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 C_ar-C(O) bond (1B), with a torsional angle [C(6)-C(1)-C=O] of 32.7(24) degrees as determined by GED and 26.0 degrees from MP2(full)/6-311++G**. Calculations also predict the presence of a second conformer, the anticlinal structure (1C), with φ = 140.0 degrees, 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 degrees 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.