Post on 04-Jan-2016
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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
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
R2PI Experimental Details
• Tonge; MacMahon; Pugliesi; Cockett. J. Phys. Chem 126, 154319 (2007)
MP2/aug-cc-pVDZ level
Previous work: FB-NH3
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
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?
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
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
Franck-Condon SimulationsExperiment
σ- type
π - type
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+
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
Reaction Pathway One - Cl atom loss
+·
TS1?
Garvey et al J. Phys. Chem . 1994, 98, 7479-7487
-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
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
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)
Advisor: Scott A. Reid
Group members:BrandonLloydAimable
Acknowledgements