Correlation models heteroatoms

Similarly, using another model based on heteroatom increments proposed by Dischler (267), it was possible to calculate a very satisfactory empirical geometry for thiazole (122) with the aid of Shoolery s correlation (268) between and C-H bond length. 

Carbon atoms are classified depending on their hybridization and whether their neighbors are carbon atoms or heteroatoms. Halogen atoms are classified by the hybridization and oxidation state of the C atom to which they are attached. O, S, Se, N, and P are classified in different ways. The model uses 120 different atom-type descriptions and has been developed with a training set of 893 compounds. Observed versus calculated log Kow showed a correlation coefficient of 0.926 and a standard deviation of 0.496. This method has been implemented in the Toolkit. Applications are shown in Figures 13.4.5 and 13.4.6 for the same compounds used to illustrate the Broto et al. method (Figs. 13.4.2 and 13.4.3). 

A DFT study of enolborane addition of o -heteroatom-substituted aldehydes has focused on the relevance of the Cornforth and polar Felkin-Anh (PKA) models for asymmetric induction.154 Using chiral substrates, MeCH(X)CHO, polar (X = F, Cl, (g) OMe) and less polar (X = SMe, NMe2, PMe2) substituents have been examined. The former favour Cornforth TS structures, the latter PKA. TS preferences have been correlated with the relative energy of the corresponding rotamer of the uncomplexed aldehyde. An in-depth study of addition of ( > and (Z)-enolborane nucleophiles to 2-methoxypropanal successfully predicts experimentally determined diastereofacial selectivities. 

In fact, very large basis set, correlated calculations suggest that the CH tautomer is favored over the NH in the gas phase by only 6.2 kcal/mol. Assuming an approximately equal mixture of the two tautomers in aqueous solution,3i3 the AMI-SMIa model is within about 1 kcal/mol of correctly predicting the relative free energies of solvation. The more general AM1-SM2 model is not quite as accurate in its consideration of heteroatom-bound protons, and it thus provides a somewhat poorer prediction in this instance, although it remains closer than any of the other models. 

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