Molecules orbital properties

Following on from the substituent constant methods, a number of other approaches have been applied to the prediction of pKa. The main prediction methods for pKa are summarized in Table 3.4. Of the methods to calculate pKa some are derived from atom and fragment values, others are derived from molecule orbital properties. Because of the problems of modeling ionization constants for molecules with multiple ionizable functional groups, the accuracy and predictivity of these methods remains questionable. 

Molecular properties and reactions are controlled by electrons in the molecules. Electrons had been thonght to be particles. Quantum mechanics showed that electrons have properties not only as particles but also as waves. A chemical theory is required to think abont the wave properties of electrons in molecules. These properties are well represented by orbitals, which contain the amplitude and phase characteristics of waves. This volume is a result of our attempt to establish a theory of chemistry in terms of orbitals — A Chemical Orbital Theory. 

The 1 1 and 2 1 complexes of chiral bis(5H-pyrroles) and bis(oxazolines) with the lithium cation were studied by means of DFT methods (B3LYP/6-31G and B3LYP/6-311+G ) also, the energetic, geometric, electronic, and orbital properties of the complexes were analyzed. To perform such a study, we have calculated first the isolated molecules and the 1 1 complexes [33]. 

Mutual donor-acceptor interaction is affected by the changes of electron density on the atoms and bonds of interacting particles. The formation of a donor-acceptor bond between molecules is caused by electron transfer from the highest occupied orbital of the donor to the lowest unoccupied orbital of the acceptor molecule. The properties and structure of the complex depend (inter alia) on the type of the participating orbitals. Usually the complex is of zwitterion character, easily decomposing under the influence of external forces to sets of atoms with properties differing from those of the original donor or acceptor. These zwitterions or their decomposition products can initiate polymerizations. 

Fluorine is a reactive, almost colorless gas of F2 molecules. Its properties follow from its high electronegativity, small size, and lack of available (7-orbitals for bonding. It is the most electronegative element and has an oxidation number of-I in all its compounds. Elements combined with fluorine are often found with their highest oxidation numbers, such as +1 for f in IF7. The small size of the fluoride ion results in high lattice enthalpies for fluorides, a property that makes them less soluble than other halides. 

It should be remembered that (a) a mirror plane m is a plane which is perpendicular to the plane of the molecule/orbital and also bisects it and (b) a C2 axis of symmetry is a line that bisects the molecule/orbital through the center and is in plane with it. We will understand these more by considering the symmetry properties of the MOs of an allyl system. 

We have looked at the orbital properties of the main group ML5 molecules in Chapter 11. Two basic structures are known, the square pyramid (17.14) and the trigonal bipyramid (17.15). A whole spectrum of geometries between the two extremes are also found in practice. The interconversion of the two geometries can occur in a simple way via the Berry pscudorotation process (17.16), a geometrical change with an obvious resemblance to the variation of the apical/basal angle, 0 (17.14) of the square pyramid shown in 17.17. 

Throughout this book we have often referred to the importance of the HOMO and LUMO in understanding a molecule s properties. Fukui reasoned that these frontier molecular orbitals might play an especially important role in concerted, pericyclic reactions. We have also noted before that it is always favorable to mix filled molecular orbitals wi th empty molecular orbitals. Combining these ideas led to frontier molecular orbital (FMO) theory, an elegant analysis of the types of reactions of importance here. Quite simply, frontier molecular orbital theory states that if we can achieve a favorable mixing between the HOMO of one reactant and the LUMO of another reactant, a reaction is allowed. If we cannot, the reaction is forbidden. 

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