Conjugate addition structure calculations

The resonance effect is greater between the 2- and 4-positions than between the 2- and 5-positions. This is explained in terms of the weak conjugative interaction arising from structure 8.195, there being no comparable interaction between the 2- and 5-positions. Additional MO calculations are described under hydrogen exchange below. 

Hamiltonian (VEH) method [23,24]. The electronic structure calculations are used for a direct comparison with the experimental results. The VEH method has been applied to many conjugated systems [25,26,27], justifying the suitability of the method in interpreting photoelectron valence band spectra. In addition to the electronic band structure, the VEH method provides accurate estimates of the location of the gap states in doped conjugated systems [28]. 

Alkyl derivatives of 1,4-butadiene, in general, undergo photosensitized cis-trans isomerism when photosensitizers that can supply at least 60 kcal/mol are used. An additional structural feature must be considered in treatment of their spectroscopy and photochemistry two conformers of the diene, the s-cis and s-trans, exist in equilibrium, and there are therefore two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-trans and s-cis conformers. Theoretical calculations suggest that the minimum energy for the excited state of conjugated dienes involves essentially an alkyl radical and an orthogonal allyl system (allylmethylene diradical) ... 

Rhodium Conjugate addition of aryl boronic acids to cyclohex-2-enone, catalysed by [Rh(diene)(OH)]2 complexes, afforded the corresponding product with the following enantioselectivities 96% ee with (508) ligand, 83% with (509), and 90% with (510). DFT calculations were used to shed light on the transition state structures and energies. " The sulfoxide-olefin ligand (511) exhibited >99% ee " ... 

The recently determined X-ray crystal structure of cyclopentacorannulene 25 provides an additional opportunity to test the performance of theory in predicting the minimum energy structures of nonplanar conjugated systems. Table 4 presents the C-C bond lengths in the crystal and a comparison with ab initio values calculated at HF/3-21G and HF/6-3IG levels. The agreement between experimental and theoretical bond lengths is very satisfactory with root mean square devia-... 

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