IMOMO schemes

The Application of IMOMO Schemes How Stable Are Benzyl and Diphenylmethyl Radicals ... 

We can thus conclude that the use of IMOMO schemes for the calculation of RSE values equates to combining absolute RSE values calculated at a high level of theory for a small core system with changes in RSE values through introduction of new substituents at some lower theoretical level. The results obtained with this strategy for radical 12 are summarized in Table 5.3. The RSE of radical 12 can be determined directly at G3(MP2)-RAD level and amounts to - -85.5kJ/mol. Using the IMOMO approach, the RSE of benzyl radical 11 at G3(MP2)-RAD level of - -61.0kJ/mol is combined with the difference of the RSE values of radicals 12 and 11 at ROMP2 level (79.8 52.5 = 27.3 kJ/mol) to yield RSE(IMOMO,... 

Morokuma and co-workers have also used an extrapolation scheme called IMOMO(G2MS) that connects the G2-type and IMOMO (integrated molecular orbital molecular orbital) approaches. In the G2 approach, the energy of the system is determined at a highly correlated ab initio method using a moderate basis set. The effect of the basis set is obtained at a lower ab initio level. In the IMOMO scheme, a high level of calculation is used in the study of a model system, while the effect of substituents is taken into account at a lower level. 

In the previous Section we obtained the formula for junction between quantum and classical subsystems Eq. (13). The control for the types of interactions which are taken into account is an important characteristic of particular QM/MM scheme. The authors of Ref. [110] have proposed a classification of hybrid schemes based on the interaction between fragments. According to it, the simplest type of model is mechanical embedding (examples of this type of modelling are the IMOMM [38] and IMOMO [59] schemes by Morokuma) when both QM and MM systems are not polarized by each other and their interaction is represented by classical force fields only. In this context the choice of parameters of intersystem interaction can be crucially important, so, they are frequently optimized [97,118]. More elaborated model is that including polarization of the QM subsystem. This polarization can be covered by including the MM charges into one-electron part of the Hamiltonian of the QM subsystem ... 

Figure 3 Schematics of the three-dimensional extrapolation scheme, IMOMO( G2 ). Use a G2-type method for extrapolation of correlation methods and basis sets, and the ONIOM scheme for extrapolation of the molecular size. The points represented by black circles are actually calculated values, and those represented by shaded circles are extrapolated values...

The reaction between acrylic acid (10) and 2,4-pentadienoic acid (11) has been used by Froese et al. as a test of the ability of the IMOMO(G2MS) scheme to predict the branching ratio among the different isomers. The results obtained using transition states obtained at the B3-LYP/6-31G(d) level are in nearly quantitative agreement with the experimental observations. 

---