AB-INITIO CALCULATIONS AND INTERNAL ROTOR - CONTRIBUTION FOR THERMODYNAMIC PROPERTIES S-298(O) AND C-P(T)S (300-LESS-THAN-T K-LESS-THAN-1500) - GROUP ADDITIVITY FOR FLUOROETHANES/

Ab initio calculations are performed on nine fluorinated ethane compou nds and thermodynamic properties (S-298(o) and C-p(T)'s 300 less-than T/K less-than 1500) are calculated. Geometries of stable rotational co nformers and transition states for internal rotation are optimized at the RHF/6-31G (6-31G(d)) and MP2/6-31G* levels of theory. Harmonic vi brational 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 hin drance potential are tabulated. Standard entropies (S-298(o)) and heat capacities (C-p(T)'s, 300 less-than T/K less-than 1500) are calculate d using the rigid-rotor-harmonic-oscillator approximation with direct integration over energy levels of the intramolecular rotation potentia l energy curve. Heats of formation are adopted from literature evaluat ion and BAC-MP4 ab initio calculations. Thermodynamic properties for f luorinated 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 CH2FC H3, CHF2CH3, and CF3CH3, 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 req uired to accurately estimate Delta H-f(298)o, S-298(o), and C-p(T)'s ( 300 less-than T/K less-than 1500) for fluoroethanes where fluorine(s) are on carbons adjacent to a carbon bonded to fluorine(s).