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

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Presentation on theme: "The Simplest Criegee Intermediate: (CH2OO): Equilibrium Structure and Possible Formation from Atmospheric Lightning Michael McCarthy, Kyle Crabtree, Oscar."— Presentation transcript:

1 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

2 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)

3 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

4 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, 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, Microwave spectroscopy and isotopic substitution: M. Nakajima and Y. Endo, 2013, J. Chem. Phys., 139, What remains to be done?

5 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

6 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

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

8 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

9 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 0.0028

10 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)

11 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

12 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

13 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 ± kcal/mol HEAT-345(Q) ± kcal/mol mHEAT-345(Q) kcal/mol

14 Speculation: Formation via Tunneling?

15 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, ), 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

16 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.

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