ralf i. kaiser department of chemistry university of hawai’i at manoa honolulu, hi 96822

Ralf I. KaiserDepartment of Chemistry

University of Hawai’i at ManoaHonolulu, HI 96822ralfk@hawaii.edu

Probing the Reaction Dynamics of

Hydrogen-Deficient Hydrocarbon Molecules and Radical Intermediates

via Crossed Molecular Beams

Introduction

CHx

C2Hx

C3Hx

C4Hx

C5Hx

HC

HC

CH

CH

CH

C

HC

HC

CH

CH

CH

HC

CHHC

HC

CH

CH

Introduction

k = 10-11 – 10-12 cm3s-1 T < 1500 K

Eact = 5 – 45 kJmol-1

H3C H

H

H

H

H

H H

methylacetylene

acetyleneethylene

benzene

H

•

H

H

H

allene

H

H

CH3

H

propylene

Objectives

Investigate the Dynamics and Energetics of Phenyl Radical Reactions

Requirements

1. Preparation of Highly Reactive Reactant Radicals C6H5(X2A1)

2. Identify Reaction Products and Infer Reaction Intermediates

3. Obtain Information on Energetics and Reaction Mechanisms

↓

Single Collision Conditions

Crossed Molecular Beams Experiments

C6H5NO C6H5 + NO

< 0.1 % He seeded

200 Hz; 2800 – 3400 ms-1

ΔT

Crossed Molecular Beams Setup

Main Chamber = 10-9 torr

Detector = 10-11 - < 10-12 torr

1. Hydrocarbon Free

Requirements

2. Extremely Low Pressures

3. Signal Maximization

+

C6H5 + C2H2

77 amu 26 amu

C8H7 103

C8H7 + H 102 + 1

C8H6 + H2 101 + 2

+

C6H5 + C2D2

77 amu 28 amu

C8H5D2 105

C8H4D2 + H 104 + 1

C8H5D + D 103 + 2

C6H5 + C2H2 C8H6 + H (m/z = 102)

C6H5 + C2H2 C8H6 + H (m/z=102)

indirect reaction via intermediate exit barrier

Emax = Ec - rG

C6H5 + C2H2 C6H5CCH (m/z = 102) + H

+

Rel

ativ

e E

nerg

y, E

(kJ

/mol

)

25

50

0

-25

-50

-175

-125

-100

-75

-150

C6H5+C2H2

-36

-169

15

-90

X. Gu, F. Zhang, Y. Guo, R.I. Kaiser, Angew. Chemie Int. Edition 46, 6866 (2007).

C6H5 + C2H4 C8H8 + H (m/z=104)

C6H5 + C2D4 C6H5C2D3 + D (m/z=107)

C6H5 + C2H4 C6H5C2H3 + H (m/z=104)

indirect via intermediateexit barrier

Rel

ativ

e E

nerg

y, E

(kJ

/mol

)

25

50

0

-25

-50

-175

-125

-100

-75

-150

C6H5+C2H4

-30

-151

10

-50

C6H5 + C2H4 C6H5C2H3 + H (m/z=104)

F. Zhang, X. Gu, Y. Guo, R. I. Kaiser, J. Organic Chem. 72, 7597 (2007).

C6H5 + H2CCHCH3 C9H10 + H (m/z=118)

C6H5 + H2CCHCH3 C9H10 + H

C6H5 + C2H3CH3 C9H10 + H

Rel

ativ

e E

nerg

y, E

(kJ

/mol

)25

50

0

-25

-50

-175

-125

-100

-75

-150

C6H5+C3H6

9

25

0

19

-130

511

24

0

F. Zhang, X. Gu, Y. Guo, R.I. Kaiser, JPCA 112, 3284 (2008).

Phenyl Radical Reactions

Acetylene Ethylene Methylacetylene Allene Propylene Benzene

80 – 185 kJmol-1

Phenyl Radical Reactions

0

20

40

60

80

100

120

C.M.

(A)

0

20

40

60

80

100

120

C.M.

0

20

40

60

80

100

120

C.M.

(D)

(B) (E)

(C) (F)

Rel

ativ

e In

tens

ity (

arb.

uni

ts)

0

20

40

60

80

100

120

C.M.

0 5 10 15 200

20

40

60

80

100

120

C.M.

Lab Angle , (degree)

0 5 10 15 20 25 30 35 40 900

20

40

60

80

100

120

C.M.

Phenyl versus Hydrogen Exchange

Phenyl Group Stays Intact

Partially Deuterated ReactantsIsomer-Selective Detection

Abstraction Reactions < 5 %

Phenyl Radical Reactions

0.0

0.2

0.4

0.6

0.8

1.0 (A)

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

(D)

(B) (E)

(C) (F)

P (

ET)

0.0

0.2

0.4

0.6

0.8

1.0

0 40 80 120 160 2000.0

0.2

0.4

0.6

0.8

1.0

Product Translational Energy ET, (kJmol-1)

0 40 80 120 160 200 2400.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

(A)0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

(D)

(B) (E)

(C) (F)T

()

0.0

0.2

0.4

0.6

0.8

1.0

0 30 60 90 120 1500.0

0.2

0.4

0.6

0.8

1.0

Center of Mass Angle , (degree)

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0

Indirect Reaction via Intermediates

Short Lived Intermediates (no ring closure)

Exit Barriers for Hydrogen Loss

Exoergic / Slightly Endoergic

Phenyl Radical Reactions

trans-1,3-butadiene 1,2-butadiene

2-butyne 1-butyne

Phenyl Radical Reactions

1-phenyl-1,3-butadiene (from 1,3-butadiene)

1-phenyl-butyne-21-phenyl-3-methylallene (from 1,2-butadiene) (from 1,2-butadiene)

1-phenyl-1-methylallene (from 2-butyne)

1-phenyl-3-methylallene (from 1-butyne)

1-phenyl-1-butyne (from 1-butyne)

Outlook

indene; C9H8 dihydroindene; C9H10

naphthalene; C10H8didehydronaphthalene; C10H6 dihydronaphthalene; C10H10

C6H5NO C6H5 + NOh (266 nm; 248 nm)

vp = 1800 – 2100 ms-1; EC = 30 – 50 kJmol-1

C2(X1g+/a3u) + C4H6 (1,3-butadiene)

C2 + C4H6 C6H5 + H Ec = 13 kJmol-1

C2(X1g+/a3u) + C4H6 (1,3-butadiene)

C2 + C4H6 C6H5 + H Ec = 36 kJmol-1

C2 + C4H6 C6H5 + H

phenylacyclic

H2CCDCDCH2 D2CCHCHCD2

C2 + C4H6 C6H5 + H

C2D + C4H6 C6DH5 + H

H2CCDCDCH2 D2CCHCHCD2

Ec = 45 kJmol-1

C2D + C4H6 C6DH5 + H

43 ± 10 %57 ± 10 %

C2D + C4H6 C6DH5 + H

rela

tiv

e e

ne

rgy

, k

Jm

ol-1

Outlook

EC = 80 – 185 kJmol-1

EC = 13 – 45 kJmol-1

EC = 30 - 50 kJmol-1

N

Outlook

Outlook

Acknowledgements