OXYGEN-18 STUDIES OF HOCO AND HONO FORMATION Oscar Martinez Jr. and Michael C. McCarthy Harvard-Smithsonian Center for Astrophysics School of Engineering and Applied Science, Harvard University

Fourier-Transform Microwave Spectrometer Capable of 5 to 42 GHz Pulsed nozzle (6Hz)supersonic molecular beam (~Mach 2) – 2.5kTorr stagnation pressure behind nozzle, – Total flow 20 sccm – Results in T rot ~1 – 3 K – DC discharge used to create radicals and ions MW-MW double resonance capability effectively extends range to ~60 GHz McCarthy et al., ApJ Suppl. Ser. (2000)

Inspiration: HOCO / HONO Recent HO 3 studies McCarthy et al. J. Chem. Phys. (2012) Yu et al. Phys. Chem. Chem. Phys., 2008 Competition in binding energies – Need [O 2 ]>>[H 2 O] Extend OH + X mechanism… {X = CO, NO, SO 2, etc…}

Background: HOCO / HONO HONO (Nitrous acid) and HOCO Important atmospheric and combustion intermediates – Additionally, all species involved in formation and destruction are high stakes players Prior work: Numerous studies – Experimental – Spectroscopic : PES, IR, Microwave – Kinetics – Crossed –beam Theoretical: Ab Initio playground – Prototypical complex-forming bimolecular reaction – Isomerization (cis-trans) – Tunneling – Proton “hopping” (aka. ‘intramolecular’ migration) – Coupling to experimental allows testing of theory and methods

Li et al. J. Phys. Chem. A 2012 HOCO PES

HOCO Synthesis: OH + CO H + CO2 Use of H 2 18 O, C 18 O, 13 CO, D 2 O, and D 2 isotopic labeling (in addition to normal counterparts) to extract mechanistic HOX-formation details Measured hyperfine lines for 1 0,1 → 0 0,0 transition of singly- and doubly-substituted cis- and trans- isomers: HOCO, H 18 OCO, HO 13 CO, HOC 18 O, H 18 OC 18 O, DOCO, D 18 OCO, DO 13 CO, DOC 18 O, and D 18 OC 18 O →HOCO

HOCO Step 1) HO + C 18 O → HOC 18 O Step 2) HOC 18 O → H + OC 18 O Step 2) H + OC 18 O → H 18 OCO → HOC 18 O Monitored evidence of OH, OD, and 18 O isotope equivalents No CO 2 (normal or isotopic) evidence (i.e. - Ne…CO 2 or H2O…CO 2 complexes)

Fractional amount of HOCO not of direct mechanism but “randomization” from secondary CO 2 reaction trans - D 18 OCO:DOC 18 O ratio (1:4) same as trans- H 18 OCO:HOC 18 O – No ratio quenching…no roaming – Roaming TS above entrance channel cis- ratios differ between use of H 2 18 O vs C 18 O reactants H 18 OCO:HOC 18 O (10:1) HOCO

Fermi contact constant, a F – Oyama et al. normal a F = -6.9 (trans) and 82.8 (cis) – trans- HO 13 CO fit results in a F = Oyama et al. J. Chem. Phys., 2011 HOCO unpaired electron orbitals

Asatryan et al. Int. J. Chem. Kin., 2007 HONO PES

HONO Measured hyperfine lines for 1 0,1 → 0 0,0 transition of trans-HONO: H 18 ONO, HON 18 O, and H 18 ON 18 O No formation preference for H 18 ONO or HON 18 O – Indirect and direct mechanisms – Roaming transition state below entrance channel

HONO [NO]*HONOH 18 ONOHON 18 OH 18 ON 18 O 2% % % % % % Relative abundances * [NO] variation vs dilute (~0.1%) H 2 18 O sample Extremes: [NO]>>[ 18 OH] and [NO]<<[ 18 OH]

HONO Large fraction of HONO formed directly (single collision) – no subsequent scrambling Significant fraction of HONO formed by processing of NO, presumably via H 18 OH + NO ↔ O 18 O ↔ OH + N 18 O N H 18 ON 18 O presence suggests N 18 O readily formed and subsequently reacts with 18 OH

Conclusions Mechanistic Details Hydrogen vagrancy dependent on transition state energies relative to reactants at entrance channel HOCO – TS above entrance channel → slow exchange HONO – TS below entrance channel → fast exchange (hopping/roaming) Isotopic work results in structure refinement

Acknowledgments