Quantum Chemistry/Example 15

Write an example question showing the determination of the bond length of CO using microwave spectroscopy

Example 15: edit

Deriving the Required Equations edit

When a photon is absorbed by a polar diatomic molecule, such as carbon monoxide, the molecule can be excited rotationally. The energy levels of these excited states are quantized to be evenly spaced. The distance between each rotational absorption lines is defined as twice the rotational constant   which can be measured via the following equation:


h = plank's constant = 6.626 ᛫10−34 J ᛫ s

c = speed of light = 2.998᛫108 m ᛫ s-1

I = moment of inertia

The energy to required to rotate a molecule around its axis is the moment of inertia  . It can be calculated as the sum of the products the masses of the component atoms and their distance from the axis of rotation squared:


Working it out for an heterogeneous diatomic molecule:



The distance from the atom to the center of mass   cannot easily be measure, however, by setting the origin at the center of mass equation can be derive for the two values that uses the bond length   as a variable:




Substituting these equations into the moment of inertia equation:



This equation can be simplified further if we imagine the rigid rotor as a single particle rotating around a fixed point a bond length away. The mass of this particle is the reduced mass   of the two atoms that make up the diatomic molecule:


Simplifying the previous moment of inertia equation, we get:


Solving for Bond Length edit

From here we have everything we need to be able to determine the bond length of a polar diatomic molecule such as carbon monoxide.

First, we must solve for the moment of inertia using the rotation constant:



As explained earlier, the rotational constant can be determined by measuring the distance between the rotational absorption lines and halfling it. In the case of   the rotational constant is   m-1 [4]. Plugging this value in we can determine the moment of inertia:


  kg ᛫ m2

Now that we know inertia, we can rearrange the equation we derived earlier in order to determine the bond length:




The exact atomic mass of   is 12.011 amu and   is 15.9994 amu [5]. As such the reduce mass is calculated to be:




  amu ᛫  


Plugging in the reduce mass back into our equation we can finally solve for the bond length of a carbon monoxide molecule:





References edit

  1. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Exercises%3A_Physical_and_Theoretical_Chemistry/Exercises%3A_Aktins_et_al./12.E%3A_Rotational_and_Vibrational_Spectra_(Exercises)
  2. https://phys.libretexts.org/Bookshelves/University_Physics/Book%3A_University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)/10%3A_Fixed-Axis_Rotation__Introduction/10.06%3A_Calculating_Moments_of_Inertia
  3. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Exercises%3A_Physical_and_Theoretical_Chemistry/Exercises%3A_Aktins_et_al./12.E%3A_Rotational_and_Vibrational_Spectra_(Exercises)
  4. https://webbook.nist.gov/cgi/cbook.cgi?ID=C630080&Mask=1000
  5. https://www.angelo.edu/faculty/kboudrea/periodic/structure_mass.htm