Studying Vibrations in Molecules

Consider the formaldehyde molecule given below.  You may want to build a ball-and-stick model of it.

What is a vibration in a molecule?  You may want to review the Molecules in Motion activity.

 

There are six possible vibrational modes of motion in formaldehyde.  Draw the structure, sketch arrows, and explain the modes in the table below. See a shockwave animation of modes and the terminology to describe them.

 

 

 

 

   
 mode: mode: mode:
 

 

 

 

   
 mode: mode: mode:

Click here to go to the Purdue University chemistry site to verify your modes of vibration you predicted above.  Record the wavenumber (energies) for each vibration that are given.

Look at the structure of phosgene given below.  Does phosgene have the same or different modes of vibration?  Explain.

Here are the vibration energies (wavenumber) as calculated using Spartan '02 (AM1 minimization) for formaldehyde and phosgene.  

mode of vibration CH2O  (X = H) COCl2  (X = Cl)
C-X symmetric stretching 3121 cm-1 586 cm-1
C-X asymmetric stretching 3085 cm-1 877 cm-1
C-O stretching (C=O) 2053 cm-1 1951 cm-1
X-C-X in-plane scissoring 1444 cm-1 328 cm-1
X-C-X in-plane rocking 1176 cm-1 475 cm-1
X-C-X out-of-plane wagging 1165 cm-1 641 cm-1

When the H's are replaced by Cl's, what happens to the energy of the vibration?  Why?

 

How does the carbonyl group (C=O) behave from CH2O to COCl2?

 

Now the wavenumber of the vibrations, n', are given in reciprocal centimeters, cm-1.  Calculate the range of wavelengths, l, in nanometers (nm) of electromagnetic radiation for the typical  energies of 400 - 4000 cm-1.    Remember that 1 m = 100 cm = 109 nm.

n' = 1/l

 

What part of the electromagnetic spectrum is involved in vibrations?

 

View the video of a single vibration in the amino acid cystine (produced in Spartan '02).  The vibration is the stretching of the S-S bond (light blue in center of molecule) at 409 cm-1.  How does it affect the molecule as a whole?

 

How does the S-S stretching energy compare to the C=O stretching energy?

 

Let's examine the behavior of a vertical spring with a mass hanging on it.  Click here to get a STELLA model to examine this.  The vertical spring and mass are similar to the bond as a spring between two atoms, the masses.  How does mass and the force constant (spring constant) influence the frequency?

 

Click here to get an interactive Excel spreadsheet for estimating the wave number, which is the energy or frequency of a stretching vibration for a specific bond.  You need to use it to answer the following three items:                                                      

1.    How does wave number change as reduced mass increases?  Reduced mass is calculated by this equation:  Reduced Mass = M1M2/(M1 + M2)  Support with data and explanation.  For this comparison M1 = 12 for carbon.

M1

M2

Energy

C

H

 

C

D

 

C

C

 

C

F

 

C

Cl

 

C

Br

 

C

I

 

 

2.    How does the wave number vary for the H-X bond, where X is a halogen?  Support with data and explanation.  Here M1 = 1 for hydrogen.

M1

M2

Energy

H

F

 

H

Cl

 

H

Br

 

H

I

 

 

3.    How does the wave number vary for carbon-carbon single, double, and triple bonds?  Support with data and explanation.  Set M1 = 12 for carbon, look only at the row for carbon, and change the setting of k to vary the bond order.

Force constant, k

Bond order

Energy

5 x 105

1

 

10 x 105

2

 

15 x 105

3

 

 

Compare your predicted energy for the C-D bond to the measured value which is 2253 cm-1.

 

Measure the IR spectra for the compound assigned to your group.  Your instructor will help you.  Compare and contrast the spectra (attach as a separate sheet of paper).

benzene cyclohexane
cyclohexanol cyclohexanone

Post-laboratory Questions:

1.    How could you tell an alcohol from a ketone, two common functional groups in organic compounds?

 

 

2.    Convert a wavenumber (n') of 3000 cm-1, which is typical of C-H stretch, to frequency or the number of vibrations per unit time.   The frequency,n, is given by n = cn', where c is the speed of light at 3.0 x 1010 cm/sec.  Frequency has units of 1/time.

 

 

 

The period (P) is the time between vibrations.  P = 1/n  Calculate the period for the vibration above.

 

 

3.    Explain the statement:  Chemical bonds are like mechanical springs.

 

 

4.    How does the force constant vary as bond order changes?

 

 

5.    How can you tell if a single, double, or triple carbon-to-carbon bond occurs in a molecule?  Site a reference or website you used to address this question.

 

Back to CHM 103 Webpage

Scott A. Sinex        Department of Physical Sciences         Prince George's Community College            5/2002