Electronic structure of a rigid cyclic diamide

Interest in the electronic structure of amides and polypeptides arises in part because optical spectroscopy such as electronic circular dichroism (CD) may be readily used to monitor conformational changes of peptides and proteins in solution. Our current, albeit somewhat incomplete, understanding of the relationship between polypeptide conformation and CD spectra has come in part from experimental and theoretical studies of model systems such as cyclic diamides. In this paper, we study the interactions between amide groups in a bridged cyclic diamide, diazabicyclo[2.2.2] octane-3,6-dione, through the measurement of its infrared (IR) and ultra-violet (UV) absorption and electronic CD spectra and through ab initio calculations of its electronic structure. The particular rigidity of the bridged cyclic diamide removes ambiguity regarding the conformation of the molecule in solution, which must remain close to the structure determined by X-ray crystallography. The theoretical study has been performed using the complete-active-space self-consistent-field and multiconfigurational second-order perturbation theory methods. Solvent effects have been modeled using a self-consistent reaction field. The results of the electronic structure calculations agree well with the observed spectra and allow the origin of the bands to be assigned. The electronic spectrum is dominated by an intense pi (nb)pi* transition, which is calculated to lie at 6.28 eV in the gas phase and 6.07 eV in solution. In addition, a number of less intense n pi* and pi (nb)pi* transitions are characterized. The results show that the electronic CD spectrum arises from n pi* and pi (nb)pi* transitions that can be considered to be localized on one amide group, while charge-transfer transitions do not make a significant contribution to the electronic CD spectrum.