The Butler Group

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Molecular Beam Studies of the the Electronic and
Nuclear Dynamics of Chemical Reactions Accessing
Radical Intermediates
The Butler Group Benj FitzPatrick Britni
Ratliff Bridget Alligood Doran Bennett Justine
Bell Arjun Raman Emily Glassman Dr. Xiaonan Tang
National Science Foundation, Chemistry Division
Department of Energy, Basic Energy Sciences
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Understanding Chemical ReactionsWhat is the
nuclear dynamics during the reaction? (vibration
and rotation in the colliding molecules)What is
happening to the electrons in the system? (do
they adjust instantaneously, or lag behind and
cause nonadiabatic suppression of the reaction
rate?)How can we get predictive ability from
first principle quantum mechanics?How does this
change our qualitative understanding of chemical
reaction rates and product branchingk(T)Ae-Ea/kT
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We use a combination of state-of-the-art
experimental techniques and theoretical
analysisMolecular Beam analysis of product
velocities and angular distributionsState-select
ive velocity map imagingElectronic structure
calculations of minima and transition states
along each reaction coordinate(e.g. G3//B3LYP or
CCSD(T) )Analyzing the change in electronic
wavefunction along the reaction coordinates.
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Many elementary bimolecular reactions proceed
through addition/insertion, so go through
unstable radical intermediates along the
bimolecular reaction coordinate
CH3O CO ? CH3OCO ? CH3 CO2
O propargyl ? products
??
H2C-CCH
H2CCCH ?
O
O
Addition mechanism forms or
then ???
H2CCCH
H2CCCH
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Traditional Crossed Molecular Beam Scattering or
Imaging Expts are a good way to probe Direct
Chemical Reactions
Eg. D2 F ? DDF ? D DF
DDF
D-D?
? F
Angular and Velocity Distribution of DF product
shows Backward Scattered DF product
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But how can one probe bimolecular reactions that
proceed through long-lived radical intermediates?
Eg. C2D HCCH?DCCCCH H
Forward/Backward symmetric product angular
distributions indicate there is a
long-lived intermediate in the reaction. But
what is happening along the reaction coordinate?
Kaiser et al., PCCP 4, 2950 (2002)
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But how can one probe bimolecular reactions that
proceed through long-lived radical intermediates?
Eg. C2D HCCH?DCCCCH H
Kaiser et al., PCCP 4, 2950 (2002)
UB3LYP/6-311G ZPVE
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O propargyl ? products
??
H2C-CCH
H2CCCH ?
O
O
Addition mechanism forms or
then ??? Testing our predictive
ability from first principle quantum mechanics
H2CCCH
H2CCCH
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O H2CCCH
H2CCC OH
HCCCH OH
c-C3H2 OH
INT1
INT2
INT2
O HCCCH H
Energy (kcal/mol)
(-60.3)
O
H2CCCO H
H2CCCH
O
INT2
H2CCCH
INT1
Chois expts probed only the OH products.
His RRKM calcs indicated propynal H dominates.
Choi (CBS-QB3)
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O H2CCCH
H2CCC OH
HCCCH OH
c-C3H2 OH
INT1
INT2
INT2
O HCCCH H
Energy (kcal/mol)
(-60.3)
O
H2CCCO H
H2CCCH
O
INT2
H2CCCH
vinyl CO
INT1
Choi (CBS-QB3)
Bowman (UB3LYP)
LM2
H2C-CHCO
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Our expts produce each radical intermediate
photolytically and disperse the radicals by
recoil ET and thus by internal energy
Eint radical hn-Do(C-Cl)-ET
193 nm
Cl
193 nm
nozzle
ionization source (electron impact at UofC,
tunable VUV at ALS)
-30 kV Al doorknob
skimmers
quadrupole mass spec.
Scintillator
PMT
Measuring the velocities of the stable radicals
and the velocities of the products from the
unstable radicals can determine the barriers to
each product channel and how product channel
branching changes with internal energy
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C-Cl fission gives H2CCHCO radicals dispersed by
internal energy
Eint radical hn Eint,prec-Do(C-Cl)-ET
(81.9)
193 nm
Cl
P(ET)
Energy (kcal/mol)
(23.6)
vinyl CO
H2CCHCO
LM2

H2C-CHCO
CCSD(T)
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C-Cl fission gives H2CCHCO radicals dispersed by
internal energy
Eint radical hn Eint,prec-Do(C-Cl)-ET
(81.9)
193 nm
Cl
P(ET)
Energy (kcal/mol)
(23.6)
vinyl CO
H2CCHCO
LM2

H2C-CHCO
CCSD(T)
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All the H2CCHCO radicals dissociate to vinyl
CO products

Energy (kcal/mol)
(23.6)
vinyl CO
H2CCHCO
LM2

H2C-CHCO
CCSD(T)
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Upper limit to barrier for H2CCHCO vinyl
CO

CCSD(T)
UB3LYP Barrier too high?
Energy (kcal/mol)
(26.7)
(23.6)
vinyl CO
(25.3)
vinyl CO
(20.0)
H2CCHCO
H2CCHCO
LM2

LM2

H2C-CHCO
H2C-CHCO
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C-Cl fission at 235 nm produces lower internal
energy H2CCHCO radicals
Cl 2P3/2
Cl 2P1/2 (Cl)
Eint radicalCl hn Eint,prec-Do(C-Cl)-ET
(81.9)
235 nm
Cl
Add these two, correcting for 0.85 Cl/Cl line
strength factor (Liyanage) to get total C-Cl
fission P(ET) for producing all radicals
Energy (kcal/mol)
(23.6)
vinyl CO
H2CCHCO

LM2
H2C-CHCO
CCSD(T)
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C-Cl fission at 235 nm produces lower internal
energy H2CCHCO radicals
Cl 2P3/2
Cl 2P1/2 (Cl)
Eint radicalCl hn Eint,prec-Do(C-Cl)-ET
(81.9)
235 nm
Cl
Energy (kcal/mol)
(23.6)
vinyl CO
H2CCHCO

LM2
H2C-CHCO
CCSD(T)
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Use 157 nm photoionization to detect all STABLE
H2CCHCO radicals
(157 235) - (157 only)
Eint radicalCl hn Eprec-Do(C-Cl)-ET
(81.9)
235 nm
Cl
Lowest internal energy at which the H2CCHCO
radicals dissociate is 121.61.5-81.9-182
3 kcal/mol
all R Cl
stable R Cl
Energy (kcal/mol)
(23.6)
vinyl CO
H2CCHCO

LM2
H2C-CHCO
CCSD(T)
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CCSD(T)
Eint radicalCl hn Eprec-Do(C-Cl)-ET
(81.9)
CCSD(T) barrier 23.6 kcal/mol
Exptl dissociation onset at ET 18
kcal/molgives Exptl barrier of 23.2 2 kcal/mol
Is this because the UB3LYP radical energy is
too low or the TS energy is too high?
UB3LYP Barrier too high.
(26.7)
(25.3)
Energy (kcal/mol)
vinyl CO
H2CCHCO

LM2
H2C-CHCO
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CCSD(T) (G3//B3LYP good too)
UB3LYP
Eint radicalCl hn Eprec-Do(C-Cl)-ET
Eint radicalCl hn Eprec-Do(C-Cl)-ET
(81.9)
(72.4)
CCSD(T) barrier 23.6 kcal/mol
Exptl dissociation onset at ET 18
kcal/molgives Exptl barrier of 23.2 2 kcal/mol
(26.7)
(23.6)
Energy (kcal/mol)
vinyl CO
(25.3)
vinyl CO
H2CCHCO
H2CCHCO


LM2
LM2
H2C-CHCO
H2C-CHCO
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CH3O CO ? CH3OCO ? CH3 CO2 Bridging
physical to organic chemistry ORBITAL
INTERACTIONS ALONG THE REACTION COORDINATE
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CH3O CO ? CH3OCO ? CH3 CO2
OH CO ? HOCO ? H CO2
23.1
k(T,P) product branching falloff behavior
JF Francisco, J. Chem. Phys. 237, (1998) 1-9.
QCISD(T) BW Wang, B. et al. JPCA 103, (1999)
8021-9. G2(B3LYP/MP2/CC) ZZ Zhou, Z. et al.
Chem. Phys. Lett. 353, (2002) 281-9. B3LYP
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Cl CH3OCO
CH3O(CO)Cl
193
Cl CH3OCO
C-Cl fission P(ET )
Einternal of CH3OCO
23.1
Do85.4 (G3//B3LYP)
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CH3OCO
CH3O CO
CH3 CO2
RRKM product branching BW TSs
1
280
CH3OCO
23.1
Do85.4 (G3//B3LYP)
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CH3OCO
CH3O CO
CH3 CO2
RRKM product branching BW TSs
Expt. branching w. CO/CO2 signal
1
280
CH3OCO
CH3OCO
23.1
Do85.4 (G3//B3LYP)
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CH3OCO
CH3O CO
CH3 CO2
RRKM product branching BW TSs
Expt. branching w. CO/CO2 signal
1
280
1
2.5
CH3OCO
CH3OCO
23.1
Do85.4 (G3//B3LYP)
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CH3OCO
CH3O CO
CH3 CO2
RRKM product branching BW TSs
Expt. branching w. CO/CO2 signal
1
280
1
2.5
CH3OCO
CH3OCO
23.1
Do85.4 (G3//B3LYP)
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CH3OCO
CH3O CO
CH3 CO2
RRKM product branching BW TSs
Expt. branching w. CO/CO2 signal
1
280
2.5
CH3OCO
CH3OCO
1
CO
H3CO
23.1
I asked KC Lau to re-calculate CH3 CO2
barrier G3//B3LYP and CCSD(T)
Do85.4 (G3//B3LYP)
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CH3OCO
CH3O CO
CH3 CO2
RRKM product branching BW TSs
Expt. branching w. CO/CO2 signal
1
280
2.5
CH3OCO
CH3OCO
1
CO
H3CO
6.0 (KC)
16.9 (KC)
23.1
-1.6 (KC)
O

C
-15.6 (KC)
-39.1 (KC)
H3CO
Glaude, Pitz, Thomson 2005 Good and Francisco 2000
Do85.4 (G3//B3LYP)
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Average RRKM product branching over internal
energies in our expt.
CH3OCO
CH3O CO
CH3 CO2
CH3OCO
2.5 0.5
EXPT.
1
32.5
CH3O CO
CH3 CO2
33
Average RRKM product branching Over internal
energies in our expt.
CH3OCO
CH3O CO
CH3 CO2
CH3OCO
EXPT.
2.5 0.5
1
PRED.
1
280
CH3OCO
CH3O CO
CH3O CO
CH3 CO2
CH3 CO2
34
Average RRKM product branching Over internal
energies in our expt.
CH3OCO
CH3O CO
CH3 CO2
CH3OCO
EXPT.
2.5 0.5
1
CH3O CO
CH3O CO
CH3 CO2
CH3O CO
CH3 CO2
CH3 CO2
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Average RRKM product branching Over internal
energies in our expt.
CH3OCO
CH3O CO
CH3 CO2
CH3OCO
EXPT.
2.5 0.5
1
PRED.
2.1
1
CH3OCO
CH3O CO
CH3O CO
CH3 CO2
CH3O CO
CH3 CO2
CH3 CO2
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Why is the cis barrier so much lower than the
trans one?
32.5
(34.2)
CH3O CO
(14.5)
O

cis barrier is 20 kcal/mol lower than trans
(CCSD(T))
C
CH3 CO2
H3C O
O

C
cis barrier is 7 kcal/mol lower than
trans Muckerman, FCC/CBS (2001)
H O
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Why is the cis barrier so much lower than the
trans one?
32.5
(34.2)
CH3O CO
(14.5)
Think about the interaction between the radical
orbital and the H3C-OCO antibonding orbital
CH3 CO2
sC-O
O

.
C
nC
H3C O
Radical energy lowers due to interaction with
sC-O orbital as H3C-OCO bond stretches
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Natural Bond Orbital analysis with Weinhold