Chemistry Reference
In-Depth Information
London dispersion forces, and/or similar interactions is not a simple task. Che-
mical bonding, whether noncovalent, covalent, ionic, or metallic, covers a
broad, continuous spectrum of electronic interactions and energies. Conse-
quently, the classification of a bond or interaction (e.g., double versus triple
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or covalent versus noncovalent
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) is sometimes open to interpretation. As a
result, there is no unique criterion or set of criteria that can be used to define
weak
interactions or
noncovalent
interactions. In the second volume of this
review series, Scheiner already notes this issue and highlighted the difficulties
associated with defining the hydrogen bond.
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Here, matters are even more
complicated because other weak interactions are also considered.
To limit the breadth of the present chapter, it focuses on the most com-
mon types of weakly bound clusters, namely those composed of neutral frag-
ments. (The following discussion also assumes the weakly bound clusters are
composed of closed-shell fragments that are in their ground electronic states
and dominated by a single Hartree-Fock (HF) reference function. It is cer-
tainly feasible to perform reliable computations on systems composed of
open-shell, excited state, or multireference fragments; however, by assuming
the monomers have a ''well-behaved'' electronic structure, we can focus on
computational methods that will accurately describe the weak noncovalent
interactions within a cluster.) Clusters containing one or two charged species
are mentioned (e.g., solvated ions or ion pairs). However, clusters with numer-
ous charged species (e.g., room temperature ionic liquids
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) fall outside the
scope of this review. This emphasis still leaves a wide spectrum of weak che-
mical interactions that bind the clusters together (as depicted in Figure 1).
Figure 1
Weak noncovalent interactions between neutral fragments cover a wide
spectrum of interactions and energies.
