Structure and Bonding:  The Flexibility of Bonds

Are chemical bonds in molecules rigid like the sticks we use in the model kits?

Would a molecule of decane, C10H22, always look like the image below?

Examine the molecule of iodine, I2, given below.  How would you describe the molecule?  What is the bond length in the molecule?

This type of vibration is called stretching and as you can see the bond length varies about an average distance.

Carbon dioxide is a linear molecule with a O-C-O bond angle of 180o.  View and move the CO2 molecule around below.  Is the bond angle constant?  Explain.

This type of vibration is called bending and as you can see the bond angle varies about an average angle.  Any type of motion within a molecule that changes the size and shape of the molecule is called a vibration. 

Look at the molecule of acetic acid, CH3COOH, given below.  Describe the motion?  Is this a vibration?

This illustrates bond rotation, which can occur about any single bond, and is considered a vibration because it changes the shape of the molecule.  

View the molecule of butane, C4H10, below and consider your answer to the question about decane.  Did your answer change?

In any molecule, stretching, bending, and bond rotation could be happening simultaneously.  Molecular motion can be very complex and demonstrates the dynamic nature of molecules.  Molecules are not empty, static, and rigid as the "wooden ball and stick" models can lead you to conclude.

To view the thermal motions in a short-chain alpha helix protein - click here.  You may want to right click and select rotation to turn it off.

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