We can observe and measure\u00a0this “singing” of bonds by applying IR radiation to a sample and measuring the frequencies at which the radiation is absorbed. The result is a technique known as Infrared Spectroscopy<\/a>, which is a useful and quick tool for\u00a0identifying\u00a0the\u00a0bonds present in a given molecule.<\/p>\n We saw that the IR spectrum of water was pretty simple – but moving on to a relatively complex molecule like glucose (below) we were suddenly confronted with a forest of peaks!<\/p>\n Your first impression of looking at that IR might be: agh!\u00a0how am I supposed to make sense of that??<\/em><\/strong><\/p>\n To which I want to say:\u00a0don’t panic!\u00a0<\/strong><\/p>\n Table of Contents<\/strong><\/p>\n In this post, I want to show that a typical analysis of an\u00a0IR spectrum is much simpler than you might think. In fact, once you learn what to look for, it can often be done in a minute or less.<\/strong> \u00a0Why?<\/p>\n With this in mind, we can simplify the analysis of an IR spectrum by cutting out everything except the lowest-lying fruit.\u00a0<\/em><\/p>\n See that forest of peaks from 500-1400 cm-1<\/sup> ? We’re basically going to ignore them all!<\/p>\n 80% of the most useful information for our purposes can be obtained by looking at\u00a0two specific areas of the spectrum<\/strong>: 3200-3400 cm-1<\/sup> and 1650-1800 cm-1<\/sup>. We’ll also see that there are at least two more regions of an IR spectrum worth glancing at, and thus conclude a “first-order” analysis of the IR spectrum of an unknown. [We might write a subsequent post which gets nittier and grittier about the finer points of analyzing an IR spectrum]<\/p>\n Bottom line: The purpose of this post is to show you how to\u00a0prioritize your time<\/strong>\u00a0in an analysis of an IR spectrum.<\/p>\n [BTW: all spectra are from the NIST database<\/a>. Thank you, American taxpayers!]<\/em><\/span><\/p>\n Confronted with an IR spectrum of an unknown (and a sense of rising panic), what does a typical new student do?<\/p>\n They often reach for the first tool they are given, which is a table\u00a0of common ranges for IR peaks given to them by their instructor.<\/p>\n The next step in their analysis is to go through the spectrum from one side to the next, trying to match every single peak to one of the numbers in the table. I know this because this is exactly what I did when I first learned IR.\u00a0<\/strong>I call it “hunting and pecking”.<\/p>\n The only people who “hunt and peck” as their first step are people who have no plan\u00a0<\/strong>(i.e. “newbies”).<\/p>\n So by reading the next few paragraphs you can save yourself\u00a0a lot of time and confusion.<\/p>\n [Hunt and peck has its place,\u00a0but only AFTER\u00a0<\/strong>you’ve looked for “tongues” and “swords”, below. Hunting and pecking is great to make sure you didn’t miss anything big – but as a first step, it’s bloody awful!]<\/em><\/span><\/p>\n In IR spectroscopy we measure where molecules absorb photons of IR radiation. The peaks represent areas of the spectrum where specific bond vibrations occur. [for more background, see the<\/span> previous post,<\/a> especially on the “ball and spring” model]<\/em>. Just like springs of varying weights vibrate at characteristic frequencies depending on mass and tension, so do bonds.<\/p>\n Here’s an overview of the IR window from 4000\u00a0cm\u00a0-1\u00a0<\/sup>to 500\u00a0cm\u00a0-1\u00a0<\/sup>with various regions of interest highlighted.<\/p>\n An even more compressed overview looks like this: (source<\/a>)<\/p>\n![\"ir](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/1-ir-spectrum-of-glucose-how-do-we-analyze-this-with-so-many-peaks-dont-panic.png\")
<\/p>\n\n
\n<\/a>1. Let’s Correct Some Common Misconceptions About IR<\/strong><\/h2>\n
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<\/a>2. Starting With “Hunt And Peck” Is Not The Way To Go<\/h2>\n
![\"for](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/2-for-gods-sake-when-interpreting-ir-spectra-dont-hunt-and-peck-with-a-table-instead-know-what-to-look-for.png\")
<\/p>\n<\/a>3. The Big Picture<\/strong><\/h2>\n
![\"table](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/3-table-of-typical-infrared-absorption-values-for-various-types-of-bonds.gif\")
<\/p>\n
![\"collection](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/4-collection-of-o-h-stretches-for-alcohols-5-examples.png\")
<\/p>\n![\"collection](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/5-collection-of-c-o-stretches-around-1700-for-aldehydes-ketones-esters-carboxylic-acids.png\")
<\/p>\n![\"ir](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/6-ir-spectrum-of-hexanol.png\")
<\/p>\n![\"ir](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/7-ir-spectrum-of-butanoic-acid.png\")
<\/p>\n![\"ir](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/8-ir-spectrum-of-hexanal.png\")
<\/p>\n![\"the](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/9-the-dividing-line-at-3000-cm-1-between-sp3-ch-bonds-and-sp2-c-h-bonds.png\")
<\/p>\n![\"triple](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/10-triple-bonds-have-distinctive-stretch-around-2050-to-2250-nitriles-alkynes.png\")
<\/p>\n![\"1](\"https:\/\/www.masterorganicchemistry.com\/wp-content\/uploads\/2019\/12\/11-1-minute-analysis-of-ir-of-glucose-has-oh-no-alkene-ch-no-c-o-double-bond.png\")
<\/p>\n