Alexander Pines

Proton‐enhanced NMR of dilute spins in solids

The NMR signals of isotopically or chemically dilute nuclear spins S in solids can be enhanced by repeatedly transferring polarization from a more abundant species I of high abundance (usually protons) to which they are coupled. The gain in power sensitivity as compared with conventional observation of the rare spins approaches NII(I+1)γI2/NSS(S+1)γS2, or ∼ 103 for S = 13C, I = 1H in organic solids. The transfer of polarization is accomplished by any of a number of double resonance methods. High‐frequency resolution of the S ‐spin signal is obtained by decoupling of the abundant spins. The experimental requirements of the technique are discussed and a brief comparison of its sensitivity with other procedures is made. Representative applications and experimental results are mentioned.

Iterative schemes for bilinear operators; application to spin decoupling

We extend the idea of iterative schemes from the single-spin to the two-spin-case. As an application we derive a new series of broadband heteronuclear decoupling sequences, called the DIPSI sequences. They give better quality decoupling of protons from carbon-13 than previous sequences like WALTZ-16 when there is scalar coupling among the protons. In the absence of such coupling, the DIPSI sequences offer the same high standard of performance as WALTZ-16, but over somewhat smaller bandwidths.

Bilinear rotation decoupling of homonuclear scalar interactions

Abstract A method for obtaining NMR spectra of organic liquids free of J -couplings is described. The coupling between a group of equivalent protons and an adjacent 13 C nets as a local decoupling field for the protons. A Pure chemical shift spectrum of ethanol is presented and possible applications to strongly coupled systems are discussed.

High resolution solid-state N.M.R.

By means of rotations around two axes inclined at zeros (magic angles) of the l = 2 and l = 4 Legendre polynomials, first-order and second-order N.M.R. broadening can be averaged away. Experiments with a double-rotor on the central (1/2 ↔ - 1/2) transition of sodium-23 in polycrystalline sodium oxalate illustrate the elimination of broadening due to second-order quadrupolar effects leading to a thirtyfold increase in resolution compared to magic-angle spinning.

Operator formalism for double quantum NMR

An operator formalism is presented which conveniently treats the interaction of a spin‐1 nucleus with a weak radio frequency field. The Hamiltonian in the rotating frame is H=−Δω Iz−ω1Ix+(1/3) ωQ[3 I2z−I (I+1)], where Δω is the resonance offset (Δω=ω0−ω), ω1 is the intensity of the rf field, and ωQ is the quadrupolar splitting. Nine fictitious spin−1/2 operators, Ip,i where p=x,y,z and i=1,2,3, are defined where p refers to the transition between two of the levels and i the Cartesian component. The operators, which are the generators of the group SU (3), satisfy spin‐1/2 commutation relations [Ip,j, Ip,k]=i Ip,l, where j,k,l=1,2,3 or cyclic permutation. Thus each p defines a three‐dimensional space termed p space. For irradiation near one of the quadrupolar satellites, for example, Δω=ωQ+δω with δω, ω1≪ωQ, it is shown that the effective Hamiltonian can be written H?−δωIx,3−√2 ω1Ix,1, i.e., a fictitious spin‐1/2 Hamiltonian in x space with effective magnetogyric ratio γ along the 3 (resonance offset) axis ...

Enhancement of Solution NMR and MRI with Laser-Polarized Xenon

Optical pumping with laser light can be used to polarize the nuclear spins of gaseous xenon-129. When hyperpolarized xenon-129 is dissolved in liquids, a time-dependent departure of the proton spin polarization from its thermal equilibrium is observed. The variation of the magnetization is an unexpected manifestation of the nuclear Overhauser effect, a consequence of cross-relaxation between the spins of solution protons and dissolved xenon-129. Time-resolved magnetic resonance images of both nuclei in solution show that the proton magnetization is selectively perturbed in regions containing spin-polarized xenon-129. This effect could find use in nuclear magnetic resonance spectroscopy of surfaces and proteins and in magnetic resonance imaging.

Proton-enhanced nuclear induction spectroscopy 13C chemical shielding anisotropy in some organic solids

Abstract Using the recently introduced technique of proton-enchanced nuclear induction spectroscopy, 13 C chemical shielding parameters are reported for a number of simple organic compounds, and some exemplary spectra are shown. The 13 C chemical shielding parameters are sensitive to the functional character of the carbon and to molecular structure. Several interesting aspects of these experiments, including molecular motion and cross-relaxation effects, are mentioned briefly.

Dynamic-Angle Spinning of Quadrupolar Nuclei

Abstract In dynamic-angle spinning (DAS), a sample spins around an axis inclined at an angle θ ( t ) with respect to the magnetic field such that the averages of P n ( cos θ ) are zero. The simplest case is where θ ( t ) assumes two discrete values θ 1 and θ 2 (complementary DAS angles) such that the averages of P 2 ( cos θ ) and P 4 ( cos θ ) are zero, thereby removing second-order quadrupolar (and dipolar) broadening. Examples of DAS complementary angles are θ 1 = 37.38 ° and θ 2 = 79.18 ° . Experimental details for DAS experiments are provided and applications to sodium-23 and oxygen-17 NMR illustrate the enhanced resolution achieved by removing the second-order broadening inherent in magic-angle spinning.

Multiple‐quantum dynamics in solid state NMR

Recently developed solid state multiple‐quantum NMR methods are applied to extended coupling networks, where direct dipole–dipole interactions can be used to create coherences of very high order (∼100). The progressive development of multiple‐quantum coherence over time depends upon the formation of multiple‐spin correlations, a phenomenon which also accompanies the normal decay to equilibrium of the free induction signal in a solid. Both the time development and the observed distributions of coherence can be approached statistically, with the spin system described by a time‐dependent density operator whose elements are completely uncorrelated at sufficiently long times. With this point of view, we treat the distribution of coherence in a multiple‐quantum spectrum as Gaussian, and characterize a spectrum obtained for a given preparation time by its variance. The variance of the distribution is associated roughly with the number of coupled spins effectively interacting, and its steady growth with time refl...

Functionalized xenon as a biosensor

The detection of biological molecules and their interactions is a significant component of modern biomedical research. In current biosensor technologies, simultaneous detection is limited to a small number of analytes by the spectral overlap of their signals. We have developed an NMR-based xenon biosensor that capitalizes on the enhanced signal-to-noise, spectral simplicity, and chemical-shift sensitivity of laser-polarized xenon to detect specific biomolecules at the level of tens of nanomoles. We present results using xenon “functionalized” by a biotin-modified supramolecular cage to detect biotin–avidin binding. This biosensor methodology can be extended to a multiplexing assay for multiple analytes.