Spin Echo Double Resonance Research Articles

Time symmetries in rotating sample NMR

The utility of time symmetries in rotating sample NMR is explored. For a rotating powder, the system is time shift independent as well as invariant to reversal of the sense of spinning. These symmetries are applied to REDOR (rotational spin-echo double resonance) experiment and an experiment for enhancement of spinning sidebands. Use of the symmetries greatly simplifies the mathematics describing rotating powder samples.

Extended dipolar modulation and magic-angle spinning

The direct detection of weak 13C15N heteronuclear dipolar couplings in magicangle spinning (MAS) NMR experiments is often difficult. A conventional MAS 13C NMR spectrum has spinning sidebands arising from both dipolar and chemical-shift interactions. Two-dimensional NMR experiments which separate dipolar couplings from the chemical-shift anisotropy simplify considerably their determination. Munowitz and Griffin introduced single-sided ( I ) and double-sided (2) two dimensional magic-angle spinning NMR experiments that have proved invaluable in the study of strong r3C‘H dipolar interactions. Unfortunately, when these techniques are applied to the substantially weaker 13C“N coupling, there are usually not enough dipolar sidebands to characterize effectively the dipolar sideband spectrum, even at low spinning speeds. A variation of the Munowitz and Griffin double-sided experiment, called odd p this yields more dipolar sidebands. The extra sidebands often contain enough information to allow a good determination of weak 13C“N dipolar coupling. Typically, however, the experiment must be done at low spinning speeds of the order of 500 Hz with the consequence that an overwhelming number of rotational sidebands can appear in the chemical-shift dimension. This is undesirable since the dipolar-modulated magnetization is actually measured by summing over sidebands in the chemical-shift dimension. The characterization of weak 13C15N couplings at slow spinning speeds in a complicated carbon spectrum can be simplified by the use of double-cross polarization (4) (DCP) as a selective filter; however, DCP is technically difficult to execute and generally does not give equal selection to spins with different spatial orientations (3). This Communication presents preliminary results on new experiments designed to measure weak heteronuclear dipolar couplings without the problems associated with ODRSE. Weak couplings can be measured at higher spinning speeds; very weak couplings can be measured at lower spinning speeds. Our experiments are extensions of rotational echo double resonance (5) (REDOR) which, in turn, was derived from spin-echo double resonance (6, 7) (SEDOR) . Extension of the dipolar evolution time in rotating solids is the principal feature of the experiments; accordingly, this class of experiments is called extended dipolar modulation (XDM) . XDM is a REDOR experiment and is specified by the number

Multiple-volume-selective proton NMR spectroscopy and spectral editing by spin-echo double resonance (VOSY + SEDOR)

A series of test experiments with the VOSY technique for volume-selective NMR spectroscopy has been carved out. A critical test of volume selection under chemical-shift-induced offsets at 4.7 T is reported. It is demonstrated that several volume elements can be probed in a single experiment so that metabolite concentrations can be related to other regions or to a calibration sample. The method can also be combined with spectral editing procedures such as the spin-echo double-resonance sequence (SEDOR).

Nuclear magnetic resonance study of Pt–Rh bimetallic clusters

The use of pulsed nuclear magnetic resonance to study supported Pt–Rh bimetallic clusters is described. Using energy-dispersive spectroscopy it has been shown that each individual cluster contains both Pt and Rh. For lower-dispersion samples (larger metal particles) the centre of the particles is pure Pt. For higher-dispersion samples (smaller metal particles) the Pt concentration in the surface layer of the clusters has been measured from observation of the dependence of the height of the Pt spin echo on the time, τ, between pulses of the echo sequence. For clusters on which 13CO has been adsorbed, the 13CO resonance frequency varies with the Pt–Rh ratio. 195Pt–13C spin-echo double resonance (SEDOR) has been employed to measure the fraction of Pt atoms in the surface layer of the cluster. By studying how the SEDOR signal depends on the frequency of the 13CO exciting pulse, it has been shown that the resonance frequency of 13CO is independent of whether the CO is bonded to Pt or Rh in a given sample. This result shows that the frequency shift of the 13CO is a non-local rather than a local phenomenon. The 13CO shift corresponds to that of pure Rh particles, even for surfaces which contain a substantial Pt content. The significance of this result is explained in terms of the mixing of the molecular and metallic wave-functions.

Local structure of dopants in compensated and singly-doped amorphous silicon

The distance between dopants and H in compensated and singly-doped a-Si has been determined using NMR spin echo double resonance. We find that there is a non-random local configuration around the dopants in these materials. In the compensated samples, 40% of the B atoms have a P in the first neighbor shell, much higher than the random value of 8% and indicative of dopant clustering. In addition, the H is not directly bonded to these dopants. This implies that the H has a higher chemical affinity for Si than for B and P in compensated a-Si. For singly-doped a-Si, the H-dopant structure is similar for the P-doped samples but different for the B-doped material, where approximately 40% of the B atoms have an H in the first neighbor shell at 1.6Å. This is to be contrasted with the P-doped samples which have no H at this short distance.

Pulsed electron nuclear double and triple resonance schemes

AbstractThe following pulsed electron nuclear double (ENDOR) and triple resonance schemes are discussed: Population transfer ENDOR, TRIPLE resonance, coherence transfer ENDOR, spin echo double resonance (SEDOR) and stimulated echo double resonance.

Na 43+ clusters in sodalite

Anion-sodalites with the general formula Na 8[(AlO 2) 6(SiO 2) 6]A 2, where A = Cl, Br or I, were reacted with the vapour of alkali metals. In all cases the ionic cluster Na 4 3+ was formed, together with a metallic cluster. The ionic cluster was investigated by electron spin resonance, electron spin echo and pulse-electron nuclear double resonance techniques. A modulation was observed in the electron spin echo envelope due to interaction with distant Na + ions. These Na + ions have hyperfine splitting constants between 0.24 and 1 MHz and are situated ∼ 5 Å from the centre of the sodium cluster.

NMR Investigation of the Local Dopant-Hydrogen Configuration in Amorphous Silicon

AbstractHydrogen's role in the doping efficiency and long term stability of hydrogenated amorphous silicon is not well understood. The introduction of dopants has been observed to cause an increase in the deposition rate and in the formation of dangling bonds. We have performed NMR spin echo double resonance measurements on heavily doped amorphous silicon samples prepared via plasma deposition in order to ascertain the dopanthydrogen structural units. The resulting dopant concentrations were 2.0 and 0.5 at. % boron and/or phosphorus in compensated and singly doped material respectively. Our results indicate no direct dopant-hydrogen bonding in the compensated or n-type samples. However, in the p-type material the data indicate that approximately 40% of the boron atoms are directly bonded to a hydrogen.

Studies on plantacyamin. II. NMR data, redox properties, reaction with nitrite and the formation of complex with plastocyanin

High-resolution 1H-NMR study of plantacyanin from cucumber was carried out. Assignments of many resonances were reached using methods of double resonance, spin-echo double resonance, NOE spectroscopy and difference spectroscopy. It was shown that the copper core of the protein is surrounded by hydrophobic amino acids (valine, leucine, isoleucine) as well as tyrosines. Planatacyanin is not autooxidable in neutral media; however, it shows weak autooxidability in acidic media. This reaction was inhibited by superoxide dismutase. Nitrite was found to be a more effective oxidant of reduced plantacyanin than in oxygen. The redox potential of plantacyanin was measured at pH 4.0–10.0. It was found that the potential (−340 mV) is unchanged over the pH range 4.0–8.5. Plantacyanin and plastocyanin in their oxidized states form at low ionic strength a complex of 1:1 stoichiometry.

NMR of platinum catalysts: Double NMR of chemisorbed carbon monoxide and a model for the platinum NMR line shape.

The authors report observation of the NMR line of {sup 195}Pt atoms in the surface layer of small platinum-metal particles on which {sup 13}CO has been chemisorbed. The surface {sup 195}Pt atoms are resolved from those of {sup 195}Pt atoms deeper in the particle by spin-echo double resonance between {sup 195}Pt and {sup 13}C. The particles, supported on {eta}-alumina, had dispersions (fraction of the atoms that are on the surface) of 26% and 76%. Comparison with {sup 195}Pt resonance in Pt carbonyls suggests that the magnitude of the Knight shift of the surface Pt is less than 0.2%. Analysis of the {sup 195}Pt spin-lattice relaxation indicates that the small surface Knight shift results from cancellation of 6s and 5d core-polarization contributions as was found theoretically by Weinert and Freeman for clean Pt surfaces. The {sup 13}C-{sup 195}Pt indirect spin coupling is found to be very similar to those in diamagnetic platinum carbonyl molecules. The results show that CO bonds via the C atom and verify that concepts from studies of large single crystals are valid for the small particles. The key features of the {sup 195}Pt line shapes in these small platinum particles are described by a simple phenomenological modelmore » of the spatial Knight-shift variation inside these particles. The model successfully describes the major structure seen in the NMR line shapes of samples with dispersions ranging from 5% to 76%.« less

A coherent nuclear quadrupole pulse and double resonance spectrometer

A coherent pulse and double resonance NQR spectrometer system, for investigations in the pure quadrupole regime, has been described. The pulsed NQR spectrometer is capable of operation typically over the range of 10-40 MHz and can be adapted with ease for spin echo investigations, relaxation time measurements and double resonance experiments such as spin echo double resonance (SEDOR) and double resonance in the rotating frame (DRRF). Besides the Hartmann and Hahn DRRF scheme, a DRRF scheme involving off-resonance irradiation of the abundant spins can also be implemented. To simplify construction, commercially available devices have been extensively incorporated into the spectrometer system. The general features of the spectrometer are presented and the performance is illustrated by a few typical signal recordings.

Probe of the Surface of a Heterogeneous Catalyst: Double NMR of Carbon Monoxide Chemisorbed on Highly Dispersed Platinum

The detailed NMR line of $^{195}\mathrm{Pt}$ atoms in the surface layer of small particles of Pt metal on which CO has been chemisorbed has been observed. The surface $^{195}\mathrm{Pt}$ are resolved from the $^{195}\mathrm{Pt}$ resonances of atoms deeper in the particle by spin-echo double resonance between $^{195}\mathrm{Pt}$ and $^{13}\mathrm{C}$. The $^{195}\mathrm{Pt}$ resonance position and $^{13}\mathrm{C}$-$^{195}\mathrm{Pt}$ indirect spin coupling are found to be very similar to those in diamagnetic Pt-carbonyl molecules. The results show that CO bonds via the C atom and verify that concepts deduced from studies of large single crystals are valid for the small particles.

Chlorine NQR on Derivatives of Chloral

Abstract The 35Cl-NQR spectrum of several derivatives of chloral, Cl3CCHO, was studied in the ranee 77 K ≦ T ≦ Tm. By use of the spin echo double resonance technique a relative assignment of the resonances to different Cl3C-groups within one crystallized compound was possible. The solid compounds studied are: High temperature phase of chloral hydrate, Cl3CCH(OH)2; parachloral, (Cl3CCHO)3, (α-and ß-isomer); the two phases of chloral hemihydrate Cl3CCHO · 1/2 H2O; chloralide (II); chloralhemithiohydrate, Cl3CCHO · 1/2 H2S. The structure of the molecules in the solid state and the fade out of the NQR resonances are discussed.

Investigation of alkali and 17O NQR spectra in chlorates and bromates by spin echo double resonance

The NQR spectra of 17O (isotopic concentration 1%–5%) and of the alkali ions in NaBrO 3, KBrO 3, RbBrO 3, CsBrO 3, NaClO 3, and KClO 3, have been studied using the spin echo double resonance (SEDOR) method. In KBrO 3 the 39K NQR frequency increases with increasing temperature in contrast to the NOR frequencies of the alkali nuclei in the five other substances. The values of e 2qQh −1 ( 17O) are slightly lower in the chlorates than in the bromates. For the bromates a linear relation between e 2 qQh −1 ( 17O) and the electronegativity (and the ionic radius) of the alkali ions has been found.

A nuclear quadrupole spin echo and double resonance study of DMn(CO) 5

Zero field nuclear quadrupole spin echo signals have been observed for 55Mn in DMn(CO) 5 in the temperature range 77–242°. The ν 2 transition occurs at 13.767 or 13.683 MHz at 77°, depending on crystal form obtained. The quadrupole coupling constant e 2 Qq h and asymmetry parameter η at 77° are 45.66 MHz and 0.07, respectively. Using the spin echo double resonance (SEDOR) technique, the quadrupole coupling constant of 2D in DMn(CO) 5 was determined to be 68.3 kHz, with η ∼ 0. Because the 55Mn 2D spins exist in the lattice as isolated spin pairs, it is possible to determine the dipolar interaction between the two spins directly, and from this, to estimate the MnD distance. The value obtained from the 2D spectrum is 1.61 ± 0.01 Å, in excellent agreement with the neutron diffraction result.

Spin echo double resonance: a novel method for detecting decoupling in Fourier transform nuclear magnetic resonance

It is shown that the combination of double irradiation and a simple two-pulse spin echo sequence is a powerful and sensitive method for detecting homonuclear decoupling in a complex spectrum.