# Discussion Questions

Published 2022-08-27

# 4.5 Discussion Questions


  1. Explain how colour can arise from molecules.

  1. Electronic transitions for molecules occur in the UV-vis regime.
  1. One limitation of UV-vis spectroscopy for molecules is the lack of specificity. Discuss what this means and why this is so.

  2. Despite its limitation(s), UV-vis spectroscopy is widely used in research labs. What are some common uses of UV-vis spectroscopy?


  1. The figure below is a simulated rotation spectrum of CO. Estimate the internuclei distance of CO.


  1. The molecule Cyanoacetylene \text{HC}_{3}\text{N} exhibits a spectral line at 9.098 GHz for the J=1\rightarrow0 transition.
  1. What is the rotational constant in units of GHz?

  2. In what catagory of the EM spectrum does this line belong to? (radio/microwave/IR/visible/UV/X-ray)

  3. What are the positions of the spectral lines corresponding to the transitions J=2\rightarrow1, J=3\rightarrow2, J=4\rightarrow3, etc.?

  4. Simulate the rotational (emission) spectrum of Cyanoacetylene \text{HC}_{3}\text{N} at T=10 K, 50 K and 100 K.


  1. Derive an expression for the most probable J transition line for rotational spectroscopy. (Hint: use Eq.(QM(6))).

Suggest how we can estimate the temperature of the observed object using rotational spectroscopy.


  1. By referring to the following figure, suggest why the range of isotopes would result in a different wavenumber.

Rovibrational Spectrum of HCl and DCl, adapted from J. Chem. Educ.
2018, 95, 12, 2293--2297
Rovibrational Spectrum of HCl and DCl, adapted from J. Chem. Educ. 2018, 95, 12, 2293--2297