The intrinsic acidity and basicity of a series of beta-chalcogenovinyl(thio)aldehydes HC([double bond]X)[bond]CH[double bond]CH[bond]CYH (X=O, S; Y=Se, Te) were investigated by B3LYP/6-311+G(3df,2p) density functional and G2(MP2) calculations on geometries optimized at the B3LYP/6-31G(d) level for neutral molecules and at the B3LYP/6-31+G(d) level for anions. The results showed that selenovinylaldehyde and selenovinylthioaldehyde should behave as Se bases in the gas phase, because the most stable neutral conformer is stabilized by an X[bond]H...Se (X=O, S) intramolecular hydrogen bond (IHB). In contrast the Te-containing analogues behave as oxygen or sulfur bases, because the most stable conformer is stabilized by typical X...Y[bond]H chalcogen-chalcogen interactions. These compounds have a lower basicity than expected because either chalcogen-chalcogen interactions or IHBs become weaker upon protonation. Similarly, they are also weaker acids than expected because deprotonation results in a significantly destabilized anion. Loss of the proton from the X[bond]H or Y[bond]H groups is a much more favorable than from the C[bond]H groups. Therefore, for Se compounds the deprotonation process results in loss of the X[bond]H...Se (X=O, S) IHBs present in the most stable neutral conformer, while for Te-containing compounds the stabilizing X...Y[bond]H chalcogen-chalcogen interaction present in the most stable neutral conformer becomes repulsive in the corresponding anion.
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