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Analysis of results from interrupted-decoupling NMR pulse sequences combined with high-speed magic-angle spinning

1990; Elsevier BV; Volume: 86; Issue: 1 Linguagem: Inglês

10.1016/0022-2364(90)90222-u

1557-8968

Roger H. Newman,

Advanced MRI Techniques and Applications

Opella and Frey ( I ) introduced a pulse sequence designed to suppress NMR signals from protonated carbon atoms in solid samples. Many modifications have appeared since then (2). All of the sequences involve a brief interruption in proton spin decoupling between the end of cross polarization and the beginning of data acquisition. Static dipolar interactions with nearby protons destroy 13C magnetization during the interruption, through destructive interference of a collection of different dipolar splittings for different orientations of C-H vectors within a powder. The technique is often known as “dipolar dephasing,” but the alternative name of “interrupted decoupling” (3) is used here because it describes the experimental design without presupposing success. Spinning the sample weakens the static dipolar interaction that is required for successful suppression of unwanted signals. Numerous papers have described interrupted-decoupling experiments successfully combined with MAS frequencies of a few kilohertz (2). This Note combines experiments with computer simulations to explore the consequences of higher MAS frequencies. Figure 1 shows the results of experiments with the pulse sequence of Opella and Frey ( I ) . Ammonium tartrate was selected for this work because the simplicity of the 13C CP/ MAS NMR spectrum minimized problems in choosing MAS frequencies without complications from sideband signals overlapping the signal from CH groups at 6 = 74. The sample was packed in a 7 mm diameter cylindrical silicon nitride rotor with Vespel caps and spun in a probe manufactured by Doty Scientific for 13C NMR at 50 MHz. Computer simulations were based on a Pascal program in which isolated CH groups were oriented in a matrix of 16 different angles of displacement from the MAS axis and 16 different initial values of the angle of rotation around the MAS axis. Dipolar splittings were then calculated for each of the 256 C-H vectors, and the accumulated phases of the 13C magnetization vectors were incremented at steps of no more than 5 ps through the interruption. Magnetization components were weighted according to the solid angle represented by each element in the matrix of C-H vectors. The choices of array size and step size were based on preliminary tests for adequate precision. A C-H bond length of 0.109 nm was used.