Controlling relative polymorph stability in soft porous crystals with a barostat

Area of Science:

• Materials Science
• Thermodynamics
• Computational Chemistry

Background:

• Soft porous crystals (SPCs) exhibit polymorphism, existing in multiple crystal forms with different pore sizes.
• SPCs respond to adsorbate pressure through distinct mechanisms: 'breathing' (unit cell volume change) or 'gate-opening' (internal rearrangement).

Purpose of the Study:

• To investigate the thermodynamic behavior of SPCs under external barostatting.
• To understand how external pressure influences the relative stability of polymorphic SPCs.

Main Methods:

• Monte Carlo simulations were employed to model SPCs.
• The study analyzed the effects of applying an external barostat to breathing and gate-opening SPCs.

Main Results:

• For breathing SPCs under constant external pressure, the free energy difference between polymorphs is consistently shifted, allowing easy control of relative stability.
• For breathing SPCs at fixed overall pressure, polymorph stability changes are more complex, analogous to gate-opening SPCs.

Conclusions:

• External mechanical forces can tune the effective free energy profile of empty SPCs.
• This provides new strategies for engineering thermodynamic properties and selectivity in polymorphic adsorbents.