Aspirin Isomer 5a

The conformation about the acid C-O bond is s-cis. The carboxy group is oriented such that the OH group is close to the ester group; E with regard to the Ph-COOH bond. The conformation about the ester C-O bond is s-trans. This structure is rather interesting as it allows for an intramolecular H-bond. The acid's hydroxyl group again is the H-bond donor but, in this case, the ester carbonyl-O is the H-bond acceptor.

Figure. The RHF/6-31G* optimized structure of isomer 5a of aspirin.

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