Results of Semi-Empirical Calculations

You can use the information obtained from semi-empirical calculations to investigate many thermodynamic and kinetic aspects of chemical processes. Energies and geometries of molecules have clear relation ships to chemical ph en om ena. 0ther quan tities, like atomic charges and Frontier Orbitals, are less defined but provide useful qualitative results. 

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In agreement with the previously reported theoretical study, the results of semi-empirical calculations showed that the formation of the Dewar isomer is favored [99H(50)1115]. Probably, the observed formation of the azirine derives from a thermal isomerization of the first photoproduct, in line with that described in the case of furan and thiophene derivatives (Fig. 11). 

For 2-aminofurans the results of semi-empirical calculations have been found to be entirely consistent with the regioselectivity of substitution and cycloaddition reactions (93JHC113, 97JOC4088, 01JCS(P1)680). 

There are indications that planar Co(II) complexes in coordinating solvents show high-spin/low-spin equilibria (16, 48). The high-spin states are stabilized by strong axial interaction (38). This is in agreement with the results of semi-empirical calculations showing that, in the base adducts, some quartet levels are even closer to the ground state than in the four-coordinated planar complexes. Their influence in EPR spectroscopy is mainly on the hyperfme parameters. This will be discussed in Chapter VII. 

The results of semi-empirical calculations on isolated chain segments and on interacting pairs of chain segments of PVDY and VDY/VY copolymers (Y=F, Cl or Br), utilizing both force field and quantum mechanical techniques, have been summarized, and used to draw as many conclusions as possible, from such a limited set of calculations, about chain packing patterns and stabilities. 

This result indicates that in strictly theoretical calculations, the f functions may almost as well be omitted unless they can be optimized for the London energy itself. For the purpose of semi-empirical calculations, however, the /A functions from the polarizability must be retained for the substitution in the London energy. The error for hydrogen atoms is only about 4 per cent, however, and there does not appear to be any reason that it would increase greatly in more complex systems. 

The conformational equilibria of three 2-substituted-l,3-dithianes 112-114 were studied by C NMR spectroscopy at various temperatures and in different solvents (Scheme 6), and both enthalpy and entropy differences were evaluated (Table 8) <1999T359>. The predominance of the axial conformers proved to be of enthalpic origin, in opposition to the entropic contribution which favors the equatorial conformers. The more polar solvent, on the other hand, stabilizes the more polar equatorial conformation. Parallel DFT calculations in the gas phase and in solution emphasized that both 2-substituents point outside the 1,3-dithiane ring system in the two conformations, for example 113-ax and 113-eq, and reproduce the experimentally observed conformational equilibria. Thus, the results of semi-empirical PM3 calculations <1997JMT(418)41> were able to be amended. 

Results from semi-empirical calculations are not as good as those from other models, but somewhat better than might have been anticipated on the basis of previous reaction energy comparisons (note, however, their favorable performance for relative CH bond dissociation energies). 

This is in contrast to the results obtained following selective excitation of the PH2 unit discussed above, and yielding a multi-step electron transfer leading to charge separation. The different outcome can be discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semi-empirical calculations [48]. Remarkably, the zinc porphyrin based array PZn-Ir-PAu, 254+, displays an efficient electron transfer with the formation of a CS state with unitary yield also upon excitation of the iridium complex. This happens because the selective excitation of the zinc porphyrin chromophore discussed above, and the deactivation of the excited state PZn-3Ir- PAu, follow the same paths as those reported in Scheme 8. 

One can consider not only open, but semi open and closed bottles also. As it follows directly from the results of semi empirical PM3-calculations the main peculiarities of construction to be described above are typical for all kinds of the (n,n)+(n,0) bottles. 

The results of semi-empirical PM3 calculations confirm the possibility of existence of the different types of nanotubes of variable diameter. 

Bands C-E in the photoelectron spectrum of COClj (Table 17.11) are assigned to ionizations from orbitals which are predominantly chlorine based and bands F and G are assigned to ionizations from the t(CO) and initio calculations (see Table 17.10) predict the (r(CO) 10a, level to be significantly more stable than the x(CO) 2b j level hence the assignment of band F to ionization from the x(CO) and band G to a(CO), in accord with Thomas and Thompson [2028]. The only evidence to support Chadwick s [349,350] counter-intuitive placing of the level above the x(CO) level are the results of semi-empirical CNDO/2 calculations. Bands H and I are assigned to ionization from the... 

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. 

In such studies the form of the N=XY fragments was also subjected to refinement It has been shown that deviations from linear structures, if they occur, should not exceed 10°. However, even such a possibility makes the choice of molecular models quite a complicated problem. The electron diffraction data have been found to be consistent with two sets of parameters for the C1N=C=0 molecule, depending on the assumption of linear or bent geometry for the N=C=0 group. The authors of prefer the bent model since it gives a better fit with the microwave data and the results of semi-empirical CNDO/2 calculations. 

A variety of different models of the interface between water and a solid phase have been used in computer simulations. As far as the solid is concerned, a basic distinction can be made between smooth solid phases without atomic structure on the one hand and corrugated surfaces on the other. The latter surfaces have been modeled as rigid (frozen) or flexible atomic lattices representing the solid phase [47-51] or as a corrugated external potential that describes the effect of the solid phase by a more or less elaborate potential function F(x,y,z) [52-56]. The generic metallic features are modeled by treating the metal phase as a medium of infinite dielectric constant or by using the jellium model (e.g.. Ref. 57-59). In several cases, the results of semi-empirical and ab initio quantum chemical calculations have been parametrized [40, 48, 55]. 

The He (I) p.e. spectra for the ylides Me3As=CH2, Me3As=CHSiMc3, and Me3As=C(SiMc3)2 have been compared with data for similar phosphorus compounds and the results of semi-empirical M.O. calculations. 

Raff used a six-body potential energy surface based on the results of semi-empirical and ab initio calculations and the thermodynamic data for reactants and products, but not adjusted to fit kinetic data for the reaction. Cross-sections for reactions (53) and (54) and for the corresponding reactions (55) and (56) of T ... 

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