Theoretical studies on stable geometries of C₇₈O₆ based on C₂v-C₇₈

Abstract

The semi-empirical AM1 and INDO/CIS methods as well as density function theory have been used to study equilibrium geometries and spectroscopic properties of the possible isomers of C₇₈O₆ based on C₂v-C₇₈. The most stable geometry at the ground state of C₇₈O₆ is found to be 28, 29, 30, 31, 52, 53, 70, 71, 73, 78, 75, 76-C₇₈O₆ (A). The C(70)-C(71) and C(75)-C(76) bonds are opened up to form annulene-like structures, whereas the other four bonds are not broken and show epoxy features in C₇₈O₆ (A). The oxygen atoms added tend to approach the sites vacated by the previous oxygen atoms since the conjugation system in C₂v-C₇₈ is destroyed at these sites. Based on the B3LYP/6-31G optimized geometries, the main absorptions in the electronic spectra and the stretching vibrations of the C=C bonds in the IR spectra of C₇₈O₆ isomers are blue shifted as compared with those of C₂v-C₇₈ calculated using the INDO/CIS and AM1 method, respectively. The chemical shifts of the carbon atoms in the epoxy and annulene-type structures of C₇₈O₆ isomers, calculated at B3LYP/6-31G level, move upfield.