3
$\begingroup$

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?

$\endgroup$
4
$\begingroup$

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

enter image description here

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.