Journal of the American Chemical Society p. 6506 - 6515 (1982)
Update date:2022-08-11
Topics:
Branca, Mario
Gamba, Aldo
Barzaghi, Mario
Simonetta, Massimo
The barrier to rotation about the carbonyl carbon-phenyl carbon bond of 3-nitrobenzaldehyde anion radical in N,N-dimethylformamide solution has been determined by a detailed line-shape analysis of the corresponding electron paramagnetic resonance (EPR) spectra at different temperatures.At 25 deg C the cis conformation is favored by 0.98 kcal mol-1 in free energy, and the free-energy barrier to the cis-trans conversion amounts to 9.35 kcal mol-1.Both enthalpy and entropy contributions to the free-energy profile are accurately evaluated (ΔH<*> = 3.91, ΔH0 = -1.13 kcal mol-1; ΔS<*> = -18.2, ΔS0 = -7.1 cal mol-1 K-1).The effects of ionic association and solvation on the barrier are discussed.The EPR spectra of 4-nitrobenzaldehyde and 2-nitrobenzaldehyde anion radicals are also reported as a function of temperature.In these cases, the barriers to internal rotation are too high to be surmounted on the EPR time scale.STO-3G ab initio MO calculations are given for benzaldehyde, 3-nitrobenzaldehyde, 4-nitrobenzaldehyde, and the corresponding anion radicals.The spin distribution in the anion radicals is calculated by McLachlan and INDO methods.
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