Molecular Structure and Torsional Potential of trans-Azobenzene. A Gas Electron Diffraction Study

The molecular structure of trans-azobenzene (Ph−NN−Ph) has been determined by gas electron diffraction. Diffraction patterns were taken at 407 K and data analysis was made using the structural constraints obtained from MP2/6-31+G* calculations. Vibrational mean amplitudes and shrinkage corrections were calculated from the harmonic force constants given by a normal coordinate analysis. Vibrational mean amplitudes were refined as groups. The torsion of each phenyl ring was treated as a large amplitude vibration. The potential function for torsion was assumed to be V (φ1, φ2) = Σi=1, 2V2 (1 − cos 2φi) /2 + V4 (1 − cos 4φi) /2, where φi denotes the torsional angle around each N−C bond. Quantum mechanical calculations were performed by taking account of two torsional motions to derive a probability distribution function, P (φ1, φ2). Because P (φ1, φ2) = N exp (−V (φ1, φ2) /kT) was found to be a good approximation at 407 K where N is a constant, it was adopted in the data analysis. The determined potential constants (V2 and V4/kcal mol-1) and principal structure parameters (rg/Å, ∠α/deg) with the estimated limits of error (3σ) are as follows: V2 = 1. 7 (6) ; V4 = 0. 6 (13) ; r (NN) = 1. 260 (8) ; r (N−C) = 1. 427 (8) ; = 1. 399 (1) ; = 1. 102 (7) ; ∠NNC = 113. 6 (8) ; (∠NCCcis − ∠NCCtrans) /2 = 5. 0 (9), where means an average value and Ccis and Ctrans denote the carbon atoms cis and trans to the NN bond, respectively. Thus, the stable form was found to be planar with C2h symmetry. The observed structure was compared with those of trans-azoxybenzene (Ph−N (−O) N−Ph) and trans-stilbene (Ph−CHCH−Ph). The stability of the liquid crystals with these types of molecular cores was discussed on the basis of the gas-phase structures of the model compounds of cores. Nearly the same results were obtained in the data analysis using the constraints from RHF/6-31G** ab initio calculations.