on a slice through the molecule reveals its internal structure, but sometimes more
relevant to reactivity is the picture seen by mapping it onto the van der Waals
surface, because this is the picture presented to the outside molecular world.
Examining the ESP interactions between a molecule and the active site of an
enzyme can be important in drug design [ 109 ]. Various applications of the ESP
are discussed by Politzer and Murray [ 324 ] and Brinck [265a]. The ESP at any point
on the van der Waals surface can be assigned a quantitative value, namely the
energy needed to move a charge (say, a proton) from infinity to that point, and some
programs will calculate the ESP at any point on the surface on which one clicks
with the mouse.
Fig. 5.47 Distribution of net charge in the water molecule (electrostatic charge, calculated with
AM1 –Chapter 6). Negative to positive: red to blue R O Y G B). (a) Slice through the plane of the
molecule; the contour lines show the decrease in net negative charge. (b) Charge in space; this
corresponds essentially to the lone pairs. (c) Charge mapped on the van der Waals surface
Fig. 5.48 The hydrocarbon pyramidane, C 5 H 4 , evidently (pyramidane has not been synthesized)
has a lone pair of electrons on its pyramidal carbon atom, like carbene (methylene), CH 2. While
the lone pair on CH 2 is no surprise (draw the Lewis structure for the singlet), a cycloalkane with an
unshared electron pair is remarkable
5.5 Applications of the Ab initio Method 369