The combined effect of quadrupolar and dipolar interactions on the excitation and evolution of triple quantum coherences in 7Li solid state magic angle spinning NMR

Abstract

Magic-angle spinning triple-quantum NMR spectra of lithium-7 provide enhanced spectral dispersion for the inherent low chemical shift range of this nucleus, while maintaining linewidths, which are free of any quadrupolar broadening to first order. Since the quadrupolar interaction of 7Li is very small, in the order of the radio frequency nutation frequencies and only moderately larger than the spinning rates, such spectra are also only marginally affected by the second order quadrupolar interaction under large magnetic fields. In the current study we demonstrate that the existence of two and more proximate 7Li spins, as encountered in many materials, affects both excitation and evolution of triple-quantum coherences due to the combined effect of quadrupolar and homonuclear dipolar interactions. We show that the generation of 7Li triple-quantum coherences using two π/2 pulses separated by one-half rotor period is superior in such cases to a single pulse excitation since the excitation time is shorter; thus the maximum signal is only marginally affected by the homonuclear dipolar couplings. When the quadrupolar–dipolar cross terms dominate the spectra, single- and triple-quantum lineshapes are very similar and therefore a true gain in dispersion is maintained in the latter spectrum. The effects of quadrupolar–dipolar cross terms are experimentally demonstrated by comparing a natural abundance and a 6Li-diluted samples of lithium acetate, resulting in the possibility of efficient excitation of triple quantum coherences over longer periods of time, and in longer life times of triple-quantum coherences.