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