The Effect of Charge Distribution On RDX Adsorption in IRMOF-10

Quantum mechanical (QM) calculations and classical grand canonical Monte Carlo (GCMC) simulations were performed to test the effect of charge distribution on hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) adsorption in IRMOF-10. Five different methods, including Mulliken Population Analysis (MPA), Löwdin Population Analysis (LPA), Natural Bond Orbital (NBO) Analysis, the electrostatic potential (ESP)[1] and charges from the MM3 potential[2] were used to assign point charges to IRMOF-10 from ground state structures obtained from QM calculations. Classical GCMC simulations of RDX in IRMOF-10 were performed using each set of point charges. As the charge distributions varied, the adsorption loading and interaction potential energies are significantly different. None of the five sets of point charges when inserted into the MM3 potential are able to quantitatively capture the energy of adsorption or its variation as the position of the RDX adsorbate is changed. One of the potential reasons for the discrepancy between the classical and QM calculations is that the partial charges were generated under different conditions than those present in the simulation of RDX in IRMOF-10. The charge distribution of RDX in the adsorbed phase is significantly different than that in the crystalline phase and the charge distribution of IRMOF-10 also changes significantly with the introduction of an RDX molecule into the cage.

References:

[1] R. Babarao and J.W. Jiang, Molecular screening of metal-organic frameworks for CO2 storage, Langmuir, vol. 24, pp. 6270-6278, 2008.

[2] M. Tafipolsky, S. Amirjalayer and R. Schmid, Ab initio parametrized MM3 force field for the metal-organic framework MOF-5, J. Comput. Chem., vol. 28, pp. 1169-1176, 2007.