Simple QSPR for Calculating Combustion Metrics by Hand

Quantitative structure–property relationships (QSPRs) provide a framework for predicting molecular properties from structural features. This work proposes a QSPR model of novel simplicity that can be executed using a standard hand-held calculator. It uses only the molecular formula (CaHbNcOdFeClfBrgIh) to predict lower heating value (ΔHlow(kJ/mol)) and molar volume (Vm(mL/mol)). With these two parameters and the molar mass, other properties are easily computed such as density (ρ(g/mL)), specific heat of combustion (ΔHmass(kJ/g)), and combustion energy density (ΔHvol(kJ/mL)). Linear regression was used to train these QSPR models against 1174 ΔHlow and 4865 Vm values from the CRC Handbook of Chemistry and Physics. The regression produced this model for lower heating value: ΔHlow(kJ/mol) = 431.4(1.3)a + 88.85(79)b + 98.2(5.4)c – 161.2(2.8)d – 193.6(4.4)e – 51.5(7.4)f – 36(16)g – 80(26)h. A second regression produced this model for molar volume: Vm(mL/mol) = 16.03(38) + 7.01(6)a + 4.80(3)b + 3.71(23)c + 5.31(11)d + 9.78(11)e + 17.56(23)f + 19.34(40)g + 27.25(88)h. These models perform well against the literature training set values for ΔHlow (R2 = 0.9979) and Vm (R2 = 0.9943). The molecular formula cannot account for structural isomers, which introduces some expected limitations. To demonstrate versatility, this model is applied to many unrelated fields such as macro-nutrient energy content, combustible sources of energy, fire suppression analysis, and the energy content of volumetric fuel blends. Overall, these results demonstrate that a simple QSPR model using only the molecular formula can serve as a computationally inexpensive tool for industrial chemistry use and related thermochemical analyses.