2.1 Quantum mechanical calculations

QM geometry optimizations were performed at the MP2 level of theory with the aug-cc-pVDZ basis set⁶⁷ for fluorinated and chlorinated species and with the aug-cc-pVDZ-PP basis set^68,69 for brominated and iodinated species. The QM vibrational spectra were calculated following QM geometry optimizations from the same model chemistry, with the resulting frequencies scaled by 0.959⁷⁰ for use as target values. Dihedral angle potential energy scans (PES) were initiated on the QM optimized geometries with the same model chemistry with only the targeted dihedral constrained while the remaining degrees of freedom were allowed to relax. Single point calculations were performed on the optimized geometries to obtain the final QM energies, molecular dipole moments and molecular polarizabilities for use as target data, where the MP2/cc-pVQZ model chemistry⁶⁷ was used for fluoro- and chloro-substituted analogs, and MP2/sdb-cc-pVQZ⁷¹ model chemistry was used for bromo- and iodo-analogs. The aforementioned QM calculations were performed using Gaussian03.⁶⁰

Single-point interaction energies for fluorinated, chlorinated and brominated analogs with water were obtained at the RIMP2 level of theory with the cc-pVQZ basis set⁶⁷ using PSI4.⁶¹ For iodinated analogs, single-point interaction energies were calculated with the sdb-cc-pVQZ basis set⁷¹ at the MP2 level of theory using NWCHEM.⁶² In the interaction energy calculations, the basis set superposition error (BSSE) was corrected using the counterpoise method,⁷² and the intramolecular geometries of the halogenated molecules were kept in their gas phase optimized geometries and water was maintained rigid in the SWM4 geometry.²¹ In calculating the QM interaction energies, when the halogen acts as a hydrogen bond acceptor, the hydrogen atom of water was directed toward the halogen in the perpendicular or linear orientation and served as a hydrogen bond donor (Figure 3a–b and 3d–e). When the halogen participates in a halogen bond interacting with water as a hydrogen bond acceptor, the acceptor atom was directed toward the halogen along the C-X direction (Figure 3c and 3f). The interaction energy profiles were calculated by varying the distance between the halogen and the interacting atom from 1.5 to 5.0 Å in intervals of 0.1 Å.

Water interaction orientations for halogenated benzenes in a, b, and c, and halogenated ethanes in d, e, and f, where water acts as hydrogen bond donors (HBD) in perpendicular and linear orientations (HBD90° and HBD180°) as well as acts as a hydrogen bond acceptor (HBA) in linear orientations (HBA180°). Halogen atom is colored in green, hydrogen in white, carbon in cyan, and oxygen in red.

Single-point interaction energies between chlorobenzene or chloroethane and the rare gases Ne and He were performed at the MP3/6-311++G(3d,3p) model chemistry.^73,74 The interaction energy profile were calculated by varying the distance between the chlorine and the rare gas atom from 1.5 to 5.0 Å in intervals of 0.1 Å with chlorobenzene/chloroethane kept in their MP2/aug-cc-pVDZ⁶⁷ optimized geometry.