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'Advances in W-band EPR spectroscopy of nitroxide radical spin labels'

 The loaded Q-value of a loop gap resonator (LGR) containing an aqueous sample at W-band is about 100. Since the gyromagnetic ratio of an electron spin is 2.8 MHz per gauss, the bandwidth of the resonator between 3 dB points at 94 GHz can be said to be 335 gauss. The spectral width of an immobilized nitroxide at 94 GHz is about 125 G. Thus, it is technically feasible to sweep the microwave frequency rather than the magnetic field through the Zeeman transitions of the spin system to acquire the complete spectrum without retuning the resonator. The LGR is an enabling technology for achieving frequency agile W-band spectroscopy. EPR experiments using microwave frequency modulation coupled with magnetic field sweep have been successfully performed. Frequency modulation at 50 kHz with a depth of modulation of 11.3 MHz frequency deviation has been carried out on 10 μM TEMPO in water. Best sensitivity was obtained at 2^nd harmonic, with lock-in detection at 100 kHz. Transfer of FM at 50 kHz to AM at 100 kHz by resonance of the spin system dominated the frequency response of the LGR, resulting in good baseline stability. This is a promising technology for saturation transfer experiments using second harmonic out-of-phase displays. The W-band LGR is also favorable for saturation recovery EPR and ELDOR experiments because of the high resonator efficiency parameter, Λ, and because incident saturating pulse power is reduced for a given value of H₁. Saturation recovery measurements of T₁ were carried out on the molecules we have studied in previous saturation recovery experiments in the frequency range of 2 to 35 GHz (J. Phys. Chem. B 108 [2004] 9524). The trend of decreasing spin lattice relaxation rate with increasing microwave frequency in aqueous samples is broken at W-band. The longest T₁ values occur in the Q-band range. Hofbauer et al. from the Freed group (Rev. Sci. Instrum. 75 [2004] 1194) report a similar finding and suggest a possible model. Established theories of paramagnetic relaxation in the liquid phase are inconsistent with the dependence of T₁ on the microwave frequency that we, as well as the Freed group, observe.