Comparative analysis of the quasiline spectra of organoboron compounds and their heterocyclic and aromatic analogs

Recently, much attention has been devoted to the study of orgunoboron compounds. This has arisen because they are rather widely used in medicine, production of plastics, and carbohydrate chemistry. The study of the electronic structure of these compounds is of considerable interest. Sevm-al works [1-3] have attempted to establish a connection between the vibronic spectra and the structure of or~anoboron compounds. In particular, in [1, 2 ] the absorption spectra of 10-hydroxy-10,9- borazarophenanth re and 10-hydroxy-10,9boroxarophenanth rene were measured in eyclohexane and ethanol solutions at 20~ On the basis of the similarity in the positions of the principal maxima in the spectra of the organoboron compounds and in the spectrum of phenanthrene we have arrived at a conclusion favoring the similarity of their r-electronic structures. Michl and Jones [3] reached the same conclusion from the presence in the spectra of the organoboron compounds of series of frequencies resembling those of aromatic analogs. However, the distribution of ~ -electron density in these compounds is known [4] to be strongly distorted in comparison with their aromatic analog-phenanthr ene - as a result of the specific properties of the heteroatoms incorporated in the molecules. This was not mentioned in [1-3]. Our intention in the present work was to observe changes in electronic structure due to the heteroatoms as displayed in the vibronic spectra. The positions of the purely electronic transitions and the vibrational frequencies active in the absorption, fluorescence, and phosphorescence spectra can be determined with great precision (up to 10-15 cm -i) from the fine structure of the spectra, obtained by Shpol'skii's method [5]. Thus we can compare the vibrome spectra of these compounds with the equivalent spectral fine structure of their hydrocarbon analogs, recorded under the same conditions. Since these compounds contain nitrogen and oxygen in addition to boron, we found it convenient to compare their spectra with the quasiline spectra of heteroeyclic compounds containing only nitrogen and oxygen as heteroatoms. We chose two compounds, namely, phenanthridine and carbazole, to represent the nitrogen heterocycle. In these compounds the nitrogen affects the molecular ; -systems differently, since in the first the lone pair lies in the plane of the molecule, while in the second the lone-pair orbital is parallel to the Pz orbitals of the neighboring carbon atoms [6]. We used the spectra of dibenzofuran [7] for comparisons with the spectra of the oxygen-containin g compounds. Thus, these compounds constitute a series: phenunthrene, phenanthridine, carbazole, 10-hydroxy-10,9borazarophenanth rene