Equatorial piperidine and the piperidine–water complex.: Rotational spectra and molecular structures Original Research Article

The rotational spectra of all mono-substituted 13C-isotopomers of equatorial piperidine and of the piperidine–water complex were analyzed and their rotational, centrifugal distortion, and quadrupole coupling constants were derived. Further a partial rS-structure of piperidine was determined and compared with an optimized structure on MP2/6-31G** level. The observed spectrum of the piperidine–water complex is in agreement with two possible structures. Only one of them is supported by dipole moment estimates from line intensities and is consistent with calculations on MP2/6-31G** level. In this structure the N⋯H–O hydrogen bond and the water molecule are located in the symmetry plane of piperidine. The free water proton is directed into the same direction as the amino proton. Common assumptions like unchanged structures of monomers, linear hydrogen bond, and symmetry restrictions were tested by ab initio calculations.