Dependence of hydrocarbon sigma CC bond strength on bond angles: The concepts of “inverted”, “direct” and “superdirect” bonds

Conformational preference analysis in C2H6 using orbital forces and non-covalent interactions; comparison with related systems

“…10 Dealing with CC bonds in hydrocarbons, Σ t was found to be linearly correlated ( r 2 = 0.96) to intrinsic bond energy (BE) in a panel of 15 bonds, 11 with BE values based on AIM calculations. 12 It allowed BE evaluations in constrained systems such as propellane, 11,13 and partition into σ/π contributions to bonding. 14 Then the sum Σ t will be exploited in different ways.…”

“…First, K-S MOs do not obey eqn (1) and their physical meaning is only the slope of their energy as a function of R. Second, more importantly, the sums of their DOF exhibit worse correlations with bonding energies. 11 In practice, K-S MO forces are generally quite close to H-F ones, according to the choice exchange-correlation functional. 13,14 2.2 Non-Covalent Interactions (NCIs)…”

Bicyclobutanes: from curiosities to versatile reagents and covalent warheads

“…3,13 Inagaki has also shown that bridgehead substituents can be a determining factor as to whether the bridgehead carbons exhibit “inverted” tetrahedral configurations (all four atoms bonded to carbon are on the same side of the plane containing the carbon) as a result of the extent to which they influence geminal delocalization between the bridgehead and side bonds. 19 Based on a mean value for the angles of the six “substituents” on both carbon atoms of the bridgehead bond of 82°, Chaquin 20 has recently classified the bridgehead bond in BCB as an “inverted bond” in which the smaller back lobes of the hybrid orbitals overlap to form the C–C σ-bond, while the larger lobes point outward.…”

“…This hybridization gives rise to a bent σ-bond with a significant amount of π character (26.1% as determined by Newton). 9 a More recently, as part of a systematic correlation of σ-bond strength to bond angle for a series of hydrocarbons, Chaquin 20 has indicated that the smaller bond angles in BCB correlate to decreased σ but increased compensatory (39.7%) π contribution (including some three-center, two electron bonds involving the methylenes). As expected, the overall bond strength does decrease as a result of strain.…”

Synthesis and Applications of Bicyclo[1.1.0]butyl and Azabicyclo[1.1.0]butyl Organometallics

“…[11] The geometric restrictions imposed upon these bicyclic species means the bridgehead atoms adopt unusually tight bond angles. [12][13] In order to satisfy the necessary geometry, the hybridization of the atoms gives rise to a central bond that is almost entirely comprised of p orbitals and bridgehead atom bonds that have increased s-character. [14][15] Analogous to the bonding observed with cyclopropane rings, these "bent" bonds place a significant proportion of electron density outside the internuclear axis.…”