Energetics and stability of azulene: From experimental thermochemistry to high-level quantum chemical calculations

•Experimental standard molar enthalpy of formation, sublimation azulene.•Mini-bomb combustion calorimetry, sublimation Calvet microcalorimetry.•High level composite ab initio calculations.•Computational estimate of the enthalpy of formation of azulene.•Discussion of stability and aromaticity of azulene.

The standard (p0 = 0.1 MPa) molar enthalpy of formation for crystalline azulene was derived from the standard molar enthalpy of combustion, in oxygen, at T = 298.15 K, measured in a mini-bomb combustion calorimeter (aneroid isoperibol calorimeter) and the standard molar enthalpy of sublimation, at T = 298.15 K, measured by Calvet microcalorimetry. From these experiments, the standard molar enthalpy of formation of azulene in the gaseous phase at T = 298.15 K was calculated. In addition, very accurate quantum chemical calculations at the G3 and G4 composite levels of calculation were conducted in order to corroborate our experimental findings and further clarify and establish the definitive standard enthalpy of formation of this interesting non-benzenoid hydrocarbon.