# For a molecule, does the term 'thermal vibration' mean the same thing as 'molecular vibration'?

When speaking of a molecule, do both terms ('thermal vibration' and 'molecular vibration') describe the same thing, being the one and only periodic motion of the atoms within the molecule?

Or, are thermal vibrations technically something different than molecular vibrations?

I've read about random motions of the atoms in molecules being the thermal vibrations. Random doesn't sound like the periodic motions though (the molecular vibrations).

Any clarification would help. Thanks so much.

• I personally associate "thermal vibrations" to how atoms vibrate in solid lattice (phonons), and "molecular vibration" with the vibration modes of a single molecule. Here's a similar question: Is molecular vibration just phonon modes for a single molecule? – user3408085 Aug 23 '18 at 16:04
• Yes. But I was wondering if there was a secondary-type vibration of the atoms (the particles themselves), other than the molecular vibration, that would be considered the random thermal motion (or vibration). Do you see what I mean? – adam3033 Aug 23 '18 at 16:14

If you consider an isolated gas molecule then vibration is just excitation of the various vibrational modes. For an isolated molecule the only way to excite vibrational modes is to absorb photons, and the only way for the excitations to decay is to emit photons.

In a gas of many molecules the principle mechanism for excitation and de-excitation of vibrational modes is collisions between gas molecules. Assuming the gas is in thermal equilibrium the populations of the various excited states will then have something approximating a Boltzmann distribution with the $T$ in the Boltzmann distribution equal to the gas temperature. Insofar as the term thermal vibration has any standard meaning (it's not a term I'm familiar with) presumably it refers to this distribution of excited vibrational states of the molecules.

The vibrational states themselves are the same states as in an isolated molecule. The thermal bit just refers to the distribution of energy between those states.

• Thank you. So it's probably just best to associate the phrasing 'thermal vibrations' with non-molecular lattice vibrations (although what you suggested above does make sense). And thermal motions could probably best refer to the resultant oscillating electrons in a solid, liquid or dense gas? – adam3033 Aug 23 '18 at 17:22
• @adam3033 yes, I guess so. I'm not sure the term thermal vibrations has any universally accepted meaning. – John Rennie Aug 23 '18 at 17:38

A thermal vibrations in a crystalline solid produce atomic displacements, which in turn can be resolved into different states of polarization such that vibrations parallel to the wave vector are longitudinal waves and the two directions at right angles to the wave vector are transverse waves.

As the rules of quantum mechanics apply to all the different atomic vibrations in the crystal, the lattice pulsates as a complete assembly in discrete energy steps of ħω(phonons).

The phonon is related to both the frequency of vibration and the temperature. If the temperature is raised, the amplitude of atomic vibration increases, and in quantum terms, this is considered as an increase in the number of phonons in the system.

The concepts of temperature and thermal equilibrium associated with crystal solids are based on individual atoms in the system possessing vibrational motion.

A molecular vibration occurs when atoms in a molecule are in periodic motion while the molecule as a whole has constant translational and rotational motion.

The frequency of the periodic motion is known as a vibration frequency, and the typical frequencies of molecular vibrations range from less than 1013 to approximately 1014 Hz, corresponding to wavenumbers of approximately 300 to 3000 cm−1.

In general, a non-linear molecule with N atoms has 3N – 6 normal modes of vibration, but a linear molecule has 3N – 5 such modes, because rotation about its molecular axis cannot be observed.[1]

A diatomic molecule has one normal mode of vibration. The normal modes of vibration of polyatomic molecules are independent of each other but each normal mode will involve simultaneous vibrations of different parts of the molecule such as different chemical bonds.

A molecular vibration is excited when the molecule absorbs a quantum of energy, E; E = hν (where h is Planck's constant).

A fundamental vibration is excited when one such quantum of energy is absorbed by the molecule in its ground state. When two quanta are absorbed the first overtone is excited, and so on to higher overtones.

Therefore the thermal vibrations and molecular vibrations are terms representing two different situations and mechanisms, however, the thermal state of the physical system can have exchanges/sharing of thermal energy quanta between the two states.

reference-

• Thank you. So with molecules, whether a solid lattice, liquid, or gas, there would just be molecular vibrations, correct? No atomic vibrations/thermal vibrations? And in a lattice of just atoms (no molecules), you would just have atomic vibrations, i.e. thermal vibrations? – adam3033 Aug 23 '18 at 19:33
• @adam3033-yes, the referred studies do point the same – drvrm Aug 23 '18 at 19:50