198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

Directional Aspects of Intermolecular Interactions 7
angles from ideality, and the number of water molecules that have been located.
Then follows a description of the protein, its amino acid sequence, including an
analysis of which parts of the backbone fold as helix, sheet, or turns. The main
entry is a list of atomic coordinates (ATOM) and information on three-dimensional
coordinates of metal ions, substrates, and inhibitors bound to the protein
(HETATM). These coordinates can readily be extracted in a suitable form for use
with a computer-graphics system.
The Inorganic Crystal Structure Database (ICSD) [7] contains information on
all compounds containing at least one nonmetallic element but no C-C or C-H
bonds (because these are covered by the CSD). Each reported crystal structure
has a separate entry. Information provided in the database includes the chemical
name, phase designation, unit cell dimensions, density, space group, and the
oxidation state of the elements. Atomic information includes three-dimensional
coordinates. Also listed are the R value, temperature, pressure, method of measurement
and the full journal reference.
2.2
Spectroscopic Methods of Studying Intermolecular Interactions
Spectroscopic methods of analysis of intermolecular interactions involve
studies of pure rotational and high-resolution vibration-rotation spectra of
weakly bound heterodimers (that is, dimers composed of two different species).
The two compounds are cooled until heterodimers form, sometimes by seeding
with argon and expanding the mixture adiabatically through a nozzle [1]. The
rotational constants measured give the principal moments of inertia from which
information on the geometry of the dimer can usually be deduced. The absolute
intensity of a rotational transition will give a measure of the intermolecular
stretching force constant and the dissociation energy of the dimer. The centrifugal
distortion constant of a rotating molecule, obtainable from the rotational
spectrum, also gives information on the stretching force constant. These measurements
make it possible to measure the strength of the intermolecular interactions.
3
Types of Intermolecular Forces
There are several types of intermolecular interactions, each of which involves
electrostatic forces of some kind or other.An example is provided by the ion-ion
interactions between cations and anions. Pure electrostatic (Coulombic) interactions
are long-range and many, such as hydrogen bonding, are directional.
Molecules can, however, be distorted by the electric fields of surrounding molecules,
even if the molecules themselves are electrically neutral.
If molecules are polar, that is, if they have a dipole moment, they may interact
with each other in a head-to-tail arrangement (a dipole-dipole interaction). If
one molecule is polar (with a dipole moment) and the other is nonpolar but
polarizable, the polar molecule may induce a dipole in the nonpolar molecule (a
dipole-induced dipole interaction). These two dipoles, one permanent and the