ralf i. kaiser department of chemistry university of hawai’i at manoa honolulu, hi 96822...
DESCRIPTION
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 + ) withTRANSCRIPT
Ralf I. KaiserDepartment of Chemistry
University of Hawai’i at ManoaHonolulu, HI 96822
Investigating the Chemical Dynamics of Bimolecular Reactions of Dicarbon and Tricarbon Molecules
with Unsaturated Hydrocarbons
Introduction
CHx
C2Hx
C3Hx
C4Hx
C5Hx
HC
HC
CH
CH
CH
C
HC
HC
CH
CH
CH
HC
CHHC
HC
CH
CH
H3C H
H
H
H
HH H
methylacetylene
acetyleneethylene
benzene
H
•
H
H
H
allene
ObjectivesInvestigate the Formation of Hydrogen-Deficient, Carbon-Bearing
Molecules via Reactions of C2(X1g+/a3u) and C3(X1g
+) with
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
Main Chamber = 10-9 torr
Detector = 10-11 - < 10-12 torr
1. Hydrocarbon Free
Requirements
Oil Free Pumps(Maglev, Scroll, DryVac)
2. Extremely Low Pressures
3. Signal Maximization
Copper GasketsCryo Cooling
(LN2; Cold Heads)
SourcesIonizer, QMS, Ion Counter
Crossed Molecular Beams Setup
Crossed Molecular Beams Experiments
72 - 175 kJmol-110 – 50 kJmol-1 peak collision energy
20 collision energies 14
9 labeling experiments 5
1,500 – 2,600 K 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
+)
p1, Cv, 2+
1.211 1.365 1.206 1.063
p3, Cs, 2A"
1.464 1.4031.080
77.089.9
81.0139.1
p2, Cs, 2A"
1.3221.407
1.529
1.340
1.078136.7 149.5
155.7
156.458.3
1.457 1.394
1.5101.072
147.1
p5, C2v, 2B1
p4, Cs, 2A'
1.463
1.6031.408
1.085
131.3
54.4
p6, Cs, 2A'
1.2351.402
1.306
1.113163.8 120.7
98.6
[0.0][118.0]
[140.2] [171.4]
[74.8][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 % indirect reaction mechanism(s) via C4H2 complexe(s)
3 – 17 kJmol-1 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
- 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+/a3u) + C2H2(X1g
+)
C2(X1g+) + C2H2(X1g
+)
forward-backward symmetric center-of-mass
angular distributions
C2(X1g+/a3u) + C2H2(X1g
+)
-123.9
-163.1
-176.8 -180.1
-123.9
C2(a3u)+C2H2(X1g+)0.0
-41.9
HCCCC(X2+)+H
-14.6
t1 t2 t3
rela
tive
ener
gy, k
Jmol
-1
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?
C2D2(X1g+)
13C2H2(X1g+)
C2HD(X1+)
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
identical CM functionscompared to non-labeled reactant
long lived diacetylene intermediate
no induced ISC
H D
13 13
Summary C2(X1g+/a3u) Reactions
1. identification of dicarbon vs. atomic hydrogen exchange pathway
+ CH3
C6H6 PES
+ C5H5JCP 113, 9622 (2000)JCP 113, 9637 (2000)JCP 115, 5107 (2001) C10H8 PES
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
1. identification of tricarbon versus atomic/molecular hydrogen
exchange
Summary C3(X1g+) Reactions
+ CH3
C6H6 PES
+ C4H5
C10H8 PES
Summary C3(X1g+) Reactions
3. borderline of direct/indirect 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 20ethylene 42 4methylacetylene 42 6 allene 42 6benzene in progress
molecule entrance barrier Eo, kJmol-1
(E) ~ [1- Eo/E]
5. assignment of intermediates
Summary C3(X1g+) 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
C4Hx (x = 1 -4)
C5Hx (x = 1 - 4)
C6Hx (x = 3, 4)
C6H6 PES
C10H8 PES
Summary
Outlook I
C4Hx 1 2 3 4 C5Hx 1 2 3 4 C6Hx 3 4
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)
experiments suggest inclusion of distinct isomers and additional molecules
Outlook IIsoft 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 )
0 20 40 60 80 100 120 140 160 180 2000
2
4
6
8
10
10 20 30 40
2
4
6
4 mA @ 80 eV, Utotal
= 2.1 V, IH= 5.2 A
Em
ssio
n C
urre
nt (m
A)
Electron Energy (eV)
10 mA @200 eV, Utotal
= 2.4 V, IH= 5.5 A
0.9 mA @ 8 eV
Acknowledgements
Xibin Gu, Ying Guo, Fangtong Zhang (UH)
Alexander M. Mebel (FIU)