Molecular orbital method heteroatoms

The hetero atom parameters have been obtained in various ways, for example by striving for a best correlation of HOMO values with ionization energies, or of polarographic reduction potentials with LUMO values. The whole subject of SHM parameters and best heteroatom parameters is now of little practical importance, since much better quantitative molecular orbital methods are now readily available. 

Molecular mechanics calculations have been carried out on a wide range of chemical compounds including hydrocarbons, heteroatomic molecules, steroids, carbohydrates and proteins. Furthermore, a variety of information has been obtained such as heats of formation, rotational barriers, and rates of reaction (18,2,19). The major advantage of this method over other computational methods is that it is reasonably fast to perform in comparison to formal molecular orbital calculations. 

In Eqs. (3) and (4), H stands for the effective one-electron Hamiltonian the integration is over the whole space. In the simple method, a number of simplifying assumptions about the values of the integrals ftjk, j, and Sjk are introduced in the case of molecules containing no heteroatoms these are known as the Hiickel approximations. It seems useful to use this designation also for molecules with heteroatoms, and in the present review this method will be referred to as the Hiickel molecular orbital (HMO) method according to Streitwieser s suggestion.4... 

The method of molecular orbitals is of great value for quantitative discussions, especially of molecules containing heteroatoms. 

To synthesize a new electronic system, tri-heteroatom-substituted cyclopropenium, we thought it useful to examine beforehand the stability of the proposed trisubstituted cyclopropenium system by quantum mechanical methods (or, in other ways). A simple Hiickel molecular orbital calculation for the tri-heteroatom-substituted cyclopropenimn cation 3) gave the results shown in Table 1, where qc and -pc-c denote the n electron density on the ring carbon atom and the n bond order between ring carbon atoms, respectively. Ai it (additional resonance energy) as the measure of stability of 3) is a value obtained as a resonance energy difference between 3 a) and 3 b). 

A recent comparison (33.) of quantities calculated from molecular electronic wave functions for pyrrole and pyrazole indicated that ARCANA values compared more favorably with large-scale ab initio values than those calculated by other non-rigorous methods in the case of orbital energies of occupied molecular orbitals, gross atomic populations of heteroatoms, and total overlap populations including negative overlap populations between nonbonded atoms. 

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