Electron nuclear quadruple resonance for assignment of overlapping spectra

Davies electron-nuclear double resonance revisited: Enhanced sensitivity and nuclear spin relaxation

Over the past 50 years, electron-nuclear double resonance (ENDOR) has become a fairly ubiquitous spectroscopic technique, allowing the study of spin transitions for nuclei which are coupled to electron spins. However, the low spin number sensitivity of the technique continues to pose serious limitations. Here we demonstrate that signal intensity in a pulsed Davies ENDOR experiment depends strongly on the nuclear relaxation time T(1n), and can be severely reduced for long T(1n). We suggest a development of the original Davies ENDOR sequence that overcomes this limitation, thus offering dramatically enhanced signal intensity and spectral resolution. Finally, we observe that the sensitivity of the original Davies method to T(1n) can be exploited to measure nuclear relaxation, as we demonstrate for phosphorous donors in silicon and for endohedral fullerenes N@C(60) in CS(2).

The solution conformation of triarylmethyl radicals

Hyperfine coupling tensors to 1H, 2H, and natural abundance 13C were measured using X-band pulsed electron nuclear double resonance (ENDOR) spectroscopy for two triarylmethyl (trityl) radicals used in electron paramagnetic resonance imaging and oximetry: methyl tris(8-carboxy-2,2,6,6-tetramethyl-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl) and methyl tris(8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl). Quantum chemical calculations using density functional theory predict a structure that reproduces the experimentally determined hyperfine tensors. The radicals are propeller-shaped with the three aryl rings nearly mutually orthogonal. The central carbon atom carrying most of the unpaired electron spin density is surrounded by the sulfur atoms in the radical and is completely shielded from solvent. This structure explains features of the electron spin relaxation of these radicals and suggests ways in which the radicals can be chemically modified to improve their characteristics for imaging and oximetry.

Improving W-band pulsed ENDOR sensitivity--random acquisition and pulsed special TRIPLE

Two approaches for improving the signal-to-noise ratio (S/N) of W-band pulsed electron-nuclear double resonance (ENDOR) spectra are presented. One eliminates base-line problems while the other enhances the ENDOR effect. High field ENDOR spectra measured at low temperatures often suffer from highly distorted base-lines due to the heating effect of the RF pulses that causes some detuning of the cavity and therefore leads to a reduction in the echo intensity. This is a severe problem because it often masks broad and weak ENDOR signals. We show that it can be eliminated by recording the ENDOR spectrum in a random, rather than the standard sequential variation of the RF frequency. The S/N of the ENDOR spectrum can be significantly enhanced by the application of the pulse analog of the continuous wave (CW) special TRIPLE experiment. While this experiment is not applicable in the solid state at conventional X-band frequencies, at W-band it is most efficient. We demonstrate the efficiency of the special TRIPLE Davies and Mims experiments on single crystals and orientationally disordered systems.