Hyperconjugation in Carbocations, a BLW Study with DFT approximation

Abstract

The geometry of ethyl cation is discussed, and the hyperconjugation effect in carbocations is evaluated at the B3LYP/6-311G(d) level. The Block Localized Wavefunction (BLW) method is used for all evaluations of the hyperconjugation, considered as the energy gained by the delocalization onto the C+ atom. This energy is defined as the energy difference between the delocalized (standard) calculation, where the electrons are freely delocalized, and a localized form where the positive charge sits on the carbon center. It is evaluated for 18 carbocations, including conjugated systems. In these cases we were particularly interested in the additional stabilization brought by hyperconjugative effects. Among other effects, the β-silicon effect is computed. Hyperconjugation amounts in several cases to an energy similar to conjugation effects.

Dismantling the Hyperconjugation of π‐Conjugated Phosphorus Heterocycles

“…The two latter methods have been used for the evaluation of the importance of the hyperconjugation in alkynes, anomeric effects, carbocation stabilities, and hydrogen‐bonding among other chemical properties.…”

Computational Study of the Donor–Acceptor Interactions Underlying the Variable Oxygen Probe

“…However, for X = NH 3 + the interaction energies are 6.61 and 8.79 kcal/mol, which is more consistent with the experimentally demonstrated ability of the C–Si bond to donate electrons more strongly by hyperconjugation (Table ). − The major NBO interactions in the equatorial and axial cyclohexane derivatives 3 and 4 are 2 × σ C–C –σ* C–X and 2 × σ C–H –σ* C–X respectivelywhich are 6.42 and 10.15 kcal/mol for X = OH, 15.81 and 22.49 kcal/mol for X = OH 2 + , and 9.75 and 13.95 kcal/mol for X = NH 3 + , respectively, consistent with the C–H bond being a stronger donor than C–C bond in these systems and in agreement with the experimental VOP slopes obtained for 3 and 4 given in Figure . The important NBO interactions in the chalcogen derivatives 5 and 6 include the conventional vicinal σ C–H –σ* C–X and σ C–S/Se –σ* C–X interactions in addition to smaller through-space homohyperconjugative n S/Se –σ* C–X interactions involving the chalcogen p-type nonbonded pair of electrons.…”

“…This agrees with previous observations that hyperconjugation in charged systems is more pronounced than in neutral systems. 24,25,41 This result suggests that the more relevant parameter to compare neutral and charged leaving group abilities in the gas phase is the proton affinities (P.A. ).…”

“…Model systems which have been studied using the variable oxygen probe, the equations obtained from plots of C–OR bond distance (Å) vs p K a (ROH), and what were predicted to be the important donor–acceptor interactions involving the σ* C–O antibonding orbital are summarized in Figure . ,− The values given represent updated values from those originally reported to include additional high-quality low-temperature crystal data harvested from the Cambridge Structural Database (CSD) and to ensure consistency with a p K a value of 14 for water rather than 15.7 used previously among other minor revisions (see the Supporting Information for the updated VOP plots). , A strong response of C–OR bond distance to the electron demand of the OR group is demonstrated for 1 , which has an oxygen lone pair (n O ) orbital antiperiplanar to the OR substituent (the n O –σ* C–OR interaction is one of the classical explanations for the structural anomeric effect, , though recent computational studies contest whether a larger proportion of the stabilization in these systems is due to electrostatic effects − or hyperconjugative interactions , ), and a strong response is also observed for 2 , which has a C–Si bond antiperiplanar to the OR substituent (this is the basis of the well-known silicon β-effect). − However, a weaker response is present in 3 , which has two σ C–C bonding orbitals, which are weaker donor orbitals, antiperiplanar to the OR bond. Though similar responses for 3 and 4 were originally observed, inclusion of more points from the CSD now suggests that C–H bonding orbitals are slightly stronger donors than C–C bonding orbitals.…”

Computational Insight into the Mechanism of Alkane Hydroxylation by Non-heme Fe(PyTACN) Iron Complexes. Effects of the Substrate and Solvent

“…72 For cyclohexane, OH rebound barriers are in the interval from 0.8 to 0.9 kcal·mol -1 while ET activation energy are in between 8.0 and 11.2 kcal·mol -1 . Finally, 2,3-DMB has a barrierless OH rebound while the ET process has an energetic cost of 6.8 kcal·mol -1 .…”