Crystal violet's shoulder

The symmetry of the crystal violet cation has been the subject of ongoing d iscussion since G. N. Lewis and coworkers called attention to the unexpecte d shoulder in its optical absorption spectrum. Strikingly-absent from this debate is a description of the X-ray crystal structure of the common chlori de salt, that compound to which the vast majority of the spectroscopic anal yses pertain,and whose large crystals account for the familiar name. This a bsence is especially curious since a preliminary structure determination wa s made as early as 1943, Here, we present single crystal structures of two forms of crystal violet chloride, a monoclinic monohydrate and a trigonal n onahydrate. In principle, the pair is well suited to address whether desymm etrization is responsible for the shoulder since the cation is asymmetric i n the monoclinic crystal, while it is axially symmetric in the trigonal cry stal. Absorption spectra of the two crystalline forms were identical and do minated by peaks assigned to aggregates. Raman spectra also could not disti nguish the hydrates from one another. Mixed crystals of benzene-1,2-dicarbo xylic acid (phthalic acid) containing mM concentrations of crystal violet w ere prepared in order to study the cation fixed in a dissymmetric medium, y et free from aggregate absorption. Through this work, and a reevaluation of the literature, we show that the two prevailing theories for the shoulder, that there are two isomers in equilibrium, and that desymmetrization gives rise to two excited states, are not mutually exclusive.