3.3. Natural Bond Orbital Charge Analysis. The NBO
analysis results involving transition metals and alkali metals are
tabulated in Tables 1 and 2, respectively. To gain more insight
into the role of the zeolitic framework atoms upon adsorption,
the total charges of the adsorbate, the cation, and the zeolite
cluster are listed separately for cluster calculations. The total
charges of the adsorbate and the cation in the bare cation
calculations with the geometric constraints imposed are also
listed for comparison. The electron configurations of the cations
are shown in parentheses.
Overall we can observe that there is a significant exchange of
electrons between the adsorbate, the zeolite, and the cation.
Transition metals lose d electrons, but electrons received on s
and p orbitals overcompensate for this loss. Alkali metals can only
receive electrons on s and p orbitals. The retrieval of electrons in
the valence s and p orbitals decreases the net charges of alkali
metal cations in this order: Li > Na > K. These exchanges are more
significant in the presence of the zeolite framework; the zeolite
enhances, in particular, the recovery of electrons in p orbitals.
A more detailed discussion can be found in the Supporting
Information. While these trends are clear, a more precise interpretation
of adsorption energy in terms of charge transfer is still
needed. Therefore, absolutely localized molecular orbital energy
decomposition analysis was carried out to quantify the contributions
of different energy components to the adsorption energies
that vary with the adsorbate and the cation.