Vibrational analysis as a powerful tool in structure elucidation of polyarsenicals : a DFT-based investigation of arsenicin A

A detailed experimental and theoretical vibrational analysis of arsenicin A, the first polyarsenicals isolated from Nature, is here reported. By exploiting the different and complementary selection rules of the Infrared (IR) absorption and of the Raman scattering transitions and by carrying out Density Functional Theory (DFT) calculations on several candidate structures we arrive to a substantiation of the structure of arsenicin A. We demonstrate that vibrational spectroscopy can be a very useful tool in structure elucidation in cases where Mass Spectroscopy (MS) measurements lead to shifty information and Nuclear Magnetic Resonance investigations are hindered by the presence in the molecule of silent nuclei and/or elements of symmetry. From one side, our approach allows to establish a reliable top-down methodology based on molecular vibrational parameters for the structural elucidation of any unknown organic compounds of medium-small size on which, today, accurate theoretical calculations can be carried out in reasonable times without recourse to great computing resources. From the other side, our investigation points out i) the great potential of Raman scattering spectroscopy, when coupled to a good level of theoretical analysis, to establish the skeleton of structurally undetermined molecules and ii) the better capability of IR absorption spectroscopy to distinguish different atom connectivities inside a given molecular skeleton.