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Updated: Mar 29, 2026

On-line Analysis of Nitrogen Containing Compounds in Complex Hydrocarbon Matrixes
Published on: August 5, 2016
CHARMM Force Field Parameters for Nitroalkanes and Nitroarenes
Jeffery B Klauda1, Bernard R Brooks1
1Laboratory of Computation Biology, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
Abstract:
New CHARMM force field (FF) parameters are developed for nitro compounds, referred to here as C27rn, for subsequent use in molecular dynamics (MD) simulations. The nonbonded terms are adjusted to best fit densities and hydration energies of nitropropane and nitrobenzene. High-level quantum mechanical calculations are used to obtain accurate conformational energies of nitroalkanes and nitrobenzene and to adjust the torsional potential of the CHARMM FF. For nitroalkanes, the calculated gauche (g) conformer of the C-C-C-N torsion is more stable than trans (t). Consequently, nitropropane MD simulations with C27rn result in 74% population of this g conformer. The C27rn FF is in excellent agreement with experiment for various bulk (density, isothermal compressibility, and heat of vaporization) and interfacial (surface tension) properties of nitropropane, nitrobutane, and nitrobenzene. MD simulations with the OPLS-AA FF for nitropropane and nitrobenzene result in similar property predictions as C27rn, except a reduced stability of the C-C-C-N g conformer.
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Acidity of 1-Alkynes
The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
Conformations of Cycloalkanes

