DFT critical points of the internal rotation potential energy surface and infrared spectra of the [1H], [2H] 2,5-pyrazinedicarboxamide

The 2,5-pyrazinedicarboxamide (PDCA), and its 2H isotopic derivative were prepared by synthesis, their infrared spectra being recorded and assigned. Within the characterization scheme, structures and spectroscopy of these compounds, in this work we present DFT quantum chemical calculations of the critical points of the internal rotation potential energy surface (in phase gas as a simplified modelization of condensed phase) gaussian 98 package has been used with the basis set 6-311G** and B3P86 correlation exchange functional. The most stable conformation of the isolated molecule corresponds to that in what both amide groups are in the plane of the pyrazine ring, presenting intermolecular hydrogen bridges between carboxylic oxygens and contiguous hydrogens of the pyrazine ring. The molecule in this conformation shows C2h symmetry. Two series of critical points have been calculated: (i) those coming from the rotation of one amide group around C-C bond, keeping the other in the most stable planar conformation; and (ii) the same rotation, but keeping the other amide group in position of secondary minimum conformation of the previous series.