Page 84 - Science at the nanoscale

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June 12, 2009
From Atoms and Molecules to Nanoscale Materials
74
6th power; while for many molecules, the repulsive force varies
with the inverse 12th power of intermolecular distance. A good
approximation to Fig. 4.10 is thus given by the Lennard-Jones
potential:
A
B
PE = −
+
12
6
r
r
Here, A and B are both constants to be determined experimentally.
The Lennard-Jones potential can also be expressed in various dif-
ferent forms other than that given in Eq. 4.10.
4.2.7
Hydrogen Bonding
Hydrogen bonding is a particular type of dipole-dipole
interaction that is important and should be given special consid-
eration. This occurs in molecules containing a hydrogen atom
bonded to electronegative atoms such as O, N or F. The dipole
generated through this bond will interact with another elec-
tronegative atom (known as a hydrogen bond acceptor) forming the
so-called hydrogen bond (Fig. 4.11). Hydrogen bonding can be
either intramolecular or intermolecular. It is among the strongest
type of secondary interactions and plays a significant role for
molecules such as water, and secondary, tertiary, etc. structures
of proteins and nucleic acids.
The simplest example of hydrogen bonding is that between
water molecules. As shown in Fig. 4.12, the oxygen atom of
the H 2 O molecule has two lone pairs of electrons available for
Figure 4.11. RPS: PSP0007 - Science-at-Nanoscale (4.10) ch04
Various types of hydrogen bonding.