A NONPARAMETRIC DETERMINATION OF THE INTERNAL-ROTATION POTENTIAL AND THE MOLECULAR-GEOMETRY OF O-CHLOROANISOLE USING GAS ELECTRON-DIFFRACTION AND DATA FROM SPECTROSCOPY AND AB-INITIO CALCULATIONS

Structural parameters of o-chloroanisole have been determined using a dynamic model from electron diffraction together with vibrational spec troscopic data and ab initio calculations. A new approach to calculate the nonparametric torsional potential of the methoxy group based on T ikhonov's method of regularization was applied. The nonparametric tors ional potential gives a two-minimum function with respect to the rotat ion energy around the C-O bond. The most stable conformation correspon ds to a planar form with the torsional angle phi = 0 degrees and anoth er to an orthogonal form with phi = 90 degrees. The energy difference between these two conformers is found to be 0.9-1.0 kcal mol(-1) and t he barrier height at phi = 65 degrees is 1.4-1.6 kcal mol(-1). These v alues are in good agreement with those calculated by the ab initio met hod. The main geometrical parameters (r(a) in Angstrom, angle(alpha) i n 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) = 1.3 58(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); angle CCOCCl = 118.7(2.2); angle COCC = 119 .9(2.5); angle COCClC = 121.5(1.1); angle COC = 117.6(2.6); angle COCC l = 119.1(2.1); angle CCO = 124.7(1.2). The structural results are com pared with those obtained for similar compounds. Stereochemical peculi arities in or tho derivatives of anisole which result in the existence of an orthogonal conformer are discussed.