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

摘要

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 77 pulse at the beginning of the t_1 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 AlPO_4-14,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.