ANOMERIC AND REVERSE ANOMERIC EFFECT IN ACETALS AND RELATED FUNCTIONS

Ab initio geometry optimizations were performed on various conformations of XH(m)-CH2-OH and XH(m)-CH2-NH2 and corresponding protonated systems with XH(m) = F, OH and NH2. By combining Fourier analysis with energy decomposition methods developed in a previous paper, the electronic part of the anomeric stabilization energy, e, could be extracted. For neutral systems, the values of eo are -2.0 for X = F, -1.0 for OH and -0.7 for NH2. The values of e(N) are -3.6 for F, -1.9 for OH and -1.0 for NH2. e(O) refers to stabilization due to O and e(N) due to N. For protonated systems the results for e(O) are -20.5* for FH+, -6.2 for OH2+, and -3.5 for NH3+ e(N) values are -30.6* for FH+, -17.3* for OH2+, and -5.7 for NH3+. The units are kcal/mol. In the cases marked by asterisk, charge-dipole complexes are formed. Energy values for the reverse anomeric effect, operative in protonated systems, are v(O) = -7.5 for OH2+, -8.0 for NH3+, and v(N) = -7.5 for NH3+. In other cases, complexes are formed. The mechanism for electronic stabilization is explained by partial pi-bonding in the CH2-OH or CH2-NH2 fragment of the molecule.