Structure of Methyl 2-Methylbenzoate: Steric Effect of a Smallortho-Substituent

Abstract

The structure and conformation of methyl 2-methylbenzoate (2) was investigated 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 same result: there is an equilibrium between two conformations around the Car−C(O) bond, the sp (2B) and ap (2C) forms, found in approximately 4:1 abundance (energy difference 3.7 or 2.7 kJ/mol, respectively). This result confirms previous analyses of the steric effects in methyl-substituted benzoic acids and their esters. The ab initio calculations predicted that the minimum energy molecules are those with the planar sp and ap conformations, with torsional angles of 0° and 180°, respectively. On the other hand, the corresponding absolute values found by GED differed by 29° from the values of 0°and 180°, that is, the average absolute torsional angles were found to be 29° and 151°. The difference between the two approaches is discussed. From the other geometrical parameters, some expected deformations were observed, in particular lengthening of the C(1)−C(2) bond and widening of the adjoining angles. In terms of structural organic chemistry, the results imply that a small steric effect doesn’t necessarily distort planarity or result in steric inhibition of resonance, even when the steric effect demonstrably influence other geometrical parameters.