Carbon-13 solid state NMR of pitch. part 2

Abstract

A set of six pitch samples, numbered TL-86-168 through TL-86-173, was investigated by carbon-13 solid state nuclear magnetic resonance spectroscopy. The samples comprised three sets of two samples; within each set the temperature employed during synthesis was either 300°C (TL-168, -169 and -170) or 350°C (samples TL-171, -172 and -173). The three sets also differed with respect to other conditions of synthesis, which were not made available to the authors. Nmr analysis revealed the following. All samples were essentially totally aromatic, exhibiting apparent aromaticity values of 1.0. As a result, no information is available concerning aliphatic functional groups. The various synthesis conditions did not measurably alter the aromaticity. The degree of condensation, and substitution patterns, may have been affected by the synthesis conditions. The ratio of tertiary to quaternary aromatic carbons vas determined independently by_ dipolar dephasing experiments as well as detailed analyses of the corresponding cross polarization relaxation time behaviour. The dipolar dephasing data show that the fraction of quaternary carbons ranges from 38% to 54% throughout the series. Differences in the temperature of synthesis did not yield well defined trends in this variable. The corresponding average structure deduced from these measurements is not affected substantially by these relatively small changes. The average number of carbons per aromatic cluster is about 15 - 18, corresponding to mixtures of rings varying in size from 2 to 4. Both proton rotating frame spin-lattice relaxation times and C-13 spin lattice relaxation times appeared- to vary systematically between the samples in a given pair, ie, they varied as a function of temperature. These differences were not large, however, and corroborated the overall conclusion that only minor structural modifications are represented by this entire set of pitches.