Semi-empirical calculations for

More recently, infrared data have found further application in conformational studies of complex quinolizine derivatives, such as the previously mentioned alkaloid cytisine 9. Comparison of experimental values for the and t co) frequencies with the theoretical values obtained from semi-empirical calculations for... 

Semi-empirical calculations for the geometry and dipole moment of tetrasubstituted bis-spiro-l,2,4-trithiolanes derived from adamantanethione were reported <2004JMT(668)179>. Satisfactory agreement with X-ray data was obtained with the PM3 method. 

Semi-empirical calculations for the simple vinyl cation C2H3+ have been reported by Hoffmann (1964) and by Yonezawa et ad., (1968). More rigorous calculations by Sustmann et ad. (1969) are based on a semi-empirical method based on the neglect of diatomic differential overlap (NDDO) calibrated to results of ab initio Hartree-Fock-Roothaan SCF calculations. Recent work by Hopkinson et al. (1971) is entirely based on a non-empirical LCAO-MO-SCF method. 

The properties of oligomers of thiophene at the ground and excited states have been investigated by semi-empirical and ab initio methods. Semi-empirical calculations for the ground state of thiophene involve the QCFF/PI -f CISD method, while ab initio calculations are carried out at the HF/6-31G and the CASSF/3-21G" levels and also at the SCF and the averaged coupled-pair functional (ACPF) levels, with (basis set I) and without (basis set II) the sulfur 3d-orbitals. 

In 1987, Comelisse and coworkers reported a semi-empirical calculation for the meta cycloaddition of benzene with ethylene, which impressively consolidates many experimental aspects of this reaction and flts well with the Mattay analysis. This calculation, performed at the CNDO/S and MNDO levels, supports an exciplex mechanism with charge polarization, in accord with Mattay s treatment and a related mechanism developed earlier in the Comelisse laboratories. According to this mechanism (equa-... 

In these semi-empirical calculations, for reasons of simplicity, one imagined the molecular forces simply as rigid, additive central forces, in general cohesion, like gravitation this presumption actually implied... 

Hence, a quick analysis might use LUMO energies from semi-empirical calculations for the analysis of the reduction of 3 (Scheme 3). Because the LUMO energy of acetone as model for site (a) in 3 is the lowest within the three electrophores, one must expect that an electron will be accepted at this site. This is indeed demonstrated in the PET reduction of 3 to a ketyl radical anionic intermediate using tri-ethylamine as sensitizer [8]. 

A major concern connected with the application of semi-empirical methods to supramolecular problems is that these methods were parameterized to reproduce molecular rather than intermolecular properties. Over the last few years there have been some efforts to improve treatment of the core parameters in semi-empirical methods that play a large role in the nonreproducibility of experimental data [127]. A special problem of semi-empirical methods is that they present an unphysical stabilization effect for short-range H H interactions see the analysis of intermolecular H H interactions in supramolecular chemistry [128] and the history of semi-empirical calculations for this purpose cited therein. 

The sources for all listed properties are either experimental observations or theoretical computations. We do not include results from semi-empirical calculations. For experimental and computational details of literature values the reader must check the original publications. Details for calculations done by us for this chapter with methods of Spartan-02 and -04 (Wave-function, Inc., Irvine, CA, USA) are given in Appendix 17.2. 

Tab. 4.15. Semi-empirical calculations for amine alkylations enhancements of dipole moments from GS to TS, energy of activation evaluated in the gas phase.

Semi-Empirical Calculation for a Preliminary Reaction of the Modification. 

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