Interpreting IR Spectra

Getting Around an IR Spectra

Because the wavelength range of IR radiation useful in vibraional spectroscopy has wavelengths which are rather small numbers (i.e about 2.5x10^-6 to 25x10^-6 meters) chemists have devised an alternative units of reciprocal centimeters (cm^-1).  This is sometimes referred to as wave-number and is the x-axis on an IR spectra.  The useful range for molecules is between 4000 cm^-1 and about 400 cm^-1.  Wave-number is really a measure of energy and higher energy is to the left and lower energy is to the right on a spectra.  Vibrational modes with high energy will show up on the left and those with lower energy will be on the right.

The y-axis is either the intensity of light absorbed or transmitted.  In an absorbance spectra the signal intensity indicates the amount of light absorbed.  The signals or features you are looking for are peaks in an absorbance spectra.  In a transmittance spectra the signal intensity shows the amount of light that passed through (transmitted) the sample.  In a transmittance spectra you/re looking for valleys (or upside down peaks).  They can be inter-converted by using A=-log(T/100).  Most FTIR will do the conversion for you.  Below are the transmittance and absorbance spectra for 1-propanol.

Absorbance Spectra (1-propanol)

Transmittance Spectra (1-propanol)

General Procedure for Interpreting IR Spectra (Beginner)

There are three regions of an IR spectra from which most of the info you will use in organic chemistry will come from.

1. O-H and N-H Stretch Region - On the left (high energy end of the spectra) look for -O-H, N-H (3600-3300 cm^-1).  The OH signal is usually broad strong absorption.   N-H are less broad and strong.
   • In the spectra above for 1-propanol a broad strong absorbance is observed in this region.
     
2. C-H Stretch Region - Look at the 3000 cm-1 part of the spectra.  In this region lie the C-H stretches.  The C-H stretches will have slightly different wave-numbers depending upon whether the H's are attached to sp³, sp² or sp carbons.  Just to the right of the 3000 cm^-1 line will lie the sp³ C-H stretches (3000-2800 cm^-1).  To the left of the 3000 cm^-1 at a slightly higher energy will lie the sp² C-H (3200-3000 cm^-1).  And further to the left of these will lie the sp C-H absorbances (3300-3200cm^-1).
   • As expected the 1-propanol spectra above has signals for sp³ C-H (to the right of the 3000 cm-1 line) but not for sp or sp² C-H.
     
3. C=O Stretch Region - Check for the existence of a carbonyl (C=O) stretch around 1700 cm^-1.
   • There are no C=O in 1-propanol, and therefore an absorbance at 1700 cm^-1 is not observed.

Why are C-H and N-H signals in the high energy end of the spectrum?
When a heavy atom (C, O, N) is bound to a light atom such as H the vibration is at a higher frequency (energy).  See Hooke's law for a harmonic oscillator.  Assume one of the m1 is infinite then examine the dependence of frequency on mass m2. 

Why are sp C-H stretches at a higher energy than sp³ C-H stretches?
Again see Hookes law.  In this case an sp C-H bond has a greater spring constant (stronger bond) than an sp³ C-H bond and therefore a higher frequency of vibration.