Molecular structure of trans-azoxybenzene determined by gas electron diffraction combined with ab initio calculations

The molecular structure of trans-azoxybenzene (Ph-N(-O)=N-Ph) has been dete rmined by gas electron diffraction. Diffraction patterns were taken at 160 degreesC and data analysis was made using the structural constraints obtain ed from RHF/6-31G** ab initio calculations. Vibrational mean amplitudes and shrinkage corrections were calculated from the harmonic force constants gi ven by normal coordinate analysis. Vibrational mean amplitudes were refined as groups. The torsion of phenyl rings was treated as a large amplitude mo tion. The potential function for torsion was assumed to be V(phi (1), phi ( 2)) = V-12(1 - cos 2 phi (1))/2 + V-14(1 - cos 4 phi (1))/2 + V-22(1 - cos 2 phi (2))/2, where phi (1) and phi (2) denote the torsional angles around the N-C and N(O)- C bonds, respectively. The potential constants (V-ij/kcal mol(-1)) and the principal structure parameters (r(g) (Angstrom), angle (p roportional to) (degrees)) with the estimated limits of error (3 sigma) are as follows: V-12 = 1.7(10); V-14 = -0.1(13); V-22 = 3.5(16); r(N-O)= r(N=N )= 1,271(3); r(N(O)-C)= 1.464(7); r(N(O)-C)-r(N-C)= 0.050 (assumed); [r(C-C )]= 1.400(1); angle ONN= 127.7(16); angle ONC = 119.0(13); angleN(O)NC = 12 1.3(13); (angle NCCcis-angle NCCtrans)/2 = 9.0(14); (angleN(O)CCcis-angleN( O)CCtrans)/2 = 2.6(23), where [ ]eans an average value and C-cis and C-tran s denote the carbon atoms cis and trans to the N atoms, respectively. Thus the stable form was found to be planar. Comparison of the structure paramet ers of trans-azoxybenzene with those of p-azoxyanisole (CH3O-Ph-N(-O)=N-Ph- OCH3), which forms a nematic liquid crystal, reveals that the methoxy group s have little effect on the structure of the core of the p-azoxyanisole mol ecule. (C) 2000 Elsevier Science B.V. All rights reserved.