Riqiang Fu

Recoupling of heteronuclear dipolar interactions in solid state magic-angle spinning NMR by simultaneous frequency and amplitude modulation

A new scheme is proposed for solid-state NMR which uses simultaneous frequency and amplitude modulation (SFAM) to restore heteronuclear dipolar interactions between two unlike spin-1/2 nuclei such as 13C and 15N under fast magic angle spinning (MAS). The carrier frequency of one of the spins is modulated cosinusoidally while its RF amplitude is modulated as a sine wave. It is shown by expts. and simulations that the scheme allows one to reintroduce the heteronuclear dipolar interactions even at very high spinning speeds which are often required for suppression of chem. shift anisotropy (CSA) in high magnetic fields. Unlike the known REDOR and TEDOR expts., the SFAM method is not sensitive to effects of finite pulse lengths. The advantages of the SFAM technique are illustrated for doubly labeled glycine (15NH213CH2COOH). [on SciFinder (R)]

Broadband decoupling in NMR with frequency-modulated ‘chirp’ pulses

In NMR of spins with I = 1/2 in an isotropic phase, it is demonstrated by simulation and expt. that virtually unlimited bandwidths can be decoupled effectively by using chirp pulses with linear frequency modulation for adiabatic inversion combined with phase-cycles and supercycles similar to those used in composite pulse decoupling. The av. radio-frequency amplitude required is modest compared to other techniques. In applications with inhomogeneous radio-frequency fields, as in magnetic resonance imaging and localized spectroscopy with surface coils, the radio-frequency amplitude can be apodized at the rising and falling edges of chirp pulses to improve adiabatic behavior. [on SciFinder (R)]

Frequency-modulated cross-polarization for fast magic angle spinning NMR at high fields: relaxing the Hartmann-Hahn condition

Two novel schemes are described for cross polarization in NMR that make it possible to achieve an efficient transfer of polarization from abundant I spins to dil. S spins under high-speed magic angle spinning spectroscopy, which is useful for systems with large chem. shift anisotropies at high fields. The frequency of one of the RF carriers is modulated sinusoidally during spin-locking while the RF amplitude is kept const. It is shown by expt. that frequency modulation greatly attenuates the crit. character of the Hartmann-Hahn matching condition, and allows one to obtain uniform excitation over a wide range of offsets. Signal intensities of 13C resonances in alanine were obtained as a function of mismatch at spinning speeds of 12 and 15 kHz and a static field strength of 14 T (600 MHz for 1H NMR) to illustrate the advantages of these schemes. [on SciFinder (R)]

Divergent double chirp pulses for refocusing quadrupolar interactions

To excite and refocus single quantum coherences in a three-level system with a quadrupolar interaction, it is possible to use frequency-modulated pulses with two diverging frequency components that are swept simultaneously through the two single-quantum transitions. This is achieved by time-dependent amplitude-modulation of the carrier frequency, so as to generate two sidebands with frequencies that are diverging (or converging) symmetrically with respect to the carrier frequency. Simulations and experiments show that such modulated pulses can be used to refocus single quantum coherences to form quadrupole echoes, using much weaker RF amplitudes than in conventional echo sequences. To visualize the evolution of the coherences in a three-level system, a graphical representation of the density operator components is proposed.

Method and apparatus for broadband decoupling in nuclear magnetic resonance with chirp pulses

Methods and apparatus for broadband decoupling in nuclear magnetic resonance with chirp and other RF pulses are disclosed. Such methods may include the step of positioning the sample in a static magnetic field. Additional steps may define a first cycle of pulses, such as chirp pulses, wherein at least two of the pulses have a different initial phase angle from each other and may define a second cycle of pulses wherein at least two of the pulses in the second cycle have an initial phase angle different from the initial phase angles of the pulses in the first cycle. Further steps may repeatedly generate a supercycle of the first and second cycles to produce pulses for inverting the longitudinal magnetization in the sample and may also detect the signals emitted by the sample in response to inversion of the longitudinal magnetization. Alternatively, the chirp pulses in the first and second cycles may be defined to sweep a bandwidth of 100 kHz or more. The first and second cycles may also be defined to include linear frequency modulated RF pulses wherein each of the RF pulses has an amplitude which is substantially constant during at least about 50% of the pulses. Apparatus for performing the disclosed methods is also disclosed. Other methods and apparatus are also disclosed.

NMR of residual protons in partly deuterated anisotropic materials with phase-alternated decoupling of deuterium spins

A new method is presented for decoupling spins with S = 1 like deuterium in anisotropic media while observing other spins such as residual protons in partly deuterated samples. The carrier frequency of a weak radiofrequency (RF) field is applied near the center of the doublet arising from the quadrupolar interaction of the S = 1 spins. The phase of the RF field is periodically reversed with intervals matching the reciprocal of the magnitude of the quadrupolar splitting. It is shown by theory and experiment that, even when the irradiating field is quite weak, the efficiency of this phase-alternated decoupling scheme is much better than for simple continuous-wave irradiation at the center of the doublet, an established technique which is usually referred to as double quantum decoupling. The phase-alternated experiment makes it possible to decouple large quadrupolar interactions with a weak RF field. A theoretical analysis and numerical simulations are presented to demonstrate the decoupling performance. Proton spectra of partly deuterated dibromobutane in a liquid crystalline solvent have been recorded to illustrate the efficiency of phase-alternated deuterium decoupling.