Computational Chemistry

7. If a species has one calculated frequency very close to 0 cm%^1 what does that
   tell you about the (calculated) potential energy surface in that region?


8. The ZPE of many molecules is greater than the energy needed to break a bond;
   for example, the ZPE of hexane is about 530 kJ mol%^1 , while the strength of a
   C–C or a C–H bond is only about 400 kJ mol%^1. Why then do such molecules
   not spontaneously decompose?


9. Only certain parts of a potential energy surface are chemically interesting:
   some regions are flat and featureless, while yet other parts rise steeply and are
   thus energetically inaccessible. Explain.


10. Consider two potential energy surfaces for the HCN⇌HNC reaction:A,a
    plot of energy versus the H–C bond length, andB, a plot of energy versus the
    HNC angle. Recalling that HNC is the higher-energy species, sketch qualita-
    tively the diagrams forAandB.

Harder Questions 43