Electron Spin Relaxation in Gases. I. Bis(perfluoromethyl) Nitroxide

Abstract

The ESR spectra of the relatively large bis (perfluoromethyl) nitroxide (BFMN) radical has been studied in the gas phase. At low pressures, i.e., below 0.1 atm, the spectrum consists of a 480-G-wide Gaussian line which presumably arises from the unresolved nuclear hyperfine and spin–rotational lines. (The rootmean-square spin–rotational coupling constant is about 3.7 G.) As the pressure is increased by adding more radical or a diluant diamagnetic gas, the line narrows and becomes more Lorentzian. At total pressures, of radical plus diluant, above 10 atm the lines are Lorentzian and their widths vary as the inverse total pressure. This behavior can be explained semiquantitatively by assuming that the angular momentum, and hence the spin–rotational interaction, is relaxed by intermolecular collisions which can be understood in terms of a simple kinetic model. The cross sections for the relaxation of the angular momentum of the BFMN radical have been measured and the ratios, eJ, of these cross sections to the kinetic or van der Waals cross sections were determined. Approximate values of eJ are 0.28, 0.45, 0.70, and 0.84 for BFMN with N2, CF4, CF3H, and BFMN, respectively. As expected, eJ increases as the size and polarity of the colliding molecule increases. At high total pressures and low radical partial pressures the spin–rotational interactions are quenched and the nuclear hyperfine splitting is observed. At high total pressure and relatively high radical partial pressures (above 0.25 atm) the hyperfine splitting is coalesced into a single-exchange narrowed line. The cross section for spin exchange between BFMN radicals is about 0.04 of the kinetic cross section; the amount of exchange is, of course, proportional to the BFMN partial pressure.