The electronic structure of TEMPO, its cation and anion

Abstract

The electronic structure of TEMPO (2,2,6,6-Tetramethylpiperidine-N-oxyl) and its cation and anion were studied experimentally using the electron spectroscopy techniques, dissociative electron attachment (DEA) spectroscopy, electron energy-loss spectroscopy, measurement of elastic and vibrational excitation (VE) cross sections and HeI photoelectron spectroscopy. The experiments were supplemented by quantum-chemical calculations of excitation energies, ionisation potential and the Franck–Condon profile of the first photoelectron band. Electron energy-loss spectra were recorded up to 12 eV and revealed a number of bands that were assigned to two valence and a number of Rydberg transitions. VE cross sections reveal a broad band in the 3–12 eV range, assigned to σ* shape resonances and signals in the 0–1 eV range, assigned to a shape resonance corresponding to a temporary capture of the incident electron in the (already singly occupied) π* orbital. Narrow threshold peaks in the VE cross sections are assigned to dipole-bound resonances. The major DEA fragment was found to be O⁻, with bands at 5.0 and 6.87 eV, assigned to core excited resonances.

Keywords:

• TEMPO
• electron energy-loss spectroscopy
• EELS
• photoelectron spectroscopy
• dissociative electron attachment
• TDDFT calculations
• Rydberg states

Acknowledgements

This work was supported by projects No. 200020-144367/1 and PZ00P2_132357/1 of the Swiss National Science Foundation and the SFB 813 (‘Chemistry at Spin Centers’) of the German Science Foundation.

Notes

^aDoublet, four roots, CAS(5,6).

^bTwo-configurational valence–Rydberg mixed state composed of π → π*(β) and π*(SOMO) → 3p single excitations.

^cState-averaged doublet/quartet; four/four roots; CAS(5,6), def2-TZVP.

^dTZVP basis set used in the PBE0 computations.

^eTaken as the first shoulder and the band maximum, respectively. Section 3.1 discusses the limits of this interpretation.