1. O(1D) Reaction with Methane Studied by
ISMS 2014
O(1D) Reaction with Methane Studied by
State-Resolved Scattering Distribution Measurements of Methyl Radicals
Toshinori Suzuki
Department of Chemistry
Graduate School of Science, Kyoto University
coworkers
Dr. Yoshihiro Ogi
RIKEN
Prof. Hiroshi Kohguchi
Hiroshima U.

2. O(1D2) + CH4  various products
100
H 23%
CH4+O(1D2)
2H+H2CO
H2O 6%
H+CH3O
H2O
+CH2(a)
H+CH2OH 50
OH 69%
CH3+OH
CH4+O(3P)
Heat of formation (kcal/mol)
H2+HCOH H2 5%
H2+H2CO
Branching ratio
(Lin et al. JCP 113 (2000) 5287)
-50
CH3OH

3. O(1D2) + CH4  OH + CH3: dual pathways
Early barrier
Excited-state PES
Collinear transition state
100
CH4+O(1D2) 3 4 5 7 6 5
abstraction 4 2 3 2 1 50 1
CH3+OH
CH4+O(3P)
CH3+OH(v)
CD3+OD(v)
Heat of formation (kcal/mol)
insertion
Methanol intermediate
No barrier (deep well)
Ground-state PES
-50
CH3OH

4. Why are there two pathways?
Barriers at collinear configuration（~1.8 kcal/mol）
OH(A2S) + H
2 A'
O(1D) + H2 a’
OH(X2P) + H
1 A" a’
Excited-state PES；
“Abstraction” a’
1 A'
Ground-state PES；
“Insertion”
Formation of H2O(X)
by O-atom insertion

5. The elusive abstraction pathway (part 1)
CH3 + OH formation occurred even in high pressure gas
and liquid argon.
W. B. DeMore and O. F. Raper, JCP, 46, 2500 (1967)
C. L. Lin and W. B. DeMore, JPC, 77, 863 (1973)
Due to abstraction? But, vibrational relaxation of CH3OH*, created by insertion, may not be fast enough.
Reaction starting from CH4-O3 vdW complex exhibited OH signal appearance times of 0.2, 0.5 and 5 ps.
C. C. Miller, R. D. van Zee, and J. C. Stephenson, JCP, 114, 1214 (2001)
Assigned to abstraction by translationally hot O(3P).

6. IR laser absorption spectroscopy of
J. Chem. Phys.
IR laser absorption spectroscopy of
CH3 product

7. Lifetime of any CH3OH* intermediate must be short+ +
Rovibrational
distribution
Vibrational
distribution 1
n =0
“hot OH”
“cold CH3”
0.8
n2=1
0.6
Relative Population
Ecoll= 21.4
kcal/mol 2
0.4 3
0.2
Ecoll= 7.0
kcal/mol 4
n OH
1000
2000
3000
N OH
Vib. Energy of CH3 (cm-1)
LIF spectroscopy
IR diode laser spectroscopy
Park & Wiesenfeld, JCP 95(1991)8166
Suzuki & Hirota, JCP 98(1993)2387

8. Rev. Sci. Instrum. 40, 1402 (1969)
J. Chem. Phys. 56, 769 (1972).

9. The elusive abstraction pathway (part 2)
Lin, Shu, Lee, Yang, JCP 113 (2000) 5287
non-state selected OH
insertion
large b
insertion
(osculating complex)
CH4
abstraction
O(1D)
Ecoll = 6.8 kcal/mol
Strong forward ?
Weak backward or

10. Crossed molecular beam scattering & ion imaging
O(1D) + CH4 → OH + CH3(v, N)
CCD camera
MCP+Phosphor
electrodes X ~
3pz IP UV
CH3+
1% O2
/rare gas
CH4
10% CH4
/rare gas
O(1D)
O2 + hv(157nm) → O(1D) + O(3P)
State-selective ionization of CH3(REMPI)
F2 excimer laser
YAG-Dye SHG

11. (2+1) REMPI spectrum of CD3 at Ecol = 6.8 kcal/mol
- Vibrational distribution - X ~
3pz IP UV n2 n1
CD3 Intensity (arb.) + 1 2 2 1 3 2 1 2 1 3 1 1 2 2 3 1
UV Two-Photon Energy (cm-1)

12. Scattering image of CD3(v = 0, N = 3)
O(1D)
FORWARD
BACKWARD
SIDEWAYS
CD3(v=0) + OD(v=5)
CD3(v=0) + OD(v=4)
Kohguchi et al.
PCCP
(2008)
CD3(v=0) + OD(v=3)

13. Forward Scattering is caused by insertion
O(1D) + CD4 → OD + CD3(v=0, Ecol = 6.8 kcal/mol
O(1D)
insertion
reactants
products
forward
broad
high
CD3
Intensity
CD4

14. Backward scattering with discrete speed distribution is by abstraction
O(1D) + CD4 → OD + CD3(v=0, Ecol = 6.8 kcal/mol
O(1D)
reactants
products
abstraction
high
discrete
“ring”
Intensity
backward
CD3
vOD 6 5 4
CD4

15. n1 is only excited by insertion pathway~
3pz IP UV 2 1
CD3 Intensity (arb.) +
Ins.
Abs.
Ins.
Ins.
Abs. 2 1 3 2 1 2 1 3 1 1 2 2 3 1
UV Two-Photon Energy (cm-1)

16. Abstraction creates rotationally cold methyl only
CD3 (v=0, Ecol = 6.8 kcal/mol
Broad forward scattering
“Insertion”
N = 3 5 7
10-12
Ring-like backward scattering
“Abstraction” 16

17. Collision energy dependencesa si
Excitation function E0
Ecol
Ecol E0 ? sa si

18. Imaging of beam velocities using m/e=18 signals
Ogi et al., Phys. Chem. Chem. Phys., 2013, 15,
1 km/s
UV laser
CD4/Rg
O(1D)/Rg
CD4/He
1680 m/s
H2O+
m/e=18
CD4/Ne
CD4/Ar
CD4/Xe
CD4/He
CD4/Ne
CD4/Ar
CD4/Xe
O(1D2)/He
O(1D2)/Ne
Beam speed (m/s)
1680±90
940±50
660±50
390±30
1890±90
880±50
O(1D)/Ne
O(1D)/He

19. Determination of Newton diagram
1 km/s
UV laser
CD4/Rg
O(1D)/Rg
CD4/He
H2O+
vCD4
m/e=18
wCD4
CD4 / Ne
CD4 / Ar
CD4 / Xe
vCM
CD4/He
CD4/Ne
CD4/Ar
CD4/Xe
O(1D2)/He
O(1D2)/Ne
Beam speed (m/s)
1680±90
940±50
660±50
390±30
1890±90
880±50
wO(1D)
O2 / Ne
vO(1D)
O(1D)/He

20. Beam velocities and collision energies
max
min

21. sa/si No clear isotope effect on abstraction barrier height
Ogi et al., Phys. Chem. Chem. Phys., 2013, 15,
CH3 (v = 0)
CD3 (v = 0)
Shuai et al.
JPCL 3(2012)1310
E0 = 0.7±0.3
E0 = 0.8±0.1
p = 2.0±0.2
p = 1.6±0.1

22. * Universal crossed beam
Isotope effect in insertion pathway More forward scattering of CH3 than CD3 !
* Universal crossed beam
18O(1D2) + CH4
Lin et al. JCP(2000)
CH3 (v=0), insertion
CD3 (v=0), insertion
CH3 (v1=1)
CD3 (v1=1)

23. CH3OH* dissociates faster than CD3OD*
rotates
faster
slower H H H D H H H D H H H D H H H D
insertion
insertion
breaks up
faster
breaks up
slower
But
less backward
more backward

24. Isotope effect: only CD3 exhibits angle-dependent recoil !
CH3 (v1=1)
CD3 (v1=1)
Velocity (km/s)
Intensity (arb. units) 1 2 3 Fw Sw Bw
6.4
kcal/mol
3.7
1.6
3.8
1.8
6.8
CH3 (v1=1)
CD3 (v1=1)
6.4
kcal/mol
6.8 Fw Sw Bw
6.8
3.7
3.8
1.6
1.8

25. CH3OH* dissociates faster than CD3OD*
rotational clock
faster
slower H H H D H H H D H H H D H H H D
Intramolecular
Vibrational
Redistribution
insertion
insertion
breaks up
breaks up
less backward
more backward
(more translational energy)

26. Calculation overestimates lifetime of methanol complex
Ab initio trajectory calculation
Yu and Muckerman JPCA 108 (2004)8615
direct process
< 450 fs
long-lived complex
> 1 ps
19 %
81 %
Scattering distribution should be
close to forward-backward symmetry.
They used RRKM to predict the branching into
different products of H/H2/OH etc. Is it OK???

27. Atom + Methane: benchmark
F, Cl, Br, O(3P) + methane
Abstraction without an intermediate
See for example
Czako and Bowman [JPCA, 118, 2839 – 2864 (2014)]
Presentation WH09 by H. Pan, WH11 by E. Volpa

28. Summary
Abstraction: no clear isotope dependence of barrier (~0.8 kcal/mol).
Abstraction: creates low J states of v=0 or n2 excited states.
Insertion: CH4 creates a shorter-lived methanol intermediate than CD4.
Insertion: energy partitioning is non-statistical in both CH4 and CD4.
Insertion: IVR does occur in O + CD4.
Insertion: CH4 exhibits a narrower forward scattering width. (Tunneling at large impact parameter or smaller momentum transfer)
Insertion: methyl radicals are rotating like cartwheel (low K), as expected.
Theoretical dynamical calculations should be refined for the ground state pathway. There is no dynamical calculation for the excites state pathway.