COMPARISON OF AM1 AND PM3 IN MOPAC6 WITH LITERATURE FOR THE THERMODYNAMIC PARAMETERS OF C-1 AND C-2 CHLOROCARBONS

Ideal gas thermodynamic properties, Delta H(f 298)degrees, S(f 298)deg rees, and Cp(T) (300 less than or equal to T/K less than or equal to 1 500), for all C-1 and C-2 and eight C-3-C-6 chlorinated hydrocarbons a re calculated by AM1 and PM3 in MOPAC6 and then compared with the avai lable literature data. The approximation method of Fitter and Gwinn is used to account for internal rotor contributions to S(298)degrees and Cp(T) (300 less than or equal to T/K less than or equal to 1500),in p lace of the torsion frequency estimation in MOPAC. AM1- and PM3-derive d S(298)degrees and Cp(T) (300 less than or equal to T/K less than or equal to 1500) are found to be consistent with the literature data (re lative deviation less than or equal to +/-5%) except S(298)degrees of C2HCl5 and Cp (300-500 K) of chloroacetylenes, Delta H(f 298)degrees o f CCl4, C2HCl3, C2Cl4, and C2Cl2 obtained by AM1 and Delta H(f 298)deg rees of CH2Cl2, CH2ClCH2Cl, CH2ClCHCl2, CHCl2CHCl2, C2Cl4, and C2Cl2 o btained by PM3 are not in good agreement with the literature values (d eviation greater than or equal to 15 kcal/mol), Delta H(f 298)degrees (kcal/mol) calculated by PM3 are corrected as follows: Delta H(f 298,e xperiment)degrees = -2.35 + 1.086 Delta H(f 298,PM3)degrees for the pa rtially chlorinated hydrocarbons; Delta H(f 298,experiment)degrees = 5 .36 + 1066 Delta H(f 298,PM3)degrees for the fully chlorinated hydroca rbons. A modified group additivity method implemented in THERM is show n to be precise for predicting Delta H(f 298)degrees, S(298)degrees, a nd Cp(T) (300 less than or equal to T/K less than or equal to 1500) of C-1 and C-2 chlorocarbons. AM1- and PM3-calculated Delta H(f 298)degr ees for chlorocarbons can be improved by linear correction equations. Comparison of estimations from group additivity, corrected MOPAC6/PM3, and literature data suggests that the modified group additivity is th e better estimation method.