Relativistic strengthening of hydrogen bonds in bihalide anions: A four-component CCSD(T) study

In this study, we systematically investigate relativistic effects in bihalide anions [XHX]- (X = F, Cl, Br, I) using four-component coupled-cluster calculations based on the Dirac-Coulomb Hamiltonian at the CCSD(T) level. Relativistic contributions are quantified by comparing results obtained with the four-component Dirac-Coulomb and Lévy-Leblond Hamiltonians, both combined with a Gaussian nuclear charge distribution model. Relativistic effects are intrinsic to molecular systems and can, in principle, modify hydrogen-bond strengths alongside solvation, nuclear quantum effects, and nuclear dynamics. We identify two distinct regimes of relativistic behavior for bihalide anions with light (F, Cl) and heavy (Br, I) halogen nuclei. Relativistic effects lead to a measurable enhancement of hydrogen bonding as reflected in rovibrational constants (vibrational frequencies and rotational constants), equilibrium geometries, thermodynamic parameters, and the electron-density distribution, in contrast to the known trend of relativistic weakening of covalent bonding in the neutral hydrogen halides HX (X = F, Cl, Br, I). The maximum relativistic strengthening of the hydrogen bond is about 0.6-0.7 kcal/mol, while the relativistic localization of electron density in the internuclear region amounts to ∼0.1%-3.0%.