Potential symmetry breaking, structure and definite vibrational assignment for azulene: Multiconfigurational and density functional results

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Abstract
A systematic ab initio determination of the geometry and harmonic force field with inclusion of correlation energy for the ground state of azulene is presented. The calculations were carried out using the unrestricted natural orbital-complete active space (UNO-CAS) method and density functional theory (DFT) using the compound Becke–Lee–Young–Parr (B3-LYP) exchange-correlation functional. The 6-31G* basis set has been used in both methods. The geometry optimized with DFT leads to a structure with approximately equal C–C bond lengths and C2v symmetry, while UNO-CAS results show an alternating bond structure with Cs symmetry and a very low (∼32 cm−1) barrier height. Agreement of the calculated vibrational frequencies with experiment is very good for both methods, except in the B2 symmetry species which contains the bond alternation mode. In this species, the density functional results agree better with experiment than those based on a multiconfigurational wave function, showing that the minimum has C2v symmetry. All vibrations are assigned for azulene and its perdeuterated analog.

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