INVESTIGATION OF COAL AND A NAPHTHALENE PITCH BY MEANS OF THE TRIPLE-ECHO MAGIC-ANGLE TURNING EXPERIMENT

The triple-echo magic angle turning 2D experiment together with short contact time and dipolar dephasing variations is used to investigate the structure of a series of coals and a naphthalene-derived pitch. The aromaticities obtained with the standard triple-echo MAT experiment at 4.7 T with a single contact time are compared with those obtained from the standard C-13 CP/MAS variable contact time experiment at a magnetic field of 2.35 T and at a sample spinning rate of about 4.1 kHz. A high correlation (R(2) = 0.987) is found between the aromaticities for these two totally different experimental approaches. By introducing the chemical shift anisotropy into the experiment, structural information which is hidden in the standard C-13 CP/MAS spectrum due to the severe overlapping of the resonance lines can now be extracted by the triple-echo MAT 2D experiments. It is found that the overlapping powder patterns for the protonated and nonprotonated aromatic carbons can be successfully separated by the short contact time and dipolar dephasing variations of the triple-echo MAT, thereby simplifying the interpretation of the isotropic shift dimension as well as simplifying the measurement of the C-13 CSA principal values in such complicated systems. The measurements of C-13 CSA principal values of protonated, bridgehead, and alkyl-substituted carbons in a naphthalene-derived pitch, a semianthracite, and an anthracite coal are presented as typical examples. Because of the sideband-free nature of the triple-echo MAT data in the isotropic chemical shift dimension, the quality of the 2D baseplane, and the technical simplicity in implementation of the experiment, the standard triple-echo MAT experiments appear to be useful for qualitative and perhaps quantitative measurement of coals at any magnetic field strength.