The anomeric effect in first-row/second-row acetal-like systems

Ab initio calculations were performed at the HF/6-31+G* level on first-row/second-row neutral, protonated, and anionic acetal-like systems. The neutral and anionic systems are of the general form XH(m)CH(2)YH(n)((-)) with XH(m) = F,a-OH, s-NH2, s-CH3; YHn = SH, PH2; and YHn- = PH-, SiH2-. For the protonated systems the general form is XH(m)(+)CH(2)YH(n), with XH(m)(+) = a-FH+, s-OH2+, s-NH3+; and YHn = SH, PH2. In all cases the hydrogens on Y were rotated about the CY bond, and geometries were optimized for each setting of the YHn dihedral angles. Following previously developed methods, the electronic part e of the anomeric stabilization was evaluated. For PH2 rotation, -e(P) (e is always negative) is smaller than for corresponding nitrogen systems, whereas for SH rotation the -e(S) parameters are larger than for corresponding oxygen systems. In the case of protonated molecules, the energy associated with the reverse anomeric effect was estimated to be -4 kcal/mol for s-OH2+CH2SH and -3 kcal/mol for s-NH3+CH2SH. For corresponding phosphorus systems they are close to -2 and -5 kcal/mol respectively. The pi-bonding model applied to neutral species shows that in the case of SH rotation the F and O systems clearly follow the trend of the pi-bonded cation CH2=SH+. For PH2 rotation, however, only the F system can be considered to correlate with such a model.