Molecular orbitals relative energies

The relative frontier molecular orbital (FMO) energies of the reagents are very important for the catalytic control of 1,3-dipolar cycloadditions. In order to control the stereochemical outcome of a reaction with a substoichiometric amount of a ligand-metal catalyst, it is desirable that a large rate acceleration is obtained in order to assure that the reaction only takes place in the sphere of the metal and the chiral ligand. The FMO considerations will be outlined in the following using nitrones as an example. 

How much more stable is the bonding molecular orbital relative to a pair of noninteracting p atomic orbitals It is difficult to provide a numerical answer in kcal mole-1 that is meaningful, but we can describe the energy in symbolic terms. First, the energy of one electron in the p atomic orbital of an sp2 hybridized carbon, as in 3, is taken as a standard quantity, a, often called the Coulomb energy ... 

Valence tautomers, benzene oxide 1 and oxepine 2 (Equation 1), as well as relative tautomeric systems, benzene sulfide-thiepine and o-xylene-2,7-dimethyloxepine, have been studied by a post-Hartree-Fock (HF) ab initio QCISD(r)/6-31G //MP2/6-31G method. In particular, the enthalpy calculated for a benzene oxide-oxepine system is 0.59 kj moF1 <1997PCA3371>. The calculated molecular orbital (MO) energies are in linear relationship to those from the photoelectron (PE) spectra <1996JCF1447>. Barrier to tautomerization for a benzene oxide-oxepine system is 29.4 kj mol-1. Protonation stabilizes the oxide form versus the oxepine <1997PCA3371>. 

Using Tg and plots (Figure 14.7), along with the resulting Morse potentials, show in a schematic fashion that bond strengths can be increased with lower electrostatic repulsions (between electrons or between nuclei), and with lower kinetic energies in the molecular orbitals relative to the atomic orbitals. 

The relative acidities in the gas phase can be detennined from ab initio or molecular orbital calculations while differences in the free energies of hydration of the acids and the cations are obtained from FEP sunulations in which FIA and A are mutated into FIB and B A respectively. 

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