The methods described in the previous sections have also been applied to identify the changes that are induced when a molecule is adsorbed onto a substrate. The example considered is the adsorption of methanol onto a zeolite catalyst. A plane wave calculation of this reaction [11,12] shows that substantial changes in charge density take place, with proton transfer from the Brø nsted acid site of the zeolite to the methanol.
Figure 2: Mulliken charges for isolated Brø nsted acid site in zeolite. Red atoms are oxygen, yellow silicon, grey aluminium and blue hydrogen. The bond population between O and H is 0.52.
Figure 3: Bond populations of O---H bonds in the adsorbed complex. Circles indicate framework oxygen atoms, X the methanol oxygen, squares hydrogen, and the triangle aluminium (actually out of the plane of the plot). Contours are valence charge density in .
Figure 4: Mulliken charges for methanol adsorbed in 8 ring of zeolite structure. Green atom is carbon, other colours as in figure 2.
First we consider the partial charges and bond populations for the isolated methanol given in table 1 and figure 1, and the isolated zeolite acid site shown in figure 2. The values confirm that there are significant bond populations between pairs of atoms generally considered to be covalently bonded and also confirm the partially ionic nature of the zeolite framework, as suggested by Shah et al. from an examination of the total charge density . Figure 3 shows the bond populations in the adsorbed complex, along with the total valence charge density. The bond populations confirm the picture given by the charge density, in particular showing a significant rearrangement of the covalent bonds. The proton is seen to be transferred from the zeolite to the methanol thus confirming the chemisorbed nature of the complex. Weak bonding is seen between both protons and oxygens of the zeolite framework, probably indicative of strong polarisation and hydrogen bonding stabilising the adsorbed complex . Proton transfer in such an acid-base interaction is generally assumed to involve charge transfer of the form:
By considering the Mulliken charges, illustrated in figure 4, we conclude that the group has an overall charge of +0.77. While this is not as large as the formal charge of +1 suggested by the equation (12), it does demonstrate the presence of substantial charge transfer from the zeolite to the adsorbate. The charges also indicate increased polarisation of the C---O bond, indicating the possibility that the methyl group has become susceptible to nucleophilic attack.