 So our original discussion was yes we have an electrical field we have a magnetic field and electrical field is actually stronger so that is the thing it is going to affect strongly with my system. Now how this interaction happens? So I have an electrical field from my electromagnetic radiation which I am defining as E. So what I am trying to do right now trying to get a parameter or a mathematical equation so that I can quantify things. So an electrical field I have and this electrical field is going to change the electronic distribution in a molecule. So can anyone name a parameter which connects to a change of the electronic distribution in a molecule, any parameter that defines electronic distribution or electronic distribution. I said dipole moment could be the option of electronic parameter. Okay good so dipole moment not exactly is connected to the electronic distribution it actually is a direct effect of its electronic distribution so the direct parameter that connects is known as the polarizability that you have learned right during the Raman spectroscopy specially. So polarizability given by this grip term of alpha and then if you combine them together what is the effect of this electric field on this polarizability then you go to this factor known as dipole moment. So dipole moment is more of an effect that it is created when we are putting an electrical field around the system. So that means we are going to create a dipole moment in a molecule when it is interacting with this electronic electrical field of an electromagnetic radiation and that is going to create the electronic transition. Okay so over here I am going to take a few seconds so electronic transition is going to happen what is electronic transition means it is the electronic redistribution what it is redistribution to from a ground state electronic wave function to a excited state wave function. Why it is happening because the electric field of an electromagnetic radiation is affecting it the effect can be parameterized with the polarizability and that is factored by this dipole moment and this dipole moment because it is creating this electronic transition is known as the transition dipole moment and now with the very minimal mathematical knowledge I have if I want to write it down the probability of the electron transition for this particular system can be written as psi excited state we are going under this influence of this transition dipole moment over here from the ground state and that I have to take from minus infinity to plus infinity that means all the space possible while this is I am actually doing this integral over a small volume and this equation most of you have seen earlier and that is what is the physical origin of that this integral this transition dipole moment is coming because of the light matter interaction and how the light electrical field is interacting with the electronic distribution of a matter or electron and this value of this integral can have two values it can be 0 it can be 1 if it is 0 then it is called a forbidden transition the transition not possible if it is 1 you can say it is a allowed transition so how do we know it is a allowed transitional forbidden transition that two ways you can do that one is looking into the molecule find out your symmetry and each of this ground state or excited state you can put it with a symmetry representation which can be found in the character table and from there you can put all those things over here and this dipole moment is generally represented by the x y or z axis on that particular character table and then you can do this calculation from the character table and find it out whether it is 0 or 1 I am not going into the details it will be covered by professor Lila in the second part the second one we can do just by using our chemical knowledge that whether it should be allowed or not and in most of the time to be honest what chemists do they do the experiment fast and find out whether it is giving you a strong absorbance or not and from there we back calculate try to find out what will be the actual molecular distribution during this electronic transition so let us look into this equation that we already know for that but before going further just to double check you understand it properly an electrical field of the electronic the electromagnetic radiation interacts because it is stronger compared to magnetic field it interacts with the electronic distribution given by the polarizability if we combine them together I get a dipole moment this dipole moment is created during the interaction between the electronic the electromagnetic radiation and the matter so it is a very transition dipole moment that means it happens only during the transition and this is the equation we use which is known as the probability of the electronic transition or some time integral dipole moment system which is actually giving you an idea whether this transition will be allowed or not now this equation we already know from our knowledge which is given by absorbance of a molecule is equal to epsilon cl and this equation is known as the beer Lambert's law so let's connect what is those things so generally what we do we generally take a sample in a cubit we pass a light we find out its intensity say it is I0 if this molecule is absorbing on the other side I am going to get a different intensity of life say I I take the log of I0 versus I and this is defined as the absorbance or a now what are those epsilon C and L all those things coming into so over here the intensity of the light changes why because the molecule is changing its electronic distribution and that needs energy and that energy is taken from this incoming light and that is shown during the outgoing light the sum of the portion of the energy is missing why because that energy is used for this electronic redistribution or what we say as electronic transition so that is the electronic transition we are talking about so it goes to the excited state and this energy is sapped now what are the factors it is going to contribute over there first say I have two samples same sample but different concentration this is much more concentrated this is less concentrated so we will be absorbing more so obviously this is going to absorb more because we have more molecules present there so more molecules means it has more chance to changes electronic distribution and more and more energy will be absorbed so that is why concentration is a factor C which is directly proportional to the absorbance now say I take the same sample same concentration but the shape of the qubit is different one is longer than the other who is going to absorb more because the light has to pass through this so this is the region it has to cover similarly this is covering this area so you can see this is covering much more larger area compared to here so it also depends how much of the sample is exposed to light and that is given by this particular term called path length that means how much length in that particular is qubit which contains my system or solution is allowed to interact with the incoming light so this is given by this term L so obviously absorbance is very proportional to L now comes the third term the epsilon value so what is this epsilon value now say I actually take two qubit same size path length same same concentration but what is different is my sample this is a sample B this is a sample A same concentration same path length are these two systems going to show the same absorbance not really the absorbance can be different and in most of the time they are different why because this is one other system is also very important what is the identity of this particular molecule because depending on the identity of the molecule whether it is A or B it is going to interact with this electromagnetic radiation differently so there should be a particular factor which is going to relate how the interaction is happening between the molecule and the electromagnetic radiation and that will be something of a molecular property and that is given by this epsilon system which is known as molecular extinction coefficient and this is actually a very unique property for each of the molecule so until there I think most of you already know but now I want to understand what this molar extinction coefficient actually is what is the physical understanding for time so let's go back to the molecule one more time so say you have two molecules this is the electronic distribution for molecule A and here comes my incoming electrical field what is going to happen this is going to change my electronic redistribution right and during that what is the actually happening between the electrical field and electronic environment so what that you have to understand this electronic distribution that molecule A has it is nothing but a wave function right it is a ground state so I can say it is actually having a wave function and over there you are bringing a different wave that is the electromagnetic wave and all together what I am going to see is a different kind of wave the wave nature is changing just very simple understanding you can say there is a small wave before you bring another small wave combine them together you get a bigger wave because the wave function is defining how the electron is distributed you can say it is going to a totally different wave function and that is what is happening so that means we can define the interaction between the electromagnetic radiation and an electrical field of a molecule through the interaction of waves and how the wave interaction can be defined it defined by this very important term oscillation how much oscillation can be transformed between them and there is going to be a term called oscillatory function which is going to define how good the interaction between this electromagnetic radiation incoming and this existing electronic field is going to happen and that will all depend mostly on this wave function because this electromagnetic radiation incoming is going to be constant for most of the molecules what is going to different molecule to molecule is this electronic distribution over here and how much it can interact with the incoming interaction and how much it can oscillate the oscillatory function so what is the oscillatory function in mathematical so this oscillatory function defining this term by a feq is given by this particular term don't need to remember it i'm just giving you the number for your sake of understanding a particular constant into new one to new two this is the frequency is between which i am measuring a particular absorbance band D new this epsilon new is the function between which a molecule can interact for an example say this is the absorbance this is the frequency so i'm measuring a frequency between new one to new two and what i see there is a band like this and over there what i'm going to do is take the area under that so how that area will look like like this or like this or like this it total sorry it totally depends how the molecule is interacting with the electromagnetic radiation while the molecular electronic distribution is behaving how much it can exchange the oscillation depending on that this area will change and this area is defined by this oscillatory function and over there the interchange is actually defined by this molecular extension coefficient value which is defined by this particular frequency so that is why for each of the band when you actually report for a molecule we always try to say what is the epsilon value molecular extension coefficient value and the next question we ask at what wavelength because it is totally wavelength dependent because the wavelength dependent means it knows that exactly which particular region of the electromagnetic radiation i have to interact and that counter intuitively saying to you that which particular region of the wave function of the existing electronic distribution is going to interact okay now this oscillatory function is the main reason why it is a molecular property because it defines how the actually in molecular level the light and matter actually interacts okay so this epsilon value will be very much it will be very much a property of that particular molecule so the concentration the path length that we found from this equation Robert Lambert's law these are can be controlled from outside but this epsilon value is a constant molar extension coefficient is the constant you cannot change it for a particular molecule is a constant so that is a good thing because with respect to that you can find out molecular properties and that we have done earlier so over here i'm going to show you different kind of electronic transitions and i'm going to tell you how the epsilon value is going to differ so again this epsilon value is coming from this equation of absorbance equal to epsilon into c into l what will be the unit of absorbance absorbance doesn't have any unit because it is a ratio of intensity of light log value of the same thing so it will cancel each other out so absorbance value doesn't have any unit c is going to have a unit of concentration molar in general that means modes for detail l is going to have a path length of a distance so generally is given in the centimeter why because practically we use a qubit of one centimeter path length size so with respect to those things what would be the units of epsilon this is unit less this is having unit of molar this is centimeter so obviously this is going to be mole inverse centimeter inverse now different transition we are talking about pi to pi star transition we are talking about they have a strong epsilon value greater than 10 000 mole inverse centimeter that means it is a very strong allowed transition now coming back to that visualization what herjit was trying earlier what you're talking about say we are talking about a carbon double bond oxygen system so it has this particular pi bond from there it is interacting with an incoming light and redistributing this electronic the electrons to its pi star orbital so that is what we mean by pi to pi star transition this electron redistributes such a way that some of them come here some of them go here they change in symmetry and all those things and you end up with this particular final stage and that is what is known by electronic transition it is very much allowed because of the symmetry parameters again it will be taught by professor biller later the symmetry allowed transitions so that is why it is actually can have a huge oscillation exchange between the incoming light and this orbital so this interaction between this incoming electrical field and the existing magnetic field depends on different factors one of the factors symmetrically allowed or not that means is there symmetry matching or not if the symmetry matches that can have a very strong interaction very strong exchange of oscillation shown by very high value of epsilon if you go to the same similar kind of molecule but n to pi star transition n means a lone pair under oxygen it is exchanging to the pi star there you see the value a little bit lower down thousand five thousand to ten thousand unit or so which shows that it is still allowed but not as allowed as pi to pi star transition why because the overall distribution of the electron is such that when you are trying to redistribute itself it is not that easy to do symmetry wise energy wise all those things and that is reflected by a little bit lower value of epsilon then comes d d transition especially the transition vendors they generally have a value between hundred to thousand that means they are allowed with respect to the spin but with respect to the orbital they are not allowed their orbital forbidden the symmetry of the orbital doesn't allow it so that is why the oscillation exchange doesn't happen that easily reflected with the epsilon value why this huge range that is because it depends that can you break the symmetry a little if you put some p orbital contribution to the d that has a little bit more probability and that increases the epsilon value if you don't have that much interaction it will have a lower value of epsilon value so by looking into these values you can have an idea how much interaction between a p orbital and the metal d orbital is happening and the p orbital interaction typically comes from the ligand so you can have a very good idea how the ligand and metal are actually interacting through the other one can come d d transition but for an example a m n plus 2 high spin system that means a d 5 system a d 5 high spin system you have a value between point one to one for some time even go to 10 this is because it is spin forbidden orbital forbidden because already there so it is very little chance of an exchange of this orbital sorry the oscillatory motions between them and that is reflected over there in the values of this manganese so that is why you can see a manganese plus 2 solution in water very light pink whereas other faster potential element give much more deep color so that is what it is actually happening there so that is what is happening during an absorbance so the main take home messages over there for this particular class is the following electronic transition when it happens it happens between a light matter interaction it happens mostly due to electrical field is actually interacting with the electronic field created by the electron itself so it is a electron distribution that is getting affected the transition is actually depends on the polarizability and the electrical field so polarizability is a property of a matter electrical field property of this light or the incoming elementary radiation and that is given by this transition dipole moment and this transition dipole moment can be used to find out what is the probability of a particular transition if i'm going to an excited state operator will be this particular moment from the ground state the detail if it is zero or one depending on that i can say it is disallowed or it is allowed and the other important thing we learn absorbance equal to epsilon cl that you already know but what this epsilon value is coming from we found it is connected to this particular factor known as oscillatory function because that is an interaction between the electrical field waveform and the electronic distribution waveform of the molecule okay so we'll stop over here we'll ask for any question any of you have and the next class will go for this transition between the molecules for an optical active molecule yes sir LMCT and MLCT lies in which range in terms of epsilon so LMCT and MLCT generally lies somewhere in between there and it depends on like how much of the LMCT and LMCT you are doing for an example it depends on that symmetry and other factors how much it is allowed for an example you have already seen that that MLCT transition if i want to see you'll find that the 4d or 5d orbital containing molecules can exchange the electronic transition better compared to a 3d why because 3d orbital electrons are much more strongly contracted much more closer to the system and that is why a change in those distribution will be trickier whereas 4d and 5d are much more diffuse it will be less control from the nuclei so that will be much more easier to change those things and that is easily reflected on their epsilon values so that is why MLCT and LMCT have a range and it also depends on which particular ligands which particular metal you are using if you are using much more diffused orbital system you are going to have much more larger MLCT and LMCT values that is for the physical understanding if you want to go a little bit more details you have to look into the symmetry of that okay sir okay thank you any more question hello sir yes sir you have told that that optical activities due to the means electric field only mainly triggers the optical activity means when we have put any optically active compound so the electric field and magnetic field both actually react both actually interact with the molecule and so i was talking about that i didn't say that optically active molecule only interact with the electrical i told you optical absorbance is mostly triggered by the electrical field i haven't come into the optical activity okay sir so absorption so whatever sir that means both interact with the electric field and the magnetic field both but the electric field is going to affect more but why optical activity is happening that is a different scenario that we'll discuss yes yes and let's focus okay