Experimental and computational study on the energetics of N-acetyl-l-cysteine

• Experimental and computational energetic study of the N-acetyl-l-cysteine (NAC).
• Ab initio calculations for the compound obtained at the G3 and G4 levels.
• ΔcrgHm° derived from the ΔfHm°ΔfHm°(cr) and the computational ΔfHm°ΔfHm°(g).
• Gas-phase molecular structure of NAC optimized at B3LYP/6-31G(2df, p) level.

The standard (p° = 0.1 MPa) molar enthalpy of formation, in the crystalline phase, of N-acetyl-l-cysteine, NAC, at T = 298.15 K, was derived from the standard molar energy of combustion in oxygen, measured by static bomb combustion calorimetry. A value of −(737.8 ± 1.9) kJ · mol−1 was obtained. For this compound, the standard molar enthalpy of sublimation, at T = 298.15 K, could not be determined due to the decomposition of the compound during the corresponding experiments.A theoretical study at the G3 and G4 levels has been carried out, and a value of −(610.7 ± 4.0) kJ · mol−1 is recommend for the enthalpy of formation in the gas phase. Using this theoretical value and the experimental enthalpy of formation in the crystalline phase, it is possible to estimate a value of (127.1 ± 4.4) kJ · mol−1 for the enthalpy of sublimation of this compound.