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Silver NyamboDepartment of Chemistry, Marquette University, Wisconsin
Reactive pathways in the chlorobenzene-ammonia dimer cation radical:
New insights from experiment and theory
69TH ISMS, June 2014
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Studying Non-Covalent Interactions using R2PI coupled with linear TOFMS
We use halobenzenes as convenient chromophores in prototypical systems:
Hydrogen bonding π – π stacking C-H/ π N-H/ π Halogen bonding
In this study: C6H5CI···NH3 dimer
Background
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R2PI Experimental Details
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• Tonge; MacMahon; Pugliesi; Cockett. J. Phys. Chem 126, 154319 (2007)
MP2/aug-cc-pVDZ level
Previous work: FB-NH3
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Vaupel; Brutschy; Tarakeshwar; Kim. J. Am. Chem. Soc. 2006, 128, 5416-5426
Previous work:
Vaupel et al- IR/R2PI-depletion spectroscopy in the N-H stretching region of NH3 complexes and ab initio calculationso Compared experimental shifts from free NH3 absorptions, assigned out of
plane structure
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Remaining open questions
1. What is the structure of the C6H5CI···NH3 dimer complex: in-plane σ- type or π – type?
2. The C6H5CI···NH3 dimer is reactive upon ionization, – What reaction pathways are possible?
3. What are the branching ratios between competing reaction pathways?
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Dimer M06-2X/aug-cc-pVTZ
CCSD(T)//M06-2X/aug-cc-pVTZ
D1 -6.4 -5.6
D2 -6.3 -8.0
D3 -6.7 -4.2
D4 -5.9 -7.3
PhCI---NH3 optimized structures and calculated BE(kJ/mol)
3.75
5
2.882
3.15
5
2.934
2.403
2.799
3.0833.083
2.434
D1
D2
D3
D4
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Calculated vertical absorptions-TD-DFT
D3D1 D2 D4
36700 36800 36900 37000 37100 37200 37300 374000.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
Cal
cula
ted
Osc
illat
or S
tren
gth
Wavenumber in cm-1
D2 D1 PhCl D3 D4
36900 37000 37100 37200 37300
Wavenumber in cm-1
Rela
tive in
tensity
PhClPhCl -NH3
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Franck-Condon SimulationsExperiment
σ- type
π - type
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The reactive 1:1 dimerClbz-(
NH 3) 3
Clbz-(NH 3
) 2
Clbz
-NH 3
Clbz
AnH
Aniline
36900 37000 37100 37200 37300
Wavenumber in cm-1
Ph-NH2+·
PhCl···NH3+·
Ph-NH3+
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The Reaction Pathways via Wheland intermediates• The reaction pathways are considered to proceed via two possible
Wheland intermediates
C6H5CI···NH3 → (C6H5CI···NH3)+· → (C6H5CI−NH3)
+· → (C6H5NH3)+ + CI· (1)
→ (C6H5NH2)+· + HCI (2)
→ (C6H4CINH3)+ + H· (3)
WH1-ipso-substitution WH2-ortho substitution
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Reaction Pathway One - Cl atom loss
+·
TS1?
Garvey et al J. Phys. Chem . 1994, 98, 7479-7487
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-5 -4 -3 -2 -1 0 1 2 3 4 5
2
4
6
8
10
12
14
16Ener
gy
(kJ/
mol)
Reaction Coordinate
Corresponding IRC Calculations for Cl and HCl loss
• IRC calculations suggest Cl loss and HCl loss occurs competitively from WH1
• We optimized a four-center TS leading to HCl loss that lies 25kJ/mol above WH1
-5 -4 -3 -2 -1 0 1 2 3 4 5
20
40
60
80
100
120
Reaction Coordinate
• IRC calculations suggest Cl loss and HCl loss occurs competitively from WH1
• We optimized a four-center TS leading to HCl loss that lies 25kJ/mol above WH1
-5 -4 -3 -2 -1 0 1 2 3 4 5
20
40
60
80
100
120
Reaction Coordinate
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0 5000 10000 15000 20000 25000 300000
5
10
15
20
Rat
io o
f ra
te c
onst
ants
for
Cl l
oss
to H
Cl l
oss
Energy in cm-1
HCl loss : Cl loss branching ratios from WH1• To quantify the branching
ratios- Microcanonical TS theory (CHEMRATE) was used
• Cl loss channel predominates due to its lower barrier
• H atom loss channel is energetically inaccessible via our 2-photon energy
• WH2 either decays back to the ionized complex or it photo-dissociate via H loss
Exp (HCl : Cl) - 1 : 5.4Calc (HCl : Cl) - 1 : 5.6
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Summary• The dimer complex was assigned to the in-plane σ-type geometry
• We identified 3 channels by which the reaction path of the dimer may follow– The predominant reaction channel is the Cl atom loss– The HCl and H atom loss
• TS linking WH intermediates to the products were found computationally for all channels and were confirmed by IRC calculations
• Microcanonical TS calculations found the branching ratio of Cl : HCl loss to be 1 : 5.6, in accurate agreement to our experimental ratio (1 : 5.4)
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Advisor: Scott A. Reid
Group members:BrandonLloydAimable
Acknowledgements