The influence of externally applied electric fields on the intramolecular O–H⋅⋅⋅O hydrogen bond in malonaldehyde was investigated using MP2/6–311++G(d,p) calculations with full geometry optimization under field perturbation. Electric fields aligned with the hydrogen-bond axis increase the electron density at the bond critical point and enhance the n(O) → σ*(O–H) charge-transfer interaction, resulting in strengthening of the hydrogen bond. In contrast, fields applied along the molecular dipole moment induce global charge redistribution and weaken the interaction. Quantum Theory of Atoms in Molecules (QTAIM) analysis shows that the hydrogen bond retains its closed-shell character across all field strengths and orientations, with strengthening under axis-aligned fields arising mainly from enhanced electrostatic polarization. Proton-transfer potential energy surface calculations reveal that fields aligned with the hydrogen bond reduce the proton-transfer barrier and increase the tunneling frequency. These results demonstrate that electric-field orientation plays a key role in modulating hydrogen-bond strength and proton-transfer dynamics

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