IMOMM methods

Three such methods have been proposed by Morokuma and coworkers. The integrated MO + MM (IMOMM) method combines an orbital-based technique with an MM technique. The integrated MO + MO method (IMOMO) integrates two different orbital-based techniques. The our own n-layered integrated MO and MM method (ONIOM) allows for three or more different techniques to be used in successive layers. The acronym ONIOM is often used to refer to all three of these methods since it is a generalization of the technique. 

We applied the QM/MM IMOMM method [41] to Rh-diphosphine catalyzed hydroformylation, to provide a quantitative theoretical characterization of the origin of regioselectivity in Rh-diphosphine systems. We focused on the experimentally characterized xantphos ligands, for which variation in electronic properties is minimal. Using the IMOMM method, which only accounts for the steric properties of ligands, was fully justified. 

We have recently broadened those investigations to study the origin of the enantioselectivity in the dihydroxylation of terminal aliphatic n-alkenes. The dihydroxylation of the series from propene to 1-decene was studied by means of the IMOMM method [97]. Experimental studies on propene, 1-butene, 1-pentene, 1-hexene and 1-decene showed that the reaction was enantioselec-tive in all cases, leading to the R product. Moreover, the results show a dependence of the enantioselectivity on the chain length it sharply increases from propene to 1-pentene, and after that the enantioselectivity remains practically constant for 1-hexene and 1-decene. The explanation for this dependence of the enantioselectivity with the chain length remained elusive. On the other hand, the -stacking interactions that were found to be critical for styrene cannot be responsible for the observed enantioselectivity for these terminal aliphatic n-alkenes because they do not have aromatic rings. 

Maseras F (2000) The IMOMM method opens the way for the accurate calculation of real transition metal complexes. Chem Commun 2000 1821-1827... 

Froese RDJ, Musaev DG, Morokuma K, Theoretical Study of Substituent Effects in the Diimine-M(II) Catalyzed Ethylene Polymerization Reaction Using the IMOMM Method, J Am Chem Soc, 120, 1581-1587 (1998)... 

The IMOMM method can in principle be applied to any combination of QM and MM methods. Both the QM and the MM level of the calculation are indicated in this text through a compact terminology of the type IMOMM(QM level MM level). 

A second possible approach to the topic of the quality of the MM description is the tuning of some parameter of the force field to the particular problem under study. Although this has seldom been done in applications of the IMOMM method to transition metal chemistry, because it can obscure interpretation of the result, there is at least one particular example where it has been helpM (12). 

The study was broadened by the same authors to the investigation of the origin of the enantioselectivity in the dihydroxylation of terminal aliphatic n-alkenes. The dihydroxylation of the series from propene to 1-decene was studied by means of the IMOMM method [71]. Experimental studies on propene,... 

The application of QM/MM methods, concretely the IMOMM method, has also been proven to be successful to determine the origin of the enantioselectivity, and to predict quantitatively the origin of the enantioselectivity. In addition, the contribution of each part of the catalyst to the enantioselectivity was also quantified. In this case, all the transition states were fully optimised. The advantage of this method is that it can be directly applied to study any reaction, because no parameterisation is needed, albeit the single calculations are more expensive. 

The ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) method is an extension of IMOMO and IMOMM that performs calculations on n different systems using n levels of calculation. Each system and calculation level constitute a layer. The IMOMO and IMOMM methods are versions of ONIOM with n = 2. ONIOM3 is the three-layered version of ONIOM and uses a real system calculated at a low level, an intermediate model system calculated at a medium level, and a small model system calculated at a high level [M. Svensson et al., J. Phys. Chem., 100,19357 (1996)]. 

Froese, R. J. Musaev, D. G Morokuma, K. Theoretical study of substituent effects in the diimine-M(II) catalyzed ethylene polymerization reaction using the IMOMM method. J. Am. Chem. Soc. 1998, 120, 1581-1587. 

Carbo et al. used a QM/MM IMOMM method [18] to study the origin of regioselectivity in the Rh-diphosphine-catalyzed hydroformylation [55]. The... 

Most of the applications presented use the integrated molecular orbital/ molecular mechanics (IMOMM) method (2), which therefore wiU be briefly described in the next section. Afterwards, two sections wiU discuss particular aspects of the calculation setup, and another section will present specific ways to analyze QM/MM results. A final section will offer concluding remarks. 

---