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Do intramolecular bonds vibrate in solids ?

  1. Nov 27, 2013 #1
    Do intramolecular bonds vibrate in solids...???

    In case of molecular solids does the Intramolecular bonds(bonds within atoms of a molecule) vibrate as they do vibrate in gas and liquid phases.

    We are familiar that motion of molecules in solids is in form of vibrations. But when we talk of that we are normally speaking about inter-molecular vibrations(phonons)

    Now, we know that intermolecular bonds are held together by weak forces like Van Der Walls' forces,London Dispersive forces,etc. While the intra-molecular forces are generally strong forces like covalent bonding,dipole-dipole forces,etc.

    "Also when we heat a solid(of molecular nature), it is easy to envisage that intermolecular vibrations increases. But on heating does intra-molecular vibrational transtions to higher vibrational energy states also takes place as on heating a gas comprised of molecules."

    "Also does the energy gap between various intramolecular vibrational states(If they exist) in solids , remains the same as the energy gap between different vibrational levels in gases and liquids.???"

    "Thanks For Reading and Paying Attention"​
  2. jcsd
  3. Nov 27, 2013 #2


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    Yes, of course there are also intramolecular vibrations in solids. They are called "optical phonons".
    As these vibrations cannot be strictly separated from intrermolecular vibrations, they no longer occur at fixed frequencies but are broardened out into bands.
  4. Nov 27, 2013 #3
    Thanks for replying Mr. DrDu. I liked your answer and agree with it. I also thought the same thing before posting this question,but needed just just a confirmation.

    But can you please elaborate on "they no longer occur at fixed frequencies but are broardened out into bands."

    I think that even bands are made up of different and definite energy levels. Am I not correct. If not please let me help to understand what you meant by "broadened in bands". What exactly you meant to tell by use of word "band".

    Thanks For Reading
  5. Nov 27, 2013 #4


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    Yes, you are right, but these energy levels are labelled by wavenumber as all molecules in the crystal move correlated. You can understand this most easily considering a dimer of two molecules: There will be two new resonance frequencies, one where both molecules move in phase and one where they are 180 degrees out of phase.
    With three molecules you get 3 distinct new resonance frequencies until you get a continuum in the limit of an infinite crystal. If the molecules interact only slightly, then instead of a single energy level E for the isolated molecule you will find for every energy inside a band E-ΔE, E+ΔE a level.
  6. Nov 29, 2013 #5
    Thanks Mr. Drdu for explaining so well. Sorry to reply too late. I was too busy in my studies. But once again thanks a lot. My doubt is clear now.
  7. Nov 30, 2013 #6
    Why are they called "optical"?
    I read a claim that "optical phonons" are possible only if atoms differ in mass, bonding strength or charge.

    Then can "optical phonons" happen in, say, solid nitrogen?

    There are very strong triple bonds in each nitrogen molecule. But both N atoms have same bonding strength, suppose they also have same mass (all N-14 or all N-15) and then also the same charge.

    Looking across the symmetry plane across the triple bond of a nitrogen molecule, is the rest of crystal symmetric as well?

    How are the intramolecular stretching vibrations of dinitrogen molecules connected to the stretching vibrations of other dinitrogen molecules around the nitrogen crystal?
  8. Nov 30, 2013 #7


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    I think they are called optical because they are responsible for bands in the near infrared.
    Of course nitrogen has optical phonons as the intramolecular bonding is much stronger than the interaction between nitrogens in different molecules.
    Concerning your last question, think of N2 molecules in a line. The molecules will hinder less if if one is extending, the neighbouring is contracting and vice versa. So this mode will have least frequency while the mode where all are in phase has highest frequency.
  9. Nov 30, 2013 #8
    Stretching of symmetric bonds has no dipole moment and therefore cannot absorb infrared. Diatomic homonuclear molecules (the ones that resist polymerization at low temperature being 1 isotope of difluorine and diiodine, 2 isotopes of dihydrogen, dinitrogen, dichlorine and dibromine, and 3 isotopes of dioxygen) cannot absorb - in sparse gas.

    In dense gas or liquid, when another molecule happens to be anywhere else than on the symmetry plane across the bond, it breaks the symmetry and therefore creates dipoles, allowing collision- or pressure-induced absorption.

    But do pressure-induced lines occur in solids? Do these solids possess any asymmetry around the intramolecular bonds, or does the presence of identical next molecule each side of the stretching bond make absorption impossible? Are optical phonons in nitrogen allowed by crystal symmetry?
  10. Nov 30, 2013 #9


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    I did not say so. But anyhow you can easily detect these vibrations e.g. using Raman spectroscopy.
    Obviously, the lattice symmetry can break the inversion symmetry, whence the stretching mode may become IR active. In solid N2, such a phase is observed for pressures higher than 1.9 GPa:
    http://physics.wsu.edu/research/high-pressure/1996/Nitrogen%20PRB.pdf [Broken]
    Last edited by a moderator: May 6, 2017
  11. Nov 30, 2013 #10
    Traditionally one distinguishes between "acoustic" and "optical" phonons.

    Acoustic phonons have a dispersion that goes to zero frequency at zero wave vector. In crystals there are 3 branches of acoustic phonons, two transverse and one longitudinal. They are involved in sound, hence the name.

    Optical phonons have a finite frequency at zero wave vector. For many crystals these frequencies fall into the visible range. Visible light has a very small wave vector compared to the Brillouin zone of typical (small molecule) crystals. Therefore single-phonon optical measurements can only "see" optical phonons. Energy scales may change with materials, but the name "optical phonons" sticks. The more complex the unit cell, the more optical phonon branches you get.
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