Ab Initio Calculations and Internal Rotor: Contribution for Thermodynamic Properties S°_(298) and C_p(T)'s (300 < T/K < 1500): Group Additivity for Fluoroethanes

摘要

Ab initio calculations are performed on nine fluorinated ethane compounds and thermodynamic properties (S°_(298) and C_p(T)'s 300 < T/K < 1500) are calculated. Geometries of stable rotational conformers and transition states for internal rotation are optimized at the RHF/6-31G~* and MP2/6-31G~* levels of theory. Harmonic vibrational frequencies are computed at the RHF/6-31G~* level of theory. Potential barriers for internal rotations are calculated at the MP2/6-31G~*//MP2/6-31G~* level. Parameters of the Fourier expansion of the hindrance potential are tabulated. Standard entropies (S°_(298)) and heat capacities (C_p(T)'s 300 < T/K < 1500) are calculated using the rigid-rotor-harmonic-oscillator approximation with direct integration over energy levels of the intramolecular rotation potential energy curve. Heats of formation are adopted from literature evaluation and BAC-MP4 ab initio calculations. Thermodynamic properties for fluorinated carbon groups C/C/F/H2, C/C/F2/H, and C/C/F3 are determined by existing thermodynamic group parameter of C/C/H3 and data on CH_2FCH_3, CHF_2CH_3, respectively: no fluorine or other halogen is on the methyl carbon adjacent to the carbon bonded to the fluorine(s). Six interaction terms in addition to the above groups are developed to account for repulsion and steric effects. Interaction terms are required to accurately estimate ΔH°_f S°_(298), and C_p(T)'s (300 < T/K < 1500) for fluoroethanes where fluorine(s) are on carbons adjacent to a carbon bonded to fluorine(s).