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)

Rotational barriers and pi conjugation effects have been studied in H2X-YR systems (X = Al, B; Y = O, S; R = H, CH3) by means of ab initio calculations at the MP4/6-311G**//MP2/6-311G** level. In non-substituted systems H2X-YH, it is shown that the rotational barrier depends on three factors: (i) pi conjugation strength between the X vacant p orbital and the Y p-lone pair; (ii) the opening ability of the Y-valence angle and (iii) a possible direct interaction between X and the Y hydrogen substituent. The conjugation stabilization has been estimated through Valence Bond calculations and is found to decrease in the order BO (22.6 kcal/mol), BS (16.6 kcal/mol), AlO (10.2 kcal/mol), and AlS (8.1 kcal/mol). Study of the methylated systems confirm the results found in unsubstituted species. The most important feature is the location of a deconjugated secondary minimum for H2B-SCH3 species. An acute B-S-C angle (69.6 degrees) is found, thus evidencing a direct H ... B agostic interaction with a four-membered (BSCH) heterocycle. To further evidence our analysis, additional calculations have been performed on carbocationic species H2C+-YR (Y = O, S; R = H, CH3) for which the H2C+ moiety is a strong pi acceptor. As expected, the C+-O and C+-S conjugation are strong (about 60 kcal/mol). Only two minima have been located for the H2C+-OCH3 species. In the sulfur case, the Potential Energy Surface (PES) is more complicated and six stationary points have been characterized. A strong agostic interaction is found for a secondary deconjugated minimum for which an hydrogen symmetrically bridges the two carbon atoms. A schematic energy profile connecting the various extrema is given for this cationic species.