Transcript Slide 1
Investigating the Chemical Dynamics of Bimolecular Reactions of Dicarbon and Tricarbon Molecules with Unsaturated Hydrocarbons Ralf I. Kaiser Department of Chemistry University of Hawai’i at Manoa Honolulu, HI 96822 [email protected] Introduction H C C HC CH HC CH HC CH HC CH C H HC CH HC CH C H C H CHx C2Hx C3Hx C4Hx C5Hx Objectives Investigate the Formation of Hydrogen-Deficient, Carbon-Bearing Molecules via Reactions of C2(X1g+/a3u) and C3(X1g+) with H H H H H H acetylene ethylene H H3C H • H H methylacetylene H allene benzene Requirements 1. Preparation of Highly Reactive Reactants C2(X1g+/a3u) and C3(X1g+) 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 Requirements 1. Hydrocarbon Free Oil Free Pumps (Maglev, Scroll, DryVac) 2. Extremely Low Pressures Copper Gaskets Cryo Cooling (LN2; Cold Heads) Main Chamber = 10-9 torr Detector = 10-11 - < 10-12 torr 3. Signal Maximization Sources Ionizer, QMS, Ion Counter Crossed Molecular Beams Setup Crossed Molecular Beams Experiments 10 – 50 kJmol-1 20 9 1,500 – 2,600 K peak collision energy 72 - 175 kJmol-1 collision energies 14 labeling experiments 5 3,000 – 3,800 K C2(X1g+/a3u) + C2H2(X1g+) TOF at m/z = 49 (C4H+) and m/z = 48 (C4+) superimposable C4H Isomer C2(X1g+/a3u) + C2H2(X1g+) 1.211 1.365 1.206 1.063 p1, Cv, 2+ [0.0] 1.078 136.7 149.5 155.7 1.407 1.322 1.340 58.3 156.4 1.529 [118.0] p2, Cs, 2A" 1.403 1.080 77.0 81.0 139.1 89.9 1.464 1.408 1.463 131.3 1.603 54.4 1.085 [171.4] [140.2] p3, Cs, 2A" 1.457 p4, Cs, 2A' 1.306 1.394 1.072 1.510 147.1 1.402 1.235 163.8 120.7 98.6 1.113 [74.8] p5, C2v, 2B1 p6, Cs, 2A' [230.6] C2(X1g+) + C2H2(X1g+) C4H(X2+) + H(2S1/2) RG = - 33.3 kJmol-1 C2(a3u) + C2H2(X1g+) C4H(X2+) + H(2S1/2) RG = - 41.9 kJmol-1 RG(experimental) = - 40 5 kJmol-1 C2(X1g+/a3u) + C2H2(X1g+) 33 3 % 3 – 17 kJmol-1 indirect reaction mechanism(s) via C4H2 complexe(s) one channel could have exit barrier C2(X1g+/a3u) + C2H2(X1g+) intensity over complete angular range indirect reaction dynamics switch from forward to backward peaking as collision energy increases could suggest multiple reaction channels C2(X1g+/a3u) + C2H2(X1g+) - H2 -H products reaction enthalpy, kJmol-1 C4H(X2+) + H(2S1/2) - 33 c-C3H2(X1A1) + C(3Pj) + 152 C4(X3u) + H2(X1g+) - 10 c-C3H(X2B1) + CH(X2) + 246 CH2(X3B1) + C3(X1g+) + 142 C2H(X2+) + C2H(X2+) + 68 C2(X1g+) + C2H2(X1g+) forward-backward symmetric center-of-mass angular distributions C2(X1g+/a3u) + C2H2(X1g+) C2(a3u)+C2H2(X1g+) 1. exit barrier 0.0 -14.6 2. shallow potential energy wells asymmetric center-of-mass angular distributions HCCCC(X2+)+H -41.9 relative energy, kJmol-1 3. switch from forward to backward impact parameter dependence ? -123.9 -123.9 -163.1 t1 -176.8 -180.1 t2 t3 Remaining Questions can heavy isotopes induce ISC? C2D2(X1g+) 13C H (X1 +) 2 2 g C2HD(X1+) symmetry or long-lived C2(X1g+/a3u) + C2D2(X1g+)/13C2H2(X1g+)/C2HD(X1+) solely atomic hydrogen/deuterium loss pathways no induced ISC C2(X1g+/a3u) + C2D2(X1g+)/13C2H2(X1g+)/C2HD(X1+) Ec = 29 kJmol-1 no induced ISC long lived diacetylene intermediate identical CM functions compared to non-labeled reactant H D 13 13 Summary C2(X1g+/a3u) Reactions 1. identification of dicarbon vs. atomic hydrogen exchange pathway + CH3 C6H6 PES + C5H5 C10H8 PES JCP 113, 9622 (2000) JCP 113, 9637 (2000) JCP 115, 5107 (2001) Summary C2(X1g+/a3u) Reactions 2. indirect 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 C2(X1g+/a3u) Reactions Summary C3(X1g+) Reactions 1. identification of tricarbon versus atomic/molecular hydrogen exchange + CH3 C6H6 PES + C4H5 C10H8 PES Summary C3(X1g+) Reactions 2. reactions have pronounced entrance barriers molecule entrance barrier Eo, kJmol-1 acetylene ethylene methylacetylene allene benzene 95 20 42 4 42 6 42 6 in progress (E) ~ [1- Eo/E] 3. borderline of direct/indirect reaction dynamics via addition of tricarbon to the -bond of the hydrocarbon 4. reactions are endo (acetylene) / exoergic 5. assignment of intermediates Summary C3(X1g+) Reactions Summary 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 3. identification of building blocks and precursors to PAHs in combustion flames C4Hx (x = 1 -4) C6H6 PES C5Hx (x = 1 - 4) C6Hx (x = 3, 4) C10H8 PES Summary Outlook I A Mechanism of Aromatics Formation and Growth in Laminar Premixed Acetylene and Ethylene Flames http://www.me.berkeley.edu/soot/mechanisms/mechanisms.html (Michael Frenklach) C4Hx 1 2 3 4 C5Hx 1 2 3 4 3 4 C6Hx 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 H2 & CO outgassing; strongly reduced CO2 background) 2. Thoriated Iridium vs LaB6 Filament (1,600 K vs. 1,200 K ) 10 mA @200 eV, Utotal= 2.4 V, IH= 5.5 A 4 mA @ 80 eV, Utotal= 2.1 V, IH= 5.2 A Emssion Current (mA) 10 8 6 6 4 4 2 2 0.9 mA @ 8 eV 10 20 30 40 0 0 20 40 60 80 100 120 140 Electron Energy (eV) 160 180 200 Acknowledgements Xibin Gu, Ying Guo, Fangtong Zhang (UH) Alexander M. Mebel (FIU)