Influence of solvation on the hydrogen bond in hydrogen malonate An ab initio and semiempirical study

Abstract

The energy difference ΔE EZ in vacuo between the closed (E) and open (Z) forms of the hydrogen malonate ion was computed at the ab initio HF/6–31G(d,p), 6–311++G(d,p), DFT B3LYP/6–31G(d, p) and B3LYP/6–311++G(d,p) levels. The effects of the polar environment were simulated by attaching discrete water molecules, by the self-consistent reaction field model (SCRF) and by the combination of both approaches. These simulations were done at the HF/6–31G(d,p) level. The SCRF approach applied to both forms of the HM tetrahydrate yields the free energy difference of −0.73 kcal mol −1, slightly favouring the open form; this contrasts the stabilization by ~20kcal mol −1 of the E form in vacuo. The metric parameters of the intramolecular hydrogen bond indicate a slight weakening of the intramolecular H-bond by the influence of hydration. Calculations at the PM3 level did not reproduce the ab initio computed ΔE EZ values.