Does 2-Methylacetophenone Comply with Steric Inhibition of Resonance? A Direct Experimental Proof of Its Nonplanar Conformation from a Joint Ab Initio/Electron Diffraction Analysis

The structure and conformations of 2-methylacetophenone (1) have been investigated by ab initio calculations carried out at the MP2(full)/6-311++G** level and by gas electron diffraction (GED). According to both methods, 1 exists predominantly as a form with the CO bond synclinal with respect to the Car−C(O) bond (1B), with a torsional angle [C(6)−C(1)−CO] of 32.7(24)° as determined by GED and 26.0° from MP2(full)/6-311++G**. Calculations also predict the presence of a second conformer, the anticlinal structure (1C), with ϕ = 140.0°, with an abundance of less than 6%, an amount hardly detectable by GED. Different DFT computational protocols both support a nonplanar form of the predominant conformer (B2PLYP) and are in contradiction (B3LYP, M052x, B98, B97-D) with this experimental finding. The GED results, supported by the calculations that involve long-range correlation, are in a good agreement with 13C NMR spectroscopic investigations, UV spectra, and dipole moment studies. However, previous claims that assumed steric inhibition of resonance caused by a significantly nonplanar conformation with ϕ close to 90° have been disproved. Steric crowding is evident from the geometrical parameters, particularly from the C(1)−C(2) bond length and from the C(1)−C(2)−C(H3) and C(2)−C(1)−C(O) bond angles. It is concluded that any explanation of reactivity by steric inhibition of resonance and by other steric factors must be supported by experimental and/or theoretical investigation of the actual molecular shape.