Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Guest-host interactions in molecular systems with emphasis in energy applications
Sotiris S. Xantheas
Chemical & Materials Sciences Division, Pacific Northwest National Laboratory
902 Battelle Blvd., Mail Stop K1-83, Richland, WA 99352
sotiris.xantheas@pnnl.gov
The objective of this research effort is to develop an understanding of the factors
controlling the affinity and selectivity of several molecular hosts to a variety of guest molecules
with a particular emphasis on energy applications. The molecular level details of the various
prototype guest/host systems, as probed experimentally via spectroscopic techniques and
obtained theoretically by various levels of electronic structure theory, play an important role in
the assessment of the accuracy of the latter. This information is subsequently used to model the
guest/host interactions in complex molecular hosts such as hydrate lattices.
Calixarenes (CAs) are cyclic oligomers built with phenol units. They are recognized as
molecular receptors and form a variety of complexes with metal ions, anions and neutral
molecules. CAs have a cavity defined by benzene rings and form endo-complexes with
G
G
Fig. 1. The two bonding
scenarios of a molecular
guest (G) to the host
Calix[4]arene (C4A).
hydrophobic molecules and cations through both CH-π and charge-π
interactions. In addition, CAs have hydroxyl groups at their lower
rim. These OH groups are strongly hydrogen (H)-bonded with each
other, resulting in the stabilization of the cone conformation. The
balance of the interaction between the guest molecule (G in Figure 1)
and either the benzene or the hydroxyl group site is very subtle. For
example, though it was expected that Calixarenes (C4A) and aliphatic
molecules form endo-complexes (i.e. the aliphatic molecule lies
inside the CA cavity), recent NMR studies revealed that the complex
exists as the exo-complex having the N + −H---O - form (i.e. the
aliphatic molecule is located outside the cavity and it is H-bonded to
the OH groups). These molecular host/guest systems are ideal test beds for probing the nature of
the underlying intermolecular interactions, by switching on and off several of its components
upon changing the molecular guest (G = Ar, H2O, NH3, C2H4, N2, CH4). When used in
conjunction with various electronic structure calculations, the measured structural (via the
interpretation of the IR spectra) and energetic results for these complexes provide important
information regarding the performance of different
levels and approximations of electronic structure
theory in reproducing the subtle balance of those
interactions.
Table 1 lists the computed energy
difference (in cm -1 ) between the more stable endo-
and exo-isomers of C4A-H2O. The structures of
these two isomers are determined from two very
different classes of interactions, namely OH-π and
hydrogen bonding interactions, two of the most
important ones in the modeling of aqueous
biological systems. High levels of electronic
211
Table 1. Calculated energy difference (in cm -1 )
between the endo- (more stable) and exo-isomers
of C4A-H2O at various levels of theory.