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Calculation of Entropy and Heat Capacity of Organic Compounds in the Gas Phase. Evaluation of a Consistent Method without Adjustable Parameters. Applications to Hydrocarbons

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journal contribution
posted on 24.05.2007 by DeLos F. DeTar
The purpose of this study has been to determine how well a consistent ab initio thermostatistical method reproduces experimental values of heat capacity and entropy. The method has been applied to calculation of heat capacity and entropy of a representative set of hydrocarbons that includes compounds consisting of multiple conformers. All Cp and S values are for the gaseous state at 1 atm; units are cal K-1 mol-1. A detailed sensitivity (error) analysis has been performed to determine the root mean square (rms) values of errors expected of the calculated values:  these are 0.27 cal for Cp and 0.36 cal for entropy. In comparing calculated values with experimental values, it is necessary to consider also the uncertainties of the experimental data. When these are included, the expected rms values of Cp(experimental) Cp(calculated) values at 298.15 K range from 0.21 to 0.73. For S(experimental) S(calculated), they range from 0.36 to 0.72. Calculations with frequencies derived with the 6-31G(d,p) basis set and scaled by 0.91 yielded rms values for Cp(experimental) − Cp(calculated) of individual compounds from 0.14 to 0.84 cal and rms values for S(experimental) − S(calculated) of individual compounds from 0.07 to 1.11 cal. Calculated Cp values for 7 out of 16 compounds agree with experimental values within the rms uncertainty estimated for the compound, and 11 fall within twice that estimate. For entropy, the calculated values for 13 of 18 compounds agree with the very limited available experimental data within the rms estimated uncertainty for the compound, and 16 of 18 fall within twice the uncertainty.