Isomerization Effects on Chemical Shifts and Spin-Spin Coupling Constants of Polyacetylene Chains: A GIAO-DFT Study

Using the gauge-including atomic orbitals approach with B3LYP exchange-correlation functional in combination with the 6-311++G(2d,2p) basis set, we have calculated the 13 C chemical shifts [Delta sigma] and (13)C-(13)C indirect spin-spin coupling constants [J(C,C)] for the series of isolated planar polyacetylene chains C(2n)H(2n+2), from n = 2 up to n = 11. For both cis and trans isomers, infinite polymer values can be estimated from converged chemical shifts in the central unit of each isomer. The theoretical model predicts the chemical shift difference between the cis and trans forms of 10.28 ppm, in very good concordance with experimental result. The (1)J(C-C) and (1)J(C=C) calculated values are more geometry-dependent but they exhibit notable regularity typical for such systems as the size of chain is increased. Variations between (1)J(C-C) and (1)J(C=C) in the central unit of the C(22)H(24) chain are estimated around 11 Hz, independent of isomeric form. It is found for both (1)J(C-C) and (1)J(C=C) that variations between the cis and trans forms are in the range of 3-4 Hz, indicating also a distinction of the isomeric form. In addition, our results show that the presence of a structural change on these conjugated backbones has marked influence on the chemical shifts. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 1616-1625, 2011