Ab initio calculations on PAHTC conformations

In analogy to the syn- and a t/-diastereomers of PAHDE, there are two metabolically possible diastereomeric forms of PAH triol carbocations, syn and anti, each with two conformers differing by the geometry of the hydroxylated ring. 

the conformational preference for the eee and eea forms in vacuo appears to be a consequence of the formation of an additional intramolecular H-bond. On the other hand, in the diaxial conformations the hydrogen atoms of the 1-OH and 2-OH groups are available for intermolecular H-bonds. Therefore the OH group orientations are expected to depend on the molecular environment. In particular, the stability of diaxial conformers should be significantly increased in a molecular environment where all possibilities for H-bonding can be utilised. 

Even after trapping an intermediate bay-region PAHTC [156] there is little prospect to experimentally determine the conformation of reactive TC intermediates. The theoretical predictions can however, be checked indirectly by calculating the relative stabilities of the diaxial and diequatorial forms of PAHDE s, for which experimental estimates are available. NMR proton spin coupling experiments indicate equilibria between the two forms, with the diequatorial favoured for a n -DE s and 

The ab initio calculations indicate that the bay-region PAHTC conformers with pseudo-diaxial orientation of the 1-OH and 2-OH group (structures 6 and 7) are energetically acceptable at room temperature and can be populated by directed conformational change, even if the pseudo-diequatorial conformers were favoured in aqueous solution [112]. This provides a quantum-theoretical rationale for the results of the AMBER force field calculations [112] presented below. 

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