Molecular and electronic structure of the dehydroalanine derivatives : The cyclic dipeptide

The molecular structure of the cyclic dipeptide of dehydroalanine(3,6-dimethylene-piperazine-2,5-dione) has been determined from three-dimensional X-ray data. C₆H₆N₂O₂ is monoclinic, space group P2₁, with Z = 2 in a cell of dimensions a = 5.261(3), b = 13.617(4), c = 4.570(3) Å, β = 104.58(5)°. Molecules are planar and they are linked in ribbons by two hydrogen bonds through the amido-groups. The vapour-phase He(I) and He(II) photoelectron spectra are also presented. The assignments are proposed by comparison with simpler related molecules and supported by ab initio calculations. Analogies with respect to the photoelectron results of the corresponding acyclic compound N-acetyldehydroalanine are discussed.