Assignment self-consistency

Semiempirical methods are widely used, based on zero differential overlap (ZDO) approximations which assume that the products of two different basis functions for the same electron, related to different atoms, are equal to zero [21]. The use of semiempirical methods, like MNDO, ZINDO, etc., reduces the calculations to about integrals. This approach, however, causes certain errors that should be compensated by assigning empirical parameters to the integrals. The limited sets of parameters available, in particular for transition metals, make the semiempirical methods of limited use. Moreover, for TM systems the self-consistent field (SCF) procedures are hardly convergent because atoms with partly filled d shells have many... 

We have seen that the ab initio self-consistent quantum mechanical functional methods such as DFT/B3LYP with the chosen 6-31+G(d,p) basis sets are well suited to calculate reasonable molecular ion structures and vibrational spectra of these ions. The results obtained by us or others have indicated that the neglect of the presence of cation-anion interactions is a reasonable approximation for a rather successful prediction of the Raman spectra. Based on such calculations, detailed and reliable assignments of the spectra can be given and information on conformational equilibria can be obtained. 

In Fenske and Hall s parameter-free SCF calculations (80-84), the He1t 1-electron operator is substituted by a model 1-electron operator that has a kinetic energy and potential energy term for each atomic center in the complex. This approach assumes that the electron density may be assigned to appropriate centers. The partitioning of electron density is done through Mulliken population analyses (163) until self-consistency is obtained. The Hamiltonian elements are evaluated numerically, and the energies of the MO s depend only on the choice of basis functions and the intemuclear distance. 

Constant k is usually assigned a value of 1.75 to 2.00. Normally, the valence state ionization energies are corrected for charge effects and the calculations cycled to self-consistency (75). Madelung correction terms have also been included by some workers (109). 

It is surprising that the ethyl group is located on N-3 of the thiatriazolium salts. Comparison with other multisite heterocycles for which the relative nitrogen nucleophilicity is known suggested the reactivity order N-4 > N-3 > N-2.6 The authors attempted to derive a reactivity rationale for the assignment from self-consistent field (SCF)-MO-CNDO calculations. Total charge densities and the frontier orbital... 

The former postulate is the more attractive in that it allows more freedom of assignment, and is the currently favoured model (Hotta and Anderson, 1959 Piette, 1960). The latter has been discussed by two groups simultaneously in terms of restricted rotations within the radical (Abraham et al., 1958 Ingram et al., 1958). However, in neither instance nor in subsequent discussion (Symons, 1959) was it possible to obtain a really self-consistent model for the whole spectrum. 

The self-consistent statistical exchange (Xa) multiple scattered wave method, which is well described in Ref. (58), has also been used by Bair a and Robaux (59) to assign the peaks in the UP-spectrum of CO/Ni. 

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