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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.

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    $\begingroup$ The most simple example I can think of is the interaction between a specific radiation (mostly IR/Near IR); in this particular case bonds are shown to behave like a spring or a Quantum Mechanical object (the second one is way truer than the first). Strictly speaking, when the energy contained inside the radiation $E_i=h\nu$ meets a equal energy gap inside a bond $e^-\to e^{-*}\iff\Delta E=E_i$, the electrons are excited and fly to the next accessible quantum state, stretching the bond. $\endgroup$ – TheVal Dec 14 '13 at 8:28
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    $\begingroup$ I'm sure pressure have a lot of influence, IR spectra are the most suitable instrument to see the shift causes from temperature and pressure variation as you stated changing this parameters change the resonance frequence of the oscillator. $\endgroup$ – G M Dec 14 '13 at 13:03
  • $\begingroup$ I know from coordination chemistry that the bond length of CO changes according to electronic effects, depending on other ligands in the complex. $\endgroup$ – tschoppi Dec 14 '13 at 13:42
  • $\begingroup$ There's a slight bending in cyclopentane. So, tension arising from cyclical molecule structure must be accounted for. This also includes molecules forced into a plane by a pi bond (ideally, by an aromatic ring) $\endgroup$ – John Dvorak Dec 14 '13 at 23:01
  • $\begingroup$ @JanDvorak am I right to say that "bending" is the alteration of the usual tetrahedral angle between bonds? But does it involve stretching of the bonds? $\endgroup$ – Herng Yi Dec 15 '13 at 2:25
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There are two distinct phenomena, which are somewhat related.

Bonds do vibrate like springs, and as you said they can store energy by changing its vibrational state. Like a string, the more energy stored in the vibrational motion, the larger the magnitude of such vibration. Different from a spring, such vibrations are associated with discrete quantum numbers, and even at absolute zero temperature, such vibration still possess a zero-point energy and can never stop.

  • Non-dynamical effect that change the statistical average of bond length

Molecular vibration often (but not always) occurs at an extremely short time frame, and the vibration itself is often (again not always) of a very small magnitude, so when we refer to bond length we often mean the average bond length over time.
Normally a molecule stays near the bond length where the energy is the lowest. There can be a few reasons for a the average bond length to change while keeping the connectivity of the molecule intact, with some examples:

  1. Each conformational isomers will have different bond lengths. Usually this is a small number. Examples of large bond length change between conformational isomers with the same bond connectivity are those with Jahn-Teller effect.

  2. By changing the vibrational state of the molecule, the average bond length can be different. Usually, because of anharmonicity, the higher the vibrational state, the higher the average bond length. Temperature change, pressure change, collision or photon excitation (especially infrared light) are the most common causes of a change in vibrational state.

  3. Through inter-molecular interactions or external forces, the potential energy surface of the molecule can be changed so that the energy minimum happens at a different distance. In this case, the bond length will change, but the energy stored in the bond will not change. This can be achieved through external electronic field, or intermolecular forces such as electrostatic force from an ion, hydrogen bonds, charge transfer, electron donator/acceptor interaction, etc.

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