In Infrared spectroscopy, stretches are easily understandable. But, how do I visualize (or conceptually understand) the difference between dihedral (torsions), out of plane bends, coplanar bends, and perpendicular bends? Can you give examples?


A molecule with N atoms has 3N degrees of freedom

3 translational modes

if it is linear, it will have 2 rotational modes; if non-linear 3

These leaves 3N-6 (3N-5 if linear) vibrational modes. Vibrational modes are further subdivided into stretching, bending or torsional motions.

  • Stretching motions can be symmetric or asymmetric
  • Bending motions are often referred to as scissoring, twisting, wagging, and rocking
  • Torsional motions refer to rotations about bonds

This diagram illustrates these various stretching and bending vibrations

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Depending on the geometric relationship between the transition moment for a specific vibrational mode and the symmetry axis for the mode, vibrational modes may be further classified as parallel or perpendicular.

Here is a link to a 31 page article (slide show) that has a lot of background and pictures on this subject. Section 4.11 (page 19) might be particularly interesting to you.

One final example that you might be interested in, formaldehyde has 4 atoms and is non-linear so it will have 6 vibrational modes. These are all pictured on page 2 in this link.


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