Ralf I. Kaiser Department of Chemistry University of Hawai’i at Manoa Honolulu, HI Probing the Reaction Dynamics of Hydrogen-Deficient Hydrocarbon Molecules and Radical Intermediates via Crossed Molecular Beams

Introduction CH x C 2 H x C 3 H x C 4 H x C 5 H x

Introduction k = – cm 3 s -1 T < 1500 K E act = 5 – 45 kJmol -1

Objectives Investigate the Dynamics and Energetics of Phenyl Radical Reactions

Requirements 1.Preparation of Highly Reactive Reactant Radicals C 6 H 5 (X 2 A 1 ) 2. Identify Reaction Products and Infer Reaction Intermediates 3. Obtain Information on Energetics and Reaction Mechanisms ↓ Single Collision Conditions Crossed Molecular Beams Experiments C 6 H 5 NO  C 6 H 5 + NO < 0.1 % He seeded 200 Hz; 2800 – 3400 ms -1 ΔTΔT

Crossed Molecular Beams Setup Main Chamber = torr Detector = < torr 1. Hydrocarbon Free Requirements 2. Extremely Low Pressures 3. Signal Maximization

C 6 H 5 + C 2 H 2 77 amu 26 amu C8H7C8H7 103 C 8 H 7 + H C 8 H 6 + H

C 6 H 5 + C 2 D 2 77 amu 28 amu C8H5D2C8H5D2 105 C 8 H 4 D 2 + H C 8 H 5 D + D

C 6 H 5 + C 2 H 2  C 8 H 6 + H (m/z = 102)

indirect reaction via intermediate exit barrier E max = E c -  r G

C 6 H 5 + C 2 H 2  C 6 H 5 CCH (m/z = 102) + H X. Gu, F. Zhang, Y. Guo, R.I. Kaiser, Angew. Chemie Int. Edition 46, 6866 (2007).

C 6 H 5 + C 2 H 4  C 8 H 8 + H (m/z=104)

C 6 H 5 + C 2 D 4  C 6 H 5 C 2 D 3 + D (m/z=107)

C 6 H 5 + C 2 H 4  C 6 H 5 C 2 H 3 + H (m/z=104) indirect via intermediate exit barrier

C 6 H 5 + C 2 H 4  C 6 H 5 C 2 H 3 + H (m/z=104) F. Zhang, X. Gu, Y. Guo, R. I. Kaiser, J. Organic Chem. 72, 7597 (2007).

C 6 H 5 + H 2 CCHCH 3  C 9 H 10 + H (m/z=118)

C 6 H 5 + H 2 CCHCH 3  C 9 H 10 + H

C 6 H 5 + C 2 H 3 CH 3  C 9 H 10 + H 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 Phenyl versus Hydrogen Exchange Phenyl Group Stays Intact Partially Deuterated Reactants Isomer-Selective Detection Abstraction Reactions < 5 %

Phenyl Radical Reactions Indirect Reaction via Intermediates Short Lived Intermediates (no ring closure) Exit Barriers for Hydrogen Loss Exoergic / Slightly Endoergic

Phenyl Radical Reactions

Outlook C 6 H 5 NO C 6 H 5 + NO h (266 nm; 248 nm) v p = 1800 – 2100 ms -1 ; E C = 30 – 50 kJmol -1

C 2 (X 1  g + /a 3  u ) + C 4 H 6 (1,3-butadiene) C 2 + C 4 H 6  C 6 H 5 + H E c = 13 kJmol -1

C 2 (X 1  g + /a 3  u ) + C 4 H 6 (1,3-butadiene) C 2 + C 4 H 6  C 6 H 5 + H E c = 36 kJmol -1

C 2 + C 4 H 6  C 6 H 5 + H phenyl acyclic H 2 CCDCDCH 2 D 2 CCHCHCD 2

C 2 + C 4 H 6  C 6 H 5 + H

C 2 D + C 4 H 6  C 6 DH 5 + H H 2 CCDCDCH 2 D 2 CCHCHCD 2 E c = 45 kJmol -1

C 2 D + C 4 H 6  C 6 DH 5 + H 43 ± 10 %57 ± 10 %

C 2 D + C 4 H 6  C 6 DH 5 + H relative energy, kJmol -1

Outlook  E C = 80 – 185 kJmol -1 E C = 13 – 45 kJmol -1 E C = kJmol -1 N

Outlook

Acknowledgements