Contributions to the Gaussian Line Broadening of the Proxyl Spin Probe EPR Spectrum Due to Magnetic-Field Modulation and Unresolved Proton Hyperfine Structure
✍ Scribed by Barney L. Bales; Miroslav Peric; Maria Teresa Lamy-Freund
- Publisher
- Elsevier Science
- Year
- 1998
- Tongue
- English
- Weight
- 144 KB
- Volume
- 132
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
A simple expression is derived to compute the total Gaussian linewidth of a Voigt line that is broadened by sinusoidal magneticfield modulation as follows:
where ⌬H pp G (H m ) is the Gaussian linewidth observed with an modulation amplitude H m /2 and ⌬H pp G (0) is the Gaussian linewidth in the limit of zero modulation. The field modulation contributes an additional Gaussian broadening of H m , where is a constant, which adds in quadrature to ⌬H pp G (0) to give the total Gaussian linewidth. Denoting the overall linewidth of the Voigt line in the absence of modulation broadening by ⌬H pp 0 (0), it is shown, both by analytical means and by spectral simulation, that the constant is equal to 1/2 in the limit of H m Ӷ ⌬H pp 0 (0); however, using values of H m as large as ⌬H pp 0 (0) leads to only minor departures from ؍ 1/2. The formulation is valid both for Lorentzian and Voigt lines and is tested for 2,2,5,5-tetramethylpyrrolidin-1-oxyl-3-carboxylic acid (3-carboxy proxyl) in CCl 4 and in aqueous buffer. This spin probe was studied because the proxyl group is the only major spin-probe moiety whose Gaussian linewidth had not been characterized in the literature. For 3-carboxy proxyl, it is found that ⌬H pp G (0) ؍ 1.04 ؎ 0.01 G independent of solvent polarity. Precision values of the 14 N hyperfine coupling constant for 3-carboxy proxyl at 9.5°C are as follows: 14.128 ؎ 0.001 G in CCl 4 and 16.230 ؎ 0.002 G in aqueous buffer. The temperature dependence of ⌬H pp G (0) and the 14 N hyperfine coupling constant are reported as empirical equations. Results of the present work taken together with previously published data permits accurate correction for the effects of inhomogeneous broadening due to unresolved hyperfine structure and modulation broadening for the majority of spin probes in common use.