 Hello everyone. Once again I welcome you all to MSB lecture series on transformative chemistry. Today let me begin my lecture that is 53rd in the series on methods of characterization with more emphasis for spectroscopic methods. So far we discussed in length about coordination compounds and to an extent about organometallic compounds. In case if you make new organometallic compounds or coordination compounds to ascertain its purity and to know in what quantity we got and what is the yield, how we can improve to get all these things to get an insight into the synthetic procedure that was used and also to improve the method we should know what kind of compounds we got it. Of course the mechanistic path and all those things we already discussed in my last series of lectures. Now let us look into the characterization of these compounds. For that one we have to use analytical instruments as well as spectroscopic instruments. So let us start with UV visible spectroscopy. Before that I shall give little bit introduction to the methods of characterization or spectroscopic methods that we have at our disposal. So how to go with the spectroscopic approach to understand the structure of a molecule and also its purity. Nowadays the spectroscopic methods are widely used not only by inorganic chemists but also you can see synthetic chemists of course. Synthetic chemists when we talk it includes both organic chemists and inorganic chemists and forensic chemists also it is equally important to analyze the samples and also to ascertain toxicity and all those things and environmental chemists also it is very very vital and of course biological chemistry is insufferable with spectroscopic methods nowadays and natural products chemists also when they make new compounds or extract new chemicals from natural resources to understand what kind of compounds they got and what impurities are there and how to purify. So spectroscopic methods come very handy that means to identify new chemical compounds that we make and natural products from plants and animals that are made and drugs, toxins and explosives when it comes to forensic materials containing soil, water or the atmosphere agricultural science and mechanisms of biochemical process to understand mechanisms of biochemical process not only biochemical processing inorganic and organic process also the spectroscopy comes very very handy and it is an important tool in the present scenario. Let us look into spectral interpretation and general process we should follow for structural recidation of an unknown sample. First what we should do is let us begin about purification of the compound let us say we have pure compound how to ascertain we have to go for CHN analysis or mass spectrometry data and NMR that would give you information about molecular formula and then NMR IR UV can also give you insight into the functional groups present in it and NMR can also provide you information about substructures for example if we have whether we have methyl group aromatic group is there and then carboxylic group is there this all information comes and of course X-ray gives a complete if it is a solid compound if we can able to crystallize and get single crystal suitable for analyzing or performing X-ray crystallography then we can get molecular structure. So then about unsaturation we can get information from molecular formula and then possible structures come from all these avenues and then we can draw all possible isomers and then once of writing all possible structures we can again go back to NMR mass and IR data to arrive at most appropriate structure. So always we should have the habit of using more than one spectroscopic method the analytical method to ascertain the structure to understand the purity of the compound also to confirm that whatever the molecule we decide that we obtained in our synthetic method. So then NMR would give you reasonable structure and ultimately single crystal X-ray analysis would tell you what it is provided it is a solid sample so that means no molecular formula. So this is how we can go through various instruments and the data from those instruments to understand what kind of molecule we have made and also the details about that molecule. So to identify molecules that we prepare we can use an array of structural information elucidated by spectroscopic methods. We want to be able to get the information as rapidly as possible and we can deal with all phases of matter mixtures and pure compounds. So in that context again spectroscopic methods are very very vital in today's synthetic world. The process of determining structure is very deductive and is very much like solving puzzle. Often if we make new compounds certainly we can enjoy analyzing and interpreting the data from spectroscopic and analytical methods we get to understand what we have made. So one or more spectroscopic experiments is carried out and by analyzing the data we can hopefully determine the structure. The most common methods for structural determination are mass spectrometry and nuclear magnetic spectroscopy NMR, nuclear magnetic resonance spectroscopy and infrared spectroscopy IR and electronic spectroscopy UV and visible. Each method provides its own special kind of data that we can apply to molecular structure determination and each spectroscopic information you know gives a very interesting and a special kind of data and when you club together all this information without any ambiguity we can understand the structure of a molecule. So contributions from different form of spectroscopy comes towards understanding the purification of a sample. Among them nuclear magnetic resonance spectroscopy plays an important role when we have NMR active nuclei in that molecule. So NMR experiments apply to nuclei that have the quantum mechanical property of a spin. I shall elaborate more when I go to nuclear magnetic resonance spectroscopy after completing this UV visible spectroscopy. So time being let us know that all nuclear spin with I value half such as 1 H 13 C 31 P 15 N or nuclear spin I equals 1 for example 2 H 14 N. They provide information about the types numbers and connectivity of particular atoms. For example NMR can show ethanol has if you take simple ethanol if you record NMR spectrum 1 H NMR spectrum for this one. So it can tell you about the presence of three groups such as C H 3 C H 2 and OH. How two types of carbons in the ratio of one is to one that information comes and then three types of hydrogen in the ratio of 3 is to 2 is to 1 that information comes and C H 3 and C H 2 groups are bonded together this information also comes. So that means that is good enough to understand yes if I have ethanol in my hand. Similarly for isopropanol if you take isopropanol so what you can get from 1 H NMR is yes it has two methyl groups one C H group and one OH group for many molecules the entire structure can be reduced. So routine NMR experiments are performed on solutions of the molecules of interest normally in deuterated solvents such as C D Cl 3 C 6 D 6 D 2 O or even acetonitrile C D 3 C N and for many we only need a few milligrams and in all these cases when we perform NMR we need very few milligrams of samples about 10 percent of NMR active nuclei should be present in it. So this depends upon the abundance of the isotopes we are looking at this is very very important if it is 100 percent abundant very small quantity is sufficient but the percentage of NMR active nuclei is small then we need more samples. NMR can be used to study mechanisms and also we can get an idea about presence of transient states or even intermediates of chemical reactions in solution especially this holds good it can provide more information when you perform a chemical reaction in solution medium the samples may be mixtures. So NMR in general is a method but it is most useful spectroscopic approach for determining the structure. So NMR is a very general method but it is a most useful spectroscopic approach for determining structure of molecules. Infrared spectroscopy also called as vibrational spectroscopy so IR spectra results from the absorption of infrared radiation that causes vibrations of the molecules the spectra are typically presented as percentage transmittance or absorption the peaks carry a lot of information such as functional group identification structure information and even symmetry. So IR is a very sensitive method and is used widespread in part due to its easy sample preparation very easy to prepare sample we can record sample in solid state in solution and also for gaseous molecules we can still record IR spectrum. The equipment used is relatively inexpensive very compact and simple to use a single spectrum can be run in just a few minutes. It does not take very long duration of time for recording a spectrum or preparation of the sample. Samples can be prepared in few minutes and also we can complete recording a spectrum within a few minutes. The presence of functional group give rise to distinct features that can be identified within well different ranges of spectrum but IR is only limited to determining the presence and identification of functional groups it ends it can give you some information about presence of those functional groups that is it it cannot go beyond and it is very difficult to understand the structure and composition of the entire molecule simply by using a recording IR spectrum. Now let me give you approximate time scale for structure determination. You see for example here I have listed various techniques and also the time scale for that particular technique. So electron diffraction whatever the process that happens up to 10 to the power of minus 20 seconds one can record that means anything that is very very fast so that can be recorded with electron diffraction and x-ray 10 to the power of minus 18 up to that and uv visible up to 10 to 10 rise to minus 15 and visible 10 rise to minus 14 and IR and Raman 10 rise to minus 13 and ESR 10 rise to minus 4 to 10 rise to minus 8 and NMR 10 rise to minus 1 to 10 rise to minus 9. So that means whatever the dynamic process that happens in the time scale of 10 to the power of minus 1 to 10 to the power of minus 9 can be identified by NMR. If the process is very slow slower than 10 to the power of minus 1 or faster than 10 to the power of minus 9 then NMR fails to give any insight into those dynamics happening in solution. First kinetics 10 rise to minus 3 to 10 to the power of 2 and physical separation if any sample is stable for more than 100 seconds then one can visually look into it and pick them hand picking can be done too and if they have different morphology one can do hand picking under a microscope. So this is very very important information as far as time scales are concerned for various techniques. So now this gives electromagnetic radiation applied in different techniques here so our attention is ultraviolet. So now let us try to look in more detail about UV visible spectroscopy. Whenever we have to study something the question pops up in our mind why should we learn this top and after all it makes sense here if I make this statement because nobody solves structure with UV any longer because just with UV you cannot solve the structure but you need proof from many other instrumental data. So that means why we should learn this top because many organic molecules have chromophores that absorb UV light and then UV observance is about 1000 times easier to detect per mole than NMR so that means very easy. Still used in following reactions where the chromophore changes useful because time scale is so fast and sensitivity is so high and kinetics especially in biochemistry and enzymeology use are depend heavily on UV visible spectroscopy for the same reason. Most quantitative analytic chemistry in RNA chemistry still conducted using HPLC UV detectors one should remember about that one. If we ask why is so important yes it is very very important because of this quantitative analytical chemistry involved in doing UV visible measurements. So one wavelength in that case what happens one wavelength may not be the best for all compounds in a mixture we have to use a range of wavelengths instead of using monochromatic we have to use a range of wavelengths so that it covers all absorption or transition that happens. So affects quantitative interpretation of HPLC peak heights this affects quantitative interpretation of HPLC peak heights. But one should know what compounds could and could not be detected by UV detector one of the best ways for identifying the presence of acidic or basic groups due to big shifts in lambda wavelength for a chromophore containing a phenol carboxylic acid etc. So what happens if the peaks are shifted towards left we call it as hypsochromic shift or we call this blue shift and if it is shifted other way around it is called bathochromic shift or it is also called as red shift. So now let us look into UV absorption process for saturated compounds. Saturated compounds anticipated electronic transitions are sigma to sigma star and sigma to pi star. Their high energy transitions accessible in vacuum lambda maximum is around less than 150 nanometer not usually observed in molecular UV visible. So now if we have double bonds and unsaturated systems less energy for to pi star as a result what happens what we see is n to sigma star and pi to sigma star transitions and non-bonding electrons are lone paced or wave length will be in the range of 150 to 250 nanometer region. And these things are most common transitions among double bonded or unsaturated systems are n to pi star and pi to pi star observed in organic molecular UV visible observed in compounds with lone paced and multiple bonds with lambda maximum will be in this range. In inorganics it is little different additionally transition between d-orbit split by presence of ligand feed. So we have seen in ligand field theory how d-orbit are split based on the influence of the ligand feed. So additionally in getting different transition I show you sigma sigma star pi to pi star and n to pi star we also come across another one that is called dd transitions. So usually dd transitions occur in visible region and besides dd transition in metal complexes we also come across charged transfer transitions. Of course in case of charged transfer transitions we come across two types of charged transfer transitions metal to ligand charged transfer transitions and also ligand to metal charged transfer transitions. And there are instances where if you have a bimetallic system metal to metal charged transfer transition also occurs. Of course in all these cases one can also anticipate ligand to ligand transition as well. So in those cases when whenever we talk about charged transfer transitions one must act as a donor and the other act as an acceptor. Any of these require that incoming protons match in energy the gap corresponding to a transition from ground to excited state. That is obvious for example if I consider highest occupied molecular orbital is here and lowest one is here or any transition for that matter that has to occur between two energy levels we have to supply the energy that matches the energy difference between those two levels. So the energies corresponds to one photon of 300 nanometer light or approximately 95 kilo calories per mole. So now I have given little bit more information about metal complexes and also about de-electronic configuration. Just you know I have segregated various de-electronic configurations into four categories and below that one I will come back to this one later. So let us look into mercury iodide this is brick red compound and it has a detain electronic configuration. And then let us look into K-mino4 potassium permanganate intense purple you all have seen here metal is in plus 7 oxen state as a result it has D0 electronic configuration. And if you look into bismuth triiodide orange red in color in this one what happens detain of course the S2 P3 electronic configurations in bismuth is in trivalent state we have two electrons are left. So now we have detain S2 electronic configuration but this is very beautiful orange red in color and then if you look into prussian blue we have intense blue color you can see and that means here I am talking about metal to ligand charge transfer transition ok and ligand to metal charge transfer transition and also we can come across as I mentioned ligand to ligand but here metal to metal charge transfer transition can be explained here for intense color of prussian blue molecule. Now look into the electronic configuration let us try to understand some similarities of groups segregated in four parts D0 detain yes D0 has no D electrons no electrons and D10 means all orbitals are completely filled ok they are usually colorless and now I have another group very interesting here so this group includes electronic configuration such as D1 D4 D6 and D9 whereas the third one in the series includes D2 D3 D7 D8 and another one fourth one is unique it has a D5 completely half filled electronic configuration. So completely half filled is fine and no electrons is fine and 10 electrons completely filled is also fine then how this second and third categories were brought together yes it is very interesting you can see one electron is there in D orbital and then if you look into this one one less than half filled electronic configuration. So one electron is in D1 and one electron less than half filled electronic configuration and one more than half filled electronic configuration and one less than completely filled electronic configuration is that clear in this group we have one electron system and one less than half filled electronic configuration and one more than half filled electronic configuration and one less than completely filled electronic configuration we have. So now you can see some similarities among these electronic configurations of D1 D4 D6 and D9 now let us look into D2 D3 D7 D8. Now two electrons are there here in the same way two less than half filled electronic configuration and two more than half filled electronic configuration and two less than completely filled electronic configuration. So now it appears like later if I have spectra from all this electronic configuration I should understand as probably in category 2 all these electronic configuration probably would show one type of spectrum or one type of absorption or one type of transitions and similarly category 3 where we have D2 D3 D7 D8 also probably show one type of so just remember the significance behind this kind of classification we will understand as we progress with this course. So now let us look into metal to ligand charge transfer MLCT we call it as if the metal is highly oxidizable the transition will occur at low energy if the metal has a low oxidation number so that means if the metal is easily oxidizable the transition will occur at low energy if the metal has low oxygen state or oxidation number. If the ligand is easily reducible with low laying empty orbitals like pi star in a case of aromatic ligands or carbon monoxide sigma star in PR3 etc that means basically metal to ligand charge transfer occurs if the metal is in lower oxygen state if it is in lower oxygen state it is electron rich and also it is in lower oxygen state and also when it is electron rich because of repulsion inter electron repulsion it is prone to oxidation that we have already seen in our oxidative addition reaction reductive elimination reaction scheme. So that is one criteria in order to see metal to ligand charge transfer transition on the other hand ligands should have empty orbitals to accept electrons given from the metal so that means metal to ligand charge transfer that obviously the name itself says that electrons are coming from metal to ligand usually when we make a metal complex ligands donate through sigma donation if the metal to ligand comes it has to be in the form of pi electrons you should remember so that means pi acceptor capable ligands can facilitate metal to ligand transfer in that case non-classical ligands such as carbon monoxide, phosphines, olefins, pyridines and all those things play a major role. Examples for metal to ligand charge transfer includes 2 to bipyridine, 110 phenanthroline, carbon monoxide, cyanide and thiocyanide and if you just look into this compound here this orange colored complex is being studied as the excited state resulting from this charge transfer has a lifetime of microseconds and the complex is a versatile photochemical reduction agent and then we can also see for example tetracharbonyl phenanthroline tungsten complex or tricarbonyl bipyridine iron complex etc. So let me stop here and come back in my next lecture discussing more about UV visible spectroscopy and now we shall discuss about ligand to metal and then move on to understanding DD transitions in more detail. Until then have excellent time and also keep reading. Reading should not be stopped as far as we are learning something and do well. Take care. Thank you for your kind attention.