(Recu le 17 fOvrier 1975, accept~ le 10 mars 1975 The crystal and molecular structure of o-formylphenylselenenyl bromide has been determined by threedimensional X-ray analysis. Crystals are orthorhombic, Pc21n, with a= 15.632, b= 4-582, c= 11 "084 ,~ and Z= 4. The refinement led to a final R value of 0.060. The molecule has a planar eis conformation. The structure is compared with that of o-formylphenyltellurenyl bromide.

The 1Hmr study of the title compounds has revealed a screw conformation, with defined interconversion processes, in good agreement with crystal structure determinations and theoretical calculations. The mesomeric effect of the heteroatom is smaller than in the anisole series, due to steric inhibitions.The 13Cmr enhances, to some extent, these conclusions. In the case of Te compounds, a heavy atom effect adds to the classical mesomeric and inductive effects to account for the experimental observations.

Abstra~t-~~Se n.m.r. spectra of twenty-six organoselenium com

+Carbony1 benzeneselenenyl compounds with COCH3, CHO and COOCH, as carbonyl functions and SeCl, SeBr, SeSCN, SeSeCN, SeCN and SeCH, as selenium-containing groups, have been studied by 'H, '"C and 77Se NMR spectroscopy. The IR C=O stretching frequencies of these compounds are also reported. If the SeCH, derivatives are excluded, the compounds mainly adopt a planar 'cis' conformation, due to an interaction between the C=O group and the selenium atom. The range of over 800 ppm for the observed Se chemical shifts makes 77Se NMR spectroscopy a powerful tool for physical organic chemists. 77

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