67 with SO2 is ≤1 x 10−16 cm3 molecule−1 s−1 , which means no observable reaction between methyl peroxy and SO2 under our experimental conditions. Therefore, when SO2 is added to the reaction cell, if the photoproduct behaves like a typical peroxy radical there should be no observable change in signal strength and if the carrier behaves like a Criegee intermediate there should be a clear reduction. Fig. 7 traces (a) and (b) show what happens to the spectra produced from Cl and Br containing precursors when 7.0 torr of SO2 is added to the reaction cell. As would be expected if CH2 ClO2 and CH2 BrO2 are the carriers, there is no discernable decrease in the signal when SO2 is added. However, Fig. 7 trace (c) shows the result with the same SO2 concentration present in the reaction cell when the photolysis of CH2 I2 is performed. There is clearly a reaction that takes place between the carrier of that spectrum and SO2 which extinguishes the spectrum. Based on the reaction rate information provided above, this behavior is consistent with the methylene peroxy Criegee intermediate.

Spectroscopic Analysis Summarizing the experimental measurements to this point, it appears that we have obtained mixed results concerning whether the carrier of the molecular spectrum in Fig. 2 is CH2 O2 or CH2 IO2 , but the experimental evidence apparently favors the former. It would be ideal to obtain a theoretical analysis of the spectrum and use internal evidence to determine its carrier. Comparison of the results of such an analysis with predictions of electronic structure calculations and other expectations for its geometric and electronic structure should establish unambiguously the carrier. When the experiments observing the effects upon the spectrum of variation of conditions in its production began, no electronic structure calculations were available to compare to a spectral analysis. However as the experiments progressed so did the calculations. We have already seen in Section 3 that electronic structure calculations now strongly indicate that no planar minimum exists for the ae3 A0 state of CH2 O2 . Since a non-planar minimum occurs, the combination of very 18 ACS Paragon Plus Environment

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small Franck-Condon factors with the intrinsic low oscillator strength of a singlet-triplet transition would make observation of the ae1 A−Xe1 A electronic transition highly unlikely even if copious amounts of CH2 O2 are present. It was also mentioned above that only a shallow well is predicted (not deep enough to support bound states). Therefore in the following, we shall examine whether predictions from electronic structure calculations are compatible with assigning the carriers of the observed spectra to CH2 XO2 . If not an alternative explanation must be found. We focus our attention on the spectrum produced by photolyzing CH2 I2 since it clearly has the best signal/noise and hence information content. Moreover this spectrum has the greatest general interest because it is produced under conditions for which CH2 O2 has been reported. Similar analyses of the corresponding spectra obtained by photolyzing Cl or Br containing precursors are presented in the supplementary material, which we will cross-reference as appropriate.

Comparison of simulations and the experimental spectrum in the origin region It is clear from the experimental spectra that complex structure appears in the origin region, and is replicated in combination bands with other normal modes, particularly the OO stretch. The strong similarity of the spectra between the OO stretch transition region and the origin region is shown clearly in Figs. 8(a) and (e) for CH2 IO2 . Based upon V(τ) we expect that most of the features which could contribute to the multiple peak structure in the regions assigned to the electronic origin and OO stretch transition are correlated with the OOCI torsional mode, due to its low frequency. To understand these features in detail, we will compare their spectral intensities calculated from to the square of the transition dipole integral, and Franck-Condon factors from our 1-D torsional calculation and eZspectrum, all corresponding to a Boltzmann population distribution at 300K. Simulations of the spectrum based on these calculations are shown in Fig. 8 and compared to the experimental spectra. The positions of the sticks on the frequency axis of the simulations are based on the vibrational frequencies from the electronic structure calculations and are listed in Table 4. (Corresponding figures for the spectra based on CH2 ClOO and CH2 BrOO calculation are



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shown in Figs. S4 and S5 of the supplementary information.) To facilitate comparison between the experiment and the simulations, the stick diagrams of all simulations are shifted to match the origin and OO stretch fundamental as appropriate. Likewise, the intensity of both the internal coordinate and the Cartesian coordinate simulations are scaled to match the experimental origin (and scaled OO stretch) intensity. For each electronic structure calculation, the internal 1-D coordinate and the eZspectrum simulations are color-coded by the torsional sequences, ∆m = m0 - m00 , where m00 and m0 provide the torsional quantum numbers in e states, respectively. In each simulation, there are multiple spectral sequences, each the Xe and A containing multiple transitions. The significant intensities of the many torsional transitions are attributable to the abundant population at 300 K of several Xe state torsional levels due to its low frequency. These simulations qualitatively explain the spectral features around the origin including the “triplet” structure which is not explicable in terms of the rotational structure. While qualitative agreement is seen with the experimental spectra for all simulations, there are some quantitative differences. For example, there are discrepancies between the simulations from the 1-D internal coordinate calculation and eZspectrum using Cartesian coordinates, even with the same electronic structure calculation. The simulation based on internal coordinates in Fig 8(c) successfully predicts the strong feature of the torsional fundamental transition 1210 which appears around 7010 cm−1 in the origin region [Fig. 8(a)] and 7890 cm−1 in the OO stretch region [Fig. 8(e)], while eZspectrum does not. This difference is probably due to the failure of the harmonic approximation in the Cartesian coordinates used in the eZspectrum calculation. For the simulations that use the transition probability considering the dependence of the electonic transition dipole moment on the torsional angle τ [Fig.8(d)], there are no qualitative differences in the intensities of transitions compared to the simulation that only uses Franck-Condon factor in 8(c). Quantitative comparisons between the intensities calculated for all species are shown in Table S3 of the supplementary information. We conclude that among all the simulations of the CH2 IOO spectra, the one based on the CCSD(T*)-F12b and CASSCF(9e,8o) levels of theory coupled with the 1-D internal torsional coordinate calculation shows the best agreement with the experimentally


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observed torsional structure.

Comparison of experimental spectrum and simulation above the origin Figs. 9 and 10 show the simulations of CH2 IOO for the complete experimental spectrum. (The corresponding simulations for CH2 BrOO and CH2 ClOO and experimental spectra are shown in Figs. S6 – S9 of the supplementary information.) For the simulations in Figs. 9 and 10, the torsion features are calculated using the 1-D internal coordinate model with the potential energy surface from the UCCSD(T*)-F12b and CASSCF(13e,12o) calculations. The frequencies of these e state to transitions are then shifted by fitting the frequencies of the torsion modes on Xe state and A match the experimental spectra. For the other normal modes, the intensity of the fundamentals and combination bands between these modes are obtained from eZspectrum calculation using the same e states, respectively. UCCSD(T*) and CASSCF electronic structure calculations for the Xe and A The torsional structure associated with each fundamental or combination band is now adjusted to match the fundamental frequencies, and the overall intensities are scaled by the predicted ratio of a given band’s intensity to that of the origin. As has been stated above, the multiple peak structure around the origin region and OO stretch region is explained by the torsional features. Other strong features in the experimental spectrum can be reproduced by simulations involving non-torsional degrees of freedom. The peak at 8023 cm−1 is assigned as the combination band of the OCI bending and OO stretch, and the corresponding OCI bend fundamental is then predicted at 7146 cm−1 , which is not clearly assigned due to the interference from other species in that region in the experimental spectrum. The peak at 7305 cm−1 and corresponding peak at 8182 cm−1 are readily assigned as the COO fundamental and its combination band with the OO stretch. The peak near 7416 cm−1 is assigned as the fundamental of the CI stretch, and its combination band with the OO stretch is assigned to the line at 8293 cm−1 . The broad shoulder structure in the 7550-7700 cm−1 region is attributed to the combination bands involving the CI stretch and the torsion, along with torsional hot band transitions associated with the OO stretch. The “experimental” frequencies so determined are compared to the predicted



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values in Table 4. (Similar results for CH2 BrO2 and CH2 ClO2 are given in Tables S2 and S3.) Finally we should note that while CH2 IO2 has both G and T conformer forms, we have found no features in the spectrum which could be clearly assigned to a transition of the T conformer. This is not surprising since as shown in Table 3, the T spectrum origin is displaced well to the blue of the G conformer, whose spectrum is quite congested in this region. Moreover, the footnote to Table 3 shows that the Boltzmann population of the T conformer would be . 10% of the G form at room temperature.

Discussion As Figs. 9 and 10 show, essentially all of the features in the observed molecular spectrum when CH2 I2 is photolyzed can be explained in terms of transitions of CH2 IO2 and provides convincing evidence to assign it as the carrier. While the vibrational frequencies are chosen to best match e state the experimental spectrum, the experimentally determined normal mode frequencies of the A (and the torsional frequency in the Xe state) for CH2 IOO, CH2 BrOO, CH2 ClOO are listed in Table 4, Table S1 and Table S2, respectively. The experimentally determined frequencies show generally good agreement with the theoretical predictions considering the uncertainty of the electronic structure calculations, and possible deviations due to the anharmonicities of the vibrations. Moreover, the computed line intensities are generally in good agreement with the observed ones. For e ← Xe transitions, the G2 calculation shows agreement 68 with the exthe origin frequency of the A perimental values, which is similar to the results in the previous studies of peroxy radicals. 68 For CH2 IO2 , where G2 values are unavailable, the UCCSD(T*)-F12b/CASSCF(13e,12o) calculation of the origin frequency agrees with experiment to within 150 cm−1 . The detailed analysis clearly explains a number of unusual, and initially perplexing, features of the spectrum observed with photolysis of CH2 I2 . The complex structure in the origin and OO stretch region arises primarily because of complex hot-band structure in the low frequency OOCI torsional mode. The fact that a number of torsional levels are populated and contribute to the spec-


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trum at room temperature is not particularly surprising. Somewhat less obvious is the significant e and Xe PESs along the torsional coordinate which gives to several non-diagonal tordifference in A sional sequences, lending even more complexity to the structure. A good example is the “triplet” structure characterizing the origin and OO stretch region. While it was initially speculated to be triplet electron spin structure of the ae3 A0 state of CH2 O2 , it is now seen to arise from a ∆m = −1 sequence in the torsional mode. The overall spectrum also is complicated by the fact that nearly all the non-hydrogenic modes are active in the spectrum, again presumably due to the significant e and Xe PESs. differences in the A Another surprising feature of the observed spectrum is that the position of its origin band reversed a well established red shift in the origin going from CH3 O2 →CH2 ClO2 → CH2 BrO2 . It e and Xe states of CH2 IO2 might be argued that increased spin-orbit induced repulsion between the A might be responsible for the reversal, but estimates from the electronic structure calculations show that likely is too small an effect. Indeed, as Table 2 shows even a relatively low-level calculation shows a reversal indicating that the reversal probably results from a subtle electronic structure effect. As detailed in the supplementary material, the corresponding spectra observed from the photolysis of CH2 Cl2 or CH2 ClI and CH2 Br2 or CH2 BrI, are similarly consistent with the (expected) carriers CH2 ClO2 and CH2 BrO2 respectively. Therefore we have established unambiguous NIR spectral diagnostics to monitor the presence and reactions of these halomethyl peroxy radicals, which are expected to play a significant role in tropospheric chemistry, particularly but not exclusively, in marine areas. It is interesting to ask why experiments monitoring the intensity of the spectrum from CH2 I2 photolysis while varying reaction conditions, gave results which appeared more consistent with the known chemistry of Criegee intermediates rather than with alkyl and haloalkyl peroxy radicals. The production of additional I in the presence of O2 is indeed a strong indicator of CH2 O2 production, but as noted earlier does not establish CH2 O2 as the carrier of the observed spectrum. Recent results from Lee and co-workers 69 indicate that in the pressure range of our experiments



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the CH2 O2 /CH2 IO2 yields is ≈2/1, i.e. comparable values. However the calculations on CH2 O2 detailed in Section 3.1.1, show that no observable ae − Xe spectrum of CH2 O2 is expected in the NIR. Another observation of interest is that of the apparently very fast self reaction of CH2 IO2 . Our apparatus was designed for spectroscopy not reaction kinetics, but our estimated room-temperature reaction rate & 1 × 10−10 cm3 molecule−1 s−1 , is very fast particularly compared with values for alkyl peroxy radicals like CH3 O2 and C2 H5 O2 which are nearly 3 orders of magnitude smaller. However as shown in Table 5 earlier reports 38,70,71 of the self-reaction rate constants of other halomethyl peroxy are considerably faster than for methyl peroxy. Indeed the reported rate constant for CH2 BrO2 is only approximately a factor of 3 smaller than the lower limit we estimate for CH2 IO2 . In addition, recent reports by Chhantyal-Pun et al. 72 and Foreman, et al. 73 are enlightening. Particularly relevant for our work is the report 72 of a cross-reaction between CH2 O2 and CH2 IOO with a very fast rate of ≈ 2 × 10−10 cm3 molecule−1 s−1 , comparable to that of CH2 O2 with HO2 and even faster than the self reaction rate of CH2 O2 . The apparent very fast self-reaction rate we measure with CH2 IO2 may in fact include cross-reaction with the co-produced CH2 O2 (see Fig. 1). Our measurements are over an insufficient pressure range to distinguish between the self reaction of CH2 IO2 and its cross reaction with CH2 O2 . Nonetheless, the existence of comparable fast reactions between CH2 O2 with CH2 IO2 or with itself indicates their chemistry may be more similar than expected. Correspondingly, Foreman, et al. have reported a relatively fast reaction of CH2 OO and the precursor, CH2 I2 , with a rate constant of 2.9×10−12 cm3 s−1 . If the reaction is similarly fast between CH2 I2 and CH2 XO2 , then it may be increasing the apparent “self-decay" rate, due to the relatively high concentration of undissociated CH2 I2 present. The final observation concerning the chemical reactivity of CH2 IO2 shows a similar pattern. The reactivity of CH2 O2 with SO2 is reported to be more than 105 faster than CH3 O2 . Monitoring the spectrum of CH2 ClO2 and CH2 BrO2 shows no reaction on our time scale and establishes that their rates are . 102 times slower than for CH2 IO2 . We can estimate a rate of reaction of SO2 with


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CH2 IO2 , by measuring the half-life of the temporal decay of the observed spectrum as a function of SO2 pressure. From our relatively crude measurements, we estimate k for the reaction to the ≈1-2×10−12 cm3 molecule−1 s−1 , this clearly much faster than other peroxy radicals measured but, nonetheless, is more than a factor of 10 smaller than the rate constant for SO2 reaction with CH2 O2 which is accepted 11 to be in the range 3.4-4.1×10−11 cm3 molecule−1 s−1 . Once again the chemistry of CH2 IO2 appears to more closely resemble that of CH2 O2 rather than alkyl peroxy radicals.

Conclusion Near-IR cavity ringdown spectra have been observed following the photolysis of dihalomethanes, CH2 XY (X or Y = Cl, Br, I) in O2 /N2 mixtures. Photolysis of CH2 I2 as a precursor is particularly interesting as this procedure has previously been shown to produce the Criegee intermediate, CH2 O2 . Under the same conditions we find a highly complex spectrum in the near-IR wavelength region generally expected for the transition from the ground singlet state of CH2 O2 to its lowest triplet state. Use of this spectrum to monitor the chemistry of the carrier indicates its reaction rates are comparable to those previously reported for CH2 O2 . To aid a definitive spectral analysis, extensive state-of-the-art electronic structure calculations e and Xe states of CH2 IO2 , another have been preformed for the ae and Xe states of CH2 O2 and the A possible product of the photolysis of CH2 I2 which would likely also have a transition in the nearIR region. The calculations on CH2 O2 strongly indicate that it is not the spectral carrier because the ae − Xe transition is predicted to be above the frequency range scanned and a large geometric change between the states diminishes the intensity of the already weak singlet-triplet transition. e − Xe transition of CH2 IO2 is predicted to be close to the However the origin frequency of the A observed spectrum and the predicted vibrational frequencies are generally consistent with those in the spectrum. To analyze the observed spectrum in detail, it is necessary to carefully consider the large-



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amplitude, low frequency OOCI torsional motion. We have performed a one-dimensional numerical calculation of its eigenfunctions based on the calculated potential along the torsional coordinate. These eigenfunctions integrated with the calculated electronic transition dipole moments predict well the intensities of the torsional hot bands and overtones. Using similar procedures, we have assigned and analyzed the spectra of CH2 ClO2 and CH2 BrO2 , which are observed in the same spectral region. The experimental origins and several vibrational frequencies for all three species are reported and compared to those from various electronic structure calculations.

Supporting Information e state vibrational frequencies for CH2 BrO2 The supporting information includes (i) calculated A and CH2 ClO2 in Tables S1 and S2 respectively, (ii) calculated frequencies and intensities of trane − Xe electronic spectrum of the CH2 XO2 in Table S3, sitions involving torsional motion in the A (iii) Figures S1-S3 which show behavior of the PES along the O-O dissociation coordinate of CH2 O2 for various calculations of the lowest triplet states; (iv) Figures S4 and S5 which show predicted and observed line positions and intensities in the origin and O-O stretch spectral regions of CH2 IO2 respectively (v) Figures S6 and S7 which show comparisons of experimental spectra with simulations in the origin and O-O stretch spectral regions of CH2 BrO2 , and (vi) Figures S8 and S9 which show the same for CH2 ClO2 .

Acknowledgment The support of National Science Foundation grant, CHE-1566246 (RD) and Department of Energy Grant DE-FG02-01ER14172 (TAM) is greatly acknowledged. We thank Mourad Roudjane and Dmitry Melnik for assistance in varous aspects of the experimental work. We also thank Anne McCoy for very helpful discussions on the proper treatment of large amplitude vibrations.


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Table 1: Harmonic frequencies and ZPEs for the singlet (left) and triplet (right) states of CH2 OO shown in Fig. 4. Modes are labeled only in terms of decreasing frequency due to the change in symmetry plane between the X˜ and a˜ states. mode

Xe1 A0 a

ae3 A0 a

9

534.5

151.0

8

631.3

384.0

7

861.4

619.0

6

954.1

987.9

5

1237.9

1074.7

4

1308.5

1137.0

3

1486.1

1456.2

2

3138.8

3146.2

1

3301.2

3303.8

Te

0

11186

ZPE (HO)

6726.8

6129.9

a Harmonic

frequencies calculated

at the (U)CCSD(T*)-F12b/VTZ-F12 level (see text).



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e states of CH2 XOO(X = Table 2: The calculated equilibrium geometries for the Xe and A Cl, Br, I) and values of the F parameters (see text) based on these geometries. The units are Angstrom for the bond lengths and degree for the bond angles and dihedral angles. CH2 ClO2 a CH2 ClO2 b CH2 BrO2 a CH2 BrO2 b CH2 IO2 a CH2 IO2 b e OO Bond Length(X)

1.321

1.324

1.321

1.323

1.320

1.322

e C-X Bond Length(X)

1.788

1.762

1.957

1.931

2.176

2.146

e OOC Bond Angle(X)

111.8

110.3

112.0

110.5

112.4

110.8

e OCX Bond Angle(X)

111.3

111.1

111.6

111.2

112.0

111.7

e OCCX Dihedral Angle(X)

85.0

79.6

86.7

82.2

88.3

82.0

e (cm−1 ) F(X)

1.703

1.673

1.427

1.399

1.284

1.263

e OO Bond Length(A)

1.381

1.441

1.379

1.359

1.388

1.421

e C-X Bond Length(A)

1.814

1.789

1.991

1.975

2.212

2.229

e OOC Bond Angle(A)

111.0

106.3

111.4

109.7

111.3

108.0

e OCX Bond Angle(A)

113.4

113.2

113.7

113.2

114.1

113.0

e OCCX Dihedral Angle(A)

74.2

75.3

74.3

73.4

72.9

73.9

e (cm−1 ) F(A)

1.630

1.613

1.362

1.362

1.213

1.219

a Calculated by B3LYP/AVTZ level of theory and basis set b

Calculated by UCCSD(T*)-F12b/VDZ-F12 level of theory and basis set for the Xe state and CASSCF/(13e,

e state. 12o)/VDZ-F12 for the A


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Table 3: The experimental and calculated zero-point-energy-corrected electronic energy differences in wavenumbers between the equilibrium geometries of the Xe e states of the CH2 XO2 radicals. and A T00

CH2 ClO2

CH2 BrO2

G

T

G

T

G

T

Exp

6814

-

6799

-

6908

-

G2

6836

7711a

6821

7678a

–

–

B3LYP/TDB3LYP

8169

9510b

8129

9229b

8169

9694b

UCCSD(T*)/CASSCF(13e,12o)

6829

–

6788

–

6775

–

a Excitation energy of T conformer, relative to G: 240, 269, – b

Excitation energy of T conformer relative to G: 269, 347, 417,


CH2 IO2


e and Xe (in Table 4: Experimental fundamental and predicted harmonic vibrational frequencies for the A parenthesis) state of the G conformer of CH2 IO2 . Experimental Mode Description

38

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Page 38 of 50

This work p-H2

Matrix 39

Computational Ar

Matrix 40

TDDFT

CASSCF(13e,12o) 120(82b)

ν12 OOCI torsion

109(80)

–

–

101(85a)

ν11 OCI bending

238c

–

–

279(250)

270(262)

ν10 COO bending

397

(490.2)

(496.2)

398(502)

423(505)

ν9

CI stretch

508

(550.5)

(567.5)

519(544)

539(575)

ν8

CH2 rocking

877

(841.6)

(839.9)

847(847)

851(860)

ν7

OO stretch + CO stretch d

–

(917.7)

(919.9)

915(930)

912(954)

ν6

OO stretch + CO stretch

d

1011c

(1085.6)

(1085.3)

1043(1124)

1072(1125)

ν5

CH2 wagging + CH2 twistingd

–

(1226.5)

(1223.2)

1233(1261)

1328(1270)

ν4

CH2 wagging + CH2 twistingd

–

(1231.8)

(1230.4)

1252(1249)

1359(1259)

ν3

HCH bending

–

(1408.9)

(1407.3)

1457(1446)

1582(1452)

ν2

CH symmetric stretch

–

(2982.4)

–

3105(3114)

3275(3118)

ν1

CH asymmetric stretch

–

–

–

3203(3208)

3377(3211)

a Calculated by B3LYP/AVTZ level of theory and basis set, which has also been reported 39 b

Calculated by UCCSD(T*)-F12b/VDZ-F12 level of theory and basis set

c

Tentative Assignments

d

The normal modes are linear combinations of multiple internal motions. The coefficients of these linear combinations vary among

different states and electronic calculations.

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Table 5: Self-reaction Rate Constants (in units of 10−12 cm3 molecule−1 s−1 ) for Peroxy Radicals RO2 at Room Temperature R

rate constant

Ref.

C2 H5

0.097

70

CH3

0.46

38

CH2 F

3.1

38

CH2 Cl

3.8

38

CH2 Br

33

71

CH2 I

&100

[present work]



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Figure 1: Schematic diagram for the reaction initiated by the photolysis of CH2 XI(X=Cl, Br, or I) in the presence of a N2 /O2 gas mixture


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Figure 2: Experimental spectrum obtained from the 248 nm photolysis of CH2 I2 in a O2 /N2 mixture. There are interferences from precursor absorption between 6940-7000 cm−1 and 8700-8750 cm−1 , interference from water between 7060-7350 cm−1 , and the 2 P1/2 -2 P3/2 transition of the iodine atom is at 7603 cm−1 . The remaining structure is attributed to a molecular photoproduct.



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Figure 3: Schematic diagram of the room temperature, moderate resolution CRDS experiment. The NIR radiation is generated via isolation of second Stokes stimulated Raman shifting of a visible dye laser output. Radicals are produced inside the ringdown cavity via photolysis of a dihalomethane in the presence of O2 and N2 , using either 193 or 248 nm light from an excimer laser.


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Figure 4: (left) Geometric parameters (Å and degrees) of the planar equilibrium structure of the X˜ 1 A0 state of CH2 OO. (right) Geometric parameters for the equilibrium structure of the a˜3 A00 state (Cs symmetry, see text). The calculated parameters are very close to those derived experimentally by McCarthy et al. 51 (values and uncertainties in square parentheses).


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e states. The potential energies are evaluated at the UCCSD(T)-F12b/DVZFigure 5: PES slices along the OOCI torsion for both Xe and A e state. To produce F12(blue) level of theory and basis set for the Xe state, and CASSCF(9e,8o) level of theory and basis set for the A the smooth curves all ab initio calculated points are fit by a summation of multiple trigonometric functions with appropriate symmetry enforced.



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Figure 6: The spectra of the photoproducts obtained from 248 nm photolysis of CH2 I2 (bottom, red trace), CH2 BrO2 (middle, black trace), and CH2 ClO2 (top, blue trace) in the presence of O2 . The blue and red traces have been offset by 15 and 7 ppm, respectively, for clarity.



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Figure 7: The experimental traces obtained from the 248 nm photolysis of CH2 XI in the absence of SO2 (top, black trace) and presence of 7.0 torr SO2 (bottom, blue trace). Blue trace was shifted -7 ppm for clarity. For traces (a), (b), and (c) X=Cl, Br, and I respectively. The delay between photolysis and probe laser is ≈10 µsec.


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Figure 8: Experimental spectra in the origin region (a) and OO stretch region (e) obtained from 0 CH2 I2 photolysis, compared to simulations. Simulations of the 12m m00 transitions are shown in trace (b) using eZspectrum computed Franck-Condon factors employing Cartesian coordinate calculations based on UCCSD(T*)-F12b/VDZ-F12 and CASSCF(13e,12o)/VDZ-F12 level of theory and basis set; (c)using Franck-Condon factors from internal coordinate calculations based on the same level of theory and basis set; (d) using the transition intensity from internal coordinate calculations based on the same level of theory and basis set, including the electronic transition dipole moment variation. The simulations are color coded by the torsional sequences, ∆m = m0 − m00 , as indicated by the insert. 47 ACS Paragon Plus Environment

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Figure 9: The experimental spectrum and stick diagram simulations for CH2 IOO in the origin(000 ) region. For each torsional transition, 0 0 00 00 12m m00 in the simulation, the color of the stick represents the torsion sequence, ∆m = m − m , with m extending over levels sufficiently populated (and accordingly scaled) to give observable structure, typically m00 = 0,1,2 and 3 for the strongest transitions. Black sticks indicate transitions involving modes other than ν12 and are so labeled.



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Figure 10: The experimental spectrum and stick diagram of Franck-Condon simulations of CH2 IOO in the OO stretch (810 ) region. The labeling scheme is the same as for Fig. 9.

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82x44mm (300 x 300 DPI)


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Studies via Near-Infrared Cavity Ringdown Spectroscopy and

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