Abstract
The gasโphase acidity of Dโglucopyranose was studied by means of B3LYP calculations combined with 6โ31G(d,p) or 6โ31+G(d,p) standard basis sets. For each anomer, deprotonation of the various primary and secondary hydroxyl groups was considered. As in solution, the anomeric hydroxyl is found to be the most acidic for both anomers, but only when the 6โ31+G(d,p) basis set is used for geometry optimization. Deprotonation of the anomeric hydroxyl induces an important C(1)๏ฃฟO endocyclic bond elongation and subsequently promotes an energetically favored ringโopening process as attested by the very small calculated activation barriers. The results also suggest that interconversion between the various deprotonated ฮฑโ and ฮฒโanomers may easily occur under slightly energetic conditions. B3LYP/6โ311+G(2df,2p) calculations led to the an absolute gasโphase acidity of ฮ~acid~G^o^~298~(ฮฑโDโglucose) = 1398 kJ mol^โ1^. This estimate matches well the only experimental value available to date. Finally, this study again confirms that the use of diffuse functions on heavy atoms is necessary to describe anionic systems properly and to achieve good relative and absolute gasโphase acidities. Copyright ยฉ 2004 John Wiley & Sons, Ltd.