The Role of Chalcogen-Chalcogen Interactions in the Intrinsic Basicity and Acidity of #beta#-Chalcogenovinyl(thio)aldehydes HC(=X)-CH=CH-CYH (X = O, S; Y = Se, Te)

摘要

The intrinsic acidity and basicity of a series of #beta#-chalcogenovinyl(thio)aldehydes HC(=X)-CH=CH-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/6-311 + 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-H …Se (X = O, S) intermolecular hydrogen bond (IHB). In contrast the Te-containing analogues vehave as oxygen or sulfur bases, because the most stable conformer is stabilized by typical X…Y-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-H or Y-H groups is a much more favorable than from the C-H groups. Therefore, for Se compounds the deprotonation process results in loss of the X-H…Se (X = O, S) IHBs present in the most stable neutral conformer, while for Te-containing compounds the stabilizing X…Y-H chlcogenc-hlcogen interaction present in the most stable neutral conformer beomes repulsive in the corresponding anion.