The Simplest Criegee Intermediate: (CH2OO): Equilibrium Structure and Possible Formation from Atmospheric Lightning Michael McCarthy, Kyle Crabtree, Oscar.

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The Simplest Criegee Intermediate: (CH2OO): Equilibrium Structure and Possible Formation from Atmospheric Lightning Michael McCarthy, Kyle Crabtree, Oscar Martinez Harvard-Smithsonian CfA Lan Cheng, Thanh Lam Nguyen, John Stanton Univ. Texas, Austin Carrie Womack MIT Talk WJ03, 69th Inter. Sym. Mol. Spectroscopy Urbana, Illinois, June 2014

H2CO2 potential energy surface circa 2012 4 stable singlet isomers, i.e. wrt dissociation or isomerization: 2 have more than one conformer formaldehyde oxide or ‘Criegee’ dioxirane (known) Dihydroxycarbene (3) formic acid (2) Thanh Nguyen (Texas)

Carbonyl Oxides (Criegee) intermediates, R1R2COO Rudolf Criegee proposed in 1940s that ozonolysis of hydrocarbons in atmosphere proceeds via the reaction: Considerable theoretical interest + biradical zwitterion H H

The simplest Criegee, CH2OO Subject of intense recent study; UV, IR, and MW spectra recorded within past few years Synchrotron photoionization mass spectroscopy: O. Welz et al. 2012, Science, 335, 204-207 Ultraviolet spectroscopy and photochemistry: Beames et al. 2012, J. Am. Chem. Soc., 134, 20045–20048 Transient infrared absorption spectroscopy: Su et al. 2013, Science, 340, 174-176 Microwave spectroscopy and isotopic substitution: M. Nakajima and Y. Endo, 2013, J. Chem. Phys., 139, 101103 What remains to be done?

1. Are other formation pathways possible? Previous work has relied almost exclusively on halogenated precursors: CH2I2 + hn  CH2I + I CH2I + O2  [CH2IOO]*  CH2OO + I 2. Some ambiguities remain in purely experimental molecular structure Derive highly precise geometry from combination of singly-substituted isotopic species and vibrational corrections calculated theoretically

Formation from CH4 + O2 (excess) ? Isotopic samples (13CH4, CH2D2, 16O18O) readily available, so full structure can be determined Measurements done in combination with new high-level cc calculations (Stanton) Ultimately all five singly-substituted species detected, and precisely characterized

Deuterium hfs 1-0 line H C O H C O H C O cis trans a b cis trans a b

Highly precise semi-experimental structures Correct experimental rotational constants for the vibrational effects calculated theoretically using 2nd-order perturbation theory; electronic contributions calculated as well Theory very good at determining shape at bottom of potential well 2 1 v=0 r0 re

Data for structural determination A-1  Ia = Smiri,a2 (likewise for B & C) Total of 27 rotational constants from 9 species used in least-squares optimization of 7 structural parameters R(C-H)cis A(HCO)cis A(COO) R(O-O) R(C-H)trans A(HCO)trans R(C-O) CH2OO 13CH2OO CH218OO CH2O18O cis-CHDOO trans-CHDOO A 77749 77073 73394 76885 61771 76637 B 12465 12089 12464 11777 12135 11310 C 10721 10430 10633 10193 10121 9839 D -0.0008 -0.0005 -0.0004 -0.0006 0.0028

Molecular Geometry CH2OO — whose rotational spectrum was entirely unknown a short time ago — arguably is now one of the best characterized reactive molecules from the perspective of structure Hybrid reSE structure [calculated] CCSD(T)

Formation from atmospheric lightning? CH4 in the presence of excess O2 and an electrical discharge readily forms CH2OO 1.4 B/y lightning strikes globally [CH4] ~ 2 ppm May represent unanticipated way to spontaneously form CH2OO in troposphere

Formation mechanism? What we do know: O2 unit conserved during formation Little HOCOH, dioxirane, or formic acid, suggesting that production is highly selective Methyl peroxide (CH3OO) radical also present under same conditions Calculations of CH3 +2O2 reaction undertaken to understand formation

Methodology A modification of the original HEAT protocol (denoted as mHEAT), which can be applied to medium-sized molecule systems, is used for calculations A comparison between HEAT & mHEAT versus ATcT data Method CH3 + O2 ==> CH3OO (Reaction enthalpy) ATcT -30.61 ± 0.24 kcal/mol HEAT-345(Q) -30.40 ± 0.25 kcal/mol mHEAT-345(Q) -30.74 kcal/mol

Speculation: Formation via Tunneling?

Reaction Mechanism for Production of Cool Criegee Fast micro-canonical equilibrium is established: CH3 + O2 <==> CH3OO Owing to very large pressure gradient in expansion, some fraction of CH3OO may be cooled by collisions or react further with O2. There is a large barrier to dissociation of vibrationally excited CH3OO (at least of 58 kcal/mol, MC Lin et al, J. Chem. Phys. 2001, 115, 195-203), implying that it may not be very important (or energized CH3OO cannot access) H-abstraction proceeds via CH3OO + O2 ==> CH2OO + HO2 Cool CH2OO is trapped in a well with the depth of 19 kcal/mol. HO2 (just produced above) can react with CH3OO: HO2 + CH3OO ==> O2 + CH3OOH

Chemical Kinetics Simulation for the Discharge CH4/CD4 + O2 Microcanonical rate constants for forming the simplest Criegee intermediates in the CH4 & CD4/O2 electronic discharge experiments.