Chemistry Reference
In-Depth Information
Figure 1.7. Force plot obtained with a silicon microfabricated cantilever with a
force constant of
25 Nm
−
1
and an ultrasharp tip of radius
R
∼
<
10 nm on a highly
ab
-oriented thin TTF-TCNQ film.
& Govers, 1980). The 2.3 eV value is lower than the measured cohesive or crys-
tal binding energy of TTF-TCNQ, which is 4.9 eV per TTF-TCNQ pair (Metzger,
1977). The cohesion energy essentially accounts for the Coulomb (Madelung term),
polarization (charge-induced dipole), dispersion (van der Waals) and repulsion en-
ergies. The fact that nanoindentation is performed on the surface of the material
facilitates sublimation of the molecules. Theoretical estimates of the cohesive en-
ergy of pentacene using first-principles pseudopotential density functional theory
(DFT) give a value of 1.3 eV per molecule (Northrup
et al.
, 2002).
The estimated
D
is about 90 K, in agreement with previous heat capacity
determinations performed on single crystals (Coleman
et al.
, 1973a). Table 1.5
shows some
D
values found in the literature for some MOMs.
1.4 Crystal engineering: synthons
As already mentioned in Section 1.1, chemists regard molecular crystals as super-
molecules. This is fully justified since molecules are built by connecting atoms
through covalent bonds and crystals are built by connecting molecules with inter-
molecular interactions. Crystal engineering can be defined as
the understanding of
