W. S. Veeman

Rotational echo 14N/13C/1H triple resonance solid‐state nuclear magnetic resonance: A probe of 13C–14N internuclear distances

A novel triple resonance magic angle spinning (MAS) NMR experiment is reported that can be used to probe carbon–nitrogen interatomic distances in polycrystalline and amorphous solids, without the need for 15N isotopic labeling. Theory is presented for an S=1 nucleus under conditions of MAS and a spin‐locking radio frequency (r.f.) field. An off‐resonance r.f. field is only effective in periods during the rotor cycle where it is of the same order of magnitude as the first order quadrupolar splitting Q. This occurs when Q changes sign (the zero crossing) and passages between the S=1 Zeeman levels result. Numerical calculations are used to determine the conditions for adiabatic passages. An adiabatic passage results in the greatest change in the density matrix, inverting the Zeeman magnetization and creating quadrupolar order; faster passages, caused by faster MAS or a larger quadrupole coupling constant (e2qQ/h), result in the loss of some Zeeman magnetization to non‐spin‐locked coherences. Applying an off‐...

Quadrupole effects in high-resolution phosphorus-31 solid-state NMR spectra of triphenylphosphine copper(I) complexes

Abstract High-resolution 31P solid-state NMR spectra of triphenylphosphine copper(I) complexes, obtained by combining proton dipolar decoupling, proton-phosphorus cross polarization, and magic-angle sample spinning, often reveal asymmetric quartets rather than single resonance lines. These splittings arise from coupling with the nuclear spin 3 2 of the quadrupolar copper nucleus. Theoretical NMR spectra of spins I = 1 2 coupled to spins S > 1 2 are presented for powders spinning at the magic angle. Scalar as well as dipolar interactions are considered for the S = 1 and S = 3 2 cases. The magnetic field dependence of the observed asymmetric quartets in bis(triphenylphosphine)copper(I)nitrate can be accounted for by assuming a combination of scalar and dipolar coupling between phosphorus and copper. The spectra allow the determination of the scalar coupling constant, the dipolar coupling constant, and the copper quadrupole constant. Also, their signs can be established.

Two-dimensional solid-state nutation NMR of half-integer quadrupolar nuclei.

The 2-dimensional solid-state nutation NMR expt. for the detn. of quadrupole parameters, as introduced by Samoson and Lippmaas (1983) is evaluated. A complete series of spectra (for spin I = 3/2, 5/2, 7/2, and 9/2) resulting from d.-matrix calcns. is presented, and some exptl. aspects of the method are discussed. Applications of the method to 27Al (I = 5/2) in spodumene and to 45Sc (I = 7/2) in Sc2(SO4)3 are shown.

A novel application of nuclear spin-echo double-resonance to aluminophosphates and aluminosilicates

Abstract It is shown that by applying a spin-echo double-resonance technique to 31 P and 27 Al in AlPO 4 -5, one can detect that 31 P is coupled to 27 Al. Although the pulse sequence used resembles that of the SEDOR experiment, it actually differs and also the mechanism of the phenomena we observe seems to differ from the mechanism of the original SEDOR experiment. The experiment described in this paper is also tried on 29 Si and 27 Al in the zeolites NaA and NaY. For NaA, there is a small but definite effect of aluminum irradiation on the 29 Si spin-echo intensity. For NaY, the signal-to-noise ratio is too small to observe an effect.

The influence of hydration on the coordination state of boron in H-Boralite studied by 11B magic angle spinning n.m.r.

The effect of hydration and dehydration on the coordination of boron in H-Boralite is investigated by 11B magic angle spinning n.m.r. The experiments show that in hydrated Boralite the quadrupole interaction of boron is negligible, implying a highly symmetrical electric field gradient around boron due to the four oxygens to which boron is tetrahedrally coordinated. In dehydrated samples a substantial quadrupole interaction (e2qQ/h = 2.55 MHz, η = 0) for boron is found. This proves that on dehydration the BO4 tetrahedron is severely distorted. The close agreement with quadrupole parameters of planar BO3 groups in borates and boron oxides suggests that in the distorted tetrahedron the boron atom is located in the plane between three oxygens.

Two-dimensional exchange NMR in rotating solids: A technique to study very slow molecular reorientations

Abstract A 2D experiment capable of detecting very slow molecular reorientations is proposed, which makes use of spinning sidebands observed in magic-angle-spinning spectra. An expression describing the spinning sideband intensities of the 2D spectrum is given. The experiment is applied to dimethylsulfone. Experimental and calculated spectra show very good agreement.

NMR images of rotating solids

It is demonstrated that NMR images may be obtained from solids rotating at the magic angle. This approach leads to line narrowing in favorable samples and is easily implemented. In very rigid systems it is suggested that imaging be performed with a combination of MAS and multiple-pulse line narrowing.

Characterization of intermediate TPA-ZSM-5 type structures during crystallization

Abstract A wide range of analytical techniques (XRD, DTA/TGA, I.R., texture, SEM/TEM,13C,27Al,29Si MAS NMR) has been used to characterize intermediate products formed during the crystallization process of the zeolite Tetrapropylammonium ZSM-5 (TPA-ZSM-5). In the beginning of the crystallization process the solid products are XRD-amorphous and consist of an aluminum-rich phase next to an increasing amount of TPA-ZSM-5 entities with dimensions of the order of the unit cell of ZSM-5. In the final stage of the crystallization process (here up to 8 days) the TPA-ZSM-5 entities recombine to a crystalline ZSM-5 framework. A large amount of framework defects are then still present in the XRD-crystalline TPA-ZSM-5 framework as evidenced by 29Si MAS NMR. These defects disappear upon calcination.

Ultraslow molecular motions in crystalline polyoxymethylene. A complete elucidation using two‐dimensional solid state NMR

The slow magic angle spinning 2D exchange NMR experiment applied to semicrystalline polyoxymethylene shows the existence of ultraslow molecular rotations in crystalline regions of the material. By simulating the two‐dimensional spinning sideband spectra, which are characteristic for the molecular motion involved, it is found that a rotation of the polyoxymethylene helical chains over 200° is responsible for the observed spectra. The activation energy is 20 kcal/mol in good agreement with values obtained for the α relaxation in dynamic mechanical and dielectric studies.

Diffusion in Pseudomonas aeruginosa biofilms: a pulsed field gradient NMR study

A Pseudomonas aeruginosa biofilm is studied with pulsed field gradient echo nuclear magnetic resonance. Although not all spectral components are assigned yet, the experimental results show that a biofilm consists of components with very different diffusion coefficients. The various biofilm components that give motionally narrowed 1H NMR signals, can be grouped into five classes with diffusion coefficients, ranging from 2 x 10(-9) to 1 x 10(-13) m2 s-1. Investigation of the diffusion behavior of water in the biofilm shows three fractions with different diffusion coefficients. Besides the highly mobile bulk water at least two other fractions with much lower diffusion coefficients are detected. It is shown that one of the fractions with the low diffusion coefficient probably arises from intracellular water. Also for another component of the biofilm, glycerol, three fractions with diffusion coefficients that differ more than a factor ten are detected. Also a group of signals exists which result from practically immobile components.