 So, we have a few definitions we come together and if I remember there are four definitions and now we can be all sure they are all actually the same thing we are saying in different ways. Let's go back. First was that it can have an optical activity that means it actually turns a plane polarized light that means it shows optical rotation. Second thing we said the molecule should be mirror image and non superimposable we have some other comments also that the molecule should not have an SN then the molecule should not have a center of symmetry and number five if you want to play the molecule should not have a sigma plane and all together what I am saying these are saying that your molecule should belong to CN and DN point group the all the things are coming from here the CN and DN point group doesn't have any of them so these three are taken care of mirror image and superimposable what I am doing sigma and CN that means I am actually basically doing a SN operation so these are already to each other saying in a different way. Second rotation what is happening because in CN and DN point group you have XRX couple together or YRY couple together ZRZ couple together that means you can have mu E and mu M both active at the same time you can create the helical motion and once you create the helical motion you can differentiate between the right hand circularly and left hand circularly poise light obviously you are going to get an optical rotation so the only thing you have to remember if you want to find out optical activity is that the point group of the molecule it belongs to CN and DN or not so that much is going to define whether the molecule is optical active or not the rest of them are actually the same thing saying in different ways so you cannot say that this is right and this is wrong all of them are right actually because they are all originating from the same thing so it doesn't matter which one actually you follow but you have to ensure that you can connect all of them together if it is okay so any question up to this point so if not we will go to the next part of it so together so far we have learned this optical activity why it is there what is the physical origin of the molecule from the molecule that it creates the optical activity and we also know that this optical activity is very important in the previous sections we have also learned that how important it is because the biology actually runs on this optical activity so we have to have some spectroscopic techniques by which we can follow this optical activity now obviously we are going to have two different ways we can follow it either through optical rotation and other thing is the ellipticity so first optical rotation how it looks like so optical rotation depends on something called again circular by referent basically it is saying the refractive index for the right hand circularly porous light is not equal to left hand side circularly porous light how the data will actually look like so first of all I am drawing over here an absorbance statement normal absorbance absorbance versus lambda so question over here what is the unit of absorbance anyone later more than 1 centimeter inverse so no we need sorry yes absorb yes absorbance you don't have a unit because it is a ratio a logarithmic ratio of intensity of the light before it hitting the sample an intensity of the light after it passing through the sample so it is that has no unit as such okay so be very careful you know this so now if it is this absorbance how the optical rotation data will look like so now it is a little bit different so what if we have done so far with optical rotation we have done the optical rotation at one particular wavelength and try to find out how much angle it is rotating right we haven't really look into if I want to say plot optical rotation versus lambda we really looked into that particular and it is actually possible to do that we can plot optical rotation versus the wavelength and how does it look like what will be the effect at the same position where I have the lambda max what is going to happen where I have no optical absorbance at all over there absorbance is zero so now optical rotation depends on speed of the light it has nothing to do with the absorbance is happening on however what is found by the scientist named cotton what he found that very uniquely the optical rotation is actually dependent on the wavelength in the following way so one particular enantiomer show a data like this the significance of the dotted line and coming later but what importantly they found at the maximum of the absorbance the optical rotation value is actually zero so over here the optical rotation is nothing but in L minus nr or a function of it right phi is dependent on that so one side I'm saying positive one side is negative so this is the zero line so this optical rotation value is zero at the maximum of the absorbance anybody suggest why it is so why the optical rotation would be zero at the maximum of the absorbance Christian do any suggestion why the optical rotation would be zero at the maximum of the absorbance okay if no answer is coming so over here you have to pass the light to ensure that you are seeing a difference between nl minus in R but if you are absorbing the light as it is happening over here and the maximum you're going to absorb the most you're not passing through the light over here so if there is no light passing through there is no point of having optical rotation right so that is why it becomes a zero value okay and not only that the two enantiomers actually show exactly same but opposite signature over the signature looks exactly same but mirror image to each other both of them cross the zero line over there so what are the two difference they are enantiomer two and enantiomer one and over here you can see it is not only having the value zero at this point but they're also crossing from negative to positive or positive to negative region so depending on which particular direction they are going it is known as positive or negative cotton so for example the blue line is coming from negative vision and going to the positive so it will be written as positive cotton effect the name of the cotton effect coming from the again from the scientist cotton the same scientist who actually wrote that famous book of inorganic chemistry cotton will concern so he first noticed that so this is known as the positive cotton effect because you can see it is over here moving towards the positive direction from the negative similarly the red one coming from positive and going to the negative so it will be known as the negative cotton effect so that is first important part optical rotation is dependent on the absorbance data or the wavelength where it is absorbing the most over there you are going to see the most that distinct change on the rotation optical rotation values they actually change their direction depending on the final destination of the optical rotation value you can say it is a positive or negative cotton effect and again the five over here it is actually nl minus nr this nl minus nr l is first r is later it is how that is scientifically uh historically has been done so anywhere you see the sign positive that means nl is greater than nr if you say the opposite that means nl is less than nr and from there you can connect which one is moving faster than the other which one is probably actually holding back so this is what it is look like when it is actually absorbing now this particular part over here i am not showing in a dotted line what does it mean so it may happen that when your absorbance value is actually zero you can have a non-zero optical rotation value it's possible not always but it is possible and this kind of phenomena where there is no absorbent band at all so in the same region there is no absorbent band at all but you have a non-zero optical rotation value so one of the enantiomer will be positive one of the enantiomer will be negative but they are non-zero and this particular phenomena is known as plane curve where absorbance is zero but your optical rotation is non-zero one of the biggest example the experiment all of you have probably done sucrose solution all of you have probably done in your physical chemistry or at least in the physics experiment where you take a sucrose solution and then add acid try to see if the sucrose is breaking down or not and over there if you remember you did this experiment put this sample in a particular glass sample and put that in an optical rotation measurement system and then shine a light and that light actually is a sodium d-lamp in general which is actually used and the wavelength is generally measured is actually around 590 nanometer and you know sucrose it's nothing but sugar sugar is a white color compound that doesn't have any absorbance in the visible region otherwise we have been eating colored sugar so sucrose is a colorless solution all its absorbance is below 350 nanometer before even the visible region starts so that means it doesn't have any absorbance in the visible region although we are measuring its optical rotation at a region where it doesn't even absorb because over there sucrose solutions show this kind of plane curve behavior okay so that is how the optical rotation value gets affected with respect to the wavelength and this full system where we are measuring an optical rotation value with respect to changing of lambda value or the wavelength value is known as optical rotatory dispersion or ORD so now if i say there is a draw the ORD data or look into the ORD data now you know exactly what you're looking into optical rotation value change with respect to the change of the wavelength that you are showing up okay and at each particular wavelength it can have different values and that is why again we are saying that optical rotation should be mentioned with respect to the wavelength because otherwise it doesn't make any particular sense okay so that is how the optical rotation value changes two important factors that you have to follow up one is the cotton effect what is positive and negative cotton effect and the other thing is the plane curve okay it doesn't always need to be absorbing at a particular point to show a optical rotation value so that is what is done for optical rotation now i want to find out what is happening to the ellipticity so again i am drawing an absorbance versus wavelength so now i am going to find the ellipticity the phi and then again it is depending on al minus ar so over here how this particular data will look like this data looks like the following for one enantiomer this is the other enantiomer so this is enantiomer two this is enantiomer one so if your particular optical signature or what do i mean a optical absorbance feature if that optical absorbance feature is actually connected to an optically active phenomena or optically active functionality only then this particular absorbance will show an ellipticity and not only that their actual data will actually mirror the optical absorbance feature they'll be exactly the same why because ellipticity is directly dependent on the absorbance that is the only phenomenon so obviously if you are comparing absorbance versus ellipticity that will be very much similar however depending on which particular enantiomer you are talking about that will be either positive or negative and if you remember we talked about the cotton effect earlier the blue one will be the positive cotton effect corollary so this is not known as positive cotton effect but what i'm saying if i am going to measure the optical rotation over here also that will show the same effect at positive cotton effect for the blue one and the negative cotton effect for the red one so if you remember the positive cotton effect shows that the value goes from negative to positive whereas for ellipticity value it remain in the positive side for the same data that is al minus ar and if it is negative that is the negative cotton effect corollary so that is how the data actually looks like can you have this kind of ellipticity data for a position where there is no absorbance the answer is no because the first thing you have to have have to have an absorbance band only then you can have a difference between the right hand and left hand circulary porous light if it doesn't absorb you cannot have a ellipticity data now this particular data of ellipticity that i'm measuring with respect to the lambda value it is known again as circular dichroism and in short we call them the cd so this is known as the cd spectroscopy and this particular data is known as the cd spectra now over here it is quite possible that in a molecule i am looking into and it is always better to show optical spectroscopy and cd data on top of each other to have a better idea so say i have an absorbance value such i have four features like that and it is quite possible that only two of them this one and say this one are actually connected to optically active molecules these two these two features and the rest of them are not so how the cd spectra will look like so cd spectra look like the following so for one enantiomer it will be like this the other enantiomer it will be like this it is also possible that one of the band is positive one of the band is negative and vice versa i'm just drawing one of the possibility i'm sorry i didn't really draw it pretty well over here it will be exactly mirror image of each other so that is how it will look like enantiomer one and this will be enantiomer two so that is how the cd spectra actually looks like so we'll stop over here because the time is almost up so in the next class we'll look into some of the real-life samples starting with some proteins and find out how the cd spectra can define us what is the overall structure of the molecule and then we'll also discuss over the data that means the optical order dispersion versus cd which will be better spectra to follow a chiral activity of the molecule should it be optical rotation or it should be electricity that thing we will discuss so before we finish it up today we discuss what is the molecular origin of a molecule where an optical activity is actually generated and secondly how we can use o rd and cd to find out the difference in the enantiomer in the o rd two important features one is the cotton effect and the second is the plane curve for ellipticity that has to be connected with an optical absorbance so unless you have an optical absorbance you cannot have cd data and it may be possible not all the bands are optically active what do i mean by that that will come into the next class optically active bands