INELASTIC NEUTRON-SCATTERING STUDY OF THE PROTON DYNAMICS IN COALS

The inelastic neutron-scattering (INS) spectra from 30 to 4000 cm(-1) of four coal samples of different rank (subbituminous A, low-volatile bituminous, semianthracite and anthracite) at 20 K are reported. For e ach coal, rather broad bands superimposed on a continuum with almost c onstant intensity over the whole frequency range are observed. This co ntinuum is assigned to recoiling free protons located between the grap hite-like planes. Band frequencies are compared with those observed in the infrared for the same samples in KBr matrices at room temperature , and consistent assignments in terms of chemical-grouping vibrations are proposed. The INS spectra of the coals are also compared with that of perylene (C20H12) The spectrum of anthracite is dominated by CH an d skeletal modes of polyaromatic entities. Coals of lower rank show ba nds due to alkyl and vinyl CH2 and CH3 groups. A comparison of INS int ensities in the CH stretching region for coals and perylene gives an e stimate of the concentration of bound protons which ranges from simila r to 40% to similar to 80%. A simple model is proposed to estimate the mean size of the graphite-like domains in the coals.