Complex self-consistent field calculations

Lanthanum to nitrogen bonds are present in the mixed COT complexes [242]. Cerium(IV) forming stable complexes with COT, Ce(COT)2 is characterized. Self-consistent field calculations showed [243] the ground state of cerocene to be almost entirely 4/ corresponding to the formula Ce3+(COT -5)2 rather than Ce4+(COT2)2 The methyl substituent in COT stabilizes the Ce(IV) complex which has a non-linear Cg-Ce-Cg angle of 176°. The two COT substituents are not staggered, and they are equidistant from cerium. 

More recently gas-phase nitration was treated theoretically with MNDO (modified neglect of diatomic differential overlap) and INDO (intermediate neglect of differential overlap) self-consistent field calculations (34). Electron transfer and radical-pair recombination were favored for the nitration of toluene and the xylenes but not for nitrobenzene, for which a classical nitration route via a tt complex was favored. The calculations could not make a distinction between the two routes in the nitration of benzene. More information is needed about these coupling reactions and how they differ in the gas and heterogeneous-solution phases. 

Cerium forms stable COT complexes only when tetravalent. The first one synthesized, the dicyclooctatetraenyl Ce(COT)2 was pyrophoric, but stable in water. Self-consistent field calculations established that the ground state in cerocene is almost entirely 4f corresponding to the formulation Ce (COT )2 rather than to Ce (COT )2 (Dolg et al. 1991). The methyl substituent in the COT ring stabilizes the structure of bis(methylCOT)cerium(IV) (Boussie et al. 1991) which presents an unusual non-linear Cg-Ce-Cg angle of 176° that may arise from intramolecular electronic effects . The COT substituents are not fully staggered and the two COT are equidistant from the Ce. 

Unlike reactive diatomic chalcogen-nitrogen species NE (E = S, Se) (Section 5.2.1), the prototypical chalcogenonitrosyls HNE (E = S, Se) have not been characterized spectroscopically, although HNS has been trapped as a bridging ligand in the complex (HNS)Fc2(CO)6 (Section 7.4). Ab initio molecular orbital calculations at the self-consistent field level, with inclusion of electron correlation, reveal that HNS is ca. 23 kcal mof more stable than the isomer NSH. There is no low-lying barrier that would allow thermal isomerization of HNS to occur in preference to dissociation into H -1- NS. The most common form of HNS is the cyclic tetramer (HNS)4 (Section 6.2.1). 

Self-consistent field molecular orbital calculations by Fenske and coworkers have confirmed that nucleophilic additions to Fischer and related complexes [e.g., (CO)sCr=CXY, (T)5-C5H5)(CO)2Mn=CXY], are frontier orbital-controlled rather than charge-controlled reactions (7-9). Interaction of the HOMO of the nucleophile with the carbene complex LUMO (localized on Ca) destroys the metal-carbon w-interaction and converts the bond to a single one. 

It is evident that the approach described so far to derive the electronic structure of lanthanide ions, based on perturbation theory, requires a large number of parameters to be determined. While state-of-the-art ab initio calculation procedures, based on complete active space self consistent field (CASSCF) approach, are reaching an extremely high degree of accuracy [34-37], the CF approach remains widely used, especially in spectroscopic studies. However, for low point symmetry, such as those commonly observed in molecular complexes, the number of CF... 

The selection of configuration state functions to be included in MCSCF calculations is not a trivial task. Two approaches which can reduce the complexity of the problem are the complete active space self-consistent-field (CASSCF) [68] and the restricted active space self-consistent-field (RASSCF) [69] approach. Both are implemented in the Dalton program package [57] and are used in this study. Throughout the paper a CASSCF calculation is denoted by i active gactive RASSCF calculation by For the active spaces of HF, H2O, and CH4... 

In earlier theoretical studies Shen and coworkers used Hartree-Fock self-consistent-field (HF) calculations with different basis sets to study water complexes of anionic ONO—0 Two stable ONO—O isomers, cis and trans, formed hydrogen bonds with... 

In earlier theoretical studies Shen and coworkers used Hartree-Fock self-consistent-field (HF) calculations with different basis sets to study water complexes of anionic ONO—O-30. Two stable ONO—O isomers, cis and trans, formed hydrogen bonds with H2O molecules at different positions. Second-order Mpller-Plesset perturbation theory (MP2) with a 6-311+G(d,p) basis set has also been applied to the study of ONO—O-, (H2O), (n = 1 or 2) complexes31. Koppenol and Klasinc studied the cis and trans conformers as well as the transition state for torsional motion of ONO—O- at the HF/6-31(d) level32. In their calculations, the trans conformer is slightly more stable than the cis form, and the rotational barrier was thought to be quite high. However, correlated methods (MP2) were also used to study this molecule, and they predict that the cis conformer is more stable than the trans conformer33,34. 

Selected structural data for 3-metallacyclobutanone complexes of palladium and platinum 5, obtained from both experimental and computational determinations, are provided in Table 5. The ab initio structures calculated at the self-consistent field (SCF) and Hartree-Fock (F1F) levels show good agreement with experimental values for the bond lengths of the metallacycle, but these computations deliver somewhat longer Pd-P bonds and smaller P-Pd-P... 

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