Selenium-77 magnetic resonance in SSe and SeSe isosteres of phenanthrene

Selenium-77 magnetic resonance spectra of sulphur-selenium and selenium-selenium isosteres of phenanthrene and some of their 2-substituted derivatives have been recorded and 48 77Se chemical shifts are presented. The two isomers of the studied compounds could be differentiated because the Se nucleus is always more shielded in the ‘trans’ than in the ‘cis’ isomer, the difference varying between 10 and 40 ppm. CNDO calculations have been performed; they allow an electronic model to be proposed which accounts, at least qualitatively, for the NMR results. They also provide an electronic interpretation of the ring opening of the Se-containing ‘cis’ isomers when treated with alkyllithium.