Electronic excitation and ionisation of 1-azabicyclo[2.2.2]octane (ABCO): evidence for a twisted structure from experiment and theory

The first excited neutral and ground cationic states of 1-azabicyclo[2.2.2]octane (ABCO) have been studied using a combination of (1 + 1′) REMPI and ZEKE spectroscopy, supported by ab initio molecular orbital calculations. In common with the related tertiary alkyl amines, analysis of both REMPI and ZEKE spectra indicate that the overriding impulse upon electronic excitation and ionisation is for a flattening of the nitrogen end of the molecule, albeit hindered by the cage. In addition, the activity of the torsion mode (19) in both REMPI and ZEKE spectra suggests geometry changes along the torsion coordinate between the three electronic states sampled in these experiments. Ab initio molecular orbital calculations conducted at the MP2/cc-pVDZ level of theory yield minimum energy geometries with CNCC dihedral angles of 6.011° and 7.86° for the S0 and D0 states, respectively, which indicates that the torsion coordinate is described by a double minimum potential. However, the degree of perturbation to the vibrational eigenvalues suggests that the zero point level lies a few wavenumbers above a small barrier and that ABCO is pseudo-twisted with a vibrationally averaged C3v symmetry.