A NEW APPROACH TO THE NONPARAMETRIC DETERMINATION OF INTERNAL-ROTATION POTENTIAL FROM ELECTRON-DIFFRACTION DATA AS APPLIED TO REINVESTIGATION OF THE MOLECULAR-STRUCTURE OF M-BROMONITROBENZENE

A nonparametric numerical algorithm is suggested for calculation of the torsional potential from gas electron diffraction data using Tikhonov's method of regularization. A new approach to determine the admissible interval of the regularization parameter using Hamilton's statistical test has been considered. Applied to this reinvestigation of m-bromonitrobenzene it was confirmed that this algorithm only allows the determination of the form of the torsional potential function near its minimum. The barrier to internal rotation was estimated to be 4.6-5.4 kcal mol(-1) as interpolated by Fourier series with proper parameterization. The most important structural parameters (r(a) in Angstroms, angle alpha in degrees) are: r(C-C)(av) = 1.399(3), r(C-N) = 1.459(16), r(N=O) = 1.244(3), r(C-Br)= 1.884(6), r(C-H) = 1.099(20), angle CCNC = 123.9(1.4), angle CNCCBr = 116.8(1.5), angle CNCC = 116.6(1.9), angle CNO = 118.8(0.8). The structure parameters are compared with those obtained previously for o-, m-, and p-bromonitrobenzenes.