Kinetics and Thermodynamics of Restricted Rotation of the Formyl Group in Nitrobenzaldehyde Anion Radicals

Article abstract of DOI:10.1021/ja00388a005

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.