Vinyl, allyl, and homoallyl selenols were easily prepared by a chemoselective reduction of the corresponding selenocyanates with aluminum hydrides. Two stable vinyl and five stable allyl conformers of both series were predicted on the potential-energy surface. The interaction of SeH or SeCN groups with the vinyl group has been investigated with UV photoelectron spectroscopy and quantum chemical calculations, using the MP2/cc-pVTZ and B3LYP/cc-pVTZ levels. In the vinyl derivatives, a surprisingly strong direct conjugation of the selenium lone electron pair and the C=C double bond was observed. On the other hand, in allyl position the selenium lone pair is independent on the C=C double bond, and the hyperconjugation between the Se-C bond and the double bond is the ruling effect. Thus is clarified the type and extent of the interaction between the SeH or SeCN group and the unsaturated moiety.

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The gas-phase acidity of CH3-CH2XH (X=S, Se, Te), CH2=CHXH (X=S, Se, Te) and PhXH (X=S, Se) compounds was measured by means of Fourier transform ion cyclotron resonance mass spectrometry. To analyze the role that unsaturation plays on the intrinsic acidity of these systems, a parallel theoretical study, in the framework of the G2 and the G2(MP2) theories, was carried out for all ethyl, ethenyl (vinyl), ethynyl, and phenyl O-, S-, Se-, and Te-containing derivatives. Unsaturated compounds are stronger acids than their saturated analogues, because of the strong pi-electron donor ability of the heteroatoms that contributes to a large stabilization of the unsaturated anions. Ethynyl derivatives are stronger acids than vinyl compounds, while phenyl derivatives have an intrinsic acidity intermediate between that of the corresponding vinyl and ethynyl analogues. The CH2=CHXH vinyl compounds (enol-like) behave systematically as slightly stronger acids than their CH3-C(H)X (keto-like) tautomers. Vinyl derivatives are stronger acids than ethyl compounds, because the anion stabilization attributable to unsaturation is greater than that undergone in the neutral compounds. Conversely, the enhanced acidity of the ethynyl derivatives with respect to the vinyl compounds is due to two concomitant effects, the stabilization of the anion and the destabilization of the neutral compound. The acidities of ethyl, vinyl, and ethynyl derivatives containing heteroatoms of Groups 14, 15, and 16 of the periodic table are closely related, and reflect the differences in electronegativity of the CH3CH2-, CH2=CH-, and CH[triple chemical bond]C- groups.