Systematic characterization of the homogeneous and heterogeneous hydrogen halide dimers

This study systematically characterizes the four homogeneous and six heterogeneous hydrogen-bonded dimers formed by hydrogen halide pairs (HX/HY where X, Y = F, Cl, Br, and I). The notation HX⋯HY indicates the direction of the hydrogen bond from the HY donor to the HX acceptor. All stationary points reported for these ten dimer systems are fully optimized utilizing the MP2 and CCSD(T) ab initio methods in conjunction with quadruple-ζ correlation-consistent basis sets augmented with diffuse functions, and their nature is verified by harmonic vibrational frequency computations. The electronic dissociation energies (De) for all ten global minima are evaluated near the CCSD(T) complete basis set (CBS) limit via extrapolation schemes. These values are 19.11, 8.32, 7.38, and 6.22 kJ mol-1 for the homogeneous dimers of HF, HCl, HBr, and HI, respectively. For the heterogeneous pairs, the lighter hydrogen halide is consistently the donor in the global minimum configuration, with De ranging from 12.23 kJ mol-1 for HCl⋯HF to 7.22 kJ mol-1 for HI⋯HBr near the CCSD(T) CBS limit. Interestingly, not all heterodimer donor/acceptor permutations correspond to minima. For example, the HCl⋯HBr configuration is identified as a local minimum at all levels of theory employed in this investigation, whereas the in-plane barrier for donor/acceptor exchange vanishes for HCl⋯HI and HBr⋯HI when larger quadruple-ζ basis sets are utilized. For the seven dimer systems containing Br and/or I, the structures, energetics, and vibrational frequencies computed using conventional valence-only electron correlation procedures are similar to those obtained using an expanded valence treatment that includes the (n - 1)d subvalence electrons associated with Br and I.