Microwave Spectrum and Intramolecular Hydrogen Bonding of Propargyl Selenol (HC≡CCH2SeH)

Abstract

The microwave spectrum of propargyl selenol, HC[triple bond]CCH(2)SeH, has been investigated in the 19-80 GHz spectral range at -30 degrees C. The spectra of five isotopologues of one conformer, which has a synclinal conformation for the C-C-Se-H link of atoms, were assigned. This conformational preference allows for the formation of a weak intramolecular hydrogen bond between the hydrogen atom of the selenol group and the pi electrons of the triple bond. This hydrogen-bonded conformer is at least 5 kJ/mol more stable than the rotamer having an antiperiplanar conformation for the C-C-Se-H chain. The microwave spectrum clearly shows that the hydrogen atom tunnels between the two mirror-image synclinal forms, but it was not possible to determine the tunneling frequency. The microwave study has been augmented by quantum chemical calculations at the MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ levels of theory. These calculations predict rotational and centrifugal distortion constants that are in good agreement with the experimental counterparts but predict a far too small energy difference of approximately 1.5 kJ/mol for the energy difference between the antiperiplanar and synclinal forms. The conformational properties and internal hydrogen bonding of HC[triple bond]CCH(2)SeH are compared with similar properties of other selenols, which are stabilized by intramolecular hydrogen bonds.