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

Abstract

The molecular structure of trans-azobenzene (Ph-NdN-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(φ 1 ,φ 2 ), where φ i 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(φ 1 ,φ 2 ). Because P(φ 1 ,φ 2 ) ) N exp(-V(φ 1 ,φ 2 )/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 (V 2 and V 4 /kcal mol -1 ) and principal structure parameters (r g /Å, ∠ R /deg) with the estimated limits of error (3σ) are as follows: V 2 ) 1.7(6); V 4 ) 0.6(13); r(NdN) ) 1.260(8); r(N-C) ) 1.427(8); <r(C-C)> ) 1.399(1); <r(C-H)> ) 1.102(7); ∠NNC ) 113.6(8); (∠NCC cis -∠NCC trans )/2 ) 5.0(9), where < > means an average value and C cis and C trans denote the carbon atoms cis and trans to the NdN bond, respectively. Thus, the stable form was found to be planar with C 2h symmetry. The observed structure was compared with those of trans-azoxybenzene (Ph-N(-O)dN-Ph) and transstilbene (Ph-CHdCH-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.

A theoretical study of the lowest electronic states of azobenzene: the role of torsion coordinate in the cis–trans photoisomerization

“…The typical bond distances, angles and rotational constants for the ground state (S 0 ) and the lowest singlet (S 1 ) and triplet (T 1 ) excited states of E-Ab, obtained at the CASSCF and B3LYP levels of theory, are reported in Table 1. For the S 0 ground state, B3LYP predicts bond distances in better agreement with experiment [20] than CASSCF. This is especially true for the central N=N double bond, which is predicted to be too short at the CASSCF level (1.239 versus 1.261 Å ), with both the 14/12 and 14/14 active spaces.…”

“…Interestingly, according to a MP2/6-31G* study [20], the NNCC angle corresponding to a minimum is different from zero and the potential-energy curve forms two minima at +19.5°and )19.5°separated by a tiny barrier of 0.4 kcal mol )1…”

“…Tsuji et al [20] determined the structure of the ground state of E-Ab using second-order Møller-Plesset perturbation theory (MP2) and density functional theory (DFT) methods. CASSCF and multireference configuration interaction with singles and double excitations (MRSDCI) calculations were also performed by Ishikawa et al [21] to study the 1 np* and 1 pp* excitations.…”

Active Intramolecular Conformational Dynamics Controlling the Assembly of Azobenzene Derivatives at Surfaces

“…The methyl part of the end group can be easily identified in the STM images, while the carboxyl moieties, which are strongly active in the formation of intermolecular H bonds, are in charge of the recognition, selection, and locking of specific molecular shapes in ordered domains. Although azobenzene is a prototype molecular switch, which undergoes a trans-cis photoisomerization in the gas phase and in solution, [7,[9][10][11] at the gold surface only the (planar) trans-CMA conformer is found.…”

Conformational property of ethoxybenzene as studied by laser-jet spectroscopy and theoretical calculations

“…In such a case, it is not appropriate to treat the ethoxy and ethyl internal rotations separately by using a one-dimensional potential function for each. Therefore, the energy level calculations were carried out by means of the two-dimensional Hamiltonian [14], …”