The C=O group is observed to be dynamic, leading to modulation of the electron density gap in the pore. PLD, defined as the largest van der Waals sphere that can be accommodated inside the pore, (Figure ) was evaluated every 50 ps at 298 K in the presence of the CO2/O2 feed. A distinct PLD distribution is observed for all three pores (Figure ), indicating that the PLD is not a single value as often assumed. Strikingly, the corresponding variance (up to ~0.24 Å) is significant, considering that the difference in kinetic diameters of CO2 and O2 is only 0.16 Å.