The vibrational structures of trans- (TAB) and cis-azobenzene (CAB) are of interest due to their importance in optoelectronic applications as well as due to the unique isomerization mechanism involving the inversion process (at the nitrogen site). In this paper, we report the equilibrium structures, harmonic frequencies, and mode assignments for TAB and CAB and their isotopic analogues, using restricted Hartree-Fock (RHF), hybrid Hartree-Fock/density functional (HF/DF), and pure density functional theoretical (DFT) methods utilizing the 6-31G* basis set. The results of the optimized molecular structure obtained on the basis of RHF and all the DFT calculations are presented and then critically compared with the experimental electron diffraction results. It is observed that best structural parameters are predicted by the hybrid HF/DF method, viz. B3LYP and B3P86 followed by the pure DFT method BP86. In the case of harmonic vibrational frequencies (unsealed) and the normal modes, it is found that the BP86/6-31G* is the most accurate. The data obtained here has been used to reassign (in contrast to the previously reported MP2 results) some of the vibrational frequencies, particularly, for the main N=N and C-N vibrations of TAB. On the basis of BP86/6-31G* force field, the infrared intensities for both TAB and CAB and their isotopomers have also been calculated. Moreover, the main differences in the vibrational spectra of the two isomers of azobenzene have been discussed from normal mode analysis.

Density functional calculations of structures, vibrational frequencies, and normal modes of trans- and cis-azobenzene

Magnetic Shielding and Susceptibility Anisotropies

Hydrazoic acid labeled with deuterium and 15N was photolyzed in solid nitrogen at 20°K. The 15N multiplet splitting positively identifies absorptions of photolysis products as those of trans‐N2H2 (at 1286 cm−1), trans‐HNND (at 1481 and 1058 cm−1), trans‐N2D2 (at 946 cm−1), and cis‐N2H2 (at 3074 and 1279 cm−1). The spectrum of trans‐N2H2 is in agreement with a planar symmetric structure and the vibrational frequencies are quite close to corresponding vibrations of ethylene.Imidogen, NH, was also observed in both solid argon and in solid nitrogen as another photolysis product. The 15N splitting in the ammonia absorptions shows that the ammonia dimer in solid nitrogen has a cyclic structure.

Infrared Detection of Diimide, N2H2, and Imidogen, NH, by the Matrix Isolation Method

Alignment effects on high resolution nmr spectra, induced by the magnetic field

Stereochemical features of some nickel(II) and cobalt(II) complexes of amino ligand—2-(2-aminoethyl) pyridine—VII

Infra-red absorption spectra, over the rock-salt region, are reported for 43 substances believed to contain the -N=N- group. In the " double bond " region two common frequencies are noted, at 1406 and 1579 cm-1 (with standard deviations of ±14 and ± 9 cm-1 respectively). Since these may arise wholly or partly from vibrations of the aromatic or heterocyclic nuclei adjacent to the azo-linkages, the conclusion is drawn that N=N stretching modes, if active in absorption, are contributors to either or both of the bands at the points mentioned.

Infra-red spectra of compunds containing the Azo-group

Molar Cotton-Mouton constants are reported for certain aromatic hydrocarbons as solutes at infinite dilution in benzene or carbon tetrachloride Relative anisotropies (ie the ratio of the anisotropy in the diamagnetic susceptibility of the molecule concerned to that of benzene) are calculated for these and other molecules for which data have been recorded previously1

JW Le Fevre

Papers

Infra-red spectra of compunds containing the Azo-group

Molecular susceptibility. The diamagnetic anisotropies of some polynuclear aromatic hydrocarbons