The Potential of ¹³C NMR in Coal Research

Abstract

Since the first published report of successful proton nuclear magnetic resonance (¹H NMR) measurements of the hydrogen distribution in a coal derivative⁽¹⁾ and the first determination of a high-resolution ¹H NMR spectrum of a coal extract,⁽²⁾ proton magnetic resonance spectrometry has been used by many authors to aid in the elucidation of the structure of coal. (For a review and major references, see Tschamler and de Ruiter.⁽³⁾) Other nuclei, such as ¹⁴N, ³³S, ¹³C, and ¹⁷O, all of which have nonzero magnetic moments and are present to some extent in coal, have received little or no attention in coal-structure research utilizing NMR. Brown and Ladner⁽⁴⁾ developed a method for applying the hydrogen-distribution data obtained by ¹H NMR to the analysis of carbon structure, with particular emphasis on estimating three important structural parameters: f _a , the aromaticity; σ the degree of aromatic ring substitution; and H_aru/C_ar a parameter used to indicate the size of the polynuclear condensed aromatic ring system. Unfortunately, the calculated values of these parameters depend strongly upon a number of a priori, though generally reasonable, assumptions concerning the coal structure. The first reports^(5,6) of successful determination of nuclear magnetic resonance of ¹³C in natural abundance prompted us to explore the use of high-resolution ¹³C NMR in coal research.^(7,8) High-resolution ¹³C NMR offers a means by which f _a can be measured directly and realistic limiting values can be placed on σ and H_aru/C_ar. The routine laboratory applications of ¹³C NMR are hampered by the low natural abundance of ¹³C nuclei and by the inherently low NMR sensitivity of ¹³C as compared to other, more receptive nuclei. Recent advances in instrumentation, particularly the use of spectral time-averaging,⁽⁹⁾ are rapidly improving this situation.