On the structure and spectroscopic properties of bispidine, N, N′-dimethylbispidine and a bis-bispidine macrocycle

The equilibrium conformations of bispidine and its N, N′-dimethyl derivative were investigated with the ab initio MP2 formalism and the density functional theory (DFT) ansatz B3LYP. For bispidine, the chair-chair (cc) (exo, endo) is the most stable conformer. On the contrary, for its N, N′-dimethyl derivative, the energy difference between the cc and chair-boat (cb) conformers is small, allowing for conformational mobility of the bicyclo rings. For tetraaza-macrocycle with four nitrogen lone pair orbitals (LPOs) pointing into the molecular cavity, the C_2h structure with stepped ethylene bridges is preferred over the twisted structure of D₂ symmetry. Mono- and di-protonated forms of this base are characterized by intramolecular asymmetric hydrogen bonds. All the theoretical conformational predictions are consistent with the available experimental results. The NMR chemical shifts, calculated by the continuous set of gauge transformations (CSGT) formalism with the DFT/B3LYP method, are fairly consistent with NMR observation. The He(I) photoelectron (PE) spectra were measured and assigned by means of ab initio outer valence Green's function (OVGF) calculations. The predominance of the lowest energy conformation of bispidine and tetraaza-macrocycle is supported by the spectroscopic results. The conformational situation for N, N′-dimethyl-bispidine is more complex. Whereas its NMR spectrum indicates a predominance of the cc conformer in THF-d₈ solution, the vibrational and PE spectra should be interpreted in terms of a mixture of the cc and cb conformers in the neat liquid and gas phase.