The rate constants for ring inversion (k r.i.) and bond shift (k b.s.) in 1 and 2 were determined by dynamic NMR spectrometry while the rate constants for bond shift and intramolecular charge transfer (k c.t.) were determined for 1 2-/2K+ and 2 2-/2K+. These processes were modeled by HF/3-21G(*) ab initio molecular orbital calculations of the ground states and of several transition states for 3, 4, 3 2-, 4 2-, 3 2-/2K+, and 4 2-/2K+. The results indicate that k r.i. and k b.s. are ca. 2.5 times greater (at 240 and 280 K, respectively) for 2 compared to 1 due to larger steric repulsions in the ground state of 2. Contrariwise, k b.s. and k c.t. are 1.7 and 166 times greater, respectively, at 280 K for 1 2-/2K+ than for 2 2-/2K+. These differences are attributed to less twisting and therefore greater π delocalization between the cyclooctatetraenyl rings and the aryl ring in the bond shift and charge-transfer transition states of 1 2- compared to 2 2-. The greater difference between 1 2- and 2 2- for k c.t. compared to k b.s. is postulated to result from looser ion pairing in the charge-transfer transition state relative to the bond shift transition state.

1,4-Dicyclooctatetraenyl-1,3-butadiyne and p-, m-, and o-di(cyclooctatetraenylethynyl)benzene (1−4, respectively) and their dipotassium salts (1 2-−4 2-) in THF-d 8 have been synthesized and studied by dynamic NMR spectroscopy. Rate constants for bond shift (k BS) in the neutral cyclooctatetraene (COT) rings of 1−4 and 1 2-−4 2- and for intramolecular charge (electron and cation) transfer (k CT) between the dianion and the neutral COT rings in 1 2-−4 2- have been determined. 13C NMR chemical shifts and AM1 π-charges as well as the values of k CT are interpreted on the basis of a stronger through-bond interaction between the COT rings in the order 1 2- ≫ 2 2- ≥ 4 2- > 3 2-. Similarly, k BS decreases in the order 1 2- ≫ 4 2- > 2 2- > 3 2- on going from 1−4 to the corresponding dianions. Analysis of the AM1 π-charge densities suggests that these differences are primarily due to a through-bond effect with an additional through-space contribution from the electric field of the dianion ring for bond shift in 4 2-.

The free energies of activation for bond shift in the carbon group substituted cyclooctatetraenes (COT−M(CH3)3) in THF-d 8 at 298 K have been determined to be 16.4, 16.2, 16.2, and 18.1 kcal/mol for M = Si, Ge, Sn, and C, respectively, and 15.6 kcal/mol for CH3−COT. These data permit an interpretation of the previously reported opposite orders for the ease of the first and second electrochemical reductions in the Si, Ge, and Sn compounds. It is postulated that the order of the first reduction potential is controlled by a decrease in overlap between the substituent and the ring π orbitals in the order Si > Ge > Sn, whereas the second reduction potential is controlled by the energy gap between the symmetric π HOMO of the COT radical anion and an interacting substituent σ* orbital of π symmetry (εσ * π − επ), which increases in the order Sn < Ge < Si. HF/3-21G molecular orbital calculations indicate that the high barrier for t-Bu−COT primarily reflects steric effects in the transition state.