Physical Chemistry

The hyperfine splitting and g-factor are two important EPR parameters that give us insight into the molecules or atoms we are looking at. In this experiment, we will measure the gfactor and hyperfine splitting in several radicals: TEMPONE and TEMPOL solutions, solid Cr(III) oxide, and the benzosemiquinone radical anion. Once the spectra have been acquired, the data can be moved via a flash drive off line, and processed using the Processing and Analysis software. The g-factor and hyperfine splitting can be measured directly in the acquisition software as well, but it is important to learn how to move and process data offline.

Goals:

• To understand the basics of EPR theory
• Practice determination of g-factor and hyperfine splitting
• Explore the effect of different nuclear interactions with the unpaired electron on the EPR spectra
• Learn to move data, and use the offline processing software

Many factors contribute to the observed lineshape and linewidth of an ESR resonance. One important factor is how quickly the "spins" relax back to equilibrium. In ESR, as in NMR, most systems lose the energy they gained thermally: interactions, such as collisions, with their surroundings and each other. The faster the system relaxes back, the broader the lines.

Magnetic field inhomogeneity can cause both line broadening and lineshape distortion. The presence of a paramagnetic species like dissolved O2 and phenomena such as spin-spin exchange can also broaden lines because they provide another effective relaxation pathway for electrons. The shape of ESR lines also gives important information about molecules and their environment. In a non-viscous liquid, small molecules such as TEMPOL can tumble freely. As the solution becomes more viscous, the molecular motion becomes hindered. The ESR spectrum of the nitroxide changes as the rotational motion becomes restricted. ESR can be used to measure rotational correlation times. The technique referred to as spin labeling is based on this phenomenon.

This laboratory exercise looks at line broadening through spin-spin exchange and students calculate the collision frequency at different solution concentrations based on the ESR linewidth. The effect of dissolved oxygen on linewidths is also discussed. The second part of this lab looks at the effect of increasing viscosity on TEMPOL lineshapes.

Semiquinone anion radicals are biologically important reaction intermediates in electron transfer reactions. Ubiquinone, part of the electron transport chain in mitochondria, and plastoquinone, which is involved in photosynthesis, are both quinones. The mechanism of the reduction of a quinone to a hydroquinone goes through a semiquinone anion radical intermediate.

The reaction is easily reversible. Semiquinone anion radicals are relatively stable, and as such can be seen with ESR without the aid of a spin trap. In this lab, look at the ESR spectrum of TEMPOL and of several semiquinone radical anions. The unpaired electron in TEMPOL resides in a π* orbital on the N-O bond, and is coupled to the ¹⁴N (I = 1). The unpaired electron in the semiquinone radical anions is a delocalized π electron. This lab illustrates how the substituents on the ring and the molecular symmetry affect the ESR spectra.