TORSION BARRIERS OF END-GROUPS IN CUMULENES .2. RESULTS OF CALCULATIONS AND DISCUSSION

Computational results are presented for the torsion barriers of end-groups in cumulenes, CnH4 up to n=20, obtained by the restricted Hartree-Fock method in a semiempirical approximation of closed and open shell theories. In this approximation we found that the barrier height is equal to 0.2 eV for infinite cumulene with equal CC bond lengths. The origin of this asymptotical barrier is connected with the fact that the field described by the Hückel Hamiltonian with equal CC bond lengths is closer to the SCF Hamiltonian for odd chains than for even ones. The reason why the closed shell theory is inapplicable for cumulene barrier calculations is given, and it is shown that an appearance of alternation of barriers in the extended Hückel theory is wrong. © 1968 Springer-Verlag.