Vibrational spectra of biphenylene. In-plane normal modes of biphenylene, [2,3,6,7-2H4]biphenylene and [2H8]biphenylene

Abstract

The Raman spectra of powders and solutions of biphenylene, [2,3,6,7-²H₄]biphenylene, and [²H₈]biphenylene are presented. The frequencies assigned to the a_g modes, together with those attributed in a previous paper to the b_1u and b_2u modes, are used as a reliable experimental basis to develop a normal coordinate analysis of the in-plane vibrations.

A 28 parameter Valence Force Field, derived from the one which works successfully in the similar case of aromatic hydrocarbons, gives a satisfactory description of the vibrational modes, reproduces the observed frequencies with an average error of less than 10 cm^–1, and supports the proposed assignment of the b_3g modes.

Raman active out-of-plane modes and lattice vibrations are discussed, and the assignment of the librational modes is proposed through Cruickshank's equation, relating the frequencies of these motions to their mean square amplitude.