An MNDO SCF MO investigation of the electronic structures and conformational preferences of model polycarbonate systems

Theoretical calculations within the MNDO SCF MO formalism have been carried out to investigate the relative heats of formation, conformation preferences and excitation energies of model polycarbonate systems. It is shown that the trans-trans and cis-trans configurations of diphenyl carbonate (DPC) are of comparable energy and have a strong energetic preference for orthogonal forms. The energy barrier for the interconversion of trans-trans and cis-trans orthogonal forms is small. Comparison of the electronic structure of DPC with simple carbonyl, carboxy ester and carbonate systems indicates that the nature of the first excited singlet of polycarbonate is of $\text{π → π}{}^1$ character in contrast to the $\text{n → π}{}^1$ assignment which has been described in the literature.