Thermochemical Properties: Enthalpy, Entropy, and Heat Capacity of C2-C3 Fluorinated Aldehydes. Radicals and Fluorocarbon Group Additivity.

Abstract

Thermochemical properties of fluorinated aldehydes are important for understanding their stability and reactions in the environment and in thermal processes. Structures and thermochemical properties of C1 to C3 fluorinated aldehydes are determined by use of computational chemistry. Standard enthalpies of formation for 30 C2- and C3-fluorinated aldehydes and 31 radicals were calculated with 11 different ab initio and density functional theory methods: CBS-APNO, CBS-4M, CBS-QB3, M06-2X, ωB97X, B3LYP, G-2, G-3, G-4, and W1U via several series of isodesmic reactions. Entropy, S°298, and heat capacities, C p( T)'s (300 ≤ T/K ≤ 1500) from vibration, translation, and external rotation contributions are calculated on the basis of the vibration frequencies and structures obtained from the B3LYP/6-31++G(d,p) density functional method. Potential barriers for the internal rotations are also from this method and used to calculate hindered rotor contributions to S°298 and Cp(T)'s using direct integration over energy levels of the internal rotational potential curves. Literature data on standard enthalpies of formation of fluorinated aldehydes are compared. Thermochemical properties for the fluorinated carbon groups CO/C/F, C/CO/F3, C/CO/F/H2, C/C/CO/F/H, C/C/CO/F2, and C/C/CO/F/H are developed. Non-next nearest neighbor terms for the strong interactions resulting from fluorine atoms on adjacent and on second nearest carbon atoms are unfortunately, needed. The required non-next-neighbor interactions significantly reduce the practical application of group additivity for thermochemical properties of highly fluorinated halocarbons.