نمونه متن انگلیسی مقاله
High-level B3LYP/6- 311G(3df,2p) density functional calculations have been carried out for a series of saturated chalcogenoaldehydes: CH(X)-CH2-CH2YH (X, Y O, S, Se, Te). Our results indicate that in CH(X)- CH2-CH2YH (X Y O, S, Se) the XH ¥¥¥ X intramolecular hydrogen bond (IHB) competes in strength with the X ¥¥¥ XH chalcogen ± chalcogen interaction, while the opposite is found for the corresponding tellurium-containing analogues. For those derivatives in which XY, X being the more electronegative atom, the situation is more complicated due to the existence of two non-equivalent XH and YH tautomers. The YH tautomer is found to be lower in energy than the XH tautomer, independently of the nature of X and Y. For X O, S, Se and Y S, Se the most stable conformer b is the one exhibiting a YH ¥¥¥ X IHB. Conversely when Y Te, the chelated conformer d, stabilized through aX ¥¥¥ YH chalcogen ± chalcogen interaction is the global minimum of the potential energy surface. Systematically the IHB and the chalcogen ± chalcogen interactions observed for saturated compounds are much weaker than those found for their unsaturated analogues. This result implies that the nonbonding interactions involving chalcogen atoms, mainly Se and Te, are not always strongly stabilizing. This conclusion is in agreement with the fact that intermolecular interactions between Se and Te containing systems with bases bearing dative groups are very weak. We have also shown that these interactions are enhanced for unsaturated compounds, through an increase of the charge delocalization within the system, in a mechanism rather similar to the so call Resonance Assisted Hydrogen Bonds (RAHB). The chalcogen ± chalcogen interactions will be also large, due to the enhancement of the X Y dative bond, if the molecular environment forces the interacting atoms X and Y to be close each other.
Weak interactions play a crucial role in the structural organization of chemical and biochemical systems and in molecular recognition. Many of these weak interactions are directly associated with the formation of inter- or intramolecular hydrogen bonds (IHBs),[1±3] while others are due to nonbonding interactions between heavy atoms. Among the latter chalcogen ± chalcogen interactions have received a particular attention. These weak bonds are responsible for the enhanced stability of chelated structures with respect to open ones, as it is the case for instance in malonaldehyde[4±8] and in thiomalonaldehyde (TMA),[9, 10] and modulate their intrinsic reactivity. Tropolone, resorcinol, and acid anhydrides are paradigmatic examples of systems where the existence or the formation of IHBs affects the intrinsic basicity and/or acidity of the system. Chalcogen ± chalcogen attractive forces also play an important role as far as the preferred conformation and reactivity of some chalcogen derivatives is concerned, as it has been shown many years ago by Adcock, Angyan et al. and B¸rgi and D¸nitz,[14±16] and more recently by Minyaev and Minkin.[17, 18] Also recently, Komatsu et al. showed that 17O a nd 77Se NMR spectroscopic data provide strong evidence for intramolecular nonbonded interaction between Se and O in hydroxy-selenenyl compounds. These interactions seem to play also an important role in some reaction mechanisms.