Which Molecules Are Suitable for Purely Rotational Spectroscopy?

[adiabatman]

Which of the following molecules can be studied by purely rotational spectroscopy?

a) NH₃ b) N₂ c) CH₄ d) SF₆ e) CS f) CS₂

The attempt at a solution[/b]

To my understanding, using only rotational spectroscopy could be used to study b) N₂ and e) CS but I am not sure about my answer. Would the lack of a dipole for N₂ make it unfit? Any help would be greatly appreciated.

 

[ehild]

Which of the following molecules can be studied by purely rotational spectroscopy?

a) NH₃ b) N₂ c) CH₄ d) SF₆ e) CS f) CS₂

The attempt at a solution[/b]

To my understanding, using only rotational spectroscopy could be used to study b) N₂ and e) CS but I am not sure about my answer. Would the lack of a dipole for N₂ make it unfit? Any help would be greatly appreciated.

http://en.wikipedia.org/wiki/Rotational_spectroscopy

The rotational spectra of non-polar molecules cannot be observed by those methods, but can be observed and measured by Raman spectroscopy.

Which of the molecules above posses dipole momentum?

ehild

 

[adiabatman]

Thank you! Now I feel confident that e) CS is the answer

 

[ehild]

What about NH3?

The NH3 molecule has a large dipole moment, and this is consistent with its geometry, a triangular
pyramid.
http://en.wikipedia.org/wiki/Ammonia

Structure

The ammonia molecule has a trigonal pyramidal shape as predicted by the valence shell electron pair repulsion theory (VSEPR theory) with an experimentally determined bond angle of 106.7°.[13] The central nitrogen atom has five outer electrons with an additional electron from each hydrogen atom. This gives a total of eight electrons, or four electron pairs that are arranged tetrahedrally. Three of these electron pairs are used as bond pairs, which leaves one lone pair of electrons. The lone pair of electrons repel more strongly than bond pairs, therefore the bond angle is not 109.5°, as expected for a regular tetrahedral arrangement, but is measured at 106.7°.[13] The nitrogen atom in the molecule has a lone electron pair, which makes ammonia a base, a proton acceptor. This shape gives the molecule a dipole moment and makes it polar.

ehild

 

[paranormal]

Your answer is correct. Both N2 and CS can be studied using purely rotational spectroscopy. The lack of a dipole moment in N2 does not make it unfit for this technique, as rotational spectroscopy can also be used for non-polar molecules. However, molecules with a permanent dipole moment, such as NH3 and CH4, may also exhibit additional features in their rotational spectra due to their vibrational modes. Therefore, while these molecules can also be studied using rotational spectroscopy, the resulting spectra may be more complex. SF6 and CS2 are both non-polar molecules and therefore not suitable for purely rotational spectroscopy.