π-Bond vs. Agostic Interaction in Three-Coordinated Alkoxy and Thiolate Derivatives of Aluminium, Boron and Cationic Carbon – An ab initio Study of H2X–YR Systems (X = Al, B, C+; Y = O, S; R = H, CH3)
✍ Scribed by Isabelle Demachy; François Volatron
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 361 KB
- Volume
- 1998
- Category
- Article
- ISSN
- 1434-1948
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✦ Synopsis
Rotational barriers and π conjugation effects have been SCH 3 species. An acute B-S-C angle (69.6°) is found, thus evidencing a direct H•••B agostic interaction with a four-studied in H 2 X-YR systems (X = Al, B; Y = O, S; R = H, CH 3 ) by means of ab initio calculations at the MP4/6-311G**// membered (BSCH) heterocycle. To further evidence our analysis, additional calculations have been performed on MP2/6-311G** level. In non-substituted systems H 2 X-YH, it is shown that the rotational barrier depends on three factors: carbocationic species H 2 C + -YR (Y = O, S; R = H, CH 3 ) for which the H 2 C + moiety is a strong π acceptor. As expected, (i) π conjugation strength between the X vacant p orbital and the Y p-lone pair; (ii) the opening ability of the Y-valence the C + -O and C + -S conjugation are strong (about 60 kcal/ mol). Only two minima have been located for the H 2 C + -angle and (iii) a possible direct interaction between X and the Y hydrogen substituent. The conjugation stabilization OCH 3 species. In the sulfur case, the Potential Energy Surface (PES) is more complicated and six stationary points has been estimated through Valence Bond calculations and is found to decrease in the order BO (22.6 kcal/mol), BS (16.6 have been characterized. A strong agostic interaction is found for a secondary deconjugated minimum for which an kcal/mol), AlO (10.2 kcal/mol), and AlS (8.1 kcal/mol). Study of the methylated systems confirm the results found in hydrogen symmetrically bridges the two carbon atoms. A schematic energy profile connecting the various extrema is unsubstituted species. The most important feature is the location of a deconjugated secondary minimum for H 2 B-given for this cationic species.