1 Note: this is an authors’ version without proof reading. DOI:10.1016/j.carres.2014.10.013. Contact Ion Pairs and Solvent-separated Ion Pairs from D-Mannopyranosyl and D-Glucopyranosyl Triflates Takashi Hosoya,* † Paul Kosma, † Thomas Rosenau † † Department of Chemistry, University of Natural Resources and Life Sciences, Department of Chemistry, Muthgasse 18, A - 1190 Vienna, Austria. * Corresponding author: Email address: takashi.hosoya@boku.ac.at Phone: +43-1-47654-6460 Abstract: The chemical nature of contact ion pairs and solvent-separated ion pairs from 2,3,4,6-tetra-O- methyl--D-mannopyranosyl triflate was investigated at the DFT(M06-2X) level of theory. Comparison of the present results to those obtained for the D-glucopyranosyl counterpart in our previous study indicated that the ion pairs from the D-mannopyranosyl triflate adopt B 2,5 -like conformations more preferably than those from the D-glucopyranosyl triflate. Similarly, 1,6- anhydro-D-mannopyranosyl dioxacarbenium ions bearing a 1 C 4 conformation were shown to be more stable than the D-glucopyranosyl counterparts. Detailed reaction mechanisms in chemical glycoside synthesis are among the hottest research subjects in carbohydrate chemistry. 1 Multiple previous studies on the mechanism have been carried out by means of experimental investigations. 2-8 and theoretical calculation, 2d,9-18 aiming mainly at improving the stereochemical outcome, i.e. /-anomer selectivity. The experimental studies focused especially on glycosyl triflate donors, which are well dissolved in organic solvents making experimental investigation easier. 2-5 Researchers have suggested that glycosylation with a triflate donor occurs through nucleophilic attacks of a glycosyl acceptor to covalent intermediates, contact ion pairs, and solvent-separated ion pairs, all of which are in equilibrium in solution. As the stereochemical outcome is determined by the stability and the reactivity of the covalent and the ion pair species, knowledge of their chemical properties, such as their energies and exact chemical structures, is of considerable importance. A better definition of