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.

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

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

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

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).

IR laser absorption spectroscopy of 1988-1990 J. Chem. Phys. IR laser absorption spectroscopy of CH3 product

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

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

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

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

(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)

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)

Forward Scattering is caused by insertion O(1D) + CD4 → OD + CD3(v=0, N=3) @ Ecol = 6.8 kcal/mol O(1D) insertion reactants products forward broad high CD3 Intensity CD4

Backward scattering with discrete speed distribution is by abstraction O(1D) + CD4 → OD + CD3(v=0, N=3) @ Ecol = 6.8 kcal/mol O(1D) reactants products abstraction high discrete “ring” Intensity backward CD3 vOD 6 5 4 CD4

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)

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

Collision energy dependence sa si Excitation function E0 Ecol Ecol E0 ? sa si

Imaging of beam velocities using m/e=18 signals Ogi et al., Phys. Chem. Chem. Phys., 2013, 15, 12946 - 12957 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

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

Beam velocities and collision energies max min

sa/si No clear isotope effect on abstraction barrier height Ogi et al., Phys. Chem. Chem. Phys., 2013, 15, 12946 - 12957 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

* 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)

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

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

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)

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???

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

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.