Applications of natural orbitals for chemical valence in a description of bonding in conjugated molecules

Natural orbitals for chemical valence (NOCV) were used to describe bonding in conjugated π-electron molecules. The ‘single’ C–C bond in trans-1,3-butadiene, 1,3-butadiene-1,1,4,4-tetra-carboxilic acid, 1,3,5,7-octatetraene, and 11-cis-retinal was characterized. In the NOCV framework, the formation of the σ-bond appears as the sum of two complementary charge transfer processes from each vinyl fragment to the bond region, and partially to the other fragment. The formation of the π-component of the bond is described by two pairs of NOCV representing the transfer of charge density from the neighboring ‘double’ C–C bonds. The NOCV eigenvalues and the related fragment-fragment bond multiplicities were used as quantitative measures of the σ- and π- contributions. The σ-component of the ‘single’ C-C bonds appears to be practically constant in the systems analyzed, whereas the π-contributions increase from butadiene (ca. 7.5%) to retinal (ca. 14%).