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.
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.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.