Molecular and electronic structure of Troger's bases

The equilibrium conformations of Troger's base and two new analogue were investigated at the MP2/6-31G(d, p) ab initio level of theory. The electronic structure of these rigid yet sharply folded molecules was investigated by means of their spectroscopic properties. The C-13-NMR chemical shifts, calculated by the CSGT (continuous set of gauge transformations) formalism with the HF-B3LYP/6-311+G(2d, p) hybrid functional model, were fairly consistent with NMR observations. The He(I) photoelectron spectra were measured and interpreted by means of ab-initio many-body OVGF (outer valence Green's function) calculations. The theoretical results satisfactorily reproduced the energies and splittings of the uppermost bands, associated with the two nitrogen lone pair orbitals and the phenyl Tr orbitals. Electron transmission spectroscopy, with the support of calculated pi* virtual orbital energies, was employed to characterize the empty molecular levels. The 2,3,8,9-tetramethoxy analogue of Troger's base exhibits a complex conformational situation. Whereas its NMR spectrum indicates a predominance of trans-trans arrangements of the two o-dimethoxy groups in CDCl3 solution, its photoelectron spectrum should be interpreted in terms of a mixture of the trans-trans and twisted conformers in the gas phase.