A Nonparametric Determination of the Internal Rotation Potential and Molecular Geometry of O-Chloroanisole Using Gas Electron Diffraction and Data from Spectroscopy and Ab Initio Calculations

J.Mol.Struct., v. 376. p. 173-181 (1996)

ABSTRACT. Structural parameters of o-chloroanisole have been determined using a dynamic model from electron diffraction together with vibrational spectroscopic data and ab initio calculations. A new approach to calculate the nonparametric torsional potential of the methoxy group based on Tikhonov's method of regularization was applied. The nonparametric torsional potential gives a two-minimum function with respect to the rotation energy around the C-O bond. The most stable conformation corresponds to a planar form with the torsional angle Д = 0 degrees and another to an orthogonal form with Д = 90 degrees. The energy difference between these two conformers is found to be 0.9-1.0 kcal/mol and the barrier height at Д = 65 degrees is 1.4-1.6 kcal/mol. These values are in good agreement with those calculated by the ab initio method. The main geometrical parameters (ra in A, angle(a) in degrees and errors given as three times the standard deviation from least squares are as follows: r(C-C)(mean)=1.398(4); r(O-C(Ph))=1.358(36); r(O-C(Me))=1.426(21); r(O-Cl)=1.733(4); r(C-H)(Ph)=1.086(6); r(C-H)(Me)=1.095(6); C-C(O)-C= 118.7(2.2); C(O)-C-C=119.9(2.5); C(O)-C(Cl)-C=121.5(1.1); C-O-C=117.6(2.6); C(O)-C-Cl=119.1(2.1); C-C-O=124.7(1.2). The structural results are compared with those obtained for similar compounds. Stereochemical peculiarities in ortho derivatives of anisole which result in the existence of an orthogonal conformer are discussed.

Laboratory of Electron Diffraction