Broadband Tunable Electron Paramagnetic Resonance Spectroscopy of Dilute Metal Complexes

Abstract

Analysis of the electron paramagnetic resonance (EPR) of transition ion complexes requires data taken at different microwave frequencies because the spin Hamiltonian contains operators linear in the frequency as well as operators independent of the frequency. In practice, data collection is hampered by the fact that conventional EPR spectrometers have always been designed to operate at a single frequency. Here, a broadband instrument is described and tested that operates from 0.5 to 12 GHz and whose sensitivity approaches that of single-frequency spectrometers. Multifrequency EPR from triclinic substitutional (0.5%) Cu(II) in ZnSO4 is globally analyzed to illustrate a novel approach to reliable determination of the molecular electronic structure of transition ion complexes from field-frequency 2D data sets.

Very Low-Frequency Broadband Electron Paramagnetic Resonance Spectroscopy of Metalloproteins

“…A schematic overview of the low-frequency version of the spectrometer setup is presented in Figure 1 with emphasis on those elements in which the present design differs in essence from the previous broadband machine. 1 …”

“…The previously developed wire microstrip cell with the sample sandwiched between a metal ground plate and a thin coiling metal wire 1 is retained, and only the resonance structure, which runs from circulator port 2 till the open RF end of the cell, is changed in that the previous ‘long’ coax stretch of up to a few meters is now replaced with a very long cable of 20–60 m. This extreme optical path is required at low microwave frequencies to ensure that the organ-pipe effect of standing waves produces sufficient resonance dips with sufficiently high loaded Q -factor ( Figure S2 ). On the other hand, the trombone-type RF phase shifter, used in the previous design to fine-tune these resonance dips, has now been removed as its internal maximum length has become negligibly small compared to the overall path.…”

“…Proof of principle was shown in the case of 0.5% triclinic Cu(II) substitutionally added to ZnSO 4 with a data set collected from circa 0.8 to 12 GHz. 1 In a subsequent study to explore applicability to metalloproteins, in particular low-spin ferric hemoproteins, I encountered two practical limitations. First, although the broadband spectrometer had an excellent resolution, its concentration sensitivity was intrinsically low and only allowed for data collection by extensive averaging on hemoproteins prepared at the very high end of their solubility.…”

Detection of L-band electron paramagnetic resonance in the DPPH molecule using impedance measurements

“…The line width in this frequency range was about 2 Oe which is consistent with the dilute nature of paramagnetic species (free radicals) in DPPH. 36 To verify the results obtained through the MI method, we measured the EPR spectra with a broad band ferromagnetic resonance spectrometer (Cryo-FMR by NanoOsc™ from Quantum Design Inc. USA). This spectrometer makes use of the lock-in technique and records the derivative of power absorbed (dP/dH) by the DPPH sample placed on top of a wave guide while H dc is swept for xed rf excitations of 2 GHz, 4 GHz, 10 GHz and 12 GHz as shown in Fig.…”

Distinguishing electron paramagnetic resonance signature of molecular hydrino

“…Spin quantification 43 was done with respect to an external standard of known S = 1/2 concentration. For this purpose, the X-band spectrum of a powder of 0.5 % (metal/metal) of 44 Cu(II) in Zn(II)SO4•1H2O was taken, which has essentially the same density (weight/volume) as the hydrino-containing Ga(O)OH powder. Gauss, and whose center is distinctly shifted from the free electron value to g = 2.0045.…”