The investigation of hydrocarbon cracking reaction energetics with composite energy methods

Hydrocarbo cracking reactions are one of the most commonly encountered reactions in the petroleum industry, and the energetics of the reactions are crucial in understanding the reaction mechanisms and predicting reaction rates. In this work, a modified composite energy method (CBS-RAD(MP2)) is created as a version of the CBS-RAD method which gives accurate energetics for hydrocarbon free radical reactions. It replaces the time consuming QCISD(fc)/6-31g* method in the geometry optimization and frequency calculation steps with MP2(full)/6-31g* level calculations. The accuracy of the new CBS-RAD(MP2) method is compared with the widely used G2, G3 and CBS-QB3 composite methods for predicting heats of reaction and activation barriers of 14 hydrocarbon cracking reactions. We find that the new CBS-RAD(MP2) method has the second least RMS error of 1.22 kcal/mol for heats of reaction calculations. For activation energy calculations, the new CBS-RAD(MP2) method has the least RMS error of 1.37 kcal/mol. Moreover, the CBS-RAD(MP2) method was found to require only 81% of the computational time required compared to the CBS-QB3 method, 32% of G3 and 15% of the G2 method, making it an attractive alternative for predicting hydrocarbon cracking reaction energetics.