Theoretical and experimental 1H, 13C and 15N NMR spectra of monomethyl substituted tetramethyl 9a H- and 4 H-quinolizine-1,2,3,4-tetracarboxylate

Abstract

The theoretical 1H, 13C and 15N NMR spectra were calculated at the Hartree–Fock level by using the polarized double zeta (PDZ) basis set of Hansen and Bouman for the geometries optimized at the same level. The theoretical chemical shifts were calculated by subtracting the isotropic shielding calculated for given quinolizine nuclei from that calculated for the reference molecules. For monomethyl substituted tetramethyl 4 H-quinolizine-1,2,3,4-tetracarboxylate, the calculated spectra fit fairly well the experimental NMR data, whereas some experimental NMR signals of 9a H-tautomers are not reproduced correctly in calculations. This discrepancy is interpreted in terms of conformational differences between frozen structures of molecules calculated ‘in vacuum’ and flexible molecules measured in polar medium.