Conformation energy, theoretical calculation

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

Theoretical calculation at the HF/6-31G level99,102 on some S-S dications and their precursors have shown that the electronic structure of 1,4-dithionia-bicyclo[2.2.0]hexane and the sp-sp conformation of the tetramethyldisulfonium dication, the difference in the energy levels of [S] — [S] and [S] + n[S]103 is decreased owing to steric strain and the order of orbitals thus corresponds to case B. In the less strained systems (l,5-dithioniabicyclo[3.3.0]octane, 1,4-dithioniabicyclo[3.2.0]heptane), the order of orbitals corresponds to case C. Interestingly, ap-ap conformer of tetramethyldisulfonium dication was reported to correspond to case A. 

It is well accepted that the high diastereospecificify of aldehyde allylboration reactions is a consequence of the compact cyclic transition structure. Theoretical calculations have shown that the chairlike transition structure shown in Scheme 1 and Fig. 1 is the lowest in energy relative to other possibilities such as the twist-boat conformation. With boronate reagents, it has also been suggested that a weak hydrogen bond between the axial boronate oxygen and the hydrogen of the polarized formyl unit contribntes to the preference for the transition structme with the aldehyde substituent in the psendo-eqnatorial position. ... 

Theoretical calculations (6-31G ) have been used to compare the energies of four possible transition states for Diels-Alder reaction of the BF3 complex of methyl acrylate with 1,3-butadiene. The results are summarized in Fig. 6.4. The endo transition state with the s-trans conformation of the dienophile is preferred to the others by about 2kcal/mol.14... 

The photoelectron spectra of 1,2,4-trioxolane and 1,2,4-trithiolane were in good agreement with theoretical calculations which assumed a half-chair conformation as a starting point. 1,2,4-Trithiolane gave the following characteristic vertical ionization potentials (eV) 8.72 ( (SS)), 9.10 ( (SS) — n(S)), 10.99 (n" (SS)-I- (S)), and 11.06 ((t(CS)) <76CJC146> (see also Section 4.16.2 for energy calculations). 

The selection of theoretical model with which to obtain the energy profile should be based on documented performance with regard to calculation of relative conformer energies and barrier heights. Full discussion has already been provided in Chapter 8. 

The situation appears decisively different in 3-formyl derivatives. Theoretical calculations (77JCS(P2)1601 79NJC473) reveal a small energy difference between X,0-cis and X,0-trans conformations in furan-3-carboxaldehyde (4, X = O), and pyrrole-3-carboxaldehyde (4, X = NH), and the X,0-cis form is... 

In mobile equilibria, the increase in the dielectric constant of the solution should enhance the population of the conformer with higher polarity (SOM 18), as is also proved by theoretical calculations. The solvation energy may lead to inversion in isomer stability on going from the vapor phase to a solution of a certain polarity, as occurs for furan-2-carboxaldehyde (72T3015). Inversion in conformer stability may also occur on going from the vapor to the liquid or solid state. 

This section presents X-ray diffraction, dynamic NMR and theoretical data on the conformations of heterocyclic eight-membered rings. Ideally, it would be desirable to know the whole conformational potential energy surface of the ground electronic state of a molecule. In practice, the lowest (and perhaps the second lowest) energy conformation can be more or less characterized, and some conformational energy barriers can be determined and calculated. 

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