Молекулалық кернеу және энергиясы

When a molecule is deformed from its lowest energy conformation by applied stress

In chemistry, a molecule experiences strain when its chemical structure undergoes some stress which raises its internal energy in comparison to a strain free reference compound. The internal energy of a molecule consists of all the energy stored within it. A strained molecule has an additional amount of internal energy which an unstrained molecule does not. This extra internal energy, or strain energy, can be likened to a compressed spring. Much like a compressed spring must be held in place to prevent release of its potential energy, a molecule can be held in an energetically unfavorable conformation by the bonds within that molecule. Without the bonds holding the conformation in place, the strain energy would be released.

The standard heat of formation (ΔfH°) of a compound is described as the enthalpy change when the compound is formed from its separated elements. When the heat of formation for a compound is different from either a prediction or a reference compound, this difference can often be attributed to strain. For example, ΔfH° for cyclohexane is 29.9 kcal mol−1 while ΔfH° for methylcyclopentane is 25.5 kcal mol−1. Specifically, Van der Waals strain is considered a form of strain where the interacting atoms are at least four bonds away from each other. The amount on steric strain in similar molecules is dependent on the size of the interacting groups; bulky tert butyl groups take up much more space than methyl groups and often experience greater steric interactions. The effects of steric strain in the reaction of trialkylamines and trimethylboron were studied by Nobel laureate Herbert C. Brown et al. They found that as the size of the alkyl groups on the amine were increased, the equilibrium constant decreased as well. The shift in equilibrium was attributed to steric strain between the alkyl groups of the amine and the methyl groups on boron.

There are situations where seemingly identical conformations are not equal in strain energy. Syn pentane strain is an example of this situation. There are two different ways to put both of the bonds the central in n pentane into a gauche conformation, one of which is 3 kcal mol−1 higher in energy than the other.

Medium sized rings (7–13 carbons) experience more strain energy than cyclohexane, due mostly to deviation from ideal vicinal angles, or Pitzer strain. Molecular mechanics calculations indicate that transannular strain, also known as Prelog strain, does not play an essential role. Transannular reactions however, such as 1,5 shifts in cyclooctane substitution reactions, are well known.

The amount of strain energy in bicyclic systems is commonly the sum of the strain energy in each individual ring.