A dipole moment study of the conformational properties of diaryl diselenides and related compounds |
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Authors: | Giuseppe C. Pappalardo Kurt J. Irgolic R.A. Grigsby |
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Affiliation: | Istituto di Chimica Generale, Università di Catania, Viale A. Doria, 95125 Catania Italy;Department of Chemistry, Texas A&M University, College Station, Texas 77843 U.S.A. |
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Abstract: | The electric dipole moments of the diaryl diselenides (RC6H4)2Se2 (R H, 4-F, 4-Br, 4-CH3, 3-F) were measured in benzene solution at 25 and 45°C. The conformations of these compounds were deduced by matching experimental moments with values calculated for a variety of possible conformations. In the dissolved state the diselenides exist at 25°C in fixed “skew” conformations characterized by dihedral angles of 75–106° between the CSeSe planes, corresponding to the conformational energy minima. At 45°C oscillations about the SeSe bonds are excited in the diphenyl and bis(4-methylphenyl) diselenides, whereas the 4-bromophenyl derivative exhibits free rotation. The fluoro compounds have temperature-independent dipole moments, suggesting “rigid conformations” with dihedral angles of 106° (4-F) and 74.4° (3-F). An analysis of the dipole moments at 25 and 45°C obtained for the compounds (RC6H4)2X2 (R H, 3-F, 4-F, 4-Br, 4-CH3; X S, Se, Te) showed that the conformational properties of these derivatives change on passing from X S to X Te. The observed variations are explicable in terms of a decreasing repulsion between the lone electron pairs of the chalcogen atoms on going from the disulfides to the ditellurides and a concomitant reduction of the energy barrier to rotations about the XX bonds. |
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