When do covalent bonds of a molecule $M$ extend or compress while no bonds in $M$ are broken? I can conceive of some possibilities but I don't know how common they are:
- Temperature changes
- Inter-molecular forces around $M$ which may stretch or compress $M$ in various directions
- The transition from $M$ into a different conformation isomer (perhaps some stretching and twisting is required to overcome the energy barrier for a large change in conformation?)
- Perhaps bonds can store energy like springs?
How likely are the above to happen, and are there other situations which induce bond deformation? Could I also have some references or topics to search on related topics? As far as I know, covalent bonds are relatively rigid and don't deform much.
Edit: one answer. Upon some literature review, it seems that rotation about $\sigma$-bonds and molecular inversion involve temporary bond deformations because the intermediate states (eclipsed conformations for the former, and the planar/linear form for the latter) involve a change in potential energy. After the transformation, the bond lengths revert to the original.