Rotational spectra of two six-membered heterocyclic N-methyl-piperidinol compounds: Conformations by OH rotation, N-methyl inversion, and ring puckering

Rotational spectra of two nitrogen containing six-membered heterocycles which are commonly used in syntheses of pharmaceuticals, namely, N-methyl-3-piperidinol (NMP3) and N-methyl-4-piperidinol (NMP4), were measured using a broadband chirped pulse and a cavity based Fourier transform microwave spectrometer. The possible conformers due to the OH rotation, N-methyl inversion, and ring puckering were investigated theoretically for these two heterocycles. The substituent position of the hydroxyl group with respective to the N atom in the heterocyclic ring has a strong influence on the preferred conformations. While one dominant conformer, favoring the OH⋯N close contact, was predicted for NMP3, several close energy conformers with OH pointing at different directions were predicted for NMP4. In contrast, only one conformer was identified for each compound experimentally. The ¹⁴N nuclear quadrupole hyperfine structures were observed for all rotational transitions and analyzed. In addition, rotational spectra of all ¹³C and ¹⁵N isotopologues of NMP4 were studied in their natural abundance, leading to a definite identification of the NMP4 conformer observed. The differences in the conformational landscapes and the OH motions in the two compounds are presented and also discussed in the context of the 1,3-diaxial interaction rule commonly used in organic chemistry.