Multiple-rotor-cycle QPASS pulse sequences: Separation of quadrupolar spinning sidebands with an application to La-139 NMR

The quadrupolar phase-adjusted spinning sidebands (QPASS) pulse sequence ha s been recently demonstrated as a useful method for obtaining quadrupolar p arameters with magic-angle spinning NMR. The sequence separates spinning si debands by order in a two-dimensional experiment. A sheared projection of t he 2D spectrum effectively yields the infinite spinning rate second-order q uadrupolar powder pattern, which can be analyzed to determine quadrupolar c oupling constants and asymmetry parameters. The RF power and spinning speed requirements of the original QPASS sequence make it an experimentally dema nding technique. A new version of the sequence is demonstrated here and is shown to alleviate many problems associated with the original sequence. New solutions to the determining equations, based on the use of multiple rotor cycles in the QPASS sequence, lead to longer delays between the nine pi pu lses, provide less chance of pulse overlap, and allow for use of weaker RF held strengths that excite only the central quadrupolar transition. A three -rotor-cycle version of the new experiment is demonstrated on the La-139 nu cleus. (C) 1999 Academic Press.