Quantum chemical calculations of linear cumulene chains

Abstract

The molecular geometry of cumulene chains H 2C n H 2 have been optimized using LCAO-SCF MO method (contracted Gaussian basis set of triple zeta valence quality TZVP and the more extended Dunning's correlation consistent basis set aug-cc-PVDZ), density functional theory at different basis set levels, and the highly parametrized MNDO(PM3) method. The bond lengths have no remarkable alternation within the chain and converge to the asymptotic limit as the value of n is getting larger. The total charges calculated by Mulliken population analysis yield unrealistic values if the split valence basis set was employed. In contrast, using NBO analysis the net charges of the individual atoms converge very rapidly to their asymptotic limits with almost zero charges on central atoms. The comparison of the different basis set results to test the reliability of proton affinities calculated from the differences in molecular enthalpies of the parent chain and the corresponding anions was carried out. The increase of the number of C C bonds in the chain decreases these differences asymptotically. The concept of ‘terminal charge repulsion’ is developed for the explanation of energy effects in long chains. The role of this concept in explanation of dependence of proton affinities on the chain length was discussed. The studied compounds are the best available building blocks in bimetallic compounds with useful properties in molecular electronics and nonlinear optics.