Lecture 12. Coupling Analysis in First-Order and Near-First-Order Systems


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Lecture 12. Coupling Analysis in First-Order and Near-First-Order Systems
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Lecture 12. Coupling Analysis in Systems
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Nowick, James
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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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I want to spend the next 3 lectures 3 classes talking really closely about first-order coupling and the reason is that there is so much to be gained by deeply understanding In a more respect as I said
on a lot of what one's going to be be doing is asking specific questions about security chemistry and being able Paris those questions is intimately linked to understanding what's going on also just in general for solving structures are being able to read spectra really read them at a level that goes beyond the level of some organic chemistry involves intimately understanding coupling so we're going to take a relatively slow path through disinfected going to go through the midterm exam only have 1 the spectra on exams that we really focus on understanding things so I was started by a kind of making the bridge between last time lecture were we talked about magnetic equivalent so we talked about no first-order systems and sold last time was sort of the bad and today is going to be the good so the bad is that I said all along that the rules that you learned in simple software organic chemistry really Roper simplifications there are very few systems that truly and in the way that you learn they should behave this in the first-order systems so first-order systems are anything like DAX systems are safe In the next systems I need to In other words there anything will you coupled protons protons within a span system are far apart in chemical shifts and if you do will have to protons that are chemically equivalent like we have been in 82 Amex system that those protons are both chemically equivalent and magnetically "quotation mark we divided this and separated from its non first-order system and these air systems in which you either have magnetically in equivalent protons that are chemically equivalent to know you have protons that are similar In chemical shifts of for example of non magnetically equivalent protons result for example 80 8 prime x x Prime systems and we talked about just how ugly those systems could be goes for like the validates system or a cinematic hell for a partner matter held higher magnetic field you look at dialectical satellite also die benzene is never going to to get better than this complex pattern of lines and then I said we have other systems like systems where the protons a similar in chemical shift and ones that are related to this for example of edX systems and the good news about many of these types of systems is that many of these non first-order systems behave very much like 1st water and that you can start to apply some type of simple rational understanding them which is more than I can say for a prime system XX Prime system on a prime BBC Prime system now sometimes these systems will look like first-order which is great because sometimes you can analyze these types of systems as 1st watering and many times you can but when I tried to show you last time was how the ones that simple way the fire simple fashion after B the operation to so what let me show you exactly and let's take the 80 the system because I think this is a great starting point and what's nice is the ADB system is going to be an archetype for many sorts of systems that although they're not first-order we can apply a first-order of analysis to war and we can start to see the distortions that occurred so a pure AX
system is 1 in which you have a doublet so it's 2 hydrogen flooded Jafar shuffled and book then that's going to be the spin systems so I'll just put on X X and Y wide to represent some other nuclei that aren't going to couple and not of course something with a hydrogen on works J. like so you would have a double-edged in aid being the biggest thing between and then another Dublin was the squiggle just a freak break in the spectrum and if those 2 indictments are far apart In chemical shifts then you're going to see them each is a simple 1 the ones that now as the distance between them become smaller in other words either you have different substituent so that instead of having them being very far apart their closer together in ppm where he simply went to a lower field spectrometer now you start to see a distortion that we would call an ADB pattern where the inner lining and so now instead of saying Walesa you effectively very very far apart beyond saying they are far apart Hi so in other words this means you have 1 here and 1 in no way over there OK now the typical way in which 1 characterizes this is the distance between these lines is the J. value the distance between these doublets and tactically 1 takes not this center of goblet but the weighted average because technically with the multiplex the position of the multiplex is not at its average but it's weighted average in other words since this line is a little bit bigger we take this center is just a hair all it's the weighted average in other words at this line is is of 4 times as it is if these lines Serena fought a three ratio and they're separated by 8 points or 0 7 ppm we'd say are right here .period forward the way over there just so long so if you call this this Delta Nu typically if Delta Nu over J is much much greater than 10 we're in a situation like this and have felt the new over J it is now less than a real called 10 and those are approximations then were sort of into this baby situation and by Delta I mean the difference in position in Burma so in other words let's say the center of this line was at 7 . 3 0 ppm at the center of this line was at 7 . 1 0 the chaos and let's just say here that our J. value it is let's say are what will work out what will work out well let's let's say that RJ value equal 17 workers now I mentioned for a moment you a very low field spectrometry imagine you're on a 100 megahertz of spectrum what's Skelton was well 730 herds there are lingering on the Delta 20 hertz so at 20 hertz these guys would be hugely closer together In fact we have a situation that walked like this at this point Delta Nu actually be just the hill farther apart because that's the way it is slated average I'm going to do shifted over Justin Justin here make the outer lines just a little bigger this would be a situation where Delta the new owners Over very small were Delta Nu is about 20 cent J is about 17 if we had the same system at 500 megahertz what with the difference in lover Delta Nu for 500 megahertz 100 Drive Select 500 hertz 500 megahertz Delta Nu please equal to 100 and so you look at the situation and at 500 megahertz should be more like this at 100 megahertz you'd be more like this and so this is your AB pattern and if they even closer they'd be like when I sketched out before where the airline would be usage and the outer line With the very tiny what what that might mean it would be like a 16 megahertz spectrometer like 1 of the freshman server software we actually have the article 100 or maybe 50 60 himself or lab it would be like this where I mentioned a situation that instead of having substituent said that put these apart at on at all . 2 ppm imagine they were separated by . 1 but the main thing to keep in mind is for any given double at no matter what the center of this debate whether I looked at it had a 500 megahertz spectrometer or at a 100 megahertz the center of this speech is going to be 7 . 3 0 and the center of this speed than we did have the sector is 7 7 . 1 0 the 17 herds more characteristic of the trans out which was actually what I was doing when I was growing is for something like this will be more like about 7 per se Florida I thought Sir questions at this point of view that OK will this center moved so if you improve the equipment so here we've have gone from this is our 100 megahertz This is our 500 megahertz and the point is the center of this for this whatever hypothetical compound this it is the center of this is always at 7 . 3 mega at 7 . 3 ppm whether match 500 megahertz are 100 but the distance between the peaks because the number of Hertzberg ppm is much smaller at 100 then at 500 the distance between the peaks here is very far it's 100 hertz apart of relatively far and over here it's it's only 20 support the bigger Indiana 1 the closer they are to gather the more they took into each other and that really is the difference between the this century is related to the ratio of the bickering that's absolutely they're absolutely well here the biggest 1 is that you have 7 . 2 9 ppm a 7 . 2 8 ppm the smaller 1 is at 7 . 3 1 ppm and by here yeah we've got these 2 lines and 1 of them is at 7 . 2 ppm and the other is at 7 . com whatever work no what's
valuable about looking and a at pattern and understand that it is it really becomes an archetype for all sorts of systems that behave very near to 1st order so we were talking before about from phenylalanine then I guess the example I gave when we were talking about spin systems was answered Tilford all alanine methyl amended and I pointed out that we had 1 so this is like a spectrum in chloroform solutions all in CDC of and we decided that we had 1 spin system over here and the multiplicity of this Proton of the N H is a double-edged because it's played by 1 coupling partner each of these protons they're known chemically equivalent so they split each other but they're going to be similar in chemical shift their similar environments so they'll both be at about 2 and a half 3 parts parts per million why do I say about 3 parts per million well the off of the fennel group so if we're methyl group off of a fan wide say 2 parts per million it's a methylene so that pushes it to like to enhance apart from millions their to a couple of electron withdrawing groups the Beta to hydrogen abated to Aqaba Nielsen story ship downfield by about another 10 about another so we'd expect them to both be at about 3 parts per million but probably not to be on top of each other so each of these is going to show loss has a D and then DVD is going to be part of what looks like it be X-pattern because this is part of and maybe MX system In this something that's far apart in from either a a and B and C and so forth and acts so we have 1 proton it's going to be way downfield and nitrogen protons are typically about 7 parts per year 1 proton it's going to be moderately downfield because it selects next to an electron withdrawing group and its Alpha to a privately-owned data to a fennel groups of this is going to be about 4 and a half parts per year and then these guys they're both going to be close the 3 parts per million so we have fall apart from the SNH far apart the and the Alpha's far apart From the beta and so this guy here is going to be selected by 3 different protons so he's going to be a D D D a fall of adjacent different we are a team were and will talk more about these were indeed if 2 of the jails are the same meanwhile the quartet if all 3 chasing same women with affirmative taller so the 1 I really want to draw draw our attention to them is these 2 hydrogen it's here because now this type of aid being pattern really can serve as an archetype for water more complex patterns that are non first-order butter close to first-order so ABX pattern is something where you have the ADB pattern in which each line is further so Imagen this type of pattern here With some level of separation but now with each of these 2 lines splitting to a and so what if she is line line and William lying and then lying lying continues to grow too this is for the ABX system so this is what we're seeing right about the future this which which of them is which is so OK so staring chemical 1 of these protons is pro are and the other Proton is pro ects their dies during the time of the year we can go ahead and so for example if I replace this With the deuterium and a thought experiment then the ranking of the the carbons because the ranking of the 4 substituent signed here becomes highest-ranked next Friday next right right and so that we would be s centers for this is the pro and this Proton is pro or if I knew the geometry here for example if I knew the fennel group of preferred the point in 1 way or another I could buy nuclear over Hauser effect experiments and the like detect certain proximity is then I would be able to get an experimental correlations were predicted collation based on say proximity and isotropic groups of 1 proton In 1 piece so right now I don't know which is which but with additional experiments this is obviously just a sketch but with additional experiments in context yes you can figure out which barristers part of which other thoughts and questions I don't think did they ever look the same height great question so right now I've made a sketch for a situation in which these are relatively near to each other in other words maybe they're separated by a 10th of a PPE and stuff the separated by a 10th of a PPE and the GAB here is going to be about 14 at 500 megahertz that would be a separation if they're separated by 10 3 ppm about 50 hertz and so Delta Nu over Jay would be about 50 to 14 about 3 but if they were very very far apart if something held these in very different magnetic environment then you would see the airlines getting bigger and the peaks becoming more alive a regular "quotation mark goblets so if they were further apart it would look more like this these guys would be bigger they would be attending into each other other questions these are really important and this is 1 of the reasons of going really slow it's not a coupling coupling is always going to be mutual and so if we call this the lets name are peaks 1 2 3 and 4 and let's name .period peaks 1 prime 2 prime 3 prime
In the 4th quarter so the
JOB it's going to be 1 minus 3 and to minus 4 they will within the limits of experimental error the the saying NJ being here will be the same within the limits of experimental error it would be 1 prime -minus 3 prime In 2 primary minus 4 per the in this case since it's an X pattern the coupling with the other protons so the coupling with the remote partner will call it a tax J A X equals 1 minus 2 In 3 miners working again those will be the same within experimental error so let's say for a moment this is 14 hurt that's also 14 hurt let's say for a moment that this is 6 hurts actually looks the Web drawn it it looks more like about 900 so let's say this is 9 hurts that's going to be about knowing that's going to be annoying hurts when limits of experimental error here this distance will also be 14 hurts as well lacked distance this distance who have drawn it looks like it's about 12 heard that looks like spam UK where 1 not under 1 minus 2 will not be equal to 2 minus and so on ,comma here J B X equals 1 1 prime minus 2 prime In 3 prime -minus 4 prior to any of you look at this pattern in you draws splitting a splitting tree diagrams we into a double sets are being changed and in each of those lines is further wet With a small so you get this pattern of lying line lying lying if I call my lines 1 2 3 In 4 1 minus 2 is the small J 3 minus for the small J 1 minus 3 as the big J 2 miners forced From the AB coupling the Germinal coupling in this particular case and the court source of the small is from the coupling to in this case this nucleus over here that's going beautiful question that's going to depend on the geometrical relationships on the cop was curve so typical EU you will not have exactly the same coupling between 1 of the 2 protons let's say the pro-West and this protons versus the other predators in this part of town and so rather than might let me put up some real data you'll see exactly the same thing but at least will be it'll be a nice nice chance for you to have a real spectrum and I know I passed this out before but we didn't work as deeply at the spectrum now let's look at it with a fresh pair of eyes let's look at it more deep voice so I didn't use this exact compound I just grabbed this right off of the old rich web page and remember you can go to SIA l . com www SIA AOL L . com to get yourself lots of suspect it's a great way to to check out your ideas and your understanding of things In so here we see a real compound I've shown you this before this is phenylalanine indeed to so unlike the example that I the hypothetical example I gave in chloroform this 1 doesn't have an here it has a meaning here and it has a carboxyl like said indeed to all those exchange and so this becomes in d to win you essentially see no coupling and don't see it and actually there's DC here so this really becomes indeed 3 plus and you don't see falling and this becomes date so this system here what remains of the 2 methylene Stanimir following so this isn't ABX
system and so you see coupling here and is you're your phenols and this is it showed D and this is your alpha proton and the history of that approach so this is a very real example of what I've sketched out and you'll notice the distance between these 2 lines does indeed match the distance between those 2 lines in other words the 1 2 4 3 tour for distance does match the 1 primed the 3 prime where to prime before time distance and you'll also notice that the 2 couplings with the alpha proton from all the different a little bit different from each other so it looks like if I had to I've already here that are coupling here this distance between 1 and 2 were 3 enforce about 6 hurts and the distance here the distance between 1 Prime to Prime 3 priming for Prime is about 8 hurts consumer notice now the alpha proton is splat into a double that of doublets so each of these is a DDE ABX pattern the ABX pattern and then you'll notice that alpha proton is indeed did manage double it reflects the 2 different couplings in other words the distance between lines 1 and 2 4 3 and 4 corresponds to this coupling to the 6 coupling and the distance between lines 1 and 3 and 2 and 4 corresponds to this coupling to the Edwards what's your questions you no I don't well up great question so the question is if the alpha proton is a doublet of double shouldn't it be leaning a lot more you notice these guys are really tempting into each other and this 1 is just barely testing it so now what this is about 300 megahertz spectrum so the distance between these 2 looks like it's about a 10th of a ppm said they're separated by about maybe it's to let's say to tenants so the distance between these 2 it is about 16 right because it's 300 megahertz so it's 3 333 100 hertz for ppm so they're separated by about 60 hertz and the J. value is about 14 so that's a case where Delta Nu over is about 4 or 5 was here the difference between the alpha proton and protons is about 1 hurts about 1 ppm about 300 hurts and J. value is 6 and 8 so this is a case member I said the big difference between the ADB and AX type of situation this is a case where Delta Nu over J is pretty big 300 versus 6 a radius of factor of well over well over 10 cent so there effectively for our part we get for it all to the other thoughts so the was so great questions so this is indeed to the most hydrogen son had a 0 Adams exchanged and so most hydrogen son had a 0 Adams it's a matter of fact I will say I can give you exceptions but I will say hydrogen is on nitrogen oxygen are replaced with you so they they get replaced with deuterium deuterium shows up in there are completely shows up not 500 megahertz by that the denied that at about 80 hurts at 80 megahertz so they don't show up in the same spectrum and the J values are so small that for all intents and purposes you don't see coupling plus the changing Korea but even without the CLA will exchange because of the Al that Amin is protonated said Munoz ammonium group and because of DC it dissolves worse phenylalanine in just pure water we wouldn't be nearly insoluble odds I
started doing here is that different types of coupling relationships have different coupling constants in what I'd like to do at this point is to talk about typical coupling constants and see how we can use them to enhance our understanding so this enrolled coupling constants generally if you needed just 1 number to keep in your head you could keep 7 hurts a 6 6 8 hurts let's say so we're talking sp 3 the sp 3 right now and if I needed number actually I'm going to just put this as a general CH 2 CH also you double bonds in the 2nd but if you need 1 number To keep in your head six-state heard 7 it is a great number to keeping your head without confirmation preference people say without a conformational bias no I mean by conformational bias well if there is a strong strongly-held relationships for example if 2 hydrogen are locked in an n-type Perry planar relationships so here we have a 180 degree the angle now value is going to be bigger it's going to be about 8 to 10 hurts so for example if you have to axial axial protons so put this is Jay access acts that would be a typical examples closely 210 hurts where you're locked into an axial relationship if you have something locked into a net tutorial relationships were now you have axial likely to Oriole were Equatorial Ecuador which now you're talking about a 60 degree died growing goal and so a typical J. acts Equatorial RJR Equatorial Equatorial is on the order of of 2 output also told Israel's Quigley's here just indicate that that's approximate 2 to 3 herds this is based exactly off of the copilot if so general way of thinking about coupling is that coupling comes from interaction of the nuclei with electrons in the bond that polarize the next bonded polarize the next if had 1 extreme of the EU have 180 did a 90 degree died he drove relationship between those 2 bonds you get no overlap of orbitals if at the other extreme you have 180 you get very good overlap in tight Perry playing a relationship and at 8 at 0 you also get a good overlap and so you see very large coupling constants at 180 words 0 and very small at 90 16 and so Kokkalis relationship is basically a relationship between the 2 your died he galangal In Jamie an assertive in general relationship would be if we go from 0 1090 2 1 aide that we go and 0 it's about 80 hurts we have kind of a "quotation mark wave going down and 90 to a minimum end up add to about 10 at 100 at about 180 careers no this is sort of for general kind of plain vanilla carbon it's modulated the the coupling constants are modulated by Electra negativity and hybridization in general elected negative substituent lead to a smaller coupling constant in general if you've got nasty to sp to bond between the 2 Adams like a double bond you have bigger J values so let me show you a couple of examples I said 1st number to keeping your head is about 7 but now if you want to think about some oddball situations you can think of like an elder hired were now you have an elected negative oxygen and you have in a speech to carbon here and our behinds Avery funny in that year's your J. coupling constant is on the water to be reheard so that sort of sort of unusual al-qaeda good to keep these are numbers that really worth keeping at your fingertips for assist Salafists were talking typically on the order of say 7 to 12 With let's say 10 hurts being typical for a trans Al-Qaeda talking maybe you have maybe 12 to 18 Hirtzer 14 to 18 herds but say 14 to 18 hurts With maybe 17 hurts being typical these are all examples of this in all couplings 1 more example following right and that sort of general 7 hertz range let's take on seen as an example on a benzene as an example were talking maybe for tho coupling maybe it's 6 to 10 herds with maybe a Hirtzer 7 hurts typical 2
couplings tend to show more variation than 3 Bylund couplings so for example if you have 2 carbons on methylene group on an S P 3 hybridized carbon with different substituent son the car business you can't see anything from say all 5 to 20 hertz depending on the Electra negativity if it's just sort of carbons on here maybe for the purpose is typical if you have an P to carbon bond you're talking maybe 0 due to hurt maybe 1 herds being difficult so all of these are example of this and all and germinal coupling in other words to bond and 3 bonds have user to HH couplings user J 3 aging if you have anything more than that if you have greater than or equal to to the and coupling we're talking long-range shuffling so for example efficiently and example warning couplings normally in saturated systems you don't see coupling but if you have a system where you have certain geometrical relationships then you may see it so we're talking about say carbon not with its neighbor but with the carbon 104 over so where does this come up usually if you have intervening double bonds so for example systems we talked a little bit about this in discussion section typical ladies let's say 0 2 3 hurts depending on the geometrical relationships madam coupling an event seen the same type of thing let's say 1 3 hurts good the only real situation that you can actually see a visible splitting were you have just as the 3 carbons is if you have a lot of relationship what's sometimes called W coupling and so usually you need or want relationship for you have a series of a Perry playing bonds this occurs for example in the north morning ring system the call W. coupling because you haven't got you like relationships to say these 2 hydrogen your boring you can see you have a series of on-site hurry .period a relationship taken W nobody can actually is loaded loaded with W. coupling so you have another WTO relationship across the ring like this and there is even a 3rd geometrical W. relationship hiding in the molecule like cells so I wanna pass out there's 1 table that's really useful in your book and I want to pass it out just because the stuff that's in the back of your book is so much better when it's passed in front of your eyes rather than simply waiting there in the back of the book undiscovered and uncared about and so this is the Appendix F I mentioned before and I just want to show you how money How many good things are hiding in this 1 little Appendix here so everything we've talked about in or is hiding hiding in this appendix so we have are a little like coupling and this is all going to I mean it's going to come up on a homework on homework if you're wondering what your typical elect couplings are your finances here if you're wondering what happens if you have a double bond next to a double bond you'll find answers here if you're wondering we've already seen in the homework we've already seen coupling across a Sadelain's and say how many bond coupling is that for bond coupling right so 1 2 3 for bonds coupling but if you ask as many people do when they come up with some of the homework assignments here but can you couple further can you get fined bond coupling the answer is right here waiting waiting for you to read about it
right over here typically aren't you still coupling across there coupling on pure Indians is going to come up on the homework and already I think why the fantasy that very soon now all of this is given in more detail in practice approach has wonderful examples for period means for fire fiends for all these types of systems offer real compounds and offers typical examples but here distilled into 2 really wonderful what appendix and really 1 page of the appendix is so much of a difference good stuff that's going to help you you out with some the problems that you were so can't think this is where I'd like to wrap it up today we're going to talk more about first-order splitting next time and we're going to walk through some examples of goblets of doublets
and triplets of doublets and doublets and triplets and upwards of doublets and prepare you for the work of the book assignment that comes later on


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