IMOMM,

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. 

Figure 4. QM/MM partition used in the IMOMM calculation of one of the possible transition states of the insertion of propene into the Rh-H bond of HRh(CO)(benzoxantphos) [88],The QM atoms are shown as "balls and sticks", and the MM atoms are shown as "tube".

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. 

Subsequently, Humbel, Sieber, and Morokuma (1996) generalized the IMOMM optuniza-tion scheme to the case where two different levels of QM theory were used instead of a QM/MM approach, and Svensson, Humbel, and Morokuma (1996) examined the relative efficacy of different combinations of levels for prototype problems. Corchado and Truhlar (1998) later proposed a refinement of that methodology to improve computed vibrational frequencies and Rickard et al. (2003) showed that a combination of MP2 and HF theories permits the calculation of high-quality NMR chemical shifts within the high-level system. 

IMOMM Integrated molecnlar orbital molecnlar mechanics... 

A computational study with the IMOMM (Becke3LYP MM3) method has been carried out on the mechanism of the enantioselective reaction of complex V(0)(L)(00H)... 

The situation for transition metal chemistry has been somehow different because, while the description of the electronic effects from the MM region may not be that important, the handling of covalent connections between the QM and MM regions is critical. The methods that have been most successful in handling this situation have been the closely related IMOMM [11] and ONIOM [12, 13]. These schemes provide a computationally economical and methodologically robust method of introducing the steric effects of the... 

Despite their relatively recent appearance in 1995, the IMOMM and ONIOM methods have been extremely productive when applied to transition metal chemistry [15]. A substantial amount of publications have been concerned with homogeneous catalysis, and they will be reviewed in this contribution. Other applications to transition metal chemistry, in structure [16, 17] and reactivity [18, 19], will not be treated here. Methodological aspects of the QM/MM approach, in connection with its application to transition metal chemistry, have been reviewed in a number of papers [20-22]. 

It is worth mentioning here that the small quantitative differences found in the energy barriers computed by the two research groups are related to the different DFT and MM descriptions used in the IMOMM(DFT MM) calculations. The particular method was IMOMM(BP86 AMBER95) in one case [36] and IMOMM(B3LYP MM3) in the other [37, 38]. One of the studies also analyzed the catalytic efficiency of similar palladium complexes [37, 38]. 

Fig. 11 Partitioning scheme for IMOMM calculations of the dihydroxylation of a styrene and b 1-decene by 0s04-(DHQD)2PYDZ. Alkenes highlighted in black...

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. 

The catalyst + aliphatic n-alkene system, for all of the olefins from propene to 1-decene, give almost 40,000 possible conformations. The most stable conformations for each of the olefins were selected (around 1,700) and their transition states were optimized at IMOMM level. The calculated enantiomeric excesses are shown in Fig. 13. Calculations are able to reproduce the observed increase in ee for short chains, and the presence of a ceiling value after which the increase in enantioselectivity is much smaller, in excellent agreement with experiment. 

The theoretical study of heme groups has become more common in recent years because of the appearance of new methodologies and the increase in computer power. In particular, the application of the IMOMM (Integrated Molecular Orbital Molecular Mechanics), a hybrid Quantum Mechanics/Molecular Mechanics (QM/ MM) scheme, has allowed the accurate study of transition metal systems [140-142]. 

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

Musaev DG, Froese RDJ, Morokuma K, Molecular Orbital and IMOMM Studies of the Chain Transfer Mechanisms of the Diimine-M(II)-Catalyzed (M = Ni, Pd) Ethylene Polymerization Reaction, Organometallics, 17, 1850-1860 (1998)... 

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)... 

Maseras F, Morokuma K (1995) IMOMM a new integrated ab initio + molecular mechanics geometry optimization scheme of equilibrium structures and transition states. J. Comput. Chem. 

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