Calculation of the C-13 NMR chemical shift of ether linkages in lignin derived geopolymers: Constraints on the preservation of lignin primary structure with diagenesis

Methodology for the calculation of C-13 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 Den sity Functional Theory employing the generalized gradient approximation. NM R shieldings were calculated using the Individual Gauge for Localized Orbit al Method. Convincing agreement was obtained between calculated and experim entally 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 pow der patterns also exhibit excellent agreement with experimental values. The se quantum mechanical calculations were applied to determine the extent of lignin primary structure preservation with diagenesis. Specifically, the ca lculations were used to assess whether inhomogeneous spectral broadening du e 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-shield ing tensor principle axis values reveals a strong correlation between aniso tropy and asymmetry with local configuration effects such as dihedral rotat ion, phenyl group rotation, and bond angle variation. These results indicat e 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 st ructure of lignin does not persist in kerogens even at relatively low therm al maturities. Copyright (C) 1999 Elsevier Science Ltd.