Transient free radicals studied by laser magnetic resonance spectroscopy

Thesis



A liquid nitrogen cooled, carbon monoxide laser magnetic resonance spectrometer was used to study mid-infrared vibration-rotation transitions in the gaseous free radical NCO, in its ̅X²Π state, at very high resolution. The use of an intracavity absorption cell made possible the observation of some transitions with sub-Doppler resolution. Developments to the spectrometer extended the range of operation of the CO laser. Most importantly, a CO laser operating on overtone transitions, Δν = 2, was operated in Oxford. The Δν = 2 CO laser operates over the range 2450-3800cm^-1 (4.08-2.63 μm), and the Δν = 1 CO laser over the range 1200-2100 cm^-1 (8.33-4.76 μm).

NCO exhibits a Renner-Teller effect in its ground electronic state, an interaction between the motion of the electrons and the bending motion of the nuclei. Vibration-rotation transitions were observed in a sequence of bands involving the excitation of the out-of-phase stretching vibration, in the region of 1900cm^-1. Some of the bands involved the excited bending vibration. The Zeeman effect behaviour of the molecular energy levels, particularly in the ²Σ vibronic states, clearly showed the manifestation of the Renner-Teller effect. The Zeeman effect in the ²Σ vibronic states was considered in detail. Many of the ²Σ LMR spectra were recorded at sub-Doppler resolution. NCO is complicated to model, and unassigned LMR spectra remain.

A harmonic Renner-Teller model was developed for the analysis. It was implemented by constructing an explicit matrix representation of the single electronic state N² effective Hamiltonian, which was diagonalised exactly. A new term in this Hamiltonian, describing centrifugal distortion corrections to the Renner-Teller coupling term, was developed for the ²Σ vibronic states in order to account for anharmonic vibronic interactions.

Authors: Gillett, D; Gillett, David Alan

• Publication date: 1994
• DOI: 10.5287/ora-yre6gzveg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Magnetic resonance; Free radicals (Chemistry); Mathematical models; Infrared spectroscopy