Ralf I. Kaiser Department of Chemistry University of Hawai’i at Manoa Honolulu, HI Investigating the Chemical Dynamics of Bimolecular Reactions of Dicarbon and Tricarbon Molecules with Unsaturated Hydrocarbons

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

Objectives Investigate the Formation of Hydrogen-Deficient, Carbon-Bearing Molecules via Reactions of C 2 (X 1  g + /a 3  u ) and C 3 (X 1  g + ) with

Requirements 1.Preparation of Highly Reactive Reactants C 2 (X 1  g + /a 3  u ) and C 3 (X 1  g + ) 2. Identify Reaction Products and Infer Reaction Intermediates 3. Obtain Information on Energetics and Reaction Mechanisms ↓ Single Collision Conditions Crossed Molecular Beams Experiments

Crossed Molecular Beams Setup Main Chamber = torr Detector = < torr 1. Hydrocarbon Free Requirements Oil Free Pumps (Maglev, Scroll, DryVac) 2. Extremely Low Pressures 3. Signal Maximization Copper Gaskets Cryo Cooling (LN2; Cold Heads) Sources Ionizer, QMS, Ion Counter

Crossed Molecular Beams Setup

Crossed Beams Experiment

Crossed Molecular Beams Experiments kJmol – 50 kJmol -1 peak collision energy 20 collision energies 14 9 labeling experiments 5 1,500 – 2,600 K 3,000 – 3,800 K

C 2 (X 1  g + /a 3  u ) + C 2 H 2 (X 1  g + ) TOF at m/z = 49 (C 4 H + ) and m/z = 48 (C 4 + ) superimposableC 4 H Isomer

C 2 (X 1  g + /a 3  u ) + C 2 H 2 (X 1  g + ) C 2 (X 1  g + ) + C 2 H 2 (X 1  g + )  C 4 H(X 2  + ) + H( 2 S 1/2 )  R G = kJmol -1 C 2 (a 3  u ) + C 2 H 2 (X 1  g + )  C 4 H(X 2  + ) + H( 2 S 1/2 )  R G = kJmol -1  R G (experimental) = - 40  5 kJmol -1

C 2 (X 1  g + /a 3  u ) + C 2 H 2 (X 1  g + ) 33  3 % indirect reaction mechanism(s) via C 4 H 2 complexe(s) 3 – 17 kJmol -1 one channel could have exit barrier

C 2 (X 1  g + /a 3  u ) + C 2 H 2 (X 1  g + ) intensity over complete angular rangeindirect reaction dynamics switch from forward to backward peaking as collision energy increases could suggest multiple reaction channels

productsreaction enthalpy, kJmol -1 C 4 H(X 2  + ) + H( 2 S 1/2 ) - 33 c-C 3 H 2 (X 1 A 1 ) + C( 3 P j )+ 152 C 4 (X 3  u ) + H 2 (X 1  g + ) -10 c-C 3 H(X 2 B 1 ) + CH(X 2   ) CH 2 (X 3 B 1 ) + C 3 (X 1  g + ) C 2 H(X 2  + ) + C 2 H(X 2  + ) + 68 C 2 (X 1  g + /a 3  u ) + C 2 H 2 (X 1  g + )

C 2 (X 1  g + ) + C 2 H 2 (X 1  g + ) forward-backward symmetric center-of-mass angular distributions

C 2 (X 1  g + /a 3  u ) + C 2 H 2 (X 1  g + ) 2. shallow potential energy wells - asymmetric center-of-mass angular distributions 3. switch from forward to backward - impact parameter dependence ? 1. exit barrier

Remaining Questions symmetry or long-lived can heavy isotopes induce ISC? C 2 D 2 (X 1  g + ) 13 C 2 H 2 (X 1  g + ) C 2 HD(X 1  + )

C 2 (X 1  g + /a 3  u ) + C 2 D 2 (X 1  g + )/ 13 C 2 H 2 (X 1  g + )/C 2 HD(X 1  + ) solely atomic hydrogen/deuterium loss pathwaysno induced ISC

C 2 (X 1  g + /a 3  u ) + C 2 D 2 (X 1  g + )/ 13 C 2 H 2 (X 1  g + )/C 2 HD(X 1  + ) E c = 29 kJmol -1 identical CM functions compared to non-labeled reactant long lived diacetylene intermediate no induced ISC HD 13

Summary C 2 (X 1  g + /a 3  u ) Reactions 1.identification of dicarbon vs. atomic hydrogen exchange pathway + CH 3 C 6 H 6 PES + C 5 H 5 JCP 113, 9622 (2000) JCP 113, 9637 (2000) JCP 115, 5107 (2001) C 10 H 8 PES

Summary C 2 (X 1  g + /a 3  u ) Reactions 2. i ndirect reaction dynamics via barrier less addition of dicarbon to the  -bond of the hydrocarbon yielding initially acyclic/cyclic collision complexes 3. reactions are exoergic 4. assignment of intermediates

Summary C 2 (X 1  g + /a 3  u ) Reactions

1.identification of tricarbon versus atomic/molecular hydrogen exchange Summary C 3 (X 1  g + ) Reactions + CH 3 C 6 H 6 PES + C 4 H 5 C 10 H 8 PES

Summary C 3 (X 1  g + ) Reactions 3. borderline of direct/i ndirect reaction dynamics via addition of tricarbon to the  -bond of the hydrocarbon 4. reactions are endo (acetylene) / exoergic 2. reactions have pronounced entrance barriers acetylene 95  20 ethylene 42  4 methylacetylene 42  6 allene 42  6 benzene in progress molecule entrance barrier E o, kJmol -1  (E) ~ [1- E o /E] 5. assignment of intermediates

Summary C 3 (X 1  g + ) Reactions

Summary 3. identification of building blocks and precursors to PAHs in combustion flames 1.conducted crossed beams experiments of dicarbon and tricarbon with small unsaturated hydrocarbons (10 – 175 kJmol -1 ) 2.inferred reaction dynamics and energetics of the reactions C 4 H x (x = 1 -4) C 5 H x (x = 1 - 4) C 6 H x (x = 3, 4) C 6 H 6 PES C 10 H 8 PES

Summary

Outlook I C4Hx 1234 C5Hx 1234 C6Hx 34 C4Hx 1234 C5Hx 1234 C6Hx 34 A Mechanism of Aromatics Formation and Growth in Laminar Premixed Acetylene and Ethylene Flames (Michael Frenklach) experiments suggest inclusion of distinct isomers and additional molecules

Outlook II soft electron impact ionization 1. Brink type ionizer made of Alloy 718 (Nickel Alloy w/o H 2 & CO outgassing; strongly reduced CO 2 background) 2. Thoriated Iridium vs LaB 6 Filament (1,600 K vs. 1,200 K ) eV

Acknowledgements Xibin Gu, Ying Guo, Fangtong Zhang (UH) Alexander M. Mebel (FIU)