Subscriber access provided by Caltech Library Clusters, Radicals, and Ions; Environmental Chemistry Stereoselectivity in Atmospheric Autoxidation Kristian H. Møller, Eric Joseph Praske, Lu Xu, John D. Crounse, Paul O. Wennberg, and Henrik Grum Kjaergaard J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01972 • Publication Date (Web): 23 Sep 2019 Downloaded from pubs.acs.org on September 24, 2019 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 24 The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 Stereoselectivity in Atmospheric Autoxidation 9 10 11 † ‡ ¶ ¶ 12 Kristian H. Møller, Eric Praske, Lu Xu, John D. Crounse, Paul O. 13 14 Wennberg,∗,¶,§ and Henrik G. Kjaergaard∗,† 15 16 17 †Department of Chemistry, University of Copenhagen, DK-2100 Copenhagen Ø, Denmark 18 19 ‡Division of Chemistry and Chemical Engineering, California Institute of Technology, 20 21 Pasadena, California 91125, United States 22 23 ¶Division of Geological and Planetary Sciences, California Institute of Technology, 24 25 Pasadena, California 91125, United States 26 27 §Division of Engineering and Applied Science, California Institute of Technology, 28 29 Pasadena, California 91125, United States 30 31 32 E-mail: [email protected]; [email protected] 33 34 35 Abstract 36 37 38 We show that the diastereomers of hydroxy peroxy radicals formed from OH and O2- 39 40 addition to C2 and C3, respectively, of crotonaldehyde (CH3CHCHCHO), undergo gas- 41 42 phase unimolecular aldehydic hydrogen shift (H-shift) chemistry with rate coefficients 43 that differ by an order of magnitude. The stereospecificity observed here for croton- 44 45 aldehyde is general and will lead to a significant diastereomeric-specific chemistry in the 46 47 atmosphere. This enhancement of specific stereoisomers by stereoselective gas-phase 48 49 reactions could have widespread implications given the ubiquity of chirality in nature. 50 51 The H-shift rate coefficients calculated using multi-conformer transition state theory 52 53 (MC-TST) agree with those determined experimentally using stereoisomer-specific gas- 54 55 chromatography chemical ionization mass spectroscopy (GC-CIMS) measurements. 56 57 58 59 1 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Letters Page 2 of 24 1 2 3 Graphical TOC Entry 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Most of the molecules central to life, including DNA, amino acids and sugars, are chiral.1,2 21 22 Typically, one chiral form of molecules exist in nature; sugars, for example, are found almost 23 24 exclusively as the D-form, while amino acids occur naturally as the L-form, and the origin 25 26 of this homochirality is a great mystery.1–3 27 28 29 30 Many biogenically- and anthropogenically-produced compounds emitted into the atmosphere 31 32 are chiral.4,5 Several studies have found that the enantiomeric composition of VOCs (includ- 33 34 ing the important monoterpenes) vary with biome, plant type, season, time of day and stress 35 36 factors.6–10 For instance, it has been observed that, in tropical regions, (-)-α-pinene domi- 37 38 nates the emissions, while (+)-α-pinene has been found to dominate in a boreal forest.6 39 40 41 42 Both experiments and field measurements have shown transfer of precursor chirality into 43 44 formed seconday organic aerosol (SOA).11–14 Studies of polymers suggest that stereochem- 45 46 istry can affect the physical properties of the aerosol, such as hydrophilicity, phase behavior, 47 48 intramolecular hydrogen bonding and molecular macrostructure.15–19 This in turn may affect 49 50 the aerosol rate of growth, their ability to act as a cloud condensation nuclei (CCN) and 51 52 radiative properties, thus affecting their influence on Earth’s climate.20–22 53 54 55 56 Enhancement of specific stereoisomers can arise from differences in rate coefficients of dif- 57 58 59 2 60 ACS Paragon Plus Environment Page 3 of 24 The Journal of Physical Chemistry Letters 1 2 3 ferent stereoisomers, i.e. stereoselectivity. A decade ago, surface stereochemical effects were 4 5 observed for heterogeneous ozonolysis (about a factor of two difference between different di- 6 7 astereomers) and were suggested as a potential route to chiral excess in prebiotic chemistry.23 8 9 It has also been speculated, that oligomerization of epoxides formed in isoprene oxidation at 10 11 aerosol surfaces could similarly be stereoselective.19 Stereoselectivity in gas-phase reactions 12 13 has so far been elusive. 14 15 16 17 We investigate the diastereomeric selectivity in unimolecular gas-phase peroxy radical hy- 18 19 drogen shift (H-shift) reactions. These H-shift reactions have been linked to the formation of 20 21 highly oxidized molecules that are important for the growth of SOA in the atmosphere.24–29 22 23 Recent global modeling suggests that at least 30 % of all isoprene molecules emitted to the 24 25 atmosphere undergo a minimum of one H-shift during its oxidation cascade highlighting the 26 27 importance of these reaction pathways.30 Peroxy radical H-shift reactions are unimolecular 28 29 reactions in which a peroxy radical abstracts a hydrogen atom from another location in the 30 31 same molecule. Organic peroxy radicals in atmospheric oxidation are typically formed from 32 33 31,32 O2-addition to a near-planar alkyl radical center. If the three substituents at the alkyl 34 35 radical are different, O2-addition will form a new chiral center. Generally, this O2-addition is 36 37 expected to form comparable amounts of the two stereoisomers leading to a racemic mixture. 38 39 For compounds with an existing chiral center, this leads to a set of four stereoisomers, as 40 41 shown in Figure 1. The enantiomers, which are inverted at both chiral centers [e.g. (R,R) 42 43 and (S,S) labeled (R*,R*) here], are mirror images of each other and thus have the same 44 45 physio-chemical properties including unimolecular reaction rate coefficients and bimolecu- 46 47 lar rate coefficients with achiral reaction partners.33 The diastereomers on the other hand, 48 49 differ at only one of the stereocenters and have different properties.33 We refer to the di- 50 51 astereomers with the same configuration at both chiral centers as (R*,R*) and those with 52 53 different configuration as (R*,S*). 54 55 56 57 58 59 3 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Letters Page 4 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Figure 1: Relation between the four stereoisomers comprising one of the structural isomers 21 of crotonaldehyde hydroxy peroxy radical, 2-OH,3-OO-CRALD. 22 23 24 We are at a point in time where these peroxy radical H-shifts are being implemented into 25 26 global atmospheric chemistry models.34,35 It is therefore imperative that the stereoselectivity, 27 28 which has been suggested by recent theoretical studies,24,30,36–39 is evaluated experimentally. 29 30 The importance of stereoselectivity in the atmosphere is two fold. Firstly, the rate coeffi- 31 32 cients can differ significantly between the different diastereomers and thus both need to be 33 34 considered.30 Secondly, it affects the stereochemistry of the formed products and may lead 35 36 to significant stereoselective enhancement. 37 38 39 40 Here, we demonstrate stereoselectivity in a pair of diastereomers formed in the hydroxyl 41 42 radical initiated oxidation of an alkene aldehyde, crotonaldehyde (2-butenal, CRALD), see 43 44 Figures 1 and 2. Using stereoisomer-specific gas-chromatography chemical ionization mass 45 46 spectrometry (GC-CIMS) measurements of the hydroxy nitrates (HN) formed by reaction of 47 48 the peroxy radicals with NO, we probe the competition between bimolecular and unimolec- 49 50 ular chemistry to quantify the rate coefficients of the peroxy H-shifts, using an approach 51 52 similar to that employed previously for other systems.40–42 As such, we provide the first 53 54 experimental demonstration of stereoselectivity in peroxy radical