AB INITIO STUDY ON THE CONFORMATIONAL BEHAVIOUR OF ETHANE-1,1-DIOL and ETHANE-1,1,2-TRIOL IN SOLUTION SAFiYE SA ˇ G ERDEM, TEREZA VARNALI* AND ViKTORYA AViYENTE Chemistry Department, Boˇ gaziçi Univerisity, 80815 Bebek-Istanbul, Turkey Ab initio optimizations at the HF/6–31G level and single-point calculations at the MP2/6–31G**//6–31G level were performed on ethane-1,1-diol and ethane-1,1,2-triol. Their conformational properties are discussed in terms of the anomeric effect, gauche effect and internal O–H interactions. The results showed a parallel behaviour with ethane- 1,2-diol. The solvent effect was taken into account using the SCRF theory with a general cavity shape which is defined by the molecular surface. © 1997 by John Wiley & Sons, Ltd. J. Phys. Org. Chem. 10, 196–206 (1997) No. of Figures: 4 No. of Tables: 9 No. of References: 12 Keywords: ethane-1,1-diol; ethane-1,1,2-triol; conformation; ab initio study Received 17 June 1996; revised 29 November 1996; accepted 17 December 1996 INTRODUCTION The study of the conformational preference of substituted ethanes has been the focus of considerable attention for a long time because many interactions that occur in them serve as a key to understanding the structures of large cyclic and acyclic compounds. In addition to many experimental and theoretical studies, a considerable amount of ab initio work has been reported in the literature on ethane derivatives. 1 The small size of these molecules allows high- level treatments, in terms of both basis sets and electron correlation. Among the substituted ethanes and propanes, poly- hydroxylated molecules have been widely studied by both experimental 2 and theoretical methods. 3 These structures are related with biological chemistry, such as carbohydrates. 4 On the other hand, carbohydrates are complex molecules whose structural features are strongly influenced by special conformational effects such as the ‘anomeric effect’ and ‘gauch effect’ or intramolecular hydrogen bonding. Whereas sugars are too large for full conformational analysis using ab initio calculations, polyhydroxyethanes are attractive models for studying these types of inter- actions. In this study, we focused out attention on the structural and conformational properties of ethane- 1,1,2-triol, ethane-1,1-diol, and ethane-1,2-diol. These are the simplest representatives of the functional groups of carbohydrates and cyclic compounds such as dioxanes and acetals, and will be of help in understanding the conforma- tional effects related to their analogues. The most dominant conformational effect controlling the structural features in carbohydrates is known as the ‘anomeric effect.’ This refers to the tendency for an electronegative substitutent at C1 of a pyranoid ring to prefer the axial rather than equatorial orientation, 5 despite the predictions based on only steric interactions. It soon became clear that this phenomenon is not restricted to carbohydrates or six-membered hetero- cycles. Molecules containing an R—O—C—O atomic arrangement prefer the gauche (axial) orientation, which is associated with variations of C—O bond lengths and O—C—O bond angles. 6 The variations have been explained as arising from interactions between the non- bonded electrons on oxygen (n p ) and antibonding C—O bond orbital (*c—O) which form the basis of the anomeric effect. There is a stereoelectronic preference for the conformation in which non-bonded electrons on one oxygen adopt an antiperiplanar orientation with respect to the C—O bond involving the other oxygen, as seen in the gauche orientation in Figure 1. This arrangement produces lengthening of the C—O (acceptor *) bond and shortening * Correspondence to: T. Varnalı. Contract grant sponsor: Boˇ gaziçi University Research Funds; Contract grant number: 89EB0599. Figure 1. Anomeric effect JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 196–206 (1997) © 1997 by John Wiley & Sons, Ltd. CCC 0894–3230/97/040196–11 $17.50