MP2/6-311++G(d,p) study of ten ionic hydrogen-bonded binary systems: Structures, normal modes, thermodynamics, and counterpoise energies

Abstract

Fully optimized structures, normal vibrational modes, and thermodynamic functions were obtained at the MP2/6‐311++G(d,p) level for the following ionic hydrogen‐bonded systems: H₃O⁺/H₂O, NH⁺₄/H₂O, NH⁺₄/NH₃, CH₃NH⁺₃/H₂O, CH₃NH⁺₃/NH₃, OH⁻/H₂O, CH₃O⁻/H₂O, cyanide/H₂O, HCC⁻/H₂O, and formate/H₂O. The calculated ΔH fell within error limits of experimental values for all but OH⁻/H₂O and the two NH₃ complexes. In the latter cases expansion of the polarization basis to (2d,2p) satisfactorily improved the model, but for OH⁻/H₂O increasing the level of correlation to MP4 was more important. Where comparison was possible, optimized structures were quite similar to those obtained at MP2/6‐31+G(d,p) by other workers. All the complexes examined here have a hydrogen bond stretching mode in the 200–350 cm⁻¹ range which may contribute to proton transfer, and whose frequency correlates qualitatively with the acid strength of the H‐bond donor. For the cyanide/H₂O system, it is not possible to choose between CN⁻/H₂O and NC⁻/H₂O as the dominant complex in the gas phase: the thermodynamic results suggest a mixture. In formate/H₂O a C_2V ‘‘cyclic’’ complex is thermodynamically favored but a ‘‘linear’’ complex is apparently observed at ∼500 K. This might well be an entropy effect, but the C_2V complex was found here to have a vibrational mode at 92 cm⁻¹ which could facilitate conversion to the linear complex at all but very low temperatures. Counterpoise calculations with ghost functions were done at MP2 for all complexes, and HF/6‐311++G(d,p) for three of the more strongly bound cases. Comparison of the MP2 and self‐consistent‐field results, along with a close examination of the virtual orbitals obtained with and without ghost functions for the components of NH⁺₄/NH₃, supports a recent contention that counterpoise calculations with correlation are highly suspect.