The epimerization process of the model sugar 2-tetrahydropyranol was studied by means of ab initio calculations. The results suggest that the rate limiting step of sugar ring opening involves a high-energy intramolecular proton transfer reaction or a low-energy process in which the proton transfer is mediated by a catalyst molecule, formic acid in the case investigated. The catalyzed process is an asynchronous concerted double proton transfer reaction, where both protons are transferred within the same elementary step but one of them is transferred much earlier than the other one along the reaction coordinate. The motion of both protons in the transition state of the catalyzed process is strongly coupled with the breaking of the C-O bond of the sugar ring. Geometry optimization at the B3LYP/6-31G* level, with additional p polarization functions located on the hydrogen atoms involved in proton transfer, appears to be suitable for further MP2/6-31G** single point energy calculations, as it provides hydrogen bond and activation energies in good agreement with those obtained from geometry optimization at the full MP2 level of theory
A theoretical study of hydrogen bonding, proton transfer and kinetic isotope effects in the dimers of 2-tetrahydropyranol and in the 2-tetrahydropyranol-H2O adducts / Morpurgo, Simone; Bossa, Mario; Morpurgo, G. O.. - In: PHYSICAL CHEMISTRY CHEMICAL PHYSICS. - ISSN 1463-9076. - STAMPA. - 3:(2001), pp. 4898-4906.
A theoretical study of hydrogen bonding, proton transfer and kinetic isotope effects in the dimers of 2-tetrahydropyranol and in the 2-tetrahydropyranol-H2O adducts
MORPURGO, Simone;BOSSA, Mario;
2001
Abstract
The epimerization process of the model sugar 2-tetrahydropyranol was studied by means of ab initio calculations. The results suggest that the rate limiting step of sugar ring opening involves a high-energy intramolecular proton transfer reaction or a low-energy process in which the proton transfer is mediated by a catalyst molecule, formic acid in the case investigated. The catalyzed process is an asynchronous concerted double proton transfer reaction, where both protons are transferred within the same elementary step but one of them is transferred much earlier than the other one along the reaction coordinate. The motion of both protons in the transition state of the catalyzed process is strongly coupled with the breaking of the C-O bond of the sugar ring. Geometry optimization at the B3LYP/6-31G* level, with additional p polarization functions located on the hydrogen atoms involved in proton transfer, appears to be suitable for further MP2/6-31G** single point energy calculations, as it provides hydrogen bond and activation energies in good agreement with those obtained from geometry optimization at the full MP2 level of theoryI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.