Electronic structure and conformational flexibility of D-cycloserine

The first comprehensive investigation of the role played by the conformational flexibility of gaseous D-cycloserine in the valence and core electronic structures is here reported. The seven most stable conformers among the twelve structures calculated at MP2/6-311++G** level of theory were assumed to properly describe the properties of the investigated compound. Taking into account 10 the contribution of these isomers, the valence photoelectron spectrum (UPS) was simulated by the Outer Valence Green‘s Function (OVGF) method. A different sensitivity to the conformational flexibility of the outermost photoelectron bands was exhibited in the simulated spectrum. The comparison of the theoretical UPS with the experimental one allowed a detailed assignment of the outermost valence spectral region. The composition and bonding properties of the relevant MOs of the most stable conformers were analyzed in terms of leading Natural Bond Orbitals (NBOs) contributions to the HF/6-311++G** canonical MOs. The C1s, N1s, and O1s photoelectron spectra (XPS) were theoretically simulated by calculating the vertical Ionization Energies (IEs) of the relevant conformers using the SCF approach. The different IE chemical shift spread of the XPS components associated to the various conformers, which is expected to affect the experimental spectra, could be evaluated in the simulated XPS, thus providing new insight into the core electronic structure. The comparison of the theoretical results with the experiment unraveled that the atomic XPS components are not mixed by the D-cycloserine conformational flexibility, and that the specific vibronic structure of the different spectral components should play a crucial role in determining the different relative intensities and band shapes observed in the experiment.