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

The molecular structure of the cyclic dipeptide of dehydrophenylalanine [3,6-bis(phenylmethylene)-piperazine-2,5-dione] has been determined from three dimensional X-ray data. C₁₈H₁₄ N₂O₂ is monoclinic, space group C2/$\text{c}$, with Z = 12 in a cell of dimensions $\text{a}$=4Ø.774(1), $\text{b}$=6.237(2), $\text{c}$=17.731(3) Å, β=107.76(5)°. Molecules are approximately planar as far as the piperazinedione ring is concerned, and they are linked in two series of hydrogen-bonded ribbons. The vapour phase He(I) and He(II) photoelectron spectra are also presented. Their assignment is proposed by comparison with related molecules and supported by semiempirical quantum mechanical calculations. Analogies and differences with respect to the photoelectron results of the cyclic dipeptide of dehydroalanine and corresponding acyclic compounds are discussed.