Periodic DFT computation of rotational barriers for linear molecules in zeolites: Validation via zero-point energies and isotopic heat difference values of adsorbed H2/D2

Abstract

A climbing image nudged elastic band (cNEB) algorithm was applied on the basis of density functional theory (DFT) calculations of the rotational barriers of eight linear molecules (H2, N2, O2, CO, CO2, NO, N2O, C2H2) adsorbed in NaY and NaCaY periodic zeolite models at the MeII cation sites (the Na or Ca metal cation in the SII site) located near the 6R windows. A specific approach is applied for molecules with positive quadrupole (H2, C2H2) and small negative quadrupole (3O2) moments while applying cNEB. The obtained T-geometry relative to the cation is the most frequent case for H2 adsorption in cationic form sieves and metalorganic frameworks (MOFs). The computed barriers for the T- and other linear (L) orientations are in good correlation with the quadrupole moments taken from literature irrespective of the dipole values (CO, NO, N2O). In the case of NaY, the Al distribution per 6R sites is also discussed which allowed finding particular favorite adsorption sites. The calculated zero-point energies relative to the obtained rotational barriers regarding the H2 - D2 pair result in a qualitative agreement with the experimental isotopic difference in the H2/D2 adsorption heats in NaY which ultimately lead to the H2/D2 separation coefficient.