Macromolecular evolution and structural defects in tectonically deformed coals

Here, the differential macromolecular evolutions and the structural defects in various TDCs (tectonically deformed coals) were profoundly investigated utilizing the C-13 NMR, FTIR, Raman, and DFT (density function theory) calculations. The increasing aromatics provides evidences for advance evolution for aromatic clusters induced by dynamic metamorphism. The variations of f(al)* (non-protonated carbons) and f(al)(H) (-CH or -CH2) indicates that the brittle could promote the aliphatic carbon diversifications while ductile deformation could significantly promote the loss of aliphatic carbons. The I-2 (= A(900-700)/A(3000-2800)) gradually increases while the I-3 (= A(1703+1745)/A(1618)) decreases. The comparisons of the D1 peak in TDCs with those in perfect-and defective graphene indicated the existences of the structural defects in TDCs. The ductile TDCs have the lowest D1 peak positions, indicating that ductile deformation can promote the macromolecular disorder degrees to the maximum extent. The gradual increase of I-D1/I-G and the good linear relationship (R-2 = 0.90) between I-D1/I-G and A(D1)/A(G) indicated the gradual increasing content of structural defects with the enhancement of the tectonic deformation. The geological conditions and the Raman results indicated the existences of the Stone-Wales defect (SW) and vacancy-like defects, including the single vacancy (SV), double vacancies (DV), and multi vacancies (MV1 and MV2) in TDCs. The evolution paths of DV (formation energy, E-f: 11.68 eV) -> MV1 (E-f: 9.74 eV) -> MV2 (E-f: 18.34 eV) and initial structures -> SW defects have witnessed the increasing defects and disorder degrees with the enhancement of the tectonic deformation without damaging the continuity of coal matrix structures, which macroscopically manifests as ductile deformation. However, the evolution paths of initial structure -> DV (E-f: 11.68 eV) and initial structure -> SV (E-f: 9.08 eV) have damaged the continuity by missing carbons, which are consistent with the brittle TDCs.