Re-evaluation of the reported experimental values of the heat of vaporization of N-methylacetamide.

The accuracy of empirical force fields is inherently related to the quality of the target data used for optimization of the model. With the heat of vaporization (ΔH(vap)) of N-methylacetamide (NMA), a range of values have been reported as target data for optimization of the nonbond parameters associated with the peptide bond in proteins. In the present work, the original experimental data and Antoine constants used for the determination of the ΔH(vap) of NMA are reanalyzed. Based on this analysis, the wide range of ΔH(vap) values reported in the literature are shown to be due to incorrect reporting of the temperatures at which the original values were extracted and limitations in the quality of experimental vapor pressure-temperature data over a wide range of temperatures. Taking these problems into account, a consistent ΔH(vap) value is extracted from three studies for which experimental data are available. This analysis suggests that the most reliable value for ΔH(vap) is 13.0±0.1 at 410 K for use in force field optimization studies. The present results also indicate that similar analyses, including analysis of Antoine constants alone, may be of utility when reported ΔH(vap) values are not consistent for a given neat liquid.