OXYGEN-18 STUDIES OF HOCO AND HONO FORMATION

Oscar Martinez Jr. and Michael C. McCarthy

Harvard-Smithsonian Center for AstrophysicsSchool 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 Trot ~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 HO3 studies

McCarthy et al. J. Chem. Phys. (2012)

Yu et al. Phys. Chem. Chem. Phys., 2008

• Competition in binding energies– Need

[O2]>>[H2O]

•Extend OH + X mechanism…{X = CO, NO, SO2,

etc…}

Background: HOCO / HONOHONO (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 + COH + CO2

• Use of H218O, C18O, 13CO, D2O, and D2 isotopic

labeling (in addition to normal counterparts) to extract mechanistic HOX-formation details

• Measured hyperfine lines for 10,1→ 00,0 transition of singly- and doubly-substituted cis- and trans- isomers:HOCO, H18OCO, HO13CO, HOC18O, H18OC18O, DOCO, D18OCO, DO13CO, DOC18O, and D18OC18O

→ HOCO

HOCO

Step 1) HO + C18O → HOC18OStep 2) HOC18O → H + OC18O Step 2) H + OC18O → H18OCO

→ HOC18O • Monitored evidence of OH, OD, and 18O

isotope equivalents• No CO2 (normal or isotopic) evidence

(i.e. - Ne…CO2 or H2O…CO2 complexes)

• Fractional amount of HOCO not of direct mechanism but “randomization” from secondary CO2 reaction

• trans - D18OCO:DOC18O ratio (1:4) same as trans- H18OCO:HOC18O– No ratio quenching…no roaming– Roaming TS above entrance channel

• cis- ratios differ between use of H218O vs C18O

reactants H18OCO:HOC18O (10:1)

HOCO

HOCO

• Fermi contact constant, aF

– Oyama et al. normal aF = -6.9 (trans) and 82.8 (cis)

– trans- HO13CO fit results in aF = 117.8

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 10,1→ 00,0 transition of trans-HONO:H18ONO, HON18O, and H18ON18O

• No formation preference for H18ONO or HON18O– Indirect and direct mechanisms– Roaming transition state below entrance channel

HONO[NO]* HONO H18ONO HON18O H18ON18O

2% 100 206 75 118

0.2% 81 112 50 112

0.02% 11 36 8 11

0.002% 3 13 3 2.5

0.0002% 0.7 4.1 0.8 1.6

0.00005% 0.2 2.9 0.4 1.1

• Relative abundances* [NO] variation vs dilute (~0.1%) H2

18O sample• Extremes: [NO]>>[18OH] and [NO]<<[18OH]

HONO

• Large fraction of HONO formed directly (single collision) – no subsequent scrambling

• Significant fraction of HONO formed by processing of NO, presumably via

H18OH + NO ↔ O 18O ↔ OH + N18O

N• H18ON18O presence suggests N18O readily

formed and subsequently reacts with 18OH

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