Quantitative aspects of solid state 13C n.m.r. of coals and related materials

✍ Scribed by Kenneth J. Packer; Robin K. Harris; Alan M. Kenwright; Colin E. Snape

• Publisher: Elsevier Science
• Year: 1983
• Tongue: English
• Weight: 480 KB
• Volume: 62
• Category: Article
• ISSN: 0016-2361
• DOI: 10.1016/0016-2361(83)90131-x

✦ Synopsis

The quantitative aspects of cross-polarization (CP), which is used in conjunction with dipolar decoupling and magic-angle rotation to obtain high resolution 13C n.m.r. spectra of coals, have been studied using a bituminous coal (82 wt% C, dmmf basis) and asphaltenes from an extract of the same coal. The condition for obtaining reliable quantitative data, that rotating frame 'l-l relaxation times (T,,, these govern the extent of CP) are much longer than the time required to polarize the carbons present (~1 ms), was met for the asphaltenes. In contrast, about half the protons in the coal have Tlp5 of w S 1 ms, these times being too short to allow CP of all the carbons. Although the aromaticities obtained for this coal were fairly constant (~~0.75) using (CP) contact times > 0.5 ms, the total peak intensity decreased markedly as the contact time was increased and was much less than that for the asphaltenes. These results indicate that not all the carbons in bituminous coals are observed by CP and, as a consequence, aromaticities reported in the literature for some bituminous coals appear to be low.