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

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

  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

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

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

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

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

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

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

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.

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

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

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)

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.

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

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

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

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

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.

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.

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

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

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

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

From Previous Experimental Studies DEA – N2 – – e hν Threshold Photodetachment Studies : Brauman and cowokers (1984): EA = 1.461 eV ∆EST = 4.3 kcal/mol McDonald and cowokers (1993): EA = 1.429 eV ∆EST = 18.3 kcal/mol Photoelectron Spectroscopy : Ellison and coworkers (1992) : EA = 1.45 eV ∆EST = 18 kcal/mol

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 = 1.40 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

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 = 3.493 eV