NEW APPROACH TO THE NONPARAMETRIC DETERMINATION OF THE INTERNAL-ROTATION POTENTIAL FROM ELECTRON-DIFFRACTION DATA - REINVESTIGATION OF M-BROMONITROBENZENE

A numerical algorithm for the nonparametric determination of the inter nal rotation potential from electron diffraction data using Tikhonov's regularization method is described The range of admissible values of the regularization parameter is estimated using Hamilton's statistical criterion. m-Bromonitrobenzene was reinvestigated using this approach . It was found that the form of the potential may be reliably establis hed only in the region of the minimum corresponding to the planar conf ormation of the molecule. Fourier series approximation of the experime ntal values of the potential in the region of the minimum gives the ro tation barrier of 4.6-5.4 kcal/mole. The following basic geometrical p arameters have been obtained (r(a) in Angstrom, angle(alpha) in deg, t he error equals triple standard deviation): r(C-C)(ave) = 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 CNCC = 123.9(1.4), angle CNCCBr = 1168(1.5), angle CNCC = 116.6(1.9), angle CNO = 118.8(0.8). The results are compared with t he data for the related compounds.