Controlling the dipole-dipole interaction using NMR composite rf pulses

Abstract

I describe composite pulses during which the average dipolar interactions within a spin ensemble are controlled while realizing a global rotation. The construction method used is based on the average Hamiltonian theory and rely on the geometrical properties of the spin-spin dipolar interaction only. I present several such composite pulses robust against standard experimental defects in NRM: static or radio-frequency field miscalibration, fields inhomogeneities. Numerical simulations show that the magic sandwich pulse sequence, a pulse sequence that reverse the average dipolar field while applied, is plagued by defects originating from its short initial and final π/2 radio-frequency pulses. Using the magic composite pulses instead of π/2 pulses improves the magic sandwich effect. A numerical test using a classical description of NMR allows to check the validity of the magic composite pulses and estimate their efficiency.

Genetic algorithm optimized triply compensated pulses in NMR spectroscopy

“…Composite pulses consist of a sequence of RF pulses that are designed to emulate the effect of a single excitation or inversion pulse. Based on the type of compensation, composite pulses are categorized as: a) broadband pulses, which compensate for offset effects [6-10]; b) composite pulses for RF inhomogeneity compensation [9, 11-13]; c) dual-compensated pulses, which operate simultaneously on both offset effects and RF inhomogeneity [14, 15]; and d) compensated pulses for zz interactions [7, 16], which include heteronuclear dipolar and scalar coupling ($I_z^1{S}_z^2$) as well as weak scalar couplings in homonuclear spin pairs ($I_z^1{I}_z^2$). However, none of these pulses are designed to compensate for RF inhomogeneity, offset effects, and zz interactions, simultaneously.…”