Study on properties of liquid ammonia via molecular dynamics simulation based on ABEEMσπ polarisable force field

Abstract

The molecular dynamics simulation has been performed to investigate the charge distribution, structural and dynamical properties of liquid ammonia at 273 K using a polarisable force field of the atom-bond electronegativity equalisation method (ABEEMσπ). One ammonia molecule in this model has eight charge sites, one N atomic site, three H atomic sites, three N–H bond sites and one lone-pair electron site. ABEEMσπ model can present the quantitative site charges of molecular ammonias in liquid and their changing in response to their surroundings. The radial distribution functions and dynamical properties are in fair agreement with the available experimental data. The first peak of g_NN(r) appears at N–N distance of ~3.50 ± 0.05 Å where most hydrogen bonds are formed. The average coordination number of the first shell is 13.0 ± 0.1 among which a central ammonia molecule intimately connects 3 ~ 4 ammonia molecules by hydrogen bonds. The power spectrum shows the vibrations of hydrogen bonds. For a reference, a simple estimation of the average hydrogen bonding energy in liquid ammonia is 6.5 ± 0.1 kcal/mol larger than 3.8 ± 0.3 kcal/mol in dimer ammonia. Our simulation results provide more detailed information about liquid ammonia.

Keywords:

• Molecular dynamics simulation
• ABEEMσπ polarisable force field
• radial distribution function
• the power spectrum
• hydrogen bonding energy

Acknowledgements

We greatly thank M. A. Ricci for providing the experimental neutron diffraction data on liquid ammonia, Anan Tongraar for providing the QM/MM results of liquid ammonia and Professor Jay William Ponder for providing the Tinker programs.