hello everybody my name is Iman welcome back to my YouTube channel today we're covering chapter 11 of MCAT organic chemistry and this chapter is all about spectroscopy now in this chapter we're going to cover three objectives first we're going to talk about infrared spectroscopy that's our first objective here we're going to introduce this technique we're going to Define intramolecular vibrations and rotations and then we're going to talk about characteristic absorptions that we should know for the mcap the second objective is about ultraviolet spectroscopy there isn't a lot we need to know about this technique for the MCAT you're not going to have to interpret any Spectra here but we are going to focus on discussing conceptual things in regards to this technique and that means we're going to talk about electron Transitions and conjugated systems and then last but not least we're going to cover nuclear magnetic resonance spectroscopy and NMR all right we're going to specifically focus on hnmr all right so hydrogen NMR so with that those are our three objectives now I want to say that there is a lot to discuss for each of these techniques in terms of really learning all the details of ir UV and NMR spectroscopy but we're only going to keep it focused on the things we need to know for the MCAT in this video all right so we're just going to cover exactly what we need to know for the mcap however if you feel like there are some things that you are curious about or you want more details or you want more practice problems or a detailed overview first before you even watch this video I would highly recommend videos from my organic chemistry 2 playlist chapter 14 in that playlist is all about infrared spectroscopy we start from the very bottom build it all the way up and chapter 15 is about NMR and we do the same thing we start from the very Basics and build it up and we're going to try to do a little bit of this but for the sake that we are prepping for the MCAT we might omit things that you might be interested in for that I would reference you to that playlist now to begin to set the stage for this chapter if you are given an unknown compound one of the most efficient ways to identify it and determine its proper properties is by using spectroscopy spectroscopy measures the energy difference between the possible states of a molecular system by determining the frequencies of electromagnetic radiation that's absorbed by the molecule these possible States they're quantized energy levels that are associated with different types of molecular motion such as molecular rotation vibration of bonds electron abs absorption and nuclear spin transitions different types of spectroscopy measure different types of molecular properties and that allows us to identify the presence of say specific functional groups and to determine the connectivity the backbone of a molecule now you watching this video because you are studying for the MCAT and you're a future doctor in the medical context spectr roscopy is important in magnetic resonance imaging MRI MRI scanners actually measure H&M Spectra of water molecules in different environments in the body and they then convert those signals into grayscale that allows for visualization of the body especially soft tissue right this is obviously important for all my future doctors watching this now with that buildup let's go ahead and get started with our first objective which is infrared spectroscopy infrared spectroscopy is a chemical analytical method that uses deferring frequencies of infrared light to detect predictable types of chemical bonds now absorption of this energy radiation of this lower energy radiation causes vibrational excitations of groups of atoms within the molecule so again IR specifically causes Bond vibrations and we see the absorptions and excitations of these Bonds in the molecule in our IR Spectra all right in our IR Spectra that we obtain now because of these characteristics because of their characteristic absorptions identification of functional groups is easily accomplished because we have a frame of reference uh where different features for different functional groups would arise in our IR Spectra so looking at this is an example of our IR Spectra we can begin to look at these features and determine what kind of groups are present in our molecule so in other words infrared spectroscopy measures molecular vibrations which can be seen as things like Bond stretching bending and a combination of different vibrational modes and to record an IR spectrum like the one that we see right here all right infrared light is going to be passed through our sample all right we're going to pass IR light through our sample and the absorbance is then measured and by determining what bonds exist within a molecule by looking at our obtained Spectra we can begin to infer the functional groups that are present in the molecule that's that sounds really cool doesn't it but how does this exactly work all right for us to really understand that we need to understand a few things all right so I'm going to run you through the things that we're going to start to discuss we're first going to try to understand all right we're going to start to understand molecular vibrations all right what does that mean what does that really mean all right and then when we understand that we can talk about the role of IR light all right and then we can start to picture how these two things come together to result in our IR Spectra this requires understanding energy gaps and Photon or light absorption all right and then we could talk about then our Spectra and our Spectra analysis all right so this is how I'm going to work you through this these are the topics and the order I'm going to work you through so that we can really begin to understand what is happening here all right so first things first understanding molecular vibrations at the heart of ir spectroscopy lies this concept of molecular vibrations molecules they're not static entities they're in constant motion and they exhibit movements like stretching where bonds between atoms lengthen and shorting uh shorten I'm going to scroll down here to show you that all right you can see bonds stretching you can see bonds break uh bending in different ways right this is where the angle between the bonds changes these movements are not random but they occur at specific frequencies that are characteristic of the type of bond and the atoms involved all right now how do molecular vibrations come about all right how do molecular vibrations come about what is the role of infrared spectroscopy well infrared light if you look at your electromagnetic Spectra all right here is infrared light all right this is the region of uh this is the region of light that we're concerned with this is the light that we are shining on our sample infrared light now in infrared light which forms part of our electromagnetic spectrum all right it this light is particularly well suited for studying molecular vibrations when infrared light passes through a sample it interacts with the molecules in the sample all right now not all frequencies of infrared light interact with a given molecule only those photons whose energy level or whose energy matches the energy required to change the vibrational state of the bond in the molecule is going to be absorbed what does that mean all right what does that mean that leads us to talk now all right about that third category of things we said we're going to talk about energy gap and Photon absorption so we're going to scroll to this page and we're going to talk about that all right this brings us to the concept of the energy gap each type of vibrational mode like stretching or bending of a bond in a molecule has a specific energy that is associated with it and the energy gap refers to the difference all right it refers to the difference in energy between two vibration States for a molecule to absorb a photon of light all right to be able to you know display in order to display a vibrational mode right for a molecule to absorb a v Photon of light and then undergo a vibrational transition the energy of the photon must be must precisely match this energy gap that you have all right there's an energy gap that determines this all right and a molecule needs to absorb a photon of light that is exactly matches this energy gap all right and when it does all right it undergoes vibrational right vibrational um energy right it it under goes vibrational transition I'm sorry all right so let me repeat this that energy gap refers to the difference in energy between two vibrational States for a molecule to absorb a photon of light and then undergo a vibrational transition the energy of the photon must precisely match this energy gap now when a photon with the right amount of energy interacts with a molecule it's going to be absorbed all right causing the molecule to jump to a higher vibrational State and this absorption of energy is what IR spectroscopy measures all right and this is why we can look at IR Spectra we could look at features and know exactly what functional groups they can refer to we understand what different energy vibrational energy levels require we know what the energy gap for different functional groups require all right and when that energy is obtained by the absorption of a photon and the molecule underg goes a vibrational transition we're going to see that feature on our IR Spectra and they're so predictable because of those energy gaps all right so now the now that we understand molecular vibrations the role of IR light and energy gaps in photon absorption let's start to talk about Spectra and Spectra analysis right the predict the the predict uh predictability of ir spectroscopy it arises from the fact that each type of group each type of bond and functional group in a molecule has a characteristic vibrational frequencies these are frequencies that are consistent for similar types of bonds across different molecules therefore when a molecule absorbs infrared light at a specific frequency it indicates the presence of a particular type of bond or functional group so let's say we shine light all right that it has a frequency range from here to here all right this is in in wave numbers we can easily convert that to two frequencies to realize all right if we are shining this kind of fre quency range of IR light on our molecule all right different groups that are present in our molecule for example like a carbonal will absorb it will absorb a specific frequency of light all right it will absorb infrared light at a specific frequency that's going to be this frequency right here all right and when a molecule absorbs infrared light at a specific frequency all right that we know that it would do that it indicates the presence of a particular type of bond or functional group so when we see this characteristic right here at this particular frequency and it looks like this kind of line shape we know that this line shape is always indicative of a carbonal bond being present okay because carbonal bonds are going to always absorb this specific infrared light all right this specific range of infrared light because it matches perfectly with the carbonal energy gap that's required all right that when a photon observes it absorbs it it will cause vibrational transition for that particular functional group and so we see it in our Spectra and we realize that we have this functional group present in our molecule all right so when a molecule absorbs infrared light at a specific frequency it indicates the presence of a particular type of bond or functional group and the result of an IR spectroscopic analysis is an IR spectrum which plots the intensity of absorption against the frequency of light and Peaks on this spectrum correspond to the frequencies at which the sample absorbs infrared light and so in short all right you have a range of in you have a range of frequencies all right of infrared light that you shine on your molecule here's your mole molecule all right for example our molecule has a carbonal group this carbonal group has a specific energy transition that that a f that that this molecule need that this functional group needs to be excited by in order to undergo a vibrational transition all right when it absorbs a photon of the right um uh energy all right when it absorbs a photon of light that is the right energy it will under go a vibrational transition and then we can see that in our IR Spectra all right and then we can tell from the features in this Spectra that we have oh this carbonal group in our molecule all right so in the case that we didn't know what this molecule was and we were to shine light on it when we look at our Spectra we're able to determine what functional groups are present in the molecule and work backwards to determine what kind of molecule we have all right so this molecule again has a carbonal functional group all right if we expose it to IR radiation we absorb we observe that the vibrational energy for that carbonal group is excited to a higher level we see this in the absorption spectra because we get this peak right here all right this is a signature peak of a carbonal group all right and so we know that this molecule has a carbonal group all right a functional group will give the same wave number range every time and a similar pattern this predict this predictability of where a functional grow group will show on an IR spectrum all right is really important because it allows us to study unknown molecules and be able to determine what functional groups are present and we're able to do this because we know that wavelengths needed to excite the vibrational modes for particular bonds and bond types makes IR spectroscopy extremely useful all right now as you can tell I really love IR spectroscopy uh because I've been babbling on about it but I just want to take a second now and reiterate the concluding remarks for those past few pages in IR spectroscopy a sample is iritated with frequencies of ir radiation and the frequencies that pass through are detected and then a plot is then constructed this is our Spectra how do we begin to understand the Spectra now that we understand kind of the theory behind IR spectroscopy all right so here I want to focus on understanding the Spectra this requires me to reiterate a couple of points that have to do with intr molecular vibrations and rotations for IR spectroscopy infrared light range runs from 700 nanom to 1 uh mm but the useful absorption for spectroscopy occurs at wavelengths of about 2,500 to 25,00 all right so uh and this is nanometers by the way 2,500 to 25,00 nanometers on an IR spectrum we use an analog of frequency that's just called wave number and so that standard range 2,500 to 25,00 nanometers corresponds to 4,000 to 400 wave number when light of these wave numbers is absorbed the molecules enter excited vibrational States all right we can see that I'm going to scroll here um scroll down actually I'm on the right page here we can see all right different types of vibrations that can occur right we have S we have stretching it could be symmetric where both are stretching in corresponding ways or it can be asymmetric you can have bending modes like scissoring wagging twisting rocking Etc all right there are other kinds like twisting and folding we are not going to concern ourselves with those now more complex vibrational patterns caused by the motion of the molecule as a whole is going to be seen in the range of 1,500 to 400 wave numbers range this is called the fingerprint region so if we come look at our Spectra right here this region right here from 1500 to 400 this is called the fingerprint region this is called the fingerprint region because the specific absorbance pattern is very characteristic of each individual molecule all right spectroscopy experts can use this region to identify a substance but you're not going to need to worry about how to interpret the fingerprint region on the mcap we are more concerned with this range right here 4,000 to 1500 wave numbers now for an absorption to be recorded the vibration must result in a change in the bond dipole moment all right this means that molecules that do not experience a change in dipole moment such as those uh composed of atoms with the same electr negativity or molecules that are symmetrical they do not exhibit absorption so for example we can't get absorption all right for O2 or br2 right because they are symmetrical and so you don't get this change in dipole moment which is really important for obtaining a a Spectra but we can for HCL or Co right these molecules are not symmetrical um they do experience a change in dipole moment um when they are EXC when a vibrational transition occurs and so we can observe those kinds of molecules using IR spectroscopy all right symmetrical symmetric bonds also like um a triple bond in acetylene this will also be silent all right so a crucial part of being able to use IR spectroscopy to explore a molecule is that the vibration must result in a change in the bond dipole moment all right and so molecules that don't experience a change in dipole moment won't be um won't exhibit absorption and that means that you won't be able to identify them on IR spectrum all right and molecules that won't experience a change in dipole moment are molecules with the same that have atoms with the same electr negativity or molecules that are symmetric fantastic now an IR spectrometer measures the percent transmit transmittance as a function of frequency all right and this plot is called an absorption Spectrum right so you can see that here transm trans mittens versus wave number this is called an absorption Spectrum all the all the signals are called absorptions absorption bands and they Point down on the Spectrum all right now the wave number again is simply the frequency of light divided by a constant all right so that's how we transition from wavelengths to wave numbers which is what we were discussing about earlier every signal whoa excuse me every signal in an IR spectrum has three characteristics wave number intensity and shape we're just going to quickly explore these characteristics and Define them all right we're going to start with wave number for every Bond the wave number of absorption is associated with uh associated with Bond stretching is dependent on two factors bond strength and the masses of the atoms sharing the bond all right and if you were to treat a bond like a vibrating spring then we can explain through hooks law all right how we get these different absorptions we don't need to worry about that for the mcap all right resonance also plays an effect right the more conjugated a system is with more resonance hybrids generally the lower the wave number all right we don't really need to understand um any of the math that you know Works us through how we associate wave numbers with groups what we need to be concerned about is the following we know that our IR Spectra can be divided into two regions the fingerprint region we're not worried about for the mcap the rest of this region is called the diagnostic region generally has fewer Peaks and it provides the clearest information this region contains all signals that are going to arise from double bonds trip triple bonds um bonds with halogens for example all right the fingerprint region it contains signals that result from the vibrational excitations of most single bonds but again we don't need to worry about that for the mcap where should we expect different groups to arise in the diagnostic region and what groups do we need to know for the mcap that's a great question I have those here in a table all right so here is a table of important signals in the DI diagnostic region that we should be familiar with for the mcap all right I personally if I was giving you advice as my friend for the mcap I would highly recommend you familiarize yourself with this table now if you are studying and you have a few days before the mcap what are the highest priority things to remember that's what I'm going to talk about now the first is obviously hydroxy group O it absorbs a broad and wide Peak around one of two frequencies around 3,300 for alcohols and around 3,000 wave numbers for carboxilic acids all right and I have that right here all right they show a broader region um in the frequency that that could happen but the things that I want you to remember is around 300 wave numbers for alcohols and around 3,000 all right centered around there for carboxilic acids the carbonal of a carboxilic acid it pulls some of the electron density out of the O Bond and that shifts the absorption to a lower wave number hence why carboxilic acid's frequency range is a little bit lower than just alcohols the second is obviously the carbonal all right we need to know the carbonal it absorbs around okay I'm going to write it here the carbonal absorbs around 1,700 wave numbers with a with a sharp deep Peak um what you notice here is a little more fine-tuned this is a carbonal group for different kinds of molecules all right like you can see um a carbonal group where it would arise for uh amides Esters Etc all right but the general thing that you should remember all right the general thing you should remember is that carbonal absorbs around 1,700 wave numbers it has a sharp deep PE Peak right there all right what you'll notice is notice how the bond between any atom and hydrogen always has a relatively High absorption frequency and how as you add more bonds between carbon atoms that absorption frequency is going to increase all right you'll notice that as we look at more Spectra as well another thing that's important for us to remember all right is um NH bonds all right NH bonds are in the same region as o bonds all right they're right here here's here's their wave number range you notice that they're Pro around the same region but they're going to have a sharp Peak o is going to have a broad Peak so the shape of the peak is going to help you distinguish between o and NH and of course this will make more sense when we tackle practice problems as well and on that note it allows us to talk about intensity and shape of Peaks as well right because we can talk about range you'll notice some groups overlap in their wave number range how do you distinguish then when they have similar ranges and that answer to that question is the intensity and the shape of the peak as well those are important just as much as where on the Spectra in terms of wave number that Peak appears so intensity is another signal characteristic of ir spectroscopy in an IR spectrum some signals will be very strong in comparison with other signals on the same Spectra all right and so that is some bonds absorb IR radiation very efficiently while other bonds are less efficient at absorbing IR radiation and this has a lot to do with the dipole moment dipole moments occur when there's a separation of charge all right in other words that dipole moment is an electric field that surrounds a bond and it acts like an antenna for absorbing IR radiation all right not to get into too much details cuz you don't need to know that for the MCAT things to remember is exactly what we talked about o and NH bonds they appear around the same range if you notice right I'm going to highlight those in Orange right here the difference is that o is going to be really broad all right it's going to be a very broad we Peak all right whereas your NH Bond all right it's going to have a sharp Peak instead of a broad one so that's going to help you distinguish between o and NH even though they appear around the same region all right and then again that goes back to shape as well right there are two main things that are going to have an effect on shape just as a little background all right and that's going to be hydrogen bonding and symmetric versus asymmetric stretching again this is not something that we have to go into in a lot of details we just need to know that for our three main groups that we need to know for the MCAT all right the shape for Co is going to be a really sharp long it's going to be a really sharp narrow Peak like this all right our uh o Peak is going to be very broad our NH Peak is also going to be a a little bit sharp all right we also should know where we're going to see um uh the CH bond for a triple bond for a double bond and for a single bond this is the regions all right and we're going to cover um what these different features look like when we tackle practice problems all right but you should definitely know the regions where these groups appear fantastic now with that being said we have covered the information on IR spectroscopy that we need to know for the mcap and we're going to do practice problems in our practice Problem video set but for now we've covered objective one we're going to move into objective two and objective two is about ultraviolet or UV Spectros opy although you will never have to interpret UV spectroscopy data on the MCAT it's still fair game for discussion for conceptual problems and so a basic understanding of how it works and when it is used is going to be enough for you to do really well on those problems on the MCAT and that's what we're going to cover here so UV Spectra is obtained by passing now ultraviolet light through a sample so notice how IR spectroscopy we passed IR light through a sample here in UV spectroscopy we pass ultraviolet light through a sample all right and it's usually diss dissolved in some inert nonabsorbing solvent and it's going to be and it's going to record the absorbance the absorbance is going to be plotted um against wavelength kind of like you see in this figure right here again we'll never need to interpret Spectra here but this is how it's plotted the biggest piece of information we get from this technique is the wavelength of Maximum absorbance which tells us the extent of conjugation within conjugated systems the more conjugated the compound the lower the energy of the transition and the greater the wavelength of Maximum absorbance why does this work well UV spectroscopy works because molecules with pi electrons or non-bonding electrons can be excited by by ultraviolet light to higher energy anti-bonding orbitals molecules with a lower energy gap between highest occupied molecular orbitals homo and lowest unoccupied molecular orbitals lumo are more easily excited and then they can absorb longer wavelengths AKA lower frequencies with lower energy now conjugated molecules or molecules with unhybridized P orbitals can also be excited by Ultra violet light all right conjugation is going to shift the absorption spectrum and it's going to result in higher maximum wavelengths AKA lower frequencies for example Benzene has three broad absorbances which Mark the energy level transitions larger conjugated molecules may may even absorb light in the visible uh range leading to color all right now because the technique for UV spectroscopy can also be used sometimes at visible wavelengths in in certain cases it's called UV Vis spectroscopy ultraviolet visible spectroscopy it all depends on the kinds of systems you're OBS observing all right and the kind of features and things you expect from the Spectra so in short ultraviolet spectroscopy measures absorption of ultraviolet light which causes movements of electrons between molecular orbitals all right to appear on a UV Spectrum a molecule must have a small enough energy difference between the homo and lumo to permit an electron to move from one orbital to the other the main takeaway here some of the main takeaways here is that the smaller the difference between Homo and lumo the longer the wavelength a molecule can absorb all right fantastic also conjugation occurs in molecules with unhybridized P orbitals and conjugation will shift the absorption spectra to higher maximum wavelengths AKA lower frequencies so like I said not much to know about ultraviolet spectroscopy for the MCAT but these are the main points that you should keep in mind um and these are kind of the conceptual things you can be tested on in regards to ultraviolet spectroscopy of course will tackle problems that you'll encounter in regards to ultraviolet spectroscopy in our practice problem set now I think I've babbled on for quite a bit and this video is probably going to be long I really like spectroscopy I am a spectroscopist I am doing my PhD in a spectroscopy group specifically multi-dimensional spectroscopy so 2dir very complicated do not recommend um but anyways I'm going to end the video here in the next video we're going to cover the third objective for this chapter which is NMR spectroscopy let me know if you have any questions comments concerns down below other than that good luck happy studying and have a beautiful beautiful day future doctors yeah