IR Spectroscopy — Transcript

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1. 0:00hey it's professor Dave, let's talk about IR spectroscopy
2. 0:10a lot of times when we do chemistry we face a very big challenge and that is that we cannot see
3. 0:14molecules, so we might do some chemical reaction, get a product, and we need some
4. 0:19way to be sure that what we got is what we think it is because we can't
5. 0:22we can't see the structure of the molecule so we haven't we have a number
6. 0:26of techniques that fall under the umbrella term of spectroscopy.
7. 0:30spectroscopy is the study of the interaction of light and matter and so
8. 0:35there's a lot of forms of spectroscopy that we can use to gather data about
9. 0:39what the structure of a molecule might be and so right now if you look at IR
10. 0:43spectroscopy IR stands for infrared as in infrared light so what we're gonna do
11. 0:48is say we do reaction we have some molecule, we wanna figure out what the molecule
12. 0:52is, we irradiate the sample with infrared light, that light is
13. 0:58going to interact in some way with the compounds in the sample and
14. 1:01then it'll eventually reach a detector so some light will be absorbed by the
15. 1:05sample, some will not, some will pass through and this is information that we
16. 1:09can use to figure out something about the structure of the molecule. the reason
17. 1:13that we use infrared light is because different molecules, we already
18. 1:17know that molecules move around, they have translational motion, we know
19. 1:22that heat energy is converted into kinetic energy of motion so molecules
20. 1:26that, say in a gas or even in a liquid are moving around with respect to one
21. 1:31another, and through conformational analysis we also know that bonds are
22. 1:35rotating all the time so there is rotational motion for all the all of the
23. 1:39sigma bonds but chemical bonds are also doing other things. as it happens they
24. 1:44can do these other things that occur specifically when they are
25. 1:47irradiated with a very specific wavelength of infrared light. they can do
26. 1:52something like a symmetric or asymmetric stretch so let's say my body is the
27. 1:56central atom and these are two atoms that it is bound to and so my arms are
28. 2:01covalent bonds and so this would be a symmetric stretch, the covalent bonds
29. 2:05can
30. 2:06actually be contracting and expanding at a particular vibration depending on
31. 2:12whether it has absorbed this particular photon of UV light. so this is a symmetric
32. 2:16stretch this would be an asymmetric stretch and we can also have a
33. 2:20symmetric bend or asymmetric bend in plane so that would look like this, this
34. 2:24would be symmetric or we could have asymmetric and we can actually have
35. 2:28twisting, we can have it like this which would be symmetric or it would be, I can't
36. 2:32even do it, it's very difficult to do but the point is we have all of these
37. 2:35different kinds of vibrational motions happening in a molecule and they occur
38. 2:40at very particular energies so they will occur when a wavelength of very specific
39. 2:47infrared light is absorbed and it will vary depending on the functional group
40. 2:51so the identity of these atoms will affect the photon absorption so this is
41. 2:58information that we're going to use to generate an IR spectrum, so let's look
42. 3:03at what that looks like right now
43. 3:07ok so what I've done here is i've drawn a sample IR spectrum so let's take a look at what
44. 3:12these look like and try to understand what we can use them for. so let's look at the
45. 3:16axes, first on the bottom here we have the wavenumber. the wavenumber is
46. 3:20telling us about the very particular energy of infrared light, so infrared
47. 3:25light is a certain band, it's a certain part of the electromagnetic
48. 3:28spectrum all the way from this wavelength to this wavelength so there
49. 3:32are many energies of infrared light in between and so we're irradiating our
50. 3:35sample with all of this infrared light of all of these different wavelengths
51. 3:39and so the wave numbers here are the way we differentiate between those
52. 3:42individual energies of IR radiation. on this axis we have the transmittance or
53. 3:49the percentage of that particular wavelength or that particular wavenumber
54. 3:53rather that reaches the detector so if we get, if we have information at a
55. 3:59hundred percent or near a hundred percent transmittance that means that
56. 4:04IR light that corresponds to that wave number is all getting through, it's all
57. 4:08getting through the sample it is not being absorbed by the sample it's gonna
58. 4:12make it to the detector and then we have peaks here where we have information so
59. 4:16let's look at this wave number
60. 4:17right here whatever is going on with with IR light of this particular
61. 4:21wavenumber it is not making it to the detector we can see that it has a very
62. 4:26low percent transmittance and if it is not making it to the detector it is
63. 4:31being absorbed by something in the sample so there is a particular
64. 4:34functional group in the molecule that is in the sample that absorbs IR light of
65. 4:39that wavenumber and that is information that tells us about what
66. 4:43kind of bonds are in that compound. so looking across here we can see
67. 4:49here's some important data, here's some important data
68. 4:53we should note that this region here around 1500 and below is called the
69. 4:57fingerprint region, there's very delicate information there, very
70. 5:02complicated to analyze and it goes beyond the scope of this tutorial so we
71. 5:05won't talk about the fingerprint fingerprint region right now but what we want
72. 5:09to understand is that we have tables, now i've only drawn, only written three
73. 5:14things up here but there's long tables of every functional group that we can
74. 5:17imagine in a molecule and we can talk about the particular wave number of IR
75. 5:23light that is absorbed by that functional group so for example we have
76. 5:27the OH sigma bond right here
77. 5:31the OH stretch occurs at this wave number so if that oxygen hydrogen covalent bond
78. 5:38absorbs infrared light of that particular wavenumber it will begin to stretch like
79. 5:43this and if it does, if it is irradiated with light of that wave number it is
80. 5:48being absorbed which means it does not get to the detector and that's why we
81. 5:53see something like this so we call this the OH stretch and so what this means in
82. 5:59terms of data that we can interpret from an IR spectrum if there is an OH stretch
83. 6:04there is an 0H group in the molecule that means there is a hydroxyl group
84. 6:09somewhere in the molecule. we can look at the spectrum and be very confident that
85. 6:13there is a hydroxyl group in this molecule. now there's a couple others
86. 6:16we will just look at the three most basic ones. the next one is called the
87. 6:20saturated CH stretch so the saturated CH stretch means that there is fully
88. 6:27saturated carbon bound to hydrogen meaning this is not an sp2 carbon, is not
89. 6:33an SP carbon, this is a carbon this is a fully saturated carbon so it
90. 6:39has the maximum bonds to other carbon and hydrogen and so this carbon-hydrogen
91. 6:44bond if it absorbs IR light of that wavenumber it will begin to stretch and
92. 6:48so it's not all making it to the detector and so that's there and most
93. 6:53organic compounds will give you a saturated CH stretch because most
94. 6:58organic compounds have some saturated carbon-hydrogen bond in it
95. 7:02okay and then lastly let's look at the carbonyl stretch. now this may vary
96. 7:08depending on whether it is a carbonyl from an aldehyde, ketone, ester, or
97. 7:12what have you
98. 7:13but it will generally occur somewhere in this region and it will look somewhat
99. 7:17like this and so if we have that stretch we know that there is a carbonyl. now
100. 7:23the last thing we wanna point out, a very common error is that people will say well
101. 7:27there's a saturated CH stretch, that's the only functional
102. 7:31group present, or they will say there's an OH stretch so there can't be some
103. 7:36other functional group, all we are trying to understand is that every functional
104. 7:40group in the molecule will be represented on the IR spectrum so let's
105. 7:44just say hypothetically we had a molecule that gave us this spectrum, we
106. 7:49would know that there is saturated carbon-hydrogen bonds, we know that
107. 7:52there's a hydroxyl group, and we know that there's a carbonyl of some kind
108. 7:55so that could look like anything, it doesn't tell us the precise structure of
109. 7:59the molecule, but it could look, say, like this it could have a hydroxyl there and it could
110. 8:05have a carbonyl there we're not sure, it's not telling us the precise
111. 8:09structure, but the reason this could be useful is that let's say we
112. 8:14do some reaction and maybe even know the empirical formula of our product and we
113. 8:19know that
114. 8:19we know that our
115. 8:21compound had as an oxygen atom. well the reason IR would be useful for that
116. 8:27is we would say: what kind of oxygen is it? is it a hydroxyl oxygen? is that an
117. 8:32ether oxygen? is it a carbonyl oxygen? and we might get an IR spectrum and know just
118. 8:38from that at least what kind of functional group the oxygen is
119. 8:41participating in. if we saw an OH stretch we would say that oxygen must be
120. 8:45a hydroxyl oxygen. if that was not present we would know that it is not
121. 8:49that, it is not a hydroxyl oxygen, so the main thing that we want to get from this
122. 8:54very basic introduction to IR spectroscopy is that functional groups
123. 8:59that absorb IR light at specific wave numbers will provide very specific peaks
124. 9:05not only are they occurring at specific wave numbers but they have
125. 9:08characteristic shapes, the OH has a nice curvy dip like this, the carbonyl is thin and long
126. 9:13and so we can take an IR spectrum and we can know roughly what kind of functional groups are in a molecule
127. 9:21thanks for watching guys, subscribe to my
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