An improved model to calculate equilibrium constants for formation of peroxy radical-water complexes

作者：Shirts Randall B^*; Kumbhani Sambhav R; Burrell Emily; Hansen Jaron C

来源：Theoretical Chemistry Accounts, 2018, 137(7): 96.

DOI：10.1007/s00214-018-2262-8

• Equilibrium constant
• Peroxy radical-water complex
• Radical-water complex
• Partition function

Recent experimental results show that the kinetics of some radical-radical reactions important for atmospheric pollution formation are faster when a radical-molecule complex forms as one step in the reaction mechanism. Calculated radical-molecule equilibria are needed to accurately describe the concentrations of complexes formed in these experiments as well as in the atmosphere. Here we report calculation of the equilibrium constant for complexation of hydroperoxy (HO2 center dot) and 2-hydroxyethylperoxy radicals (HOCH2CH2O2 center dot) with one water molecule by directly calculating the canonical partition function of the reactant and product species. We demonstrate the accuracy of the calculation using formation of the water dimer as a test case. Ab initio calculations provide the binding energy, rotational constants, and vibrational frequencies of both monomers and complexes. We demonstrate the failure of the harmonic approximation in the partition function for describing the low-frequency vibrational modes of the complexes. Instead, we model one dissociative hydrogen bond mode using a Lennard-Jones 6-3 potential and the other low-frequency vibrational modes using one-and twofold hindered rotors. The contributions of weakly bound states of the long-range dipole-dipole potential (Lennard-Jones 6-3) and of vibration-rotation coupling are not as important as the contribution of twofold hindered rotors. We also discuss methods for including multiple hydrogen-bonding configurations (local minima) when calculating equilibrium constants for formation of complexes.

• 出版日期2018-6-19