Crystal and Molecular Structure of Guaiazulene and Its Potassium Crown Ether Salt: A Conglomerate

The Karplus–Fraenkel theory of electron spin densities was put to the test in an early paper on monomeric azulene (±) radical ions by combining cyclic voltammetry and electron spin resonance. The results were then used to make a comparison with the predicted bond-angle variation of the σ–π interaction parameters (Q’s). Unfortunately, the structure of the azulene molecule monomer was not known at the time since it is a tightly held dimer. Recently, the mononuclear anion radical of guaiazulene was prepared by some of us, and the data from the synthesis, solution NMR, and crystallographic data were combined into a single coherent unit, comparing all of the relevant observations from the individual experiments. The resulting amalgamation and interpretation of the combined data are presented below.