Early time detection of OH radical products from energized Criegee intermediates CH2OO and CH3CHOO

doi:10.1016/j.cplett.2014.02.049

Chemical Physics Letters

Volume 598, 8 April 2014, Pages 23-27

https://doi.org/10.1016/j.cplett.2014.02.049 Get rights and content

Highlights

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Early time detection of OH products from unimolecular decay of Criegee intermediates.
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Doppler broadened line profiles for OH products from CH₃CHOO intermediates.
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Predominant translational excitation of products from CH₃CHOO intermediates.
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Substantially different decay mechanisms for energized CH₂OO and CH₃CHOO.

Abstract

The OH radical products from unimolecular decay of energized CH₂OO and CH₃CHOO Criegee intermediates are detected by laser-induced fluorescence shortly after they are formed utilizing 248 nm photolysis of diiodo precursors and subsequent reaction with O₂. The OH radicals from both CH₂OO and CH₃CHOO are produced with average rotational energies of ∼1–2 kcal mol⁻¹. A different mechanism for unimolecular decay of CH₃CHOO is evident from noticeably broader OH A-X (1,0) line profiles, which is attributed to Doppler broadening. Modeling of the Doppler profiles suggests that most of the available energy is channeled into translational excitation of the products.

Graphical abstract

Introduction

Ozonolysis of alkenes is known to be a significant non-photolytic source of OH radicals in the troposphere [1]. These highly exothermic reactions proceed by cycloaddition of ozone across a CC double bond to form an internally excited primary ozonide, which rapidly decomposes to carbonyl and carbonyl oxide species, the latter known as the Criegee intermediate. The Criegee intermediate is produced with sufficient energy to undergo further unimolecular decay, yielding OH radicals and other products of atmospheric importance [1]. Alternatively, the energized Criegee intermediate can be collisionally stabilized, react with other trace species in the atmosphere, such as NO₂, SO₂, HO_x and H₂O [2], [3], and/or undergo solar photolysis [4], [5], [6].

In the past, both indirect [7], [8], [9], [10], [11], [12], [13], [14] and direct [15] methods have been implemented to detect the OH radicals arising from ozonolysis of different alkenes. The focus of these studies has been the OH yields from ozonolysis. The indirect methods involve the use of an OH scavenger, such as cyclohexane [7], [8], which is reactive toward OH radicals but not with ozone or alkenes. OH yields are then determined by comparing the loss of scavenger with the loss of alkenes. In general, the OH yields are found to be highly dependent on the alkene structure. For example, the OH yield from ozonolysis of ethene, which generates the simplest Criegee intermediate CH₂OO exclusively, is just ∼0.13. By comparison, the OH yield from ozonolysis of E-2-butene, which generates only the alkyl-substituted Criegee intermediate CH₃CHOO, is ∼0.59. More generally, empirical structure–activity relationships have been proposed for estimating OH yields from ozonolysis of alkenes [11], which are in good agreement with experimental results.

Laser-induced fluorescence (LIF) on the OH A²Σ⁺-X²Π band system [16] has also been used for direct detection of OH radicals formed from ozonolysis of alkenes [15]. For a series of alkene–ozone reactions investigated under flow cell conditions, on timescales of tens of milliseconds, the OH yields were found to decrease with increasing total pressure, indicating a competition between unimolecular decay and collisional stabilization of the internally excited Criegee intermediates. Extrapolation to atmospheric pressure resulted in OH yields that were significantly lower than those determined in indirect scavenger studies.

Although the OH radicals generated from Criegee intermediates have been detected for many years [7], [8], [9], [10], [11], [12], [13], [14], [15], the stabilized Criegee intermediates themselves, e.g. CH₂OO and CH₃CHOO, have only recently been observed by photoionization [17], [18], ultraviolet [4], [5], [6], [19], infrared [20], and microwave [21], [22], [23] spectroscopies. Most of these recent studies prepare CH₂OO or CH₃CHOO by photolysis of CH₂I₂ or CH₃CHI₂ and subsequent reaction of the alkyl iodide radical with O₂. This laboratory has detected OH radicals from the unimolecular decay of Criegee intermediates concurrently with the carbonyl oxides using 1+1′ resonance enhanced multiphoton ionization [24] with time-of-flight mass spectrometric detection [4], [6]. The OH ion signals, after scaling to the respective Criegee intermediate CH₃CHOO and CH₂OO ion signal, are observed with a 4 to 1 ratio, consistent with the trend of IUPAC preferred OH yields from ozonolysis of E-2-butene and ethene, which are 0.64 and 0.16, respectively [25]. Sander and coworkers have also monitored OH as a proxy for CH₂OO concentration in bimolecular reactions [26].

The degree of initial internal excitation of the Criegee intermediate is a critical factor in generating OH products since substantial barrier(s) must be overcome along the unimolecular reaction pathways [27]. Alkene–ozone reactions that occur in the atmosphere are highly exothermic reactions with up to 60 kcal mol⁻¹ released to products [1]. A comparably high degree of internal excitation of CH₂OO results from 248 nm photolysis of CH₂I₂ and subsequent reaction with O₂ as detailed below.

A recent ion imaging study [28] in this laboratory on the photodissociation of CH₂I₂ at 248 nm showed that the CH₂I fragments are generated with an average internal energy of ∼36 kcal mol⁻¹ with an ∼8 kcal mol⁻¹ breadth (FWHM), corresponding to 85% of the available energy in the CH₂I + I^{∗ 2}P_1/2 channel (0.46 quantum yield) [29]. A similar partitioning is inferred for the CH₂I + I ²P_3/2 channel, resulting in an average internal energy for CH₂I fragments close to ∼54 kcal mol⁻¹ and likely a similar breadth. The subsequent reaction of this bimodal distribution of CH₂I fragments with O₂ is expected to give rise to highly internally excited CH₂OO intermediates via a near thermoneutral reaction [30]. Detailed information on the energy disposal in the 248 nm photodissociation of CH₃CHI₂ and the thermochemistry for the subsequent reaction of CH₃CHI with O₂ to form CH₃CHOO are not yet available.

The focus of this study is to use LIF to detect the early time production of OH radicals arising from unimolecular decay of Criegee intermediates CH₂OO and CH₃CHOO. The energy released to OH products as internal and/or translational excitation is examined through the OH X²Π product state distribution and Doppler linewidth. We anticipate very different energy partitioning in the OH products from CH₃CHOO than CH₂OO because the presence of α-hydrogens in the predominant syn conformer of CH₃CHOO opens up a more facile unimolecular decay pathway, shown schematically in Figure 1, than available for CH₂OO or anti-CH₃CHOO [1], [31]. Finally, the experimental results are compared with theoretical predictions of the reaction coordinate associated with unimolecular decay of syn-CH₃CHOO to interpret the results.

Section snippets

Experimental methods

The Criegee intermediates are generated in a similar manner as previous experiments in this laboratory [4], [5]. The diiodomethane (CH₂I₂) or 1,1-diiodoethane (CH₃CHI₂) precursor (Sigma Aldrich >98% purity) vapor is entrained in 20% O₂/Ar carrier gas (20 psi) and pulsed through a quartz capillary tube (1 mm ID, 26 mm length) into the vacuum chamber. The precursor is photolyzed by 248 nm radiation from a KrF excimer laser (Coherent, Compex102; loosely focused to ∼1 mm) aligned perpendicular to the

Results

Photolysis of the diiodo alkyl precursors occurs in the collisional region (x/D ∼ 1) of a pulsed supersonic expansion, enabling subsequent reaction of the alkyl iodide radicals with O₂ in the carrier gas to produce energized Criegee intermediates. Unimolecular decay of the Criegee intermediates results in OH X²Π radical products, which are probed rapidly after they are formed (within 300 ns) using a spatially overlapped UV laser and LIF detection. The short time delay between the lasers permits

Discussion

We estimate the excess energy available to the OH + CH₂CHO products by assuming that the internal excitation of CH₃CHOO is similar to that of CH₂OO when generated from photolysis of the diiodo precursor and reaction with O₂. The internal excitation of CH₂OO is derived from knowledge of the photodissociation dynamics of CH₂I₂ at 248 nm [28] and the essentially thermoneutral reaction of CH₂I + O₂ → CH₂OO + I [30]. By analogy, we anticipate that 248 nm photolysis of CH₃CHI₂ will yield a bimodal

Conclusions

The rotational and/or translational energy distributions of OH radical products from energized CH₂OO and CH₃CHOO Criegee intermediates are examined at early time within 300 ns of being formed. The energized Criegee intermediates are produced by 248 nm photolysis of CH₂I₂ or CH₃CHI₂ precursors and subsequent reaction with O₂ in the collisional region of a supersonic expansion. In both cases, the OH radicals are observed with average rotational energies of ∼1–2 kcal mol⁻¹. The OH A-X (1,0) P₁ line

Acknowledgements

This research was supported, in part, through the National Science Foundation under Grant CHE-1112016. J.M.B. acknowledges support through the Dreyfus Postdoctoral Program in Environmental Chemistry.

References (37)

J.H. Lehman et al.
Chem. Phys. Lett.
(2013)
D. Johnson et al.
Chem. Soc. Rev.
(2008)
L. Vereecken et al.
Phys. Chem. Chem. Phys.
(2012)
J.M. Anglada et al.
Phys. Chem. Chem. Phys.
(2011)
J.M. Beames et al.
J. Chem. Phys.
(2013)
J.M. Beames et al.
J. Am. Chem. Soc.
(2012)
F. Liu et al.
J. Phys. Chem. A
(2014)
R. Atkinson et al.
J. Geophys. Res. Atmos.
(1992)
R. Atkinson et al.
Environ. Sci. Technol.
(1993)
A.A. Chew et al.
J. Geophys. Res. Atmos.
(1996)

R. Gutbrod et al.

Int. J. Chem. Kinet.

(1997)

A.R. Rickard et al.

J. Phys. Chem. A

(1999)

P. Neeb et al.

J. Phys. Chem. A

(1999)

S.E. Paulson et al.

J. Phys. Chem. A

(1999)

S.E. Paulson et al.

J. Phys. Chem. A

(1999)

J.H. Kroll et al.

J. Phys. Chem. A

(2001)

J. Luque, D.R. Crosley, SRI International Report No. MP 99-009,...

O. Welz et al.

Science

(2012)

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Citation Excerpt :
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Editors ChoiceEarly time detection of OH radical products from energized Criegee intermediates CH2OO and CH3CHOO

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental methods

Results

Discussion

Conclusions

Acknowledgements

Chem. Phys. Lett.

Chem. Soc. Rev.

Phys. Chem. Chem. Phys.

Phys. Chem. Chem. Phys.

J. Chem. Phys.

J. Am. Chem. Soc.

J. Phys. Chem. A

J. Geophys. Res. Atmos.

Environ. Sci. Technol.

J. Geophys. Res. Atmos.

Int. J. Chem. Kinet.

J. Phys. Chem. A

J. Phys. Chem. A

J. Phys. Chem. A

J. Phys. Chem. A

J. Phys. Chem. A

Science

Editors Choice
Early time detection of OH radical products from energized Criegee intermediates CH₂OO and CH₃CHOO