Theoretical studies on the conformations and properties of phenylazonaphthalenes

Abstract

The conformations of phenylazo-2-naphthalene (I) and phenylazo-1-naphthalene (II) have been studied using ab initio methods and density functional theory. The rotational potential energy surfaces about the C-N bonds were calculated for both the trans and the cis forms at the PM3 and HF/STO-3G levels. The PM3 method was found to be incapable for locating the energy minima correctly. The geometries of rotamers obtained from the HF/STO-3G surface were fully optimized at the HF/6-31G* and B3LYP/6-31G* levels. Single-point MP2/6-31G* calculations have also been carried out at the HF/6-31G* geometries. The vibrational spectra, standard thermodynamic functions, heats of formation, and equilibrium molar fractions have been obtained. According to the calculated results, I is comparatively more stable than II. The trans forms have much lower energies than the cis forms, implying that I and II mainly exist in the trans planar forms. The energy gap (ΔE) between the frontier molecular orbitals (MOs) of the cis forms are about 0.7 eV higher than those of the trans ones, indicating that change in geometry from trans to cis form will cause a hypsochromic effect.