Relative energies of isomers

One of the main aims of quantum mechanical methods in chemistry is the calculation of energies of molecules as a function of their geometries. This requires the generation of potential energy hypersurfaces. If these surfaces can be calculated with sufficient accuracy, they may be employed to predict equilibrium geometries of molecules, relative energies of isomers, the rates of their interconversions, NMR chemical shifts, vibrational spectra, and other properties. Carbocations are ideally suited for calculations because relative energies of well-defined structural isomers are frequently not easily determined experimentally. It should, however, be kept in mind that theoretical calculations usually refer to isolated ion structures in the gas phase. 

By lowering the temperature, it is often possible to slow down the rate of intramolecular processes of this kind, and hence to obtain NMR spectra consistent with expectation. It is then possible to measure the rate constant for the process, and hence the activation barrier. In the case of PF5, this barrier amounts to 18 kJ mol-1. This kind of experiment gives much useful information about molecular flexibility, and about the relative energies of isomers. 

Why is it not valid in all cases to obtain the relative energies of isomers by comparing their MM strain energies ... 

Table 5.9 Reaction energies and relative energies of isomers (HF/3-21G< ) and HFI6-31G )...

Reactants Products Reaction energy, or relative energy of isomers, calculated Exp kJ mol 1... 

Figure 12.4 DFT-calculated relative energies of isomers 3b and 9b and of the corresponding Pd(ll) complexes 7b and 8b. ...

The LDA and the Local Spin Density approximation (LSD), when spin is considered, have been successful in determining molecular structure and many one-electron properties or expectation values. It is very well known now that the LDA and LSD underbind core electrons and overbind atoms in a molecule. Energies are not as good as those obtained by correlated ab initio methods, although the relative energies of isomers and activation barriers which do not involve bond-breaking can be quite accurate. It was observed,... 

We also have used MICROMOL to examine energetics of simple chemical reactions and to show relative energies of isomers. Protonation reactions are ideal for student projects, and a strong connection can be made with interstellar chemikry. For example, the student can be asked to complete electronic structure studies on CO, HCO+, and HOC+. Another example that forces the student to appreciate intermolecular forces is provided by examination of the potential energy curve along the reaction coordinate for linear F -I- HCN and FH -I- Such an example is simple enough to be done without too much... 

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