# Relevance of strain energy, total energy and stability of molecule?

Would anybody please highlight the difference between total strain energy of a molecule, total energy of a molecule and what we can infer about the stability of the molecule using the total strain energy of the molecule as well as the scope of using such inferences made.

Would anybody please highlight the difference between total strain energy of a molecule, total energy of a molecule

Strain energy is usually defined as the sum of all angle and steric strains that a certain molecule possesses, compared to an unstrained analogue molecule.

For example, here is a table that compares various cyclic alkanes to cyclohexane: wikipedia

In this way, stain energy can be thought of as a positive contribution to potential energy that destabilizes a molecule above the stability that its ideal bonding would suggest.

Total energy of a molecule could be defined in lots of ways, often it is done as a $$\Delta H_f$$, which could be positive or negative, and compares the potential energy of a molecule (i.e. how strong the bonds are, stronger means lower potential energy) to the elements from which it is formed. However, this will also take into account any intermolecular forces present and these are not inherent to a single molecule, but to a collection of molecules in standard states.

We could also define the energy of a molecule by thinking of the sum of the bond enthalpies, which define how strong the bonds are compared to unbonded atoms (specifically by breaking bonds homolytically, forming radical species, in the gas phase). The larger the value, the stronger the bonds are and the lower the potential energy of the molecule.

In computational chemistry, the total energy of a molecule can theoretically be calculated using quantum mechanical means. This is done using energy minimization software. There is a good post here about the theory in this process. This value will take into account the bond enthalpy/strength and strain within a molecule.

and what we can infer about the stability of the molecule using the total strain energy of the molecule as well as the scope of using such inferences made.

In general, the greater the strain energy and the higher the total energy, the less stable/more reactive the molecule.

• Are there any rules to determining the analogue molecule used for comparison? – user79161 May 18 at 16:03