Perturbations theory with bond additivity

The calculational approach used here involving fourth-order fipller-Plesset perturbation theory with bond additivity corrections (BAC-MP4), has been shown to be a powerful tool for determining heats of formation of molecular species and for analyzing possible reaction pathways. For NH2 and NH reacting with 0 and O2 we have been able to distinguish between likely and unlikely reaction products based on the stability of various reaction intermediates compared to the reactants and products. 

To aid the modelers in developing improved reaction mechanisms as well as to aid the experimentalists in their interpretation of the data, we have calculated the energetics of molecular intermediates and products arising from these reactions. Our approach was to use the highly accurate fourth order M ller-Plesset perturbation theory (24) with bond-additivity corrections. (2 )... 

The topic of interactions between Lewis acids and bases could benefit from systematic ab initio quantum chemical calculations of gas phase (two molecule) studies, for which there is a substantial body of experimental data available for comparison. Similar computations could be carried out in the presence of a dielectric medium. In addition, assemblages of molecules, for example a test acid in the presence of many solvent molecules, could be carried out with semiempirical quantum mechanics using, for example, a commercial package. This type of neutral molecule interaction study could then be enlarged in scope to determine the effects of ion-molecule interactions by way of quantum mechanical computations in a dielectric medium in solutions of low ionic strength. This approach could bring considerable order and a more convincing picture of Lewis acid base theory than the mixed spectroscopic (molecular) parameters in interactive media and the purely macroscopic (thermodynamic and kinetic) parameters in different and varied media or perturbation theory applied to the semiempirical molecular orbital or valence bond approach [11 and references therein]. 

Notice that, since e, < ej, SE is negative, that is, Ll is stabilized. The antibonding orbital 4 2 is destabilized by an amount A 2 This is set by an equation analogous to equation (5) except that the ordering of ei and Cj in the denominator is reversed. An important feature of perturbation theory, and indeed when one does a molecular orbital calculation at any level which that includes overlap, is that the antibonding combination ( 2) is destabilized more than the bonding combination (fl i) is stabilized, that is, I A 2 I > IA fi I Therefore the addition of two electrons to Ll in (1) is associated with stabilizing the formation of an M L... 

The next two papers (6.3 and 6.4) deal with the application of an ab initio quantum mechanical method (the Mpller-Plesset perturbation theory) to large binary clusters formed by water with methane or methanol. The molecules of methane or methanol were selected because they represent two extreme types of molecules 1) methane, an entirely hydrophobic molecule and 2) methanol, which has both hydrophobic and hydrophilic parts and, in addition, can form H-bonds with water. These calculations allow one to analyze the changes in the H-bond network of water in the vicinity of both molecules when they are inserted into pure water. These two cases might be helpful in understanding much more complex molecules such as proteins. 

The regioselectivity of the addition of phosphorylated anions to a/5-unsaturated ketones has been studied in some detail, - and has been discussed in terms of perturbation theory. Most products arise from nucleophilic attack at the C==C bond. The condensation of phosphonates with aromatic aldehydes may be carried out in a two-phase system in the absence of a typical phase-transfer catalyst. Among those aldehydes and ketones used successfully in alkene syntheses with phosphonates are (97), (98), (99), and (100). ... 

---