Abstract:Configuration interaction, vibronic coupling and thermal population of excited states are all expected to occur in the alkyl cyclooctatetraene anions. The importance of these effects in determining the electron resonance spectra of these species is investigated theoretically and the results compared with experiment. It is concluded that vibronic coupling plays an important role.
“…The eight equivalent protons have a coupling constant of −3.209 G, which is well within the range expected for a planar anion radical. , Calculation 4 and experiment ,,, also agree that, unlike the tub-shaped neutral COT, the radical anion (COT -• ) is planar. The necessary Jahn-Teller distortion from D 8 h symmetry comes from a b 2g distortion leading to a pair of rapidly interconverting D 4 h equilibrium geometries with alternating bond lengths (Figure ) 1 Pictorial representation of the Jahn-Teller distortion of D 8 h COT anion radical to the bond length alternating and bond angle alternating D 4 h geometries.…”
A series of new alkoxycyclooctatetraene ethers was synthesized and their anion radicals generated in
hexamethylphosphoramide. The EPR measured spin densities can be explained in terms of the odd electron
occupying MOs, Ψ4‘ = [1/2√(n
2 + 1)](nψ1 + ψ2 − nψ3 − ψ4 + nψ5 + ψ6 − nψ7 − ψ8) and Ψ5‘ =
[1/2√(n
2 + 1)](nψ1 − ψ2 − nψ3 + ψ4 + nψ5 − ψ6 − nψ7 + ψ8), where n increases with the electron-releasing nature of the alkoxy group. The anion radicals of n-propoxycyclooctatetraene and ibuprofoxycyclooctatetraene exhibit a small (ca. 0.35 G) splitting for the γ proton on the sp3 hybridized carbon. Further,
the EPR pattern for the tert-butoxycyclooctatetraene anion radical is consistent with a relatively smaller value
for n (in Ψ4‘ and Ψ5‘). These observations are consistent with a rather unusual homohyperconjugative interaction
involving overlap of the σ-bond to the γ-hydrogen and the π-system of the conjugated COT ring.
“…The eight equivalent protons have a coupling constant of −3.209 G, which is well within the range expected for a planar anion radical. , Calculation 4 and experiment ,,, also agree that, unlike the tub-shaped neutral COT, the radical anion (COT -• ) is planar. The necessary Jahn-Teller distortion from D 8 h symmetry comes from a b 2g distortion leading to a pair of rapidly interconverting D 4 h equilibrium geometries with alternating bond lengths (Figure ) 1 Pictorial representation of the Jahn-Teller distortion of D 8 h COT anion radical to the bond length alternating and bond angle alternating D 4 h geometries.…”
A series of new alkoxycyclooctatetraene ethers was synthesized and their anion radicals generated in
hexamethylphosphoramide. The EPR measured spin densities can be explained in terms of the odd electron
occupying MOs, Ψ4‘ = [1/2√(n
2 + 1)](nψ1 + ψ2 − nψ3 − ψ4 + nψ5 + ψ6 − nψ7 − ψ8) and Ψ5‘ =
[1/2√(n
2 + 1)](nψ1 − ψ2 − nψ3 + ψ4 + nψ5 − ψ6 − nψ7 + ψ8), where n increases with the electron-releasing nature of the alkoxy group. The anion radicals of n-propoxycyclooctatetraene and ibuprofoxycyclooctatetraene exhibit a small (ca. 0.35 G) splitting for the γ proton on the sp3 hybridized carbon. Further,
the EPR pattern for the tert-butoxycyclooctatetraene anion radical is consistent with a relatively smaller value
for n (in Ψ4‘ and Ψ5‘). These observations are consistent with a rather unusual homohyperconjugative interaction
involving overlap of the σ-bond to the γ-hydrogen and the π-system of the conjugated COT ring.
“…Since there is only one electron in the degenerate antibonding molecular orbitals of planar [18] -. , there is (as in the case of [6]annulene) a necessary J−T distortion from D 6 h symmetry that leads to two different structures. 2a, Group theory says that one will have D 2 h symmetry and the other will have C 2 h symmetry, and they are both found on the same vibronic potential energy surface (“Mexican hat,” e.g., Figure ). …”
The anion radicals of [18]annulene and of [6]annulene undergo Jahn−Teller distortions from the D
6
h
conformations of their respective neutral molecules. The conformation of lowest energy for the anion radical
of benzene has a C2
v
symmetry, and that for the anion radical of [18]annulene is D
2
h
. The very rapid pseudo-rotation of the benzene C2
v
system causes the six protons of the benzene anion radical to appear as if they
were equivalent in the EPR experiment. Fast pseudo-rotation rendering the uniform distribution of spin density
in the [18]annulene anion radical system is prevented by the nonplanarity of the radical. Hence, the EPR
spectrum of the [18]annulene anion radical must be interpreted in terms of a static (on the EPR time scale)
D
2
h
conformation, which pseudo-rotates over a C
2
h
transition state. B3LYP calculations support these
conclusions.
“…2, 2a), while the second is mainly a CCH in-plane bending (Fig. 2, 2b), The result of each of these vibronic distortions is to reduce the molecular syrrmetry to D4h• Moss 7 has argued that the CC stretching mode of b2g symmetry is the most likely to cause vibronic effects, because it strongly affects the CC bond integrals. This sytem 13 is rather similar to the four-membered ring discussed by Herzberg.…”
mentioning
confidence: 98%
“…The Jahn-Teller-active vi brat ions of COT 7 include modes of symmetry types b 1 g and b2g• These are illustrated in Fig. 2.…”
mentioning
confidence: 99%
“…15 ESR spectrum of CH3-cor 7 Top: experimental. Bottom: computer simulated.Approximate carbon or hydrogen motions for the b 1 g and ~g vibrational modes.INDO potential energies vs. b 2 g CC stretching distortion for cor:, CH 3 -cor~ and cH 3 o-cor: (a) cor 7(b) cH 3 -cor7 (c) CH30-cor:…”
The radical anions of methyl-, methoxy-, fluoro-, cyclopropyl-, and cyanocyclooctatetraene were generated by electrolytic reduction of the neutral compounds in liquid ammonia. The electron spin resonance spectrum of each radical anion is assigned, and a clean even-odd alternation of the spin densities is observed in each. This alternation can be correlated with the electronic properties of the substituent. For the radical anions of methyl-, methoxy-, and cycl opropyl cycl ooctatetraene, the temperature dependence of the coupling constants is reported. The results are accounted for in terms of vibronic mixing and Jahn-Teller distortions, and are discussed in relation to INDO calculations.
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