Page 77 - Physical chemistry understanding our chemical world

44 INTRODUCING INTERACTIONS AND BONDS

Figure 2.3 The water molecule has a ‘V’ shape. Experiments show
−
d O
that gaseous water has an O–H length of 0.957 18 ˚ A; the H–O–H angle
◦
is 104.474 . Water is polar because the central oxygen is electronega-
tive and the two hydrogen atoms are electropositive. The vertical arrow
+ +
d d indicates the resultant dipole, with its head pointing toward the more
H H negative end of the molecule
H
H
O H O
H
O H
H
O
H O H
H H H
O O
H
H
H O H
O
H
H
H
Figure 2.4 Water would be a gas rather than a liquid at room temperature if no van der Waals
forces were present to ‘glue’ them together, as indicated with dotted lines in this two-dimensional
representation. In fact, water coalesces as a direct consequence of this three-dimensional network
of dipole–dipole interactions. Note how all the O–H ·· · O bonds are linear
short distance because the influence of a dipole is not large. In practice, we find that
the oxygen atoms can interact with hydrogen atoms on an adjacent molecule of water,
but no further.
The interactions between the two molecules helps to ‘glue’ them together. It is a
sobering thought that water would be a gas rather than a liquid if hydrogen bonds
(which are merely a particularly strong form of van der Waals forces) did not promote
the coalescence of water. The Earth would be uninhabitable without them. Figure 2.4
shows the way that liquid water possesses a three-dimensional network, held together
with van der Waals interactions.
Each H 2 O molecule in liquid water undergoes at least one interaction with another
molecule of H 2 O (sometimes two). Nevertheless, the interactions are not particularly
−1
strong – perhaps as much as 20 kJ mol .
Whereas the dipoles themselves are permanent, van der Waals interactions are
not. They are sufficiently weak that they continually break and re-form as part of a
dynamic process.

How is the three-dimensional structure maintained
within the DNA double helix?
Hydrogen bonds

DNA is a natural polymer. It was first isolated in 1869 by Meischer, but its role
in determining heredity remained unrecognized until 1944, by which time it was