1. Photoelectron Spectroscopy of Substituted Phenylnitrene Anions
Neloni R. Wijeratne, Maria Da Fonte and
Paul G. Wenthold*
Department of Chemistry
Purdue University,
West Lafayette, IN 47907

2. Physical Organic Chemistry of Phenylcarbene and Phenylnitrene
Phenylcarbene and phenylnitrene as reactive intermediates !
Platz M. S., Acc. Chem. Res. 1995, 28, 487

3.   Electronic Structure of Phenylcarbene (PC) and Phenylnitrene (PhN)
Non Bonding Molecular Orbitals (NBMO)
Electronic
Nitrene Calculated
Carbene Calculated
Electronic state
Configuration
Energies (kcal/mol)
Energies (kcal/mol) 3 A
0.0
0.0
Radical
11 2 1 A
18.5
30.0 2 2 1 1 A
36.9
4.7
Ionic 1 2 2 1 A
High
High 1
Karney, W. L.; Borden, W. T., J. Am. Chem. Soc. 1997, 119, 1378

4. Would Introduction of Substituents on the Ring
effect the Reactivity of Phenylnitrene?
It should not effect the radical states of the nitrene, but will
effect the ionic states.
To change the reactivity we have to change the structure of
singlet state from open-shell to closed-shell.
Introduction of electron donating group stabilizes the closed-
shell singlet state.
EDG = Electron Donating Group

5. Objectives of this Study
Introduction of chlorine to the phenylnitrene ring and study
how it will effect the electronic structure of phenylnitrene.
Determination of electron affinity of Chloro-substituted
phenylnitrenes.
Chloro-substituted Phenylnitrene
Anion – N Cl

6. What happens in Photoelectron Spectroscopy ?
Energy in : E = hν
Energy out : eKE + eBE
hn = eKE eBE

7. Advantages of using PES
Selection Rule : ∆S = ± 1
doublet
triplet
singlet

8. Potential Energy Surface Diagram for Phenylnitrene
E = hν
Observed Spectrum
1eKE
1C6H5N
∆EST (C6H5N.) = 15 kcal/mol
3eKE
eBE E
eKE
3C6H5N
EA (C6H5N.) = 1.45 eV
C6H5N–
Travers, M. J.; Cowels, D. C.; Clifford, E. P.; Ellison, G. B., J. Am. Chem. Soc. 1992, 114, 8699

9. Experimental How to get a Photoelectron Spectrum ?
Generation of ions in the ion source
2. Mass Analysis
Photodetachment
4. Electron kinetic energy Analysis

10. Time-of-Flight Negative Ion Photoelectron Spectrometer (NIPES)
Source
Photoelectron (PES)
spectrum
Mass Spectrum
Nd: YAG laser hν
Mass Analysis
Photodetachment Region
M. C. Da Fonte, MSc. Thesis, Purdue University, 2007.

11. Electron Binding Energy (eBE) Spectrum of
Phenylnitrene Anion at 355 nm
2.10 eV
1.45 eV
Photoelectron Count
3.0
2.5
2.0
1.5
1.0
Electron Binding Energy / eV –
Travers, M. J.; Cowels, D. C.; Clifford, E. P.; Ellison, G. B., J. Am. Chem. Soc. 1992, 114, 8699

12. Photoelectron Spectra of Chloro-subsituted Phenylnitrene anions
DEA
– N2 hν
– e– – –
Photoelectron Count
3.0
2.5
2.0
1.5
1.0
Electron Binding Energies/eV
EA = 1.79 eV
EA = 1.82 eV
EA = 1.72 eV

13. Expected Electron Affinity Trends due to
Chlorine Substitution at the Ring
Inductive effects
Stabilizes the ion, as it is distance dependent ortho is more
favored.
Therefore the EA trend would be,
ortho > meta > para
Resonance effects
Destabilizes the ion.
meta >> ortho = para
Net electron affinity trend for chloro-subsituted PhN
EA (meta) > EA (ortho) > EA (para)

14. All calculations were done using B3LYP/aug-cc-pVTZ level of
Measured and Calculated Thermochemical Data for Chloro-Substituted Phenylnitrenes
Isomer
All calculations were done using B3LYP/aug-cc-pVTZ level of
theory.

15. Relative Energies of Phenylnitrene and Chloro-Substituted Phenylnitrenes anions
x 2.5 –

16. Singlet State of Chloro-substituted Phenylnitrene
Also the ∆EST measurements suggest that the singlet is the open-shell state. ∆
Measured
Measured
Measured
Measured
Calculated
Calculated
Calculated
Calculated E E E E
for
for
for
for
Calculated
Calculated
Calculated
Calculated ∆ E E E E
for
for
for
for ST ST ST ST ST ST ST ST
Isomer
Isomer
Isomer
Isomer ∆ * * E E E E
open
open
open
open - - - -
shell singlet
shell singlet
shell singlet
shell singlet
closed
closed
closed
closed - - - -
shell singlet
shell singlet
shell singlet
shell singlet ST ST ST ST
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol)
(kcal/mol) o o o o - - - -
chlorophenylnitrene
chlorophenylnitrene
chlorophenylnitrene
chlorophenylnitrene 14 14 14 14 2 2 2 2 m m m m - - - -
chlorophenylnitrene
chlorophenylnitrene
chlorophenylnitrene
chlorophenylnitrene 15 15 15 15 1 1 1 1
14.3 p p p p - - - -
chlorophenylnitrene
chlorophenylnitrene
chlorophenylnitrene
chlorophenylnitrene 14 14 14 14 2 2 2 2
13.7
32.6
32.6
32.6
32.6
* Johnson, W. T. G.; Sullivan, M. S.; Cramer, C. J., Int. J. Quant. Chem. 2001, 85, 492

17. Kekulé Structures of ortho and para -subsituted Phenylnitrenes
ortho and para isomers having smaller ∆EST than meta isomer indicate that there is a isomeric dependence due to a resonance interaction between the ring/nitrene and substituent.
This makes it possible to have quinoidal structures for open-shell singlets of ortho and para isomers.
Open-shell singlet

18. Would Introduction of Substituents on the Ring
effect the Reactivity of Phenylnitrene?
It should not effect the radical states of the nitrene, but will
effect the ionic states.
To change the reactivity we have to change the structure of
singlet state from open-shell to closed-shell.
Introduction of electron donating group facilitates the closed-
shell singlet state.
EDG = Electron Donating Group

19. Schematic MO Diagram showing the effect of Chlorine Substitution
In the open-shell the anti-bonding
orbital is singly occupied.
The reduction in the electron-
pair repulsion is achieved by
delocalization of the spin in
the -orbital.

20. Summary The experimental electron affinities for the ortho, meta and
para-subsituted chloro phenylnitrenes are 1.79 ± 0.05, 1.82 ±
0.05 and 1.72 ± 0.05 eV, respectively, and are in good
agreement with theoretical predictions.
Chlorine substitution of the phenyl ring does not effect the
open-shell nature of the lowest singlet state.
The singlet-triplet energy gaps are not significantly affected by
the chlorine substitution, but subtle changes are observed in
bands for the ortho- and para-singlet states, likely due to the
resonance interactions in the open-shell singlet.

21. Acknowledgement Dr. Paul G. Wenthold JAFCI
Former Wenthold Group Members : Maria Da Fonte
Dr. Daniel Goebbert
Financial Support : NSF
ACS-PRF

22. Electronic Structure of Phenylcarbene (PC) and Phenylnitrene (PhN)   
Non Bonding Molecular
Orbitals (NBMO)
PhC
PhN
Radical
Ionic
Electronic
Nitrene Calculated
Carbene Calculated
Configuration
Energies (kcal/mol) 3 A 2
0.0 1
18.5
30.0
36.9
4.7
High
Electronic state 2
11 2
Karney, W. L.; Borden, W. T., J. Am. Chem. Soc. 1997, 119, 1378

23. Electronic Structure of Phenylcarbene (PhCH)σπ σ2 π2
Triplet GS
Singlet ES
Energy
kcal/mol
0.0
~ 5

24. Kekulé Structures of ortho and para -subsituted Phenylnitrenes
Closed-shell
singlet
Open-shell singlet
ortho
para

25. From Previous Experimental Studies
DEA
– N2 –
– e hν
Threshold Photodetachment Studies :
Brauman and cowokers (1984): EA = eV
∆EST = 4.3 kcal/mol
McDonald and cowokers (1993): EA = eV
∆EST = 18.3 kcal/mol
Photoelectron Spectroscopy :
Ellison and coworkers (1992) : EA = 1.45 eV
∆EST = 18 kcal/mol

26. Why do Spectroscopy of Phenylnitrene Anion ?
Energy scale of the NIPES needs to be calibrated using a previously studied system.
EA = 1.45 eV at 355 nm
eKEtriplet = 2.05 eV
eKEsinglet = eV
Ion and radical structure of Phenylnitrene is similar to that of the substituted phenylnitrene therefore they will show similar photodetachment.
Phenylnitrene anion can be easily generated via dissociative electron attachment of phenylazide.
DEA
– N2 –
– e hν

27. Photoelectron Spectrum of Phenylnitrene Anion at 355 nm
Results
Photoelectron Spectrum of Phenylnitrene Anion at 355 nm –
Open-shell Triplet Origin = 2.05 eV
Photoelectron Count
Open-shell Singlet Origin = 1.40 eV
0.5
1.0
1.5
2.0
2.5
Electron Kinetic Energy / eV
For the 3rd harmonic, 355 nm, hν = eV