Molecular-orbital calculations interconversion

Only a few molecular orbital calculations regarding the structures, conformational changes and bonding of S molecules have been published. Fimdamental reactions of sulfur rings like interconversions are only poorly understood, and more examples for the formation of homocyclic derivatives directly from the corresponding S parent molecules are likely to be found. These few comments show that much more work has to be done before the chemistry of elemental sulfur can be regarded as well known . 

Pertinent reviews published during 1980 cover cyclometallation of P-donor ligands, CO insertion into metal-carbon o-bonds, mechanistic features of catalytic CO hydrogenation reactions, and stoicheiometric reactions of transition-metal carbene complexes. Other articles of interest deal with the stability of metal-carbon bonds, " transition-state geometry for insertion of metals into C-H bonds, organic synthesis using Group VIII metal complexes, and C-H bond activation by transition metals. " Molecular orbital calculations on the interconversion of metal bis(olefin) and metallocyclo-pentane complexes have been reported. ... 

Next, examine the equilibrium structure of acetamide (see also Chapter 16, Problem 8). Are the two NH protons in different chemical environments If so, would you expect interconversion to be easy or difficult Calculate the barrier to interconversion (via acetamide rotation transition state). Rationalize your result. Hint Examine the highest-occupied molecular orbital (HOMO) for both acetamide and its rotation transition state. Does the molecule incorporate a n bond. If so, is it disrupted upon rotation ... 

Julg et al.24 employed an approximation based on the self consistent field molecular orbital method to evaluate the average energy per atom for various structures. They calculate that whereas the normal b.c.c. structure is more stable for clusters containing more than 106 atoms, smaller clusters prefer to take up pentagonal symmetry. However, these authors make an important point, namely, that the calculated energies for different structures are very similar. Interconversion of different structures will be facile, and external factors such as the method of deposition, level of impurities, support effects, etc., may cause the less stable structure to grow. For example, impurities on... 

The distinction between atomic orbitals and basis functions in molecular calculations has been emphasized several times now. An illustrative example of why the two should not necessarily be thought of as equivalent is offered by ammonia, NH3. The inversion barrier for interconversion between equivalent pyramidal minima in ammonia has been measured to be 5.8 kcal mol However, a HF calculation with the equivalent of an infinite, atom-centered basis set of s and p functions predicts the planar geometry of ammonia to be a minimum-energy structure ... 

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