1. The lab manual states that \"the number of vibrational modes (different types

Question

1. The lab manual states that "the number of vibrational modes (different types of vibrations) in a molecule is 3N-5 for linear molecules and 3N-6 for non-linear molecules. Why do you not "see" four absorptions for the CO2 molecule? 2. How many degrees of unsaturation (sometimes called sites of unsaturation) are there in the following molecules? (a) benzene (C6H6) (c) aspirin (CoHsO4) (b) 2-methyl butane (CsH12) (d) caffeine (CsHioNO) 3. Using the strategy employed in com/2016/1123 identify the functional group most likely to account for the four most intense absorptions in the following spectrum of 2-methyl cyclohexanone.

Explanation / Answer

1. Number of vibrational modes for linear molecules is 3N-5 for linear molecules and 3N-6 for non-linear molecules where N is the number of atoms.

As CO2 is a linear molecules, Number of vibrations = 3*3-5 = 4

But we see only 3 because , when the molecule is place in x-axis, the bend or the vibrational stretch is same in y and Z axes but deforming in different direction and is said to be degenerate accounting for the 4th vibration.

Also tule of mutual exclusion states that, in centrosymmertic molecules like CO2 vibrations that are IR active are Raman inactive and viceversa. For CO2, there are 1 Raman band and two IR bands.

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