Simulating CH4 physisorption on ionic crystals

Abstract

We use quantum chemical techniques to evaluate the electrostatic and polarization components of the interaction between a rigid CH4 molecule and a lattice of point charges representing the MgO(100) surface. We find that CH4 positioned above Mg adopts an edge-down configuration in which two H atoms are oriented downward towards the MgO(100) surface and point at O ions in the surface layer. The CH4–MgO(100) electrostatic interaction is substantially less favorable (but is still attractive) for the face-down configuration in which three H atoms point downward. Neither configuration is energetically favorable for CH4 molecules positioned above O ions. We show that for edge-down CH4 molecules above Mg, the electrostatic component of the CH4-substrate interaction varies considerably as the CH4 molecule rotates about the surface normal; the polarization component of the interaction, by contrast, is nearly constant during this rotation. We show that a point-charge model for the CH4 charge distribution, in which the C and H atoms carry effective partial charges, predicts that the CH4-surface electrostatic interaction should be more favorable for face-down CH4 molecules than for edge-down CH4 molecules, in disagreement with the quantum chemical results. We show that this is because the point-charge model poorly represents the high-order electric multipoles of CH4.