 Hello everyone, I once again welcome you all to MSB lecture series on transformative chemistry. Let us continue discussion on spectroscopic methods to begin with we started discussion on UVV is the spectroscopy. So let me continue from where I had stopped in my previous lecture. I was telling about different electronic configurations which are spin allowed and leopard allowed or spin forbidden or leopard forbidden with appropriate examples. Now let us look into individual electronic configurations to identify ground terms. Now if you see here for D1 ground state is a 2D state and then T2G and EGR spectroscopic states for D. So of course DR will be split into T2G and EG in an octahedral and of course these things also originated from mullican symbols. If you see the table that I have provided you will come to know the D state how it splits a triply degenerate and doubly degenerate. So you are all familiar with from crystal field theory T2G and EG and when we look into F orbitals that will split into 3 levels in an octahedral field to triply degenerate capacity of 6 electrons and this to 14 electrons. So A2G is single so this is how it splits and we have T1G, T2G and A2G. And of course the corresponding you know values in DQ is also given here this is for F orbital and this for DR this is the D state and this is the F state essentially it is same. So now spectra let us look into spectra of D1 and D9 species. Now in a free gaseous metal ion DR will degenerate and no DD transistors are observed but in a complex degeneracy is lost and mixing also takes place and now splitting into T2G and EG in case of octahedral complexes and T2 and D in case of tetrahedral complexes. So I have given a typical spectrum here and also I am discussing the examples of hexachlorotitanate 3 minus or hexa aqua titanium 3 plus both are D1 system because titanium is in plus 3 state in both the cases. If you just look into titanium this hexa aqua complex spectrum it looks like this with absorption maxima around 20,300 centimeter minus 1 and this is happening because of the promotion of one electron in T2G to EG level. So magnitude of delta O depends on the nature of the ligands and affects the energy of electronic transitions and hence the frequency of absorption maxima that means this one represents the gap between these two. Of course you know that one how this gap varies with the ligands when we discussed in depth classification of ligands and also we saw this one in crystal field theory and also ligand field theory or molecular orbit theory how this CFSE varies with ligand field strength. So that is reflected in UV visible spectra of corresponding complexes. Now I try to bring some similarities between these electronic configurations D1, D4, D6 and D9 and D2, D7, D3, D8. This is I once again repeat one electron one more than half field one less than half field one less than completely filled and now we have two electrons two more than half field two less than half field two less than completely filled. So that means here in this case D1, D6, D4, D9 we see exclusively one DD transition where in case of D2, D7, D3, D8 we see only three DD transitions which are of course leopard allowed because of mixing and they have been allowed. You should remember D1, D4, D6, D9 show exclusively one transition D2, D3, D7, D8 show three transitions and now spectrochemical series ligand field strength all play major role in deciding the values for this one absorption. Extent of splitting is related to ligand positions in the spectrochemical series. For example, if you see here the comparison with titanium 3 plus kept constant and ligands are varied and of course we are going from low field to high field. Now you can see in case of minimum gap is there in case of D1 system of Chloro that is 13,000. Now with Chloro 18,900 with water better than F 20,300 I showed you that spectrum in the previous slide and hexasino titanate the gap is very huge and as a result more energy is needed that is reflected in this value 22,300. So now let us consider a D9 system, D9 system you cannot get better example than hexa aqua copper 2 plus splitting of DRP smaller to D1 case here. In D1 one electron is there in T2J level in D9 one hole is there in EJ level so that means we are bringing that whole formula here. So in D1 promotion of one electron from T2G to EG is very similar to promotion of one hole from EG to T2G in case of D9 electronic configuration. Promotion of hole means basically what happens say one hole is there electron comes there and the hole will appear in T2G that means electron is promoted from T2G to EG that is what it says. This further simplification and to bring similarity between D1 and D9 system. So now the D9 is inverse of D1 energy diagram holds good that means if we reverse the energy levels and do the electron transition that is very similar to D1 that is what is done here and we should not get confused with tetrahedral splitting you know this is actually octahedral complex for D9 considering the whole promotion. As a result what happens you take D1 and reverse it it becomes D9 similarly that is what I have shown exactly reverse of that of octahedral field. In case of tetrahedral it is opposite of the octahedral field 2D it becomes E and T2. Now let us look into all electronic configurations with similarities D1, D6, D4 and D9 I have seen here of course I have already mentioned remember we are considering only high spin complexes in all the cases high spin complexes where tetrahedral or octahedral whatever the electronic configuration from D1 to D9 we are considering high spin complexes. So now D1, D6, D9 for tetrahedral and octahedral is given here. So tetrahedral complexes are always high spin is it true yes may be exception in some cases let us not worry about this at this juncture. Now see D1 and D6, D4 and D9 octahedral and D4 and D9 and D1 and D6 tetrahedral have similarities so all these electronic configurations of octahedral and tetrahedral can be combined into a single diagram called Orgel diagram which describes the qualitative way of the effect of electronic configuration with one electron one more electron than half field shell one less electron than half full shell and one less than half field shell. Now you can write a common Orgel diagram to show all possible transitions for both octahedral and tetrahedral for configurations D1, D6, D4 and D9 so this is how you can represent through one Orgel diagram. So now it is very easy if you remember this one we should be able to see the transition which is a ground term from where the electron is going from which ground term to the which excited state which higher state or Homo to Lomo state you should be able to tell this Orgel diagram is good for high spin complexes of both octahedral and tetrahedral for these four configurations electronic configurations. Similarly we can also do analysis for D2, D3, D7 and D8 now you consider D2 and D8 have similarities because two electrons and two holes and this one in an octahedral field we have this electronic configuration and the electron goes to the excited state would have 1, 1 in this fashion. So now two possibilities are there electron it may be promoted from DXZ or DYZ to DZ square or DX square minus Y square and you should remember less energy is needed to promote an electron to DZ square than DX square minus Y square again you try to bring this one the similarity with John Taylor distortion where we have tetragonal elongation and tetragonal compression it is like tetragonal elongation is preferred tetragonal compression is not preferred where we will be having four weaker bonds and two stronger bonds whereas in case of tetragonal elongation two longer bonds are two weaker bonds and four stronger bonds are there in the same sense less energy is needed to promote an electron to DZ square then DX minus Y square that means we can have this electronic configuration or we can have this electronic configuration if we have this one more energy transition whether this one is less energy transition because when the electron is promoted DZ square is much lower energy compared to DX square. So electrons are spread around in all three directions XYZ reducing the electron repulsion when you put electron to DZ square on the other end if you put electrons to more electrons to XY plane as it is a more electron repulsion will be there in the XY plane because we have four ligands in it that is it. So in both the cases electrons are promoted and another high energy state will be formed thus four energy levels will be there when four energy levels are there you can anticipate three transitions. So that means for D2 ground term is 3F and four exhalation states will be there and ground state contains two electrons with parallel spins but these states contain electrons with opposite spins and are ignored. So you can ignore these terms then we will be left with only this term and this term these two terms are there they can have transitions. Now PR bits are not split that will remain so that F will be split into A2G, T1G and T2G thus three transitions are possible we have 1, 2, 3, 4 are there so you can see 1, 2 and 3, 3 transitions are possible. So that means D2, D8, D7, D3 show three transitions whereas D1, D4, D6, D9 show only one transition in DD spectrum. So now let us consider hexa aqua vanadium 3 plus complex here we are seeing three transitions and it is a D2 system but if you look into the spectrum looks like only two transitions why that is happening ligand fills in the water results in transitions occurring close to crossover point. So we are introducing another term called crossover point I shall explain that one later this is a crossover point here they are very similar in energy this is a 3T1GP and 3T2GPF and they are not resolved they overlap as a result one broad pierce along with this one high energy one that means instead of three you can see two because the second and third have very similar energies first and second has very similar energies as a result they are not resolved that means vanadium 3 plus ion with three different ligands will show three distinct peaks. So in this case what happens if you have three different ligands what would happen then they will be well separated as a result you can see three distinct transitions whereas in case of hexa aqua or maybe homolyptic total complexes it is likely that we may end up seeing only two transitions because other two are very spacely closed. In a nickel 2 plus a D2 system with two holes in EG and promoting one or two electrons to EG means transferring the holes to T2G level 3P is not split again and degenerate and only square pener geometry P is split but whereas in octahedral geometry it is not split in tetrahedral also it is not split 3F is split into again three states and will be inverted and 3A2G will be the ground term now we had this higher energy term in case of D2 now in D8 3A2G will be lowest in energy and according to this reverse it make it upside down you can see the energy levels of D8 system. D7 is similar to D2 and D3 is similar to D8 in an octahedral environment so now chromium 3 a D3 system is expected to show three peaks all these cases can be combined into a single diagram called again Orgel diagram which describes qualitative way of the effect of electronic configuration with the two electrons and two more electrons than a half field shell two less electrons than a full shell two less than a half field shell so now we can write one more Orgel diagram and I have shown electronic configurations to bring similarities two electrons and then we will be having two holes here and three electrons and then we have three holes and then tetrahedral also same thing so that means we have some similarities. So now all these things put together for both tetrahedral and octahedral in a very similar way we wrote for D1, D6, D4 and D9 we can also write here for D2, D7, D3 and D8 and this is how it looks like but you can see one notice this is little bit gone upwards and this is little bit gone downwards if you see the symmetry is very similar so I will come to that one later this is for D2 octahedral, D7 octahedral, D3 tetrahedral, D8 tetrahedral and here it is opposite D8 octahedral, D3 octahedral, D7 tetrahedral was here octahedral is tetrahedral and D2 tetrahedral exactly opposite happens and if it is ground state is this one ground state is this one for tetrahedral and then if it is ground state is this one and ground state is this one for D2 and D8 octahedral is this one One is ground state for D8 tetrahedral this one is the ground state. So you can very nicely analyze and remembering this Argel diagram should be a problem for just we have two Argel diagrams to explain all DD transitions except D5, D0 and D10. So you can see the transition one transition and the second transition and the third transition. You can see here this is the electronic spectrum of D8 system hexa-quan equal to 2 plus you can see here three transitions are labeled here why that is currode I shall discuss here. So now we have two 3 T1G states one each for 3P as well as 3F state. So we have to mention the Brockett parenthesis both T1G states are currode because they have the same symmetry and they interact with each other. That means inter-electron repulsions lower the energy of the lower state and increases the energy of the higher state when they comes together like this they are supposed to inter-cross energy of the lowest one is lowered and energy of the higher one is increased. So this effect is much more marked on the left of the diagram because two levels are closing energy. That means if the lines have been straight they would have crossed each other which implies that at crossover two electrons in one atom have the same symmetry and the same energy this is impossible prohibited by non-crossing rule. As a result what happens you can see that kind of anomaly. So instead of going straight it is deflected upward to avoid this one and the same way this one was going straight to inter-cross it does not cross but it comes down. As a result what happens there is a marked difference in the absorption wavelengths for both observed and theoretically predicted. The state of same symmetry cannot cross each other. So combined Orgel diagram I have shown here if it is gone something like this you can see something like this they would have crossed at this point to avoid this crossing this is not permitted it is going up and it is coming low. As a result what happens when we look into experimentally this will be having lower energy and this will be having higher energy than the theoretical values. Let us compare those values here. So the mixing of our inter-electronary repulsion which causes the bending of the lines is expressed by Raqqa parameters B and C and B and C can be calculated from linear combination of exchange integrals and coulomb integrals. But they are obtained empirically from the spectra of the free ions free gaseous ions. So now we will see here so no mixing and with mixing and then you can see here no mixing means you can see higher energy this is the energy and this is the one when you mixing this much drops and whereas this much increases here. So that means now we have to make correction and we have to compensate so that the experimental value matches with the theoretical one. For theoretical prediction we have to make some corrections using Raqqa parameters. For transition between the same multiplicity states B is enough to explain the position of the bands you should remember the transition between the same multiplicity set the 2s plus 1 value same if the transition occurs then only B is good enough to explain the position of the bands for different multiplicity we need both B and C. In D3 for V2 plus ions separation between 4f and 2g is 4b plus 3g because here this is also different this is also different as a result we have to use both and B value is approximately this one and C is 4 times B. So that you should be able to add this correction in the spectrum here. You can write these corrections and then add a subtract then once it will try to match. Let us consider this Chromium 3 plus B and C are known and B is 918 and C is 4133. So now the possible transition observed are 14900 and 22700 34400 with this one predicted once are here 14900 no problem because it is not affected this one is not affected and whereas this one predicted and it is coming down so it comes down little bit and now it goes up here in this one observed one. So now we have to make this correction here for correction due to the mixing of P and F terms energy of 41g increases by an amount x and this one decreases by an amount x. So now we will see that so if we add these values the B relates to free ion and the apparent value of B prime in a complex is always less than that of free ion value because electron on the metal can be delocalized into molecular orbit discovering both metal and the ligands. Molecular orbit theory talks about delocalizing the electrons between metal and ligand and use of B prime improves the agreement this delocalization is called nephrolactic effect. We also see these things in case of cyanide complexes why cyanide complexes also show very different strong field nature is because of nephrolactic effect which is given by beta equals B prime over B, beta increases as delocalization increases always less than 1 and then B prime can be anywhere between 0.07 to 0.09 B. If all the transients are then used using this formula we should be able to match the predicted one. So now with correction added you can see of course here no correction is needed with correction what happens it comes very close to it corrected 1, 7, 300 is fine I mean no issues and now it is 300 less and here it is 300 more that means now when the value is 34400 400 can be acceptable correction and then here is 300 is acceptable whereas here it is exactly the same. So this is how we can make correction to the theoretical values to match that one with observed values obtained from electronic spectra of corresponding complexes. So now spectra of D5 is a high spin complex for example if you take Mn2 plus R iron 3 plus examples are given here in these cases DD transients are strictly a formidable ground state has you know 6s term with 11 excited states they are listed here transition problems are extremely low of course you know if the electron goes like this it has to sit something like this here so that is not permitted. So that is the reason DD transients have spin from done in case of D5 in some cases what happens reversal of both the spins and whereas here reverse of only one spin is there and therefore they may be very very weak and they can be totally ignored. So in an octahedral field these force splits into 10 states now these force will be split into 10 states because G will split into 3 plus 3 plus 2 plus 1 you should remember and then F will be split into 331 and D will split into 3 and 2 and P does not split. So that is the reason we will be having about 10 states plus 11 states will be there that you can see in this one here. So this how the spectrum looks like manganese 2 hex aqua manganese has a D5 high spin electronic configuration all the orbits are occupied with one electron each none of the possible DD transition is spin allowed. In case of D5 system none of the transits are spin allowed since for any transition the spin of the electron must be reversed both higher energy is contained already one electron according to policy exclusion principle no 2 electron can have the same spin that is the problem that is the reason spin rule says it should be 0 delta S equals all possible transits are very weak and as a result hex aqua manganese complex is pale in color the ligands in a complex vibrate about mean positions. So the crystal field strength of DQ varies about a mean value thus the energy for a particular transition varies about a mean value and hence observations are broad because of several vibration rotation transits also occurring simultaneously when an electron is excited from one ground state to one state to another one. The bands are extremely weak in this cases you can see that is reflected in this value epsilon value 0.02 to 0.03 and for allowed transitions it is 5 some bands are broad and some are sharp spin allowed bands are invariably broad all spin allowed transits are invariably broad and spin forbidden are short the same thing I have shown here. So now this is the one for D5 electronic configuration I have shown here for G is split into 3 states 3, 3, 2, 1 they have similar energy and D is split into 2 state triple and double and 4p is singular state and f is split into triple generate, triple generate and 1 and now transition would takes place here and this whatever the transition we saw here are listed here with corresponding energy. So Argel energy level diagram for manganese D5 for TD omitted G for the D you can only use this one but reverse it and omitted G so it becomes Argel diagram for tetrahedral geometry. So now let us look into Tanube-Sugano diagrams simple Argel diagrams versus TS diagrams we have to see the difference Argel diagram look very simple whereas TS diagrams look little complicated when you look into it you will come to know and Argel diagrams treat only high spin complexes especially weak field complexes whereas TS diagram can accommodate spectral information for both weak field and strong field complexes and Argel diagram is good for spin allowed transitions when the number of observed peaks is greater than or equal to the number of empirical parameters DQ modified rocker parameters B and bending constant X. So although there is a provision to add low spin states to Argel diagram TS diagrams are commonly used to interpret spectra of both weak and strong field ligands can make provision in case of Argel diagrams also to allow low spin states because once you start writing we may have to add other three or four electronic configurations and incorporating them depending upon how many electrons are there and how many holes are there that should be a problem however TS diagrams are more polished and more refined to interpret spectra of both weak field and strong field ligands for high spin as well as low spin complexes. TS diagrams show how the energy levels change with DQ but they differ in several ways. Ground state is always taken as Opsissa X axis or horizontal axis and provides a constant reference point other energy levels are plotted relative to this that means the ground state is considered as X axis and low spin terms that is states where the spin multiplicity is lower than the ground state are also included in order to make the diagram general for different metal ions with the same electronic configuration and all so to allow for different ligands having different extent of ligand field strength both of which affect DQ and as well as B and B prime the axis are plotted units of energy by B versus DQ by B. So here it compensates so that you can consider everything all kind of ligands only thing is different diagram is required for different electronic configuration. So that means to accommodate all these parameters you have to have one diagram for one electronic configuration D1 you should have one electronic configuration that can explain everything similarly you should have D91 more. So now I have shown here a typical D6 system here so hexa fluorocobalt 3, D7S2 is that 3 electrons are D6 system, D6 system one weak field ligand one strong field ligand both are shown here. So D2 Venedium 3 plus no fundamental difference between strong and weak fields D6 there is discontinuity at 100 DQ yes you can see here 100 DQ that corresponds to 20 you can see here discontinuity is there at this point pairing of electrons occurs the reason why discontinuity is there pairing occurs and then high spin complex becomes low spin complex. To the left we have high spin complexes weak field ligands to the right we have low spin complexes as a result the one which was ground term become excited term. So now we can see here this phi T2G comes here now and instead this A1G becomes ground term here once after crossing this value so one should look into it carefully in all diagrams TS diagrams I am explaining only one example here to the right we have low spin complexes free ion ground term phi D in octahedral field. So singlet 1I of high energy is there this is all there you can see they are listed. So A1G is important this state is greatly stabilized by the ligand and drops rapidly in energy as ligand free strength increases. So this one is greatly and it becomes ground state ground term here and now this one this was all the way here it becomes excited state now. If you take hexafluoro cobaltate 3 minus high spin blue in color can show you one peak at 13,000 centimeter minus 1 whereas in case of this one low spin 2 transits are seen instead of 3 here we can see of course it is D6 here obviously we will see only 1 whereas here you can see 2 because they are very closely spaced. Let me stop here and maybe in my next lecture I shall start discussing about NMR NMR also I am not going to go into detail tell you little bit about basics and jump into explaining or interpreting spectra of simple compounds to multi nuclear complexes. So where you will come across you know phosphorus coupling, selenium coupling, platinum coupling, rhodium coupling very interesting and also platinum and except for rhodium and phosphorus many of them have isotopes which are low abundant. For example if you look into a platinum 195 platinum is there 96 platinum is there 195 is about 34 percent rest is NMR inactive and similarly if you go for selenium it is only 7.6 percent 77 l is NMR active rest is NMR inactive. In those cases we observe peaks called satellite peaks I should make you familiar with writing or drawing spectrum or sketching and also look how they can be split with the different couplings and other things until then have an excellent time reading whatever I discussed it so far. Once again I thank you for your kind attention see you in my next lecture there is going to be 57th lecture.