Quantitative Structure–Property Relations (QSPRs) for Predicting the Standard Absolute Entropy (S298 K°) of Gaseous Organic Compounds

Predicting gas phase entropy of select hydrocarbon classes through specific information-theoretical molecular descriptors

The usefulness of five specific information-theoretical molecular descriptors was investigated for predicting the gas phase entropy of selected classes of acyclic and cyclic compounds. Among them, total information on atomic number (TI^Z), graph vertex complexity (H^V) and total information on bonds (TIB^AT), considered together showed the best correlation along with a low standard deviation (r² = 0.97, s = 21.14) with gas phase entropy values of 130 compounds. The multiple regression equation treating these three indices as independent variables was statistically highly significant which was evident from the F-statistics. In particular, very small difference between r² and r²-pred values indicates that the regression model is not overfitted and is, therefore, suitable for prediction purposes. When truly used as a training set to predict (from regression equation) 40 additional compounds we get a very high correlation (r² = 0.975), which remains almost identical (r² = 0.97) for the combined data set of 170 compounds. The three indices appear to be useful descriptors producing correlation that remains stable with the change in the size of the data set. Also, the information-theoretical measures appear to capture an additive-cum-constitutive nature of gas phase entropy yielding an acceptable statistical fit.

Correlation model to estimate the thermodynamic properties

A technique for constructing the correlation dependencies of the thermodynamic properties of similar compounds was proposed. This technique is based on the Taylor expansion in the supposed analytic relationship between properties. The constructed correlation dependencies were used to estimate the thermodynamic properties of the condensed bromides and iodides of hafnium HfGn (G [Formula: see text] Br, I and [Formula: see text], 2, 3, 4) and the compounds formed in the PbO–SiO2 system. Standard formation enthalpy [Formula: see text], entropy [Formula: see text], heat capacity [Formula: see text] and temperature dependencies [Formula: see text] at temperatures T[Formula: see text][Formula: see text]k to 3000[Formula: see text]k were estimated. The standard estimation error of the thermodynamic properties of the compounds in the PbO–SiO2 system does not exceed 2% for all the estimated properties. For condensed halides, the standard estimation error was (1) [Formula: see text]% for enthalpy, (2) [Formula: see text]% for entropy, (3) [Formula: see text]% for heat capacity.