Hydrocarbon/Water Interactions: Encouraging Energetics and Structures from DFT but Disconcerting Discrepancies for Hessian Indices.

Abstract

In this work, ab initio electronic structure computations have been used to systematically examine the structures and energetics of nine small hydrocarbon molecules interacting with water. Full geometry optimizations and harmonic vibrational frequency calculations were performed on 30 unique dimer configurations with the MP2 method and a triple-ζ correlation consistent basis set (cc-pVTZ for H and aug-cc-pVTZ for C and O, denoted haTZ). Three different estimates of the CCSD(T) complete basis set (CBS) limit interaction energies were determined for all 30 MP2 optimized hydrocarbon/water structures, and they never deviate from their mean by more than 0.07 kcal mol(-1). MP2 and CCSD(T) interaction energies are virtually identical (within 0.05 kcal mol(-1)) for dimer configurations primarily exhibiting CH···O and OH···C type interactions, but MP2 overbinds appreciably in some dimers that exhibited OH···π type interactions, by as much as 0.3 to 0.4 kcal mol(-1) (or ≈10%) for the unsaturated cyclic hydrocarbons examined (1,3-cyclobutadiene, 1,3-cyclopentadiene, and benzene). Four density functional theory (DFT) methods (B3LYP, B97-D, ωB97X-D, and M06-2X) were also applied to all 30 systems with the haTZ basis set to compare optimized structures, energetics, and numbers of imaginary vibrational frequencies (ni). The B97-D, ωB97X-D, and M06-2X functionals provide quite reasonable structures and energetics, which is consistent with other studies. This work, however, finds that all 4 DFT methods examined struggle to reliably characterize these potential energy surfaces (PESs). For example, the values of ni from the DFT frequency calculations differed from the corresponding MP2 results for approximately one-third of the stationary points located.