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Lecture 11. Magnetic Equivalence, Spin Systems, and Pople Notation


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today's lecture is basically going to show you that a lot of the rules that you learned in soft organic chemistry like the and plus 1 rule ah simplifications and we'll be seeing a lot of examples that looked really weird because I want to show you where the simplifications break down and then next time we'll get 2 more sort of the typical rules what are so as I said I wanted to begin with this the notion of magnetic equivalents in what they began with the definition will save magnetic equivalents you let's say that 2 protons from the nuclei In other words we normally we're talking about proton NMR but cost 54 inch phosphorus is so whatever are magnetically equivalent if they are chemically equivalent the remember to nuclei that were chemically equivalent work nuclei that were interchangeable by symmetry operation a rapid process like rotation about a bond so we saw lots of examples of chemically equivalent nuclei by symmetry and then we saw how when you have for for example ,comma center methylene group is no longer economically equivalent so chemically equivalent the subset of chemically equivalent and they have the same geometrical relationship to all other nuclei In the spin system and so this brings up 1 other 1 other concept and that's the question of what is it's been system in a spin system is just a complete set of then underlying complete meaning all of the nuclei it was so complete set of nuclei in which members are coupled part of this is let's start with the the idea the spin system and let's just do this by by example so for example if we have Ethel properly the CH 3 CH 2 0 CH 2 CH 2 CH story we have in the molecule to spin systems we have once been system comprising the Ethel In another spin system comprising the Pro Bowl grew so in other words the Ethel group is a set of nuclei obviously we're talking about the Proton since for all intents and purposes the no see the teams in this fragmented so we have these hydrogen Cindy's hydrogen and at least 1 member is coupled to every other members that make up the set together In the probable group we have the methyl group the methylene group and the other methylene group and there's coupling among them in other words the CH 3 years coupled to the CH to the hydrogen is of the CH 2 were coupled to each other the hydrogen is CH rear the couple but that doesn't count the hydrogen is of the CH 2 were coupled to the next stage 2 and what's important so we each of these is complete meaning it takes in all the couple nuclei what's important is we don't have coupling between the FO spin system and the probable spin systems they constitute 2 separate sets so we can consider the Ethelred we can consider the Pro Bowl and there's no direction the let's try another example but state asks until phenylalanine and will take the methyl amid the aware the spin systems in this molecule here the 2 methyl on and and villains overruled him L of his house OK for all intents and purposes the protons are not coupled sir To the fan also what would you do for for the spin system here separately so I'm going to revise this OK so we have the panel that's going to be 1 spin system what we do here in the middle the alpha and beta What about the NH there we saw an example where said you have indeed to well when you exchange so it's deuterium there which although it has almost been for all intents and purposes you can is but what about this Senate is that going to be J. coupled member and it's a different than alcohol alcohols exchange and and members alcohols can exchange work and not under laboratory timescale if I throw them indeed too well will exchange but on the animal timescale that NIH stays there were not doing this in the future 0 so what should I do with this little part of life the said that all becomes the spinning systems and what about the very end of the molecule OK good so these guys in here now tell you right now so we have spin systems in the molecule we have the the methyl group we have the NHC the alpha and beta protons and we have the fennel group and then we have I think we have the natural admitted groups of force consisting of a molecule we can look at each of these separately tell tell you there's a minuscule light in detectable East small and Austria have a see it if you squint right later on coupling between these hydrogen bends over up but for all intents and purposes you can say that the Federal Group is not a couple Over here so that all intents and purposes we have this is an isolated spin systems will group the alpha beta and NH the
mantle on other thoughts are up it was what yeah I was young yup and so in chloroform solution indeed to all these would eventually washed out but in chloroform solution what we'd see for this and age is a "quotation mark it would be a little bit broad this 1 is going to have 3 coupling partners in chloroform solution where this hasn't exchange earned him so we see you there and double of Dublin double it's a truckload of double it's a double of triplets and we'll talk more about that and if you're not familiar with terms this NH would appear as a quartet AT and chances are it would be broadened out a little bit member I mentioned this nitrogen Quadra polar coupling so a couple of ways this can appear so it can appear as a 1 2 3 2 3 2 1 quartet slightly broadened the can just do this nitrogen Quadra polar broadening hasn't gone below that encompasses the whole thing or it can appear as something where if you don't see the wings of the Quartet and you just see old debt you might say although it looks like in Dublin has and sold depending on the Quadra poll of broadening from the nitrogen this methyl group in turn is not going to have a significant quadruple followers effects it's going to be displayed in to establish so this will be a quartet the war broad quartet or something that looks like a single led if it's furry broadened the method for of there are outlets scenario tackle this this notion of the same geometrical relationship so let's let's look at this molecule let's take to 6 died coral 1 interview benzene so as far as chemical equivalents goes we have 2 types of protons we have the Proton it's apparent to the church pew group and the proton that's matter to the groups of these 3 constitute a spin systems colorings don't count there quadruple nuclei and essentially not not spin in the church fuel group is magnetically isolated it's its own system so we look at this and we say alright we have 2 protons that are the same as far as chemical equivalents they're interchangeable by cemetery operation and now we ask this geometrical question do they have the the same relationship to all other nuclei in this system and this hydrogen says Pollock I'm worth tho To this hydrogen and this hydrogen says all war-crime were felt to it also so these 2 all are magnetically equivalent as well as chemically equivalent now there is a way of naming systems where you have different types of protons will give a different letter to each type of non chemically equivalent Proton so for example will use letters like a and B and and and X Y and the general idea is if the protons are closest in chemical shift will use letters that right next to each other in the alphabet 80's and B's and C's give far apart chemicals ship will use letters that are far apart on the alphabet letters like gays and acts were a 10 M and so depending on whether these protons are close chemical shift 2 the center Proton or whether the foreign chemical shift will either call this an aide to be Spain's system of were in 82 Annex spin yet technically only once there far apart are truly first-order but even if they're close there's some very regular patterns that you can see if they're far apart in chemical shift and by far apart what I mean is the separation of the peak centers in Hertz is many many times the coupling constant so like a typical Worthville coupling constant is about 7 hurts so with the peaks are far apart like 10 times as far apart like 70 hertz a 100 hertz at 200 hertz apart then they will end up being 80's and taxes now remember at 500 ppm 1 ppm is 500 hurts so in other words if these guys are about two-tenths of a ppm apart no three-tenths of a ppm apart we would call this an aide to text spent and what we'd expect it would be to see a double 4 the 2 1 the outside because they're being supplied by the 1 in the middle and a triplex and saw this draw little squiggly to indicate these are far apart in the spectrum in a Triplette Mike sells for the center of hydrogen I guess technically the trip would be shorter than the Catholic stomach the up if they're close together and I'm going to actually start in just a moment with the archetypical NEB system gives their closer together what you will say so if it is indeed a need to be what you'll see is as texts like attending inward depending on how far in other words the lines of the doublet instead of being equal in will become unequal and With the bigger aligned toward its J. coupling partner and the lines of the Triplette he will b similarly distorted and so that the inner lining is a little bigger then the outer 1 I was like to think of these as sort of attending in toward each other and that would be what it would look like as the aid to the system let's try another example in I'll take dive floral methane you're remember fluorine is spin activists spent over half its magnitude gyrations about 90 per cent of that of a prime so it shows up a million miles away where is your protons are resonating at 500 megahertz your flooring made resonating at 400 70 of 160 America so they are far far far away from each other but the Jake coupled to each other so collectively the hydrogen is an a for instance the cutest spins as has been system if I want to remember my geometry because we're going to ask what type of stent system that it is it's a territory roll molecule so the geometrical relationship of this hydrogen to the flooring it is the same as the geometrical relationship of this hydrogen in other words we would call this spin system and 82 X to spin system we have to hydrogen is that chemically equivalent they're interchangeable by cemetery operations and to Florence better chemically equivalent they're interchangeable by cemetery operation reflection and the hydrogen is if you test everyone
has the same geometrical relationship to all other nuclei susceptible to this isn't the 2 next year now it's just that they're far away as a set of these guys are more than a few tenths of a ppm away we recall this suspects in this case and there were 2 of them in some circumstances we call them checks to Sofia look at the H 1 Annamari spectrum of worthy F-1 animal are you would expect the H 1 in a more spectrum to show up as a dress want contrast this example with another example floor Rafael L How would we characterize that system what did the F Y OK we've let's start with this issue of chemical equivalents so are the 2 hydrogen chemically equivalent to each other OK so guys are chemically equivalent of the 2 Florence yeah bulk chemicals "quotation mark myself were going to use the same letter but we have a problem Eldena not magnetically equivalent so we need to introduce another term when we have hydrogen instead chemically equivalent but not magnetically equivalent will use or nuclear and general will use price so what we'll do is we'll call this an EIS 8th prime the next text problems businesses and the big difference is that while a system that it's an aide to X 2 systems is first-order and even these types of systems call pseudo first-order this is a spin system that is distinctly not first-order and I want to show you the difference between them the OK so on the top I have and he's a very old spectrum visas Specter from a book there were taken at 16 megahertz of the probably from the 1960's The taken on the CW instrument CW instruments are non 48 transform instruments not used anymore but these away-goals far just artifacts of it being a CW instruments at all worried about that but the main thing here it is the each of protons factor is exactly what you would expect it to travel the Floro ethylene if you look at and you say what's going on there is no simple description of this this pattern this type of pattern can be calculated that indeed are in a more spectrometers have software that all calculated that can be calculated by computer program by Back of Beyond calculations for simple systems but basically it defies a simple description why are the hydrogen leak which was 1 here but it's not the hydrogen here it's the Florence splitting the hydrogen so the dye Flora methane is a triple-A because the 2 hydrogen was 1st played by the 2 flooring In the case of the diverse flora and fully the problem is the adage that we use that hydrogen that are the same ones don't splitting each other really replies most rigorously 2 hydrogen instead a magnetically equivalents but hydrogen center chemically equivalent but not magnetically equivalent kind of sort of do there are many circumstances where for all intents and purposes you don't see any effect and these are cases that all call pseudo first-order cases and what I want to show you today is no all the stuff that we were stopped for chemistry for coupling really doesn't rigorously applying for lots and lots of common systems now the 1st thought when you look at this is OK well this is this is Bob yeah this is dived for ethylene it's sad not a common system so let's take a common system it's 1 that you're going to see In the course of your graduate career most like so this is the IOC Gulf allies commonly a plastic used as a plasticizer in all sorts of plastics if you go to the store and you buy yourself a water bottle and it says late free that's probably saying the see plastics listed as Valade free that saying it doesn't have this plasticizer this compound added this compound to make plastics pliable Tiger and tubing is great with water however because it contains tho foul if you use it on your manifold methylene chloride ths vapor will dissolve into the tight on dissolve into the dynamics of Valle di looted sniffle dribble into your reaction flask and when you work your spectrum up you'll see the spectrum you work compound up you'll see expected for this and you'll say what that what's going on now let's take a look at the molecule and figure out what sort of spin system it so we have the October groups which separated and we have the groups however we characterize the Vinci the here did a pride the indeed war if the far apart chemical shift a higher X X Prize so in other words if these guys are within the two-tenths of a ppm of each other with typical couplings you might call it a prime Bibi prime if there were more than a couple tenths of the year the price caught a text from and what that says is your rules of simple coupling may not apply and so many a graduate student has taken the reaction mixture then go on and seeing the following patented it income to their adviser for the group made and said What the heck is going on this is the IOC validate and again it defies a simple description of this isn't a
prime next ex-prime system and it doesn't matter how high field you go to the spectrum of die-off both Valley is going to look like the only difference as we change field strength is to see how this line these lines Insider a little bigger than these lines of men outside if we went to a lower field spectrometer they'd be a little more anything if we went to a higher spectrometer they'd be a little bit worried but no matter what the spectrum of Fowler is going to look like and that should freak you out just because it says all that stuff you learned of software chemistry kind of sort of applies but the people of the there's nothing Seldon neoprene is great career manifold the 1 that's even better so that has more in Europe but is more permeable to a year in so it's OK the 1 that's really good is butyl rubber which Sierra I think it's in my trial the concern of will might real copolymers anyway that's particularly good for manifold lines but stay away from tighter and tighter gun is meant for a ship are right about now that I've messed with you a little bit for your own good relays under the maladministration you for a little bit let's let's take a look and see when the rules of software chemistry applied and when they win they don't don't necessarily apply rigorously and let's play with the implications of this are itself I like it if I look at Coral ethane now my 1st thought Naldo Newman projection on it my 1st thought is Wait a 2nd well the methyl group we talk about geometrical relationships and this is confusing because this hydrogen Paris hydrogen and cited as author wrote to these hydrogen but of course there's rapid rotation so let's see what has so remember I said Rafael process used in the whole thing is now so we'll call these hydrogen and 1 2 3 and will call these guys foreign fighters have just mentioned a series of presentations and so you have 3 equal wrote emerged and although H 5 is and tied to H 2 In the 1st row we have this 2nd equal wrote Amir they're all equivalent they're all equally populated were age 2 it is go stage 5 and each 1 is a tight H 5 in this 3rd were 3 is on the rise so in the end it all evens out and if we wanted described this spin system how would we describe it then In and which take a take a guess we've talked about the chemical shifts of dyke Cora things they exon what specifically would reuse for numbers have many hydrogen are the enough group 3 it's about so what sort of spin system the today 3 Excuse spin system and all of these spin systems were there are no crimes are truly first-order and while the bees will really get things that are closer deviate from first-order but if they're on top of each other gets really messy but if there'll be separated you basically and coordinate the type of system all of these spin systems are 1st first-order means the simple rules of coupling count of a number of different types of neighbors worked out perfectly so an aide to ax to spin system you can trust is going to give you a traffic lights and other Kirkland case and die for mistake the trip other trip is at 400 16 megahertz worries 1 of 500 megahertz but you get to hear from him in the case of here we expect a triple at the Quartet and everything's hunky-dory now let's take a look at the different compound I'll take a look at the role quarrel methane and we're going to do the same thing only Minford jet will put the chlorine on the back of on the front will cause each 1 of these 2 these H 3 in the flush and and just going to mention the remembers so we have to go short alone and 1 entire Road America so we can consider the go Schroeder Mercer's appear but the separate from the entire road Amir all of them course Inter converted but let's just look at the entire road for now because the entire road and equivalent to the 2 go Schroeder hours and you'll see the conundrum that we end up the the
problem we end up with an entire in this here you can say well H 1 and H 2 here's 1 1 is gauche and then the other debts to to be ghosts you can say let's consider them as a affair here we say OK well the bromine sanitize these 2 finally have the same relationship they're interchangeable 5 symmetry so each 1 and H 2 are symmetrical H 3 and H for a symmetrical but the problem particularly to the entire Odom is that when you apply this test and say What's the geometrical relationship it's different and so you look at the entire road and you say this is going to be in a prime X ex-prime were a prime b The prime Spoon system and the problem is this is that situation that's truly first-order now the good news on this is most of the time and I've just given you some of the ugliest examples most of time knowing 1st water systems can be approximated is 1st water systems in other words you ask yourself wars what this spectrum should look like and they'll say Oh well they told media and plus 1 role it should be a triplet and a triple and that's largely true but I'm going to show you cases that break down from now I want to show you the sort of break down from from first-order too what 5 it's called pseudo first-order so in other words if we take a simple ABC system a simple MAX system of let's say I do bromine bromine chlorine according to this is a simple AX system if they're far apart you say a doublet as of late McDonald to operate the indicate that there are far apart and if their closest to closer together we see what's called an a pattern where the 2 tending to each other and if they're very close see NAB pattering with the inner lines very and if you're really close you might even be mistaken for quartet it's not it's anyway so most of the time you can get away approximating non first-order systems as sort of pseudo first-order system In other words often non first-order systems will show behavior that's very much like you would expect with just the notion that in your lines may become a wall bigger but as we saw in our example would die at both Allied you can have some very very big deviations and so what I want to do now is show you really a catalog of typical deviations because once you see them and once you see when they come up I think you'll be much less freaked out by things that occur so the scary thing about the diagram that I made on the right hand for is any time you have a methylene chain technical life it is not the 1st order every pair of methylene every methylene the pair of hydrogen is technical way they are chemically equivalent but not magnetically equivalent of course if there's a stereo center in the molecule they're not chemically equivalent then it's like an aide to but an 18 the X systems but in the case of just a plain methylene chain without Asturias center they are chemically equivalent but not magnetically normally you can get away and you say OK they taught me as a staff worthy and plus 1 role I expect to see a trip would only works pretty well I'm gonna show you some cases where we see some very very big deviations and I want to show you where these things come up so this is the sort to different molecules that I've worked with 1 of these has a horrible chain connecting and actually inoperable the group to fennel groups and so you look at the protons on this change and you say it OK they all look this file methylene Mamet's kind of reasonable looks like a triple-A you can call an apparent Triplett if you like by the 1 that's right next to the ah silliness and faring very bulky usually ends up looking very very funny you see this pattern that has what kind of looks like a triplet except in the center is further splits and the 1 over here in the middle also looks a lot like the doesn't look quite like this 1 see the thing is weird the to change if you've got a mix of entire Ghost :colon from Morrison got no and died but also some ghosts basically it averages out enough that it behaves like a 1st workers system it behaves like you were taught it should itself however if it's heavily biased toward the aren't icon from her then just as we saw in dialectical following this really funny splintered you see the same thing and this group is very bulky here so in other words when you're looking at this you have the isolated Group and then you have the hydrogen hands and it
is almost completely locked in the end I can't former and so you really really end up seeing this ends up being and a prime and Prime x ex-prime spin system that's what it technically is is so technically it's on first-order but we get that affect full force over here and you'll notice these types of patents come up again and again so here is a very different compound where you still never tried probable child try methylene chain but now you're bulky group is this TMS Group and the hydrogen that's next year Boelke TMS group again gives this exact same pattern completely different molecules but exact same pattern of Noland first-order behavior I would just call speak a multiplex so I recall all of these
multiplex and I would simply list their range I guess fall this kind of trip .period we need to explain why it was like that exactly I mean we have to understand this is a non first-order system and that normally we can get off and we can get away describing on first-order systems is first-order but you really can't vote Larry show you some other non first-order behavior so OK so most of the time takes promo helping another molecule with a chain in it and remember most of the time you can get away describing things 1st orders she'll look at your peril of propane in italics bring of a triple-A you have a sex that you have a truck flipped beautiful just what you would expect from some more
chemistry now another sort of breakdown that occurs is when protons end up lumped on top of each other even when they're not chemically equivalent to you go from Bromo propane which has a beautiful trip led to Bromo butane and you say OK and still has a beautiful Triplette we see all of our resonances dispersed you go to
promote pentane and then these 2 protons the protons at the beta and gamma position are starting to get very close to each other in Utrecht where just starts to look lot like you can already see there's some tainting in there that's history behavior that's that's perfectly normal but now using the methyl was starting to fatten them and by the time you get out these guys are really really love on top of each other so by the time you get out state Bromo hexane where now these 2 protons are 3 protons are really really let's see we've got Alpha Beta but see beta OK so now we end up with these guys really I want on top of each other and now you notice er trip really is breaking down and it doesn't look like a clean triple you could still call on apparent Triplette but it's much uglier than you would expect and this is exactly what I'm talking about going on gone first-order behavior you get up to Bromo octane and now you see it even more so said this is what I'm talking about for Nolan 1st quarter do you 2 overlaps another word you have all of these guys in the chain of overlapping with each other and all often what we will call this is virtual couplings In other words win hydrogen overlapping the methyl group is coupling to the adjacent methylene but you can say in effect it's also coupling to these others down the chain because they're right on top of each other culture and you'll see this effect it's extremely
pronounced and you've already seen this before any of you who seen a spectrum of ths has seen this behavior let shall you 6 7 against said no problem you're single and you have were chemically equivalent protons in the chain all for show up at the same chemical shift they don't let each other you go to the loo Tariq Khan said and you'd say OK even though remember none of this is a truly first-order systems none of these compounds with chains are a first-order system you'd say that doesn't want that it looks like what they thought means of working story I see a Triplette for the hour 2 CH Jews and I see a quintet for the Inter 1 everything should be hunky-dory then you've come to a don't like acid and you say what the the heck is going on and what the problem is this business of virtual coupling which is just 1 way the it's an on first-order system when you have these 2 methylene is right on top of each other this methylene group looks at it and says well on coupled to 1 but virtually coupled to the other and nail these break down into nonsensical pattern it's here and it's reciprocal so this 1 looks in it as well I'm coupled to this but I have my neighbor and his coupled to that alone together and it's nothing about the length of the chain it's all about this issue of overlap so you go up from a difficult passage of that come out like acid 1 more carbon and everything is back to being hunky-dory in other words you look at your router methyl lanes and need to look at their neighbors and even tho it's not a true first-order system they say OK were fined were not overlapping ill-behaved largely like a first-order system and this is what I'm talking about where you go ahead and use a normally you can get away with this staff were level analysis is normally you can get away with treating things as first-order versatile first-order systems but watch out because like this example unlike arise feeling examples were really playing on thin ice and as I said these types of patterns come up again and again so Tetra hydro fuel ran has exactly the same principle as on a debt they guess that you have of methylene you have this issue virtual coupling and so you have this hydrogen is virtually coupled the 1 next to the oxygen is virtually cut to the other 2 and everything's reciprocal and so you see this pattern here and you see this pattern of other place so Will during good and as human beings at pattern recognition and I've shown you 3 types 4 types of patents today I've shown you the pattern of of 80 were tho I substituted symmetrically dies substituted aromatic and we sought for italic acid were tobacco phallic but you'd also see if author died or been seeing or any other Rothfeld compound I showed you the distortion patterns for manifold that occur as I've shown you the pattern that occurs in an extended naphthalene chain when it's locked in a tight confirmation and I've shown you this pattern that you get when you get virtual coupling in the middle of the chain to keep those in mind because you will see them again in the course of the graduate who
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Title Lecture 11. Magnetic Equivalence, Spin Systems, and Pople Notation
Title of Series Chemistry 203: Organic Spectroscopy
Part Number 11
Number of Parts 29
Author Nowick, James
License CC Attribution - ShareAlike 3.0 USA:
You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor and the work or content is shared also in adapted form only under the conditions of this license.
DOI 10.5446/19253
Publisher University of California Irvine (UCI)
Release Date 2011
Language English

Content Metadata

Subject Area Chemistry
Abstract This is a graduate course in organic spectroscopy, focusing on modern methods used in structure determination of organic molecules. Topics include mass spectrometry; ultraviolet, chiroptical, infrared, and nuclear magnetic resonance spectroscopy.

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