The Butler Group

Benj FitzPatrickBritni RatliffBridget AlligoodDoran BennettJustine BellArjun Raman

Emily GlassmanDr. Xiaonan Tang

Molecular Beam Studies of the the Electronic and Nuclear Dynamics

of Chemical Reactions:Accessing Radical Intermediates

National Science Foundation,

Chemistry Division

Department of Energy,

Basic Energy Sciences

Understanding Chemical Reactions:

What 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 branching

k(T)=Ae-Ea/kT

We use a combination of state-of-the-art experimental techniques and theoretical analysis

Molecular Beam analysis of product velocities and angular distributions

State-selective velocity map imaging

Electronic 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.

O + propargyl products

H2C=C=CH H2C-C=CH

Addition mechanism forms or then ??? H2CCCHO

H2CCCHO

Many elementary bimolecular reactions proceed through addition/insertion,

so go through unstable radical intermediates along the bimolecular reaction coordinate

CH3O + CO CH3OCO CH3 + CO2

Traditional Crossed Molecular Beam Scattering or Imaging Exptsare a good way to probe “Direct” Chemical Reactions

Angular and Velocity Distribution of DF product shows Backward Scattered DF product

Eg. D2 + F D…D…F D + DF

D…D…F

D-D F

But how can one probe bimolecular reactions that proceed through long-lived radical intermediates?

Eg. C2D + HCCHDCCCCH + H

Forward/Backward symmetricproduct angular distributions indicate there is a long-livedintermediate in the reaction.

But what is happening along thereaction coordinate?

Kaiser et al., PCCP 4, 2950 (2002)

But how can one probe bimolecular reactions that proceed through long-lived radical intermediates?

Eg. C2D + HCCHDCCCCH + H

Kaiser et al., PCCP 4, 2950 (2002) UB3LYP/6-311+G** + ZPVE

O + propargyl products

H2C=C=CH H2C-C=CH

Addition mechanism forms or then ???

Testing our predictive ability from first principle quantum mechanics

H2CCCHO

H2CCCHO

H2C=C=C=O + H

c-C3H2 + OH

H2CCC: + OHHCCCH + OH

O ||HC=CCH + H

O + H2CCCHE

nerg

y (k

cal/m

ol)

Choi (CBS-QB3)

INT2

INT1

INT1

INT2

INT2

(-60.3)

H2CCCH

O

H2CCCH

O

His RRKM calcs indicated propynal + H dominates.Choi’s expts probed only the OH products.

H2C=C=C=O + H

c-C3H2 + OH

H2CCC: + OHHCCCH + OH

O ||HC=CCH + H

vinyl + CO

O + H2CCCH

Ene

rgy

(kca

l/mol

)

Choi (CBS-QB3)

+ Bowman (UB3LYP)

INT2

INT1

INT1

INT2

INT2

LM2

(-60.3)

H2CCCH

O

H2CCCH

O

H2C-CHCO

193 nmnozzle

skimmers

ionization source(electron impact at UofC, tunable VUV at ALS)

-30 kV Aldoorknob

quadrupole mass spec.

Scintillator

PMT

Eint radical = hDo(C-Cl)-ET

Our expts produce each radical intermediate photolytically and disperse the radicals by recoil ET and thus by internal energy

193 nm

Cl

CC

C

H

H

H

O

ClCC

C

H

H

H

.O

Measuring the velocities of the stable radicals and the velocities ofthe products from the unstable radicals can determine the barriers to each product channel and how product channel branching changes with internal energy

vinyl + COEne

rgy

(kca

l/mol

)

LM2H2C-CHCO

H2C=CHCO

50 100 150 200 250

m/e = 35 (Cl +)

20o, 15.0 eV

Time-of-Flight, t (μ )s

0 10 20 30 40 50

High translational energy C-Cl fission

ET(kcal/mol)

produces lowest internal energy

radicals

(23.6)

C-Cl fission gives H2CCHCO radicals dispersed by internal energy

Eint radical = h + Eint,prec-Do(C-Cl)-ET(81.9)*

193 nmCl

CC

C

H

H

H

O

ClCC

C

H

H

H

.O

P(E

T)

CCSD(T)*

vinyl + COEne

rgy

(kca

l/mol

)

LM2H2C-CHCO

H2C=CHCO

50 100 150 200 250

m/e = 35 (Cl +)

20o, 15.0 eV

Time-of-Flight, t (μ )s

0 10 20 30 40 50

High translational energy C-Cl fission

ET(kcal/mol)

produces lowest internal energy

radicals

(23.6)

C-Cl fission gives H2CCHCO radicals dispersed by internal energy

Eint radical = h + Eint,prec-Do(C-Cl)-ET(81.9)

193 nm

Cl

CC

C

H

H

H

O

ClCC

C

H

H

H

.O

P(E

T)

CCSD(T)*

vinyl + COEne

rgy

(kca

l/mol

)

LM2H2C-CHCO

H2C=CHCO

(23.6)

All the H2CCHCO radicals dissociate to vinyl + CO products

CCSD(T)

CC

C

H

H

H

.O

50 100 150 200 250 300

m/e = 27 (CH2CH+)

20o, 12.0 eV

Time-of-Flight, t (μ )s

CC

C

H

H

H

.O

CC

C

H

H

H

.O

50 100 150 200 250 300

m/e = 28 (CO +)

20o, 15.0 eV

Time-of-Flight, t (μ )s*

*

vinyl + COEne

rgy

(kca

l/mol

)

LM2H2C-CHCO

H2C=CHCO

(23.6)

CCSD(T)

vinyl + CO

LM2H2C-CHCO

H2C=CHCO

(26.7)

CC

C

H

H

H

.OCC

C

H

H

H

.O

CC

C

H

H

H

.O

UB3LYPBarrier too high?

**

*

(25.3)(20.0)

Upper limit to barrier for H2CCHCO vinyl + CO

vinyl + CO

Ene

rgy

(kca

l/mol

)

LM2H2C-CHCO

H2C=CHCO

(23.6)

C-Cl fission at 235 nm produces lower internal energy H2CCHCO radicals

Eint radical+Cl = h + Eint,prec-Do(C-Cl)-ET(81.9)*

235 nmCl

CC

C

H

H

H

O

ClCC

C

H

H

H

.O

CCSD(T)*

Cl 2P3/2 Cl 2P1/2 (Cl*)

0 10 20 30 40

R + Cl 2P3/2

ET (kcal/mol)

R + Cl 2P1/2

Add these two, correcting for 0.85Cl*/Cl line strength factor (Liyanage)to get total C-Cl fission P(ET)for producing all radicals

vinyl + CO

Ene

rgy

(kca

l/mol

)

LM2H2C-CHCO

H2C=CHCO

(23.6)

C-Cl fission at 235 nm produces lower internal energy H2CCHCO radicals

Eint radical+Cl = h + Eint,prec-Do(C-Cl)-ET(81.9)*

235 nmCl

CC

C

H

H

H

O

ClCC

C

H

H

H

.O

CCSD(T)*

Cl 2P3/2 Cl 2P1/2 (Cl*)

0 10 20 30 40

all R + Cl

ET (kcal/mol)

vinyl + COEne

rgy

(kca

l/mol

)

LM2H2C-CHCO

H2C=CHCO

(23.6)

Use 157 nm photoionization to detect all STABLE H2CCHCO radicals

Eint radical+Cl = h + Eprec-Do(C-Cl)-ET(81.9)*

235 nmCC

C

H

H

H

O

ClCC

C

H

H

H

.O

CCSD(T)*

(157 + 235) - (157 only)

0 5 10 15 20 25 30 35 40E

T (kcal/mol)

18 kcal/mol

Cl

Lowest internal energy at which the H2CCHCO radicals dissociate is: 121.6+1.5-81.9-18=23 kcal/mol

all R + Cl

stable R + Cl

vinyl + CO

Ene

rgy

(kca

l/mol

)

LM2H2C=CHCO

Eint radical+Cl = h + Eprec-Do(C-Cl)-ET(81.9)*

CCSD(T)

*

CCSD(T) barrier = 23.6 kcal/mol

(26.7)

UB3LYPBarrier too high.

(25.3)

Expt’l dissociation onset at ET =18 kcal/molgives Expt’l barrier of 23.2 ±2 kcal/mol

H2C-CHCO

Is this because the UB3LYP radical energy is too low or the TS energy is too high?

vinyl + CO

Ene

rgy

(kca

l/mol

)

LM2H2C-CHCO

H2C=CHCO

(23.6)

Eint radical+Cl = h + Eprec-Do(C-Cl)-ET(81.9)*

CCSD(T) (G3//B3LYP good too)

*

CCSD(T) barrier = 23.6 kcal/mol

Expt’l dissociation onset at ET =18 kcal/molgives Expt’l barrier of 23.2 ±2 kcal/mol

vinyl + CO

LM2H2C=CHCO

*

(26.7)

(25.3)

H2C-CHCO

Eint radical+Cl = h + Eprec-Do(C-Cl)-ET

UB3LYP

(72.4)*

CH3O + CO CH3OCO CH3 + CO2

Bridging physical to organic chemistry

ORBITAL INTERACTIONS ALONG THE REACTION COORDINATE

CH3O· + CO CH3OCO CH3 + CO2

-40

-30

-20

-10

0

10

20

30

0.0

5.8 (JF)6.4 (BW)4.9 (ZZ)

-15.1 (JF)-15.0 (BW)-14.6 (ZZ)

22.1 (JF)17.3 (BW)12.4 (ZZ)

-38.0 (JF)-37.5 (BW)-38.0 (ZZ)

CH3OCO

CH3O + CO

CH3 + CO2JF: 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

OH + CO HOCO H + CO2

k(T,P)product branching

falloff behavior

23.1

-40

-30

-20

-10

0

10

20

30

0.0

5.8 (JF)6.4 (BW)4.9 (ZZ)

-15.1 (JF)-15.0 (BW)-14.6 (ZZ)

22.1 (JF)17.3 (BW)12.4 (ZZ)

-38.0 (JF)-37.5 (BW)-38.0 (ZZ)

CH3OCO

CH3O + CO

CH3 + CO2

CH3O(CO)Cl 193

Cl + CH3OCO

Cl + CH3OCO*

C-Cl fission P(ET )

Do=85.4 (G3//B3LYP)

Einternal of CH3OCO

0 10 20 30 40 50E

T (kcal/mol)

0 50 100 150 200

m/e=35, (Cl+)

19.5o, 14.8 eV

Time-of-Flight, t (μ )sec

Cl+CH3

*OCO (15%)

Cl+CH3

*OCO (85%)

23.1

-40

-30

-20

-10

0

10

20

30

0.0

5.8 (JF)6.4 (BW)4.9 (ZZ)

-15.1 (JF)-15.0 (BW)-14.6 (ZZ)

22.1 (JF)17.3 (BW)12.4 (ZZ)

-38.0 (JF)-37.5 (BW)-38.0 (ZZ)

CH3OCO

CH3O + CO

CH3 + CO2Do=85.4 (G3//B3LYP)

CH3OCOCH3O + CO CH3 + CO2

RRKM product branching BW TSs

CH3OCO 1280

23.1

-40

-30

-20

-10

0

10

20

30

0.0

5.8 (JF)6.4 (BW)4.9 (ZZ)

-15.1 (JF)-15.0 (BW)-14.6 (ZZ)

22.1 (JF)17.3 (BW)12.4 (ZZ)

-38.0 (JF)-37.5 (BW)-38.0 (ZZ)

CH3OCO

CH3O + CO

CH3 + CO2Do=85.4 (G3//B3LYP)

CH3OCOCH3O + CO CH3 + CO2

RRKM product branching BW TSs

CH3OCO 1280 CH3OCO

50 100 150 200

m/e=28 (CO+)

19.5o, 15.4 eV

CH3OCO* -> CH

3O + CO

CH3OCO -> CH

3O + CO

Time-of-Flight (μ )s

50 100 150 200

m/e=44 (CO2

+)

19.5o, 14.8 eV

OCOCl -> CO2 + Cl

CH3OCO* -> CH

3 + CO

2

CH3OCO -> CH

3 + CO

2

Time-of-Flight (μ )s

Expt. branching w. CO/CO2 signal

23.1

-40

-30

-20

-10

0

10

20

30

0.0

5.8 (JF)6.4 (BW)4.9 (ZZ)

-15.1 (JF)-15.0 (BW)-14.6 (ZZ)

22.1 (JF)17.3 (BW)12.4 (ZZ)

-38.0 (JF)-37.5 (BW)-38.0 (ZZ)

CH3OCO

CH3O + CO

CH3 + CO2Do=85.4 (G3//B3LYP)

CH3OCOCH3O + CO CH3 + CO2

RRKM product branching BW TSs

CH3OCO 1280 CH3OCO

50 100 150 200

m/e=28 (CO+)

19.5o, 15.4 eV

CH3OCO* -> CH

3O + CO

CH3OCO -> CH

3O + CO

Time-of-Flight (μ )s

50 100 150 200

m/e=44 (CO2

+)

19.5o, 14.8 eV

OCOCl -> CO2 + Cl

CH3OCO* -> CH

3 + CO

2

CH3OCO -> CH

3 + CO

2

Time-of-Flight (μ )s

1Expt. branching w. CO/CO2 signal

2.5

23.1

-40

-30

-20

-10

0

10

20

30

0.0

5.8 (JF)6.4 (BW)4.9 (ZZ)

-15.1 (JF)-15.0 (BW)-14.6 (ZZ)

22.1 (JF)17.3 (BW)12.4 (ZZ)

-38.0 (JF)-37.5 (BW)-38.0 (ZZ)

CH3OCO

CH3O + CO

CH3 + CO2Do=85.4 (G3//B3LYP)

CH3OCOCH3O + CO CH3 + CO2

RRKM product branching BW TSs

CH3OCO 1280 CH3OCO

50 100 150 200

m/e=28 (CO+)

19.5o, 15.4 eV

CH3OCO* -> CH

3O + CO

CH3OCO -> CH

3O + CO

Time-of-Flight (μ )s

50 100 150 200

m/e=44 (CO2

+)

19.5o, 14.8 eV

OCOCl -> CO2 + Cl

CH3OCO* -> CH

3 + CO

2

CH3OCO -> CH

3 + CO

2

Time-of-Flight (μ )s

1Expt. branching w. CO/CO2 signal

2.5

23.1

-40

-30

-20

-10

0

10

20

30

0.0

5.8 (JF)6.4 (BW)4.9 (ZZ)

-15.1 (JF)-15.0 (BW)-14.6 (ZZ)

22.1 (JF)17.3 (BW)12.4 (ZZ)

-38.0 (JF)-37.5 (BW)-38.0 (ZZ)

CH3OCO

CH3O + CO

CH3 + CO2Do=85.4 (G3//B3LYP)

CH3OCOCH3O + CO CH3 + CO2

RRKM product branching BW TSs

CH3OCO 1280 CH3OCO1

I asked KC Lau to re-calculateCH3 + CO2 barrier

G3//B3LYP and CCSD(T)

Expt. branching w. CO/CO2 signal

H3C…OC=O

2.5

23.1

-40

-30

-20

-10

0

10

20

30

0.0

5.8 (JF)6.4 (BW)4.9 (ZZ)

-15.1 (JF)-15.0 (BW)-14.6 (ZZ)

22.1 (JF)17.3 (BW)12.4 (ZZ)

-38.0 (JF)-37.5 (BW)-38.0 (ZZ)

CH3OCO

CH3O + CO

CH3 + CO2Do=85.4 (G3//B3LYP)

CH3OCOCH3O + CO CH3 + CO2

CH3OCO 1280 CH3OCO1

6.0 (KC)

-15.6 (KC)

16.9 (KC)

H3C…OC=O

-1.6 (KC)

-39.1 (KC)H3C…O

C

=O

RRKM product branching BW TSs Expt. branching w. CO/CO2 signal

2.5

Glaude, Pitz, Thomson 2005Good and Francisco 2000

23.1

-20

-10

0

10

20

30

40

50

60

70

ET

(kcal/mole)A

0.0trans-CH

3OCO

CH3O + CO

CH3 + CO

2

21.6

15.6

-23.5

14.0

0.2cis-CH

3OCO

8.1

CH3 + CO2

CH3O + CO

Average RRKM product branchingover internal energies in our expt.

CH3OCOCH3O + CO CH3 + CO2

CH3OCO 2.5 ± 0.51EXPT.32.5

-20

-10

0

10

20

30

40

50

60

70

ET

(kcal/mole)A

0.0trans-CH

3OCO

CH3O + CO

CH3 + CO

2

21.6

15.6

-23.5

14.0

0.2cis-CH

3OCO

8.1

CH3 + CO2

CH3O + CO

Average RRKM product branchingOver internal energies in our expt.

CH3O + CO

CH3 + CO20

0.2

0.4

0.6

0.8

1

25 30 35 40 45 50 55E

int of CH

3OCO (kcal/mole)

CH3OCOCH3O + CO CH3 + CO2

CH3OCO1EXPT.PRED. CH3OCO 1280

2.5 ± 0.5

-20

-10

0

10

20

30

40

50

60

70

ET

(kcal/mole)A

0.0trans-CH

3OCO

CH3O + CO

CH3 + CO

2

21.6

15.6

-23.5

14.0

0.2cis-CH

3OCO

8.1

CH3 + CO2

CH3O + CO

Average RRKM product branchingOver internal energies in our expt.

CH3O + CO

CH3 + CO2

CH3OCOCH3O + CO CH3 + CO2

CH3OCO1EXPT.

0

0.2

0.4

0.6

0.8

1

25 30 35 40 45 50 55E

int of CH

3OCO (kcal/mole)

CH3 + CO2

CH3O + CO

2.5 ± 0.5

-20

-10

0

10

20

30

40

50

60

70

ET

(kcal/mole)A

0.0trans-CH

3OCO

CH3O + CO

CH3 + CO

2

21.6

15.6

-23.5

14.0

0.2cis-CH

3OCO

8.1

CH3 + CO2

CH3O + CO

Average RRKM product branchingOver internal energies in our expt.

CH3O + CO

CH3 + CO2

CH3OCOCH3O + CO CH3 + CO2

CH3OCO1EXPT.PRED. CH3OCO 2.11

0

0.2

0.4

0.6

0.8

1

25 30 35 40 45 50 55E

int of CH

3OCO (kcal/mole)

CH3 + CO2

CH3O + CO

2.5 ± 0.5

-20

-10

0

10

20

30

40

50

60

70

ET

(kcal/mole)A

0.0trans-CH

3OCO

CH3O + CO

CH3 + CO

2

21.6

15.6

-23.5

14.0

0.2cis-CH

3OCO

8.1

CH3 + CO2

CH3O + CO

Why is the cis barrier so muchlower than the trans one?

H3C … OC

=O

32.5

cis barrier is ~20 kcal/mol lower than trans (CCSD(T))

H … OC

=O

(34.2)

(14.5)

cis barrier is ~7 kcal/mol lower than transMuckerman, FCC/CBS (2001)

-20

-10

0

10

20

30

40

50

60

70

ET

(kcal/mole)A

0.0trans-CH

3OCO

CH3O + CO

CH3 + CO

2

21.6

15.6

-23.5

14.0

0.2cis-CH

3OCO

8.1

CH3 + CO2

CH3O + CO

Why is the cis barrier so muchlower than the trans one?

Think about the interactionbetween the radical orbital and

the H3C-OCO antibonding orbital

H3C … OC

=O

nC

*C-O

Radical energy lowers due to interaction with *C-O orbitalas H3C-OCO bond stretches

32.5 (34.2)

(14.5)

.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Natural Bond Orbital analysis with Weinhold