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Association ReactionStudies of Alkali Metal Ions and
Dimethoxy ethane (DXE)
Hideya Koizumi and P. B. ArmentroutUniversity of Utah
+
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Objectives• R. Dunbar has measured BDEs by association
reactions in an FT-ICR • Here, we investigate the kinetic energy
dependence of association reaction by GIBMS• Alkali metal cation and DXE association reaction
is chosen because the only product is the long-lived association complex
• We want to develop methods to obtain the BDE for the association reaction cross section
• The results test our use of statistical theory
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Potential Energy SurfaceM+DXE*
M+ + DXE
Association
UnimolecularDissociation
C ID
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Guided Ion Beam Mass Spectrospcopy(Efrim)
10 cm
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SchematicsAssociation Reaction
M+ + DXEAssociation
M+(DXE)*
Long lived Complex
Dissociation
The lifetime of the complex can be calculated from the energy dependent unimolecular decay rate constant k
Required conditions for obtaining good experimental data1) Complex has to be long lived (more than flight time )to be detected2) No collisional stabilization by multiple collisions
(pressure dependence)
Basically, M+DXE(E) = LGS(E) exp[-k(E)]
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Pressure Dependence
Na++ DXE
Energy(CM, eV)0.1 1
Cro
ss S
ectio
ns(1
0-16 cm
2 )
0.1
1
10
100
Energy(Lab, eV)0.1 1
0.100 mTorr 0.059 mTorr 0.026 mTorr 0.000 mTorr
Na+DXE
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M+ + DXE
Energy (CM, eV)0.1 1
Cro
ss S
ectio
ns (1
0-16 cm
2 )
0.1
1
10
100
Li+
Na+
K+
LGS
2.50 (0.19)
1.64 (0.04)
1.23 (0.04)
BDEs (eV) from CID values and theory (More et al.)
1.35
1.79
2.65
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Modeling the cross sectionsIn all models, the only adjustable parameter is E0
• Statistical distribution of total angular momentum (AM)(by Rodgers, Ervin, Armentrout)
Usual method used for CID (Fix n and 0 to give LGS)Problem: AM is not conserved
2) Explicit conservation of orbital AMUse impact parameter distribution
It conserves AM for the complex (w/ stationary target approx.)Approximation L’ = L’’ (No Coupling to rotations); |J | << |L |
3) Rigorous Statistical Theory (Phase Space Theory)(by Chesnavich, Bowers) Explicitly conserve angular momentum (Coupling allowed)
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Distribution of Total Angular Momentum of the Energized Molecule
J
J Dis
tribu
tion
StatisticalOrbital AMConservation& PST
Centrifugal acceleration
Jmax
Deceleration
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Na+ + DXE
Energy (CM, eV)0.1 1
Cro
ss S
ectio
ns (1
0-16 cm
2 )
0.1
1
10
100
Energy (Lab, eV)0.1 1
Na+DXE
LGS
CID 1.64 (0.04) eV
Statistical 1.78 ± 0.12
Orbital AM conservation
1.75 ± 0.11
PST 2.20 ± 0.17 !
! Means work in progress
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Na+ + DXE
Energy (CM, eV)0.1 1
Cro
ss S
ectio
ns (1
0-16 cm
2 )
0.1
1
10
100
Energy (Lab, eV)0.1 1
Na+DXE
LGS
Na+
E0=1.64eV
E0=1.75 eV
E0=1.86 eV
Very conservative Error chosen for time being
ModelOrbital AM conservation
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K+ + DXE
Energy (CM, eV)0.1 1
Cro
ss S
ectio
ns (1
0-16 cm
2 )
0.1
1
10
100
Energy (Lab, eV)0.1 1
K+DXE
LGS
K+
CID 1.23 (0.04) eV
Statistical 1.28 ± 0.10
Orbital AMConservation 1.28 ± 0.10
PST 1.62 ± 0.08 !
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Li+ + DXE
Energy (CM, eV)0.1 1
Cro
ss S
ectio
ns (1
0-16 cm
2 )
1
10
100
Energy (Lab, eV)0.1 1
Li+DXE
LGS
CID 2.50 (0.19) eV
Statistical 2.45 ± 0.35
Orbital AM conservation
2.40 ± 0.3
PST 3.00 ± 0.4 !
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CID Theory Statistical Orbital AM conservation
Phase Space Theory (!)
Li+ +DXE
2.50(0.19)
2.65 2.45(0.35)
2.40(0.3)
3.00(0.4)
Na+ +DXE
1.64(0.04)
1.79 1.78(0.12)
1.75(0.11)
2.20(0.17)
K+ +DXE
1.23(0.04)
1.39 1.28(0.10)
1.28(0.10)
1.62(0.08)
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SummaryEnergy dependence of the cross section is well
charactorized with all modelThe association reaction analysis (Statistical and
Orbital AM conservation) nearly reproduce the bond dissociation energy obtained by CID
The dynamic range of this analysis is established to be fairly large
Work In ProgressBetter implementation of Statistically rigorous PSTin this example.
Oblate & TS switching & etc
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Acknowledgement
• Group Members• Prof. P. B. Armentrout• Prof. K. M. Ervin• Thank you for listening!!This work is supported by NSF#
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One Possible reason of poor performance of statistically rigorous PST
• Maybe the complex rotating fastFragments of TS (large |L |) stop rotating • Better approximated by TTS1) Barrierless TS switching (K dependent)
( Chesnavitch & Bowers)1) Note (Tightness of TS is depend on |L |) Allow TS switch for even small K but large L
K
J
L|L | large enough thatTS is better considered to be Tight TS
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What about Sphere approximation?
Sphere is very good approximation!
(Chesnavich&Bowers)
atom-oblateatom-prolate
What about Centrifugel distortion of M+DXE complex?Our calculation shows that change of rotational energy is much less than 5% in all case [Li < Na < K]
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QuestionDoes rotational phase space dynamically restricted?
PST always prefer to dissociate with low value of orbital angular momentum no matter how fast the complex rotate.