How to interpret IR spectra - ChemistryScore (2024)

Infrared (IR) spectroscopyis a very useful method for detecting the characteristic bonds of manyfunctional groupsthrough theirabsorption of infrared light.

If you shine infrared light on a molecule, it is possible that the molecule absorbs energy from light. Absorbed energy can cause a bond tostretchorbend. We call that astretchingorbending vibration.These vibrations occur only at specific frequencies, which correspond to the frequency of IR light. When the frequency of IR light matches the frequency of a particular vibrational mode, the IR light is absorbed, and you can tellwhich frequenciesare absorbed by looking at yourinfrared spectrum.Different kinds of bonds vibrate at different frequencies, so they absorb different frequencies of IR light, so it is possible to determine the functional groups present.

At first glance, the IR spectra look very complicated, but the only three things you need to know are:

1. regions of the spectrum,
2. one number (1500), and
3. location and shape of the peaks.

Let’s considers all of these.

IR spectra can be divided into two main regions:

1. Diagnostic region– generally has fewer peaks and provides the clearest information. This region contains all signals that arise from all bonds in a molecule.
2. Fingerprint region–contains signals resulting from the vibrational excitation of most single bonds (stretching and bending).

Since the fingerprint region generally contains many signals and is more difficult to analyze, we can ignore it.It benefits us when we have similar compounds, for example, the same bonds (functional groups) but a different number of them.Such spectra will be the same in the diagnostic region, but in the fingerprint region won’t.Thus this region is called a fingerprint because each compound has a unique pattern of signals in this region, much the way each person has a unique fingerprint.

How will you distinguish these regions except the look of the peaks?

Using the value of thewavenumbers.

The abscissa of our IR diagrams shows the wavenumbers,and the boundary is at avalue of 1500 cm‾¹.So we can also draw thelineat a value of 1500 cm‾¹ when we interpret spectrum.

When we look at the following table of the characteristic stretching wavenumber values for the bonds, we can see that the most absorbing in the region above 1500 cm‾¹ and up to 3650 cm‾¹.This table shows some of the bonds and areas in which they appear.Some? So there are more?Of course, but this is enough to start. You’re probably confused with so many values in this table but don’t worry, you’ll easily remember them.You don’t have to learn all of these numbers right away because it will soon become a routine.

The third point explains everything. Now everything will be much much easier.

Now, all you need to learn is thelocation and shape of the peaks. Here are typical infrared absorption values for various types of bonds:

The most common signals are shown in the picture above, those you need to master first, but I will add even more (marked with *) in the text that also often appears in the spectra.

1. If start at 1500 cm‾¹, the first thing we encounter is avery sharp needlea signal that is acarbon-carbon double bond.*
2. In the region around 1700 cm‾¹, we can see alittle bit thicker finger-likecarbon-oxygen double bond.
3. A little bit further, we might have thearomatic overtonesthat look likefangs.Their size is not clearly defined. They can be bothshortandlong.*
4. Then, we can see avery sharp needle-likesignal that could correspond totriple bonds both carbon-carbon and carbon-nitrogen.
5. Now, we have a signal that is really hard to interpret, and this is analdehyde. Sometimes this signal occurs but actually, we don’t have an aldehyde. Because it is necessary to check ¹H NMR if this compound is present or not.
6. A little before 3000 cm‾¹ we expectsp³ hybridized carbonatom attached tohydrogen.Sometimes they are smaller or larger than other signals.
7. And a little bit after 3000 cm‾¹ we expectsp² hybridized carbonatom attached tohydrogen.
8. After sp³ and sp² comes thesp hybridized carbonatom bonded tohydrogen.If we interpret that we have some of these hybridized carbon atoms, we must confirm them with the previously mentioned signals. In other words, if we say that we have signal number 8. we also have signal number 4. And if we have signal number 7. we also have signal number 1.
9. Then, further past 3000 cm‾¹, we encounter with thelargesignal of ahydroxyl group. The OH stretch shows up in this region is a large singlet because it’smore acidic.
10. In the same region as the OH, we might have anNHstretch. The NH may show asinglet, adoubletor even atripletsometimes which depends on whether we have aprimaryorsecondaryamine. These signals aremore shorterthan OH signal.
11. In the region where the triple bond exists, we can have ashorter versionof that which is theNH Bend. This occurs forprimary amines and amides.
12. And for the end,carboxylic acids. This signal appears from OH stretch to between an aldehyde and a triple bond.This signal isvery wide and short.