No abstract

No abstract

No abstract

ABSTRACT:A 250 MHz electron paramagnetic resonance (EPR) spectrometer was constructed to be an engineering test facility for in vivo EPR imaging of physiological samples and for protein structure determination. Innovations relative to prior low-frequency EPR spectrometers include a four-coil, air-core magnet and gradient coils, a crossed-loop resonator, dynamic Q-switching to decrease dead time in pulsed EPR, and a narrow-band bridge based on circulators. The automatic frequency control system uses a signal separate from the EPR signal to make the frequency control independent of the radiofrequency (RF) phase. The design incorporates multiple excitation and signal paths to facilitate testing of a variety of resonators, two magnets, and both a locally built console described here and a Bruker console. Plug-in cards in the bridge facilitate using reflection or crossed-loop resonators in continuous wave or pulsed EPR modes. In the locally built console there is a microprocessor-controlled interface unit to handle magnetic field modulation and scan, tuning display, and other functions.

In rapid scan EPR the magnetic field is scanned through the signal in a time that is short relative to electron spin relaxation times. Previously it was shown that the slow scan lineshape could be recovered from triangular rapid scans by Fourier deconvolution. In this paper a general Fourier deconvolution method is described and demonstrated to recover the slow scan lineshape from sinusoidal rapid scans. Since an analytical expression for the Fourier transform of the driving function for a sinusoidal scan was not readily apparent, a numerical method was developed to do the deconvolution. The slow scan EPR lineshapes recovered from rapid triangular and sinusoidal scans are in excellent agreement for lithium phthalocyanine, a trityl radical, and the nitroxyl radical, tempone. The availability of a method to deconvolute sinusoidal rapid scans makes it possible to scan faster than is feasible for triangular scans because of hardware limitations on triangular scans.

ABSTRACT:The crossed-loop resonator (CLR) uses two orthogonal lumped-element resonators [one to excite the spins and one to detect the electron paramagnetic resonance (EPR)] to isolate the signal from the microwave source. It eliminates the need for a circulator. The high isolation provided by the CLR reduces the energy stored in the resonator that detects the signal, thereby reducing the intensity of the resonator ring down after the pulse, which decreases the instrument dead time. Overcoupling and synchronous switching of the Q's of the two resonators between high and low states were used to further reduce dead time and maximize the EPR signal following a pulse. Each section of a 250 MHz resonator that accommodates 1 inch diameter samples had a critically coupled Q of 950 when Q-switching was not installed. With Q-switching, free induction decays and electron spin echoes for 0.2 mM aqueous solutions of triarylmethyl radicals were obtained with dead times of a few hundred nanoseconds. Typically 50 -60 dB isolation was achieved with various samples.

Electron spin lattice relaxation rates (1/T1 ) were measured as a function of temperature at two or three microwave frequencies for three S = 1/2 species in temperature ranges with different dominant relaxation processes. Between 10 and 50 K the contribution from the direct process to the relaxation rate was substantially greater at 94 than at 9.5 GHz for a vanadyl porphyrin doped into zinc tetratolylporphyrin. For bis(diethyldithiocarbamato)copper(TI) doped into the diamagnetic Ni(II) analog the relaxation rate between 25 and 100 K is dominated by the Raman process and exhibits little frequency dependence between 9.2 and 94 GHz. For 4-hydroxy-2,2,6,6-tetramethylpiperidinoloxy (tempol) doped into a diamagnetic host the relaxation rate between about 40 and 100 K is dominated by the Raman process. In this temperature range, relaxation rates at 3.2, 9.2, and 94 GHz exhibit little frequency dependence. Above about 130 K, the relaxation rate for tempol decreases in the order S-band > X-band > W-band. The relaxation rates in this temperature range fit a model in which ]IT, is dominated by a thermally activated process that is assigned as rotation of the methyl groups on the nitroxyl ring.

No abstract