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 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)
• Competition in
binding
energies
– Need
[O2]>>[H2O]
•Extend OH + X
mechanism…
{X = CO, NO, SO2,
etc…}
Yu et al. Phys. Chem. Chem. Phys., 2008
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
HOCO PES
Li et al. J. Phys. Chem. A 2012
HOCO
• Synthesis:
OH + CO
H + CO2
→
HOCO
• 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 cisand trans- isomers:
HOCO, H18OCO, HO13CO, HOC18O, H18OC18O,
DOCO, D18OCO, DO13CO, DOC18O, and
D18OC18O
HOCO
Step 1) HO + C18O → HOC18O
Step 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)
HOCO
• 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 unpaired electron orbitals
Oyama et al. J. Chem. Phys., 2011
• Fermi contact constant, aF
– Oyama et al. normal aF = -6.9 (trans) and 82.8 (cis)
– trans- HO13CO fit results in aF = 117.8
HONO PES
Asatryan et al. Int. J. Chem. Kin. , 2007
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%) H218O 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
H
18OH + 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