Molecular Vibrations: Mechanism, Energy Source & More

In summary, molecules have an intrinsic vibrational energy that cannot be utilized and is present even at absolute zero. This energy is separate from the bond energy that is affected by temperature and pressure. The zero point energy of molecules only exists at absolute zero and is not measurable at higher temperatures.
  • #1
I've heard molecules are constantly moving or vibrating. What is the mechanism behind this? Where does the energy come from, and go? Is it always a function of temperature/pressure?
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  • #2
nuby said:
What is the mechanism behind this?

Generally speaking these oscillations are driven by coulomb forces. For more details you may want to read about quantum oscillator.

Where does the energy come from, and go?

Molecule that is excited just 'has' its energy, and its energy doesn't change. You may say that this energy is stored in vibrations.

Edit: it is a little bit more complicated, as the energy can be also stored in electrons occupying different orbitals, let's assume that electrons are not moving fom one orbital to another for now. Besides, to excite electron we usually need much more energy than to exctite bond vibrations.

Molecule can exchange energy with surroundings through collisions or interacting with radiation.

Is it always a function of temperature/pressure?

For a given molecule (to some extent for a given bond) frequency of the possible vibrations is independent on the surroundings. however, which frequencies dominate depends on the molecule energy, which in turn is a function of the temperature.
  • #3
It may be useful to consider why molecules can't stay still. This phenomena is somewhat tricky to understand because it arises fully from quantum mechanics.

First, consider the Heisenberg Uncertainty Principle. This principle states that we cannot know the exact momentum and position of a particle simultaneously. If we know the exact position of a particle at a certain time, then we cannot know its precise momentum (i.e. speed and direction) and vice versa.

So, consider an unmoving particle. Since it is not moving, we know that it's momentum must be zero. Furthermore, because it is not moving, we would also know it's precise location. Therefore, an unmoving particle violates the Heisenberg Uncertainty principle and cannot exist even at absolute zero. This requirement for particles to have momentum (and therefore kinetic energy) at absolute zero is what is known as the infamous zero point energy.
  • #4
Ygggdrasil: No offense, but that's a pretty useless answer.
It explains why molecules aren't still at Absolute Zero, which is an unattainable temperature. It doesn't really explain the why-and-hows of thermal energy in general.
  • #5
Nah, Ygg can be right - it is 100% not clear what the OP asks about. Could be the original question is about the zero point energy. Could be it is not.
  • #6
Ok, so here's my understanding now. Molecules just have an intrinsic vibrational energy that can be considered stored energy.. This energy can't be utilized, and is just "there"... It has no special name.

Then there is a bond energy "vibrational", (seperate from above) that is a function of pressure/temperature.

Then a molecules zero-point-energy only exists when the molecules are at 0K ... Above 0K zero-point energy doesn't exist (or not measurable?)

Is this accurate?
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1. What is the mechanism behind molecular vibrations?

The mechanism behind molecular vibrations is the movement of atoms within a molecule, which is caused by the interaction of the atoms' nuclei and electrons. This movement is driven by the energy stored in chemical bonds.

2. What is the primary source of energy for molecular vibrations?

The primary source of energy for molecular vibrations is the absorption of infrared (IR) radiation. When IR radiation is absorbed by a molecule, it causes the bonds between atoms to stretch and bend, resulting in molecular vibrations.

3. How are molecular vibrations detected?

Molecular vibrations can be detected through various spectroscopic techniques, such as infrared spectroscopy, Raman spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. These techniques measure the absorption or emission of radiation by the molecule, which can provide information about its vibrational modes.

4. How do molecular vibrations affect molecular properties?

Molecular vibrations can affect various properties of a molecule, including its shape, bond strength, and reactivity. For example, changes in the vibrational energy of a molecule can lead to changes in its bond lengths and angles, which can affect its overall shape and stability.

5. How do molecular vibrations play a role in chemical reactions?

Molecular vibrations play a crucial role in chemical reactions by providing the necessary energy for bonds to break and form. Vibrational energy can also affect the rate of a reaction, as molecules with higher vibrational energy are more likely to collide and react with other molecules.

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