 Today, we are going to study the application of electron parametric resonance spectroscopy to photochemical research, so that is photochemistry and EPR. So, what happens when molecules absorb light, molecules are exposed to light, what happens? So, when excited by light of course, molecule will go to excited state provided the light is absorbed by the molecule. Now, most of the molecules in the ground state are singlet, all electrons are paired. So, the first excitation usually brings the molecule to the singlet state and from singlet state the molecule can go to triplet state. And in triplet state there are two unpaired electrons and you know that unpaired electron means the system is paramagnetic, so here two electrons are unpaired, so it will be paramagnetic species, so the triplet can give rise to EPR spectra. Similarly, the excited molecule under chemical reaction and chemical reaction sometimes produces free radical, free radical also have unpaired electrons and their paramagnetic, so they can give rise to EPR spectra. So, we have two types of possibilities, one is triplet, other is free radicals and both can be studied by EPR spectroscopy. Today, we will concentrate on this aspect and not discuss the EPR spectroscopy of triplets. Now free radicals because they have unpaired electrons usually they are very reactive, they always try to react with something else. So, it is very difficult therefore to capture them and see their EPR spectrum, so it is not going to be easy task to capture this thing in the EPR spectrometer. So it is possible that molecules absorb light and does some photochemistry and they adjust the spectrometer is not able to detect them, so then not seeing EPR spectrum does not necessarily mean the reaction is not taking place, but if you do see this thing, suppose EPR spectrum is seen, then there is an ambiguous evidence that radical is found and whatever reaction mechanism one can think of is at least partly supported by the observation of this. What I am trying to say is that if you do not see the EPR spectrum of any radical, it does not mean it is not taking place, so observation is a definitive proof, not observation is not the proof that it does not happen. So that is why I have written in the slide that if detected they give ambiguous evidence. So because these are very short lived reactive radicals, how does one really detect them and try to capture them even if they are short lived. So we have to find some way of doing the experiment unlike the examples that you have seen in the EPR lectures, almost all the radicals that we showed are reasonably stable radical, so that you can scan the magnetic field very slowly and get the spectrum. Radicals do not die during the recording, here because they are short lived some special techniques need to be adopted. The three ways one can do the experiment here shown in the slide, one is that continuous photolysis, let the reaction we carried all the time and light is on the sample and we can hope to get some steady state concentration of the radical and that can be seen in a steady steady EPR spectrometer. Another possibility is called spin trapping, in this spin trapping technique this transient radical that is radicals which do not live for long time, they react with some other molecule to produce a relatively stable radical. So this stable radical can be seen in the steady state EPR spectrometer and third one is called time resolved EPR spectroscopy where we try to capture the radical as soon as they are created and we will see later how that is done. So this continuous photolysis, so here to capture the radical as they are formed we have to keep on continuing the photolysis and try to record the spectrum, so how that is done. Here first I need to sunlight on the sample and this should be done in situ that is we cannot carry the this excitation somewhere else and bring the sample to the spectrometer and hope to get the spectrum that is not possible, it has to be done inside the spectrometer to be specific it has to be done inside the micro cavity of the spectrometer, so how is it carried out, for that here is a design that let us say this is the micro cavity and this is the iris tunic screw, so we could put a sample tube and let the sample which is kept here reaction mixture can be taken through a pump and this is a low temperature bath and through this liquid goes out here and then again it can be brought back to this reservoir. Now here there is a hole which is kept here in from the cavity and light can enter through this either laser light or even steady state lamp could be inserted here, so remember the cavity looks like this, there is iris hole and this goes to the wave guide here, so for this end was blocked, but we could open a hole here and allow the light to go through but the trouble is that if you allow light to go through and hole is drill here the micro can also leak out of that, so that will reduce the key of the cavity, so that is not very desirable, so what is done here is that some manufacturer instead of opening a hole they make a grid type of arrangement here, these tiny gaps are kept there, so that so far as the micro is concerned which wavelengths are fairly big, they find that this is almost a continuous metal plane, so it will not escape, so this is one way of but the allowing the light to go through this this holes are thin, but the trouble of doing this is that much of the light is not penetrating, they are blocked by this grills here, so another technique is to actually have a hole here and let another piece of small wave guide which is mounted here, so that the wavelength of this is such that this really cannot sustain within this, this is much smaller dimension this one, so this almost act like a tube which blocks the wave from entering here, so that way there could be a small slight loss of the cavity cube, but nevertheless this will work very well particularly because the since the hole is clear all the light will go through the micro cavity and this thermostat at low temperature bath and having a liquid to go through this is to have some control the temperature, because at lower temperature radicals could live for somewhat longer time giving a making it easier to record the IPA spectrum. Another possibility is that here the there is a special tube which is used here which looks like this, so sample liquid sample enters here and then comes out and through this it goes out here and this is kept inside a standard variable temperature dr insert through which cold nitrogen gases passed through that, so that sample is cooled by this cold gas here and if you flow this liquid is done slowly then it will acquire some sort of steady state temperature which is decided by the flow of this liquid flow of this not liquid nitrogen cold nitrogen gas and the temperature is measured by this heater sensor assembly here and the previous one temperature is measured by the thermocouple here, here is an example now having describe how the experiment is done, this experiment involves exciting this paravenzo quinone this and we have signed light and see what happens, so the light is on all the time, so the sample is flowing through this and we are recording the steady state IPA spectrum the way we have described earlier, the experiment is very simple now except for this special modification on the cavity and the flow system that is used to continuously replace the sample, so here signal that is seen here is given in this slide, so this is triplet triplet another triplet triplet this triplet triplet overall you can see that this gap is same as this gap we can now guess that without making any careful measurement this gap is also same as this gap and this individual triplet also have similar hypotenuse splitting and overall this intensity of this and this and this follow 1 is to 2 is to 1, so that means this radical has one spin that is one proton now I can say proton because the chemicals are made up of basically proton magnetic nuclei, so this gives one proton of one number one another proton which you know two of them and another one which is one, so this gives a triplet that is this one is to 2 is to 1 this one gives a doublet, so this is mistake but this also gives a triplet, so this will be triplet, so this is the type of radical which the radical should have this type of protons, so here is the structure shown which is consistent with this spectrum and that is, so you see that one this is one proton which gives a doublet splitting and this and this are equivalent protons, so they also give they give rise to a triplet and another proton here and another one here these two are also equivalent but they are different from this, so they give rise to a triplet, so here this coupling constant due to one pair is different quite different from the coupling constant of the other one, so that is the way the spectrum looks like, so this definitely shows that radical must be same, this is the only possible radical that is consistent with the spectrum and knowing the chemical nature of the reactants nothing else can possibly give rise to similar if your spectrum, so the observation is very very definitive, so if instead of now this isopropanol if you take ethanol this is what happens it will again looks that the similar pattern triplet triplet triplet triplet triplet triplet, so this must be same radical as this one but in addition some smaller lines here this one this one this one, so what are these things, so little hump is here, so here then one can get some idea by measuring the relative heights of this intensities, so even though now that I see little bit here and gap between this and this is same as between this and this and between this and this, so there should be a partner of this little line somewhere here which even if I do not see it I have to assume that is there and the intensity ratio of this turns out to be 1, 4, 6, 4, 1, so the sum of the radical is also found here along with this radical and knowing the again the water server chemicals are there the most intelligent guess will be that this corresponds to here, so this is a neutral semi-colonial radical and which is there when this dissociates to make it anion radical then it will have this one pair of protons and another pair of protons now here because of the delocalization or 4 protons become equivalent and then I can get this sort of spectrum which is consistent with this, so for isopropanol I get this radical only but in ethanol this radical is predominantly seen but this is also seen to a certain extent. Another example here we use this acetone plus isopropanol and cyan light, so by the way here the light that is used here that light is absorbed by this one this is the one which absorbs in your region this does not absorb, so this goes to excited state and does photo chemistry and produce with this radical. Similarly here this carbonyl group here of acetone that absorbs the UV light and that goes to excited state and reacts with this and produces some radical and this is the result of that, so here this lines are seen now 7 lines 1, 2, 3, 4, 5, 6, 7 and if you see it is may be it is not very perceptible but they do not quite look like one transition there, this is what I discussed in one of the earlier lectures is that one says be careful if there are overlap lines are there or not, here one can see this sort of discontinuities are there, so there is an indication that some other proton is also involved in coupling that of course one can improve the resolution and get a more direct evidence of that, so here after measuring the intensities of all the lines one gets this sort of numbers and that is consistent with 6 equivalent protons and if you consider this little doublet then the another proton which gives this small doublet and the radical which is consistent with this if your signal is this one, so here so naturally here one can write the reaction mechanism that this excited state of this takes this hydrogen atom from here and produces this radical and so often that if you remove this hydrogen atom now not the proton you bring it here then this also becomes free radical, so two of this thing forms there, so that is the way the steady state photolysis and we can look at the radical directly if of course the circumstances are such that they are favorable then we can get this radical directly there of course the condition for this is that we are doing the continuous photolysis by shining light all the time and detect the spectrum, but radicals being short lived they also dying at the same time, so whatever the steady state concentrations that are built up here they will contribute to the observed spectrum, so if the radicals do not have sufficient steady state concentration it may be almost impossible to see the spectrum in this fashion. So here comes the other technique that even if that do not live for long time if I allow the radicals to react with something else to produce some other radical and that radical has sufficiently long life time then we could probably detect those radical the secondary radical and try to understand the reaction mechanism and probably make some guess about what the original radical that was, so this technique is called spin trapping appear, it trap the free radical, so idea is very simple that R is transient radical some trapping agent, so this reacts with this to give a another species which is trapped radical and this is has sufficient long lifetime and may be easier to detect, these are some common trapping agent and often they are the nitroso compound a no group, this is a diamagnetic species the radical comes here and attacks here to form a radical this could presumably be suppose should have sufficient long life time and easy to detect another one is of this kind, so this radical comes and attacks here produce a radical of this kind then this could have a long lifetime then you can see it, so one of the common trapping agent is this molecule called phenyl N mutile nitrone or called PBN it is similar to this, so phenyl group is here, so here is example for example that if ethyl radical is coming and attacking here then the radical that form is NO dot, so what will be the spectrum that this oxygen has the radical centre, so nitrogen nearby, so this will be triplet because of nitrogen spin 1 and then adjacent that is this single proton which is coming from this trapping agent, so that gives a doublet, this is the type of splitting one gets for this sort of trapped radical and since the actual radical is sitting somewhere here which is rather far from this radical centre, this spectrum does not depend very much on the type of radical which is seen in the spectrum, so it will often almost always be a triplet and is split by doublet because of this one, there could be very small difference of the coupling constants here, nevertheless by enlarge all the trapped radical give similar appear spectrum, so it is not a very characteristic evidence of what radical was trapped, all one can say that some radical is trapped and one can think of forming a reaction mechanism, the another common trapping agent is called DMPO this is structure of this, now here the second same ethyl radical comes and attacks here this place then this is the trapped radical, this can be detected in appear spectrum, here again the radical centre is near nitrogen, so this gives triplet line, then there is another proton here which also give a doublet, but here this splitting constant due to this is somewhat more sensitive to the presence of this radical because it is very near that one, that is hybrid coupling constant of beta proton is reasonably sensitive to this, so it will be easier to make better intelligent guess about the type of radical by seeing how much of this splitting constant has changed, here is an example, the parabenzoquinone, the same experiment that we did in the steady state one and we got only the evidence of this radical without having any knowledge of what the other radical was parabenzoquinone ethanol and DMPO it gives some lines of this kind here, so here you see this is doublet, some say doublet, may or may not be there, so you have to make some careful observation and see that these are the signals coming from this adduct, this is the CH3, CH2O radical which adduct with this to give a radical of this kind and other one is this one when we use ethanol, sorry this is isopropanone, so this splitting constant is this very large splitting constant due to this for one trap radical and somewhat smaller splitting constant due to this radical, so this value I write here, this nitrogen splitting constant is 13.64 gauss, this beta coupling constant and similarly adduct with 14.80 gauss and A is beta is equal to 21.8 gauss 0 gauss, so here you see the difference in the hyperbolic coupling because it is quite appreciable when this adduct is formed and this adduct is formed, so that way DMPO is a much better trapping agent for identifying the radical that is trapped there, but problem with spin trapping is that as I said earlier it is very difficult to often identify the radical that is trapped unambiguously, sometime the trapping agent itself may react with the light and then produce some radicals with solvent and sometime the trapped radical may not be the radical that one has in mind while formulating the reaction mechanism and finally even if the spin trapping technique yields no signal it does not mean the radical is not forming, finally quickly go through this time resolved EPR spectroscopy where we try to capture the EPR spectrum of a radical which is right at the time of their formation and before they react, so for that we use easily pulse laser light or electron beam pulses and spectra are detected at different times after the generation radical that is why it is called time resolved EPR and naturally it needs a fast response EPR spectrometer and pulse source of light or electron because you have to compete with the reaction of the radical. So, this is the one way I said earlier is that we use direct detection EPR spectrometer and we do not use the field modulation technique simply get the output from the preamplifier, but to improve the signal noise ratio we do signal averaging and so signal averaging is done in this way they suppose the laser is operated in repetitive mode and every time micro signal looks very similar, so we try to get let us say a small portion of the voltage at given time after the laser here given blue block here and average this many many many times there is a device called box car average or it is job is just that take a sample of the voltage at given time and then average it out. So, if you plot this voltage at this time after the laser pulse while scanning the magnetic field I can get the time resolved EPR spectrum of the species at this time after the laser pulse here is an example the duroquinone is resolved in this triethylamine solvent and then a laser UV laser light was shown on that and this is the type of radical spectrum I get. So, this corresponds to the duroquinone anion radical of this one this is the duroquinone anion radical the in here as I said earlier as a direct vision technique give the absorption spectrum, but here the way we have adjusted the spectrometer that it is suppose to give the absorptive signal, but everything comes in the opposite direction that is very strange that as if the signal is in the emissive form it is indeed very strange. So, we will not try to explore why it is so, but we will postpone it for later discussion we can do another experiment that instead of getting the spectrum as a function magnetic field we can hold the magnetic field at any place we like and then see the micro signal as a function of time now and average the whole thing by using a very fast transient analog to digital converter. So, transient digital converter will capture the signal in digits at sort of sampling rate of certain given by the user. So, this various digital values are added to generate the average value of signal by adding several of this transients. So, here is an example so, again do you recognize signal as a function of time it forms the negative direction again slowly goes to near 0. So, this time is of the order of few microsecond if we increase this time to 700 microsecond one can clearly see that signal first starts emissive and then goes to absorptive and then goes to 0. So, presumably something is happening here and then radical is decaying in this fashion. So, this is the two dimensional spectrum one can create by getting similar spectrum at different magnetic field and so, one axis is magnetic field axis other time axis. So, the example that is so, earlier in the steady state photolysis acetone and isopropanone gives the absorptive spectrum of this kind this is the derivative signal, but if we imagine that this is the way the absorptive signal is and this experiment using time visual spectroscopy gives complete different type of signal intensities. This is steady state spectrum here the time visual spectrum you see some of them are emissive some are absorptive and in doublet signals much seen much more clearly here. So, naturally something unusual information is contained in this time visual visual spectroscopy. So, it seems the therefore, the spin population does not follow the Boltzmann distribution Boltzmann distribution ensure the lower level or more popular than the higher level, but here things are all getting quite different. So, this set of information one can therefore, get in experiment of this kind and this phenomenon is called electron spin polarization right now we simply learn that it is possible to detect such phenomenon and why it is so will therefore, involve a further discussion which will take up in a subsequent lecture.