The methods described in the previous sections were applied by Dr R. Shah to identify the changes that are induced when a molecule is adsorbed onto a substrate [46, 39]. The example considered is the adsorption of methanol onto a zeolite catalyst, chabazite. A plane wave calculation of this reaction [50, 3] showed that substantial changes in charge density take place, with proton transfer from the Brønsted acid site of the zeolite to the methanol.
Figure: Mulliken charges in electronic units for isolated Brønsted
acid site in zeolite. Red atoms are oxygen, yellow silicon, grey
aluminium and blue hydrogen. The bond population between O and H is
0.52e.
Figure: Bond populations in electronic units for O--H bonds in the
methanol-chabazite 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 show
valence charge density in 
.
First, consider the partial charges and bond populations for the
isolated methanol given in Table 
and Figure
, and the isolated zeolite acid site shown in Figure
. 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 [50]. Figure 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
[3]. Proton transfer in such an acid-base interaction is
generally assumed to involve charge transfer of the form:
Figure: Mulliken charges in electronic units for methanol adsorbed
in 8 ring of zeolite structure. Green atom is carbon, other colours as
in Figure .
By considering the Mulliken charges, illustrated in Figure
, we find that the 
group
has an overall charge of +0.77e. While this is not as large as the
formal charge of +1e suggested by Equation , 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.
