Dipole moments of methyl- and trifluoromethyl-substituted methyl benzoates

Abstract

Apparent dipole moments in benzene of 1.3-bistrifluoromethylbenzene and of the methyl esters of 2,4-dimethyl-, 2,5-dimethyl-, 3,5-dimethyl-, 2,4-bistrifluorome~hyl-, 2,5-bistrifluoromethyl-, 3,5-bistrifluoromethyl-, and 3trifluoromethyl-benzoic acid have been determined. The values obtained can be explained either by rotation of the ester group out of the plane of the ring or, in the case of the 3-, and 2.4-substituted esters, by the existence of cis-and trans-isomers.

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

“…The results at the MP2(full)/6-311ϩϩG** level unambiguously prefer an equilibrium of two planar conformations sp (2B, φ ϭ 0°) and ap (2C, φ ϭ 180°), with the geometrical parameters as given in Table 1. (11)ϪO (10) From the energy difference of 3.7 kJ/mol (Table 1), the population of the sp rotamer 2B can be estimated to be 82%, in fair agreement with the previous estimate of 75% which follows from the dipole moments. [17] A value of 4.9 kJ/mol was obtained [19] for the parent acid 1B and 1C at the RHF/6-31ϩG** level.…”

“…More recently, this theory was used for interpreting the NMR chemical shifts, [5,6] dissociation constants, [2,7] electronic spectra, [6,8] and common reactivities [4] of alkyl-substituted benzoic acids; it was further applied to the properties of their esters, such as NMR shifts, [5,8] electronic spectra, [9] IR spectra, [7,10] and dipole moments. [11] The values of φ were calculated from various experimental quantities and were also supported by lowlevel theoretical calculations. [5,12] The variable angle φ served as a generally accepted explanation of stronger or weaker effects in individual compounds.…”

“…Previous direct experimental studies of 2 in solution reached conflicting conclusions; either a nonplanar [4,8] or planar [10] conformation was preferred, or both possibilities were allowed. [11,17] In this communication, we determined the conformation of 2 as an isolated molecule by the concerted use of the gas-phase electron diffraction technique and by ab initio calculations at the MP2(full)/6-311ϩϩG** level, augmented by a Monte Carlo conformational search. [20] Results and Discussion…”

Infrared spectra and conformation of methyl-substituted benzoic acids

“…Although no reasonable band separation was achieved even in this case, the measurements of asymmetry are convincing and the compounds can be classified safely ( Table 2). All compounds with two conformers (7)(8)(9)(10)(11)(12)(13)(14) have AF ! 4% and a !…”

“…1,2 Most studies have identified the steric effect with the steric hindrance to resonance and its intensity has been assumed to depend on the torsional angle f = O=C-C=C. For instance, the NMR chemical shifts, 3,4 the dissociation constants in water, 5-7 the electronic spectra 4,7 and the dipole moments of esters 8 were explained in terms of the value of the angle f, which increases with steric hindrance. Some typical values of f are given in Table 1.…”