Double-quantum homonuclear correlation magic angle sample spinning nuclear magnetic resonance spectroscopy of dipolar-coupled quadrupolar nuclei.

Abstract

A double-quantum homonuclear correlation nuclear magnetic resonance experiment for dipolar-coupled half-integer quadrupolar nuclei in solids is presented. The experiment is based on rotary resonance dipolar recoupling and uses bracketed spin-lock pulses to excite double-quantum coherence and later to convert it to the zero-quantum one. A central-transition-selective pi pulse at the beginning of the t1 evolution period differentiates coherence transfer pathways of double-quantum coherences arising from coupled spins and from a single spin, so that the latter can be efficiently filtered out by phase cycling. The experiment was tested on an aluminophosphate molecular sieve AlPO4-14, a material with a variety of aluminum quadrupolar coupling constants, isotropic chemical shifts and homonuclear distances. In a two-dimensional spectrum aluminum dipolar couplings with internuclear distances between 2.9 and 5.5 A were resolved. Although the experiment requires an application of weak radio-frequency fields, frequency offsets did not affect its performance crucially.