Modern valence-bond description of chemical reaction mechanisms: the 1,3-dipolar addition of diazomethane to ethene

Spin-Coupled Generalized Valence Bond Theory: New Perspectves on the Electronic Structure of Molecules and Chemical Bonds

Spin-Coupled Generalized Valence Bond (SCGVB) theory provides the foundation for a comprehensive theory of the electronic structure of molecules. SCGVB theory offers a compelling orbital description of the electronic structure of molecules as well as an efficient and effective zero-order wave function for calculations striving for quantitative predictions of molecular structures, energetics, and other properties. The orbitals in the SCGVB wave function are usually semilocalized, and for most molecules, they can be interpreted using concepts familiar to all chemists (hybrid orbitals, localized bond pairs, lone pairs, etc.). SCGVB theory also provides new perspectives on the nature of the bonds in molecules such as C₂, Be₂ and SF₄/SF₆. SCGVB theory contributes unparalleled insights into the underlying cause of the first-row anomaly in inorganic chemistry as well as the electronic structure of organic molecules and the electronic mechanisms of organic reactions. The SCGVB wave function accounts for nondynamical correlation effects and, thus, corrects the most serious deficiency in molecular orbital (RHF) wave functions. Dynamical correlation effects, which are critical for quantitative predictions, can be taken into account using the SCGVB wave function as the zero-order wave function for multireference configuration interaction or coupled cluster calculations.

Anatomy of bond formation. Bond length dependence of the extent of electron sharing in chemical bonds from the analysis of domain-averaged Fermi holes

We demonstrate that domain-average Fermi hole (DAFH) analysis, which has previously been used at the Hartree-Fock level, remains useful after the proper introduction of electron correlation. We perform a systematic investigation of the variation of the picture of bonding with increasing bond length in simple diatomic molecules such as N2 and LiH. Alongside values of a shared-electron distribution index (SEDI), this analysis provides further insight into the geometry dependence of the extent of electron sharing in polar and non-polar systems. We also use DAFH analysis, with correlated wave functions, to evaluate the (potential) multicentre bonding in the electron-deficient and electron-rich molecules CH2Li2 and CH2N2, respectively.

The unusual electronic mechanism of the [1,5] hydrogen shift in (Z)-1,3-pentadiene predicted by modern valence bond theory

The combination of modern valence bond theory in its spin-coupled (SC) form [SC(6)/6-31G*] and an intrinsic reaction coordinate calculation (MP2/6-31G* IRC) is used in order to obtain a model for the electronic mechanism of the gas-phase [1s,5s] hydrogen shift in (Z)-1,3-pentadiene. It is shown that this reaction follows an unusual heterolytic mechanism consistent with the C(s) symmetry of the transition state and involving the simultaneous movements of three well-defined orbital pairs. One of these is responsible for the sigma bond which initially attaches the migrating hydrogen to carbon 5 and, later on, to carbon 1. The second one realises the pi bond between carbons 3 and 4 which during the course of the reaction moves over carbons 2 and 3, while the third pair, initially involved in the pi bond between carbons 1 and 2 has no other choice but to embark on a long-range journey across the ring, ending up over carbons 4 and 5. While at first sight, it might appear that an electronic mechanism of this type would preclude the existence of an aromatic transition state, we have been able to show that the electronic structure of the transition state for this sigmatropic hydrogen shift has much in common with a hitherto apparently unknown alternative modern valence bond description of benzene involving 'antipairs' and so it can be considered to be aromatic.

1,3-Dipolar Reactions Involving Corannulene: How Does Its Rim and Spoke Addition Vary?

[reaction: see text] Corannulene undergoes 1,3-dipolar reactions with the dipoles, diazomethane, nitrile oxide, and nitrone through its rim and spoke pi bonds; the rim addition yields "one possible" adduct whereas two "regioselective" adducts are formed by spoke addition. Mechanisms of these reactions have been investigated at the B3LYP/6-31G(d) level. Computations show that both rim and spoke additions prefer concerted pathways that lie 2-5 kcal/mol lower in energy than stepwise paths. Stepwise additions can take place in two ways and the activation energies of these two modes differ by 1-2 kcal/mol. A close inspection of the energy profiles reveals that rim addition is more favorable kinetically and thermodynamically than spoke addition in view of lower activation energy and higher exothermicity observed for rim addition. The rim bond of corannulene is more flexible for distortion and also has a stronger double bond (i.e. pi-character) than the spoke bond and this facilitates rim addition over spoke addition. Deformation energy analysis also confirms the above through higher deformation in corannulene from the spoke addition when compared to rim addition. In the spoke addition, regio1 reaction is kinetically more favored than regio2 reaction. Attempts to react corannulene in an endohedral fashion have led to the exohedral adduct. Computed activation energies suggest that corannulene acts as a deactivated dipolarophile compared to ethylene. Even more striking is the observation that rim and spoke double bonds in corannulene are part of the local aromatic system but it shows remarkable reactivity compared to benzene despite the loss of aromaticity during the reaction. This is well indicated by computed NICS values. Inclusion of acetonitrile as solvent through the PCM model increases the reaction rate and exothermicity.