IUPAC definition : Strain is present in a molecular entity or transition structure if the energy is enhanced
because of unfavourable bond lengths, bond angles or dihedral angles ('torsional strain')
relative to a standard. It is quantitatively defined as the standard enthalpy of a
structure relative to a strainless structure (real or hypothetical) made up from the
same atoms with the same types of bonding. (The enthalpy of formation of cyclopropane
is 53.6 kJ mol-1, whereas the enthalpy of formation based on three 'normal' methylene
groups, from acyclic models, is -62 kJ mol-1. On this basis cyclopropane is destabilized
by i ca. /i 115 kJ mol-1 of strain energy.);
Strain is present in a molecular entity or transition structure if the energy is enhanced
because of unfavourable bond lengths, bond angles or dihedral angles ('torsional strain')
relative to a standard. It is quantitatively defined as the standard enthalpy of a
structure relative to a strainless structure (real or hypothetical) made up from the
same atoms with the same types of bonding. (The enthalpy of formation of cyclopropane
is 53.6 kJ mol-1, whereas the enthalpy of formation based on three 'normal' methylene
groups, from acyclic models, is -62 kJ mol-1. On this basis cyclopropane is destabilized
by i ca. /i 115 kJ mol-1 of strain energy.)