Structure of methyl 9-methylbenzoate: Steric effect of a small ortho-substituent

The structure and conformation of methyl 2-methylbenzoate (2) was investiga ted by ab initio calculations carried out at the MP2(full)/6-311++G** level of theory, and by gas electron diffraction (GED). Both methods gave the sa me result: there is an equilibrium between two conformations around the C-a r-C(O) bond, the sp (2B) and ap (2C) forms, found in approximately 4:1 abun dance (energy difference 3.7 or 2.7 kJ/mol, respectively). This result conf irms previous analyses of the steric effects in methyl-substituted benzoic acids and their esters. The ab initio calculations predicted that the minim um energy molecules are those with the planar sp and ap conformations, with torsional angles of 0 degrees and 180 degrees, respectively. On the other hand, the corresponding absolute values found by GED differed by 29 degrees from the values of 0 degrees and 180 degrees, that is, the average absolut e torsional angles were found to be 29 degrees and 151 degrees. The differe nce between the two approaches is discussed. From the other geometrical par ameters, some expected deformations were observed, in particular lengthenin g of the C(1)-C(2) bond and widening of the adjoining angles. In terms of s tructural organic chemistry, the results imply that a small steric effect d oesn't necessarily distort planarity or result in steric inhibition of reso nance, even when the steric effect demonstrably influence other geometrical parameters.