Abstract:
Methodology for the calculation of 13C NMR shieldings on molecular organic fragments, representative of monomers in a type III kerogen, is presented. Geometry optimization of each molecular fragment was carried out using Density Functional Theory employing the generalized gradient approximation. NMR shieldings were calculated using the Individual Gauge for Localized Orbital Method. Convincing agreement was obtained between calculated and experimentally derived isotropic chemical shielding values over a broad frequency range. Shielding calculations employing the localized orbitals/local origin method resulted in nearly identical results. NMR chemical shift static powder patterns also exhibit excellent agreement with experimental values. These quantum mechanical calculations were applied to determine the extent of lignin primary structure preservation with diagenesis. Specifically, the calculations were used to assess whether inhomogeneous spectral broadening due to both functional group variation and local configurational variability may inhibit the detection of otherwise significant quantities of alkyl-aryl ethers in lignin derived geopolymers. Determination of the chemical-shielding tensor principle axis values reveals a strong correlation between anisotropy and asymmetry with local configuration effects such as dihedral rotation, phenyl group rotation, and bond angle variation. These results indicate that a range of 9 ppm in the isotropic chemical shift can be ascribed to local configuration. Consequently, an upper limit of 5% alkyl-aryl-linkages may go undetected using NMR spectroscopy on lignin-derived geopolymers at the liginite-sub-bituminous transition. It is concluded that the primary structure of lignin does not persist in kerogens even at relatively low thermal maturities.