Revisiting the mean-field picture of dipolar effects in solution NMR

Abstract

For more than three decades, the classical or mean-field picture describing the distant dipolar field has been almost always simplified to an effective field proportional to the local longitudinal magnetization, differing only by a scale factor of 1.5 for homomolecular (identical resonance frequency) and heteromolecular interactions. We re-examine the underlying assumptions, and show both theoretically and experimentally that the mathematical framework needs to be modified for modern applications such as imaging. We demonstrate new pulse sequences which produce unexpected effects; for example, modulating an arbitrarily small fraction of the magnetization can substantially alter the frequency evolution. Thus, matched gradient pulse pairs (a seemingly innocuous module in thousands of existing pulse sequences) can alter the time evolution in highly unexpected ways, particularly with small flip angle pulses such as those used in hyperpolarized experiments. We also show that specific gradient pulse combinations can retain only dipolar interactions between unlike spins, and the dipolar field can generate a secular Hamiltonian proportional to I x .

Multiple echoes due to distant dipolar fields in NMR of hyperpolarized noble gas solutions

“…The final aim of this work is to establish a method for absolute magnetization determination without resorting to external references, the latter being always loosely defined for transiently polarized spin systems. Several steps forward are reported in the present paper and in particular we demonstrate that finite size effects, which have been fully disregarded for several decades until very recently [42], appear of key importance. Finally the large range of dipolar fields accessible with our experiments has allowed us to observe strong distortions of the echoes for large dipolar fields (significant relative to the applied gradients).…”