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DFT calculations for the equilibrium isotope effect for deuterium substitution at the anomeric centre C α  in 2-( p -nitrophenoxy)tetrahydropyran with continuum solvation show significant variation in the range of relative permittivity 2 ≤ ε ≤ 10. One-dimensional scans of potential energy (with implicit solvation by water) or of free energy (from QM/MM potentials of mean force with explicit aqueous solvation with a hybrid AM1/OPLS method) for heterolysis of the bond between C α  and the nucleofuge do not show a transition state. A two-dimensional free-energy surface that considers also the distance between C α  and a nucleophilic water indicates a pre-association D N *A Nint  ‡  mechanism with a transition state involving nucleophilic attack upon an ion-pair intermediate, and this is supported by good agreement between the mean values of the calculated and experimental α-D KIEs. However, the magnitudes of the standard deviations about the mean values for the making and breaking C–O bonds suggest that the transition state is rather plastic, with C α –O nu ≈2 ± 0.4 Å and C α –O lg ≈3 ± 0.5 Å. Not only is nucleophilic solvent assistance necessary, but there is also evidence for electrophilic assistance through specific hydrogen bonding to the nucleofuge.

Computational simulation of mechanism and isotope effects on acetal heterolysis as a model for glycoside hydrolysis