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Lecture 16. The Importance of 13C Chemical Shifts in Structure and Stereochemistry Determination

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Automatisierte Medienanalyse

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here I will I will I believe in get started then I guess write so well so I think I want to finish up what I'll call basic Connemara spectroscopy today and in other words things that things that all sort of fall at the the level of basic interpretation structures and turn things that will get on the mid-term I think probably were pick up next time is going to be introducing 2 Deanna Martin an hour or next homework said that Monday's but the 1 after that will start using to the more spectroscopy so what I wanted to to do today was to talk more about carbon 13 chemical shifts and I gave you when we talked last time I gave you this sort of general information that just like proton NMR carbon atom are aliphatic standard the outfield aromatics tend to be downfield things that are next electron withdrawing groups particularly oxygen tend to be we will call at midfield sort of in that 50 to 70 range I also indicate and we said that the Rangers all lot lot bigger at about 20 times bigger in ppm for 13 NMR another words now the Hyde ch is at roughly 10 ppm whereas in carbon Amman the high carbon always at roughly 200 ppm obsessive like 20 times bigger now see 13 shifts have a vigor range and there's also more richness In other words when we talked about proton and a mob was pretty easy and we were able to come up with some really simple your packet beyond below calculations were you could typically pegged the chemical shift to within a few tenths of a ppm we talked about a few next to a master you know let's review next to an oxygen figure you're going to be about 2 ppm downfield where you'd normally be if you're next to of benzene ring or a double bond or carbon figure you're about 1 ppm for downfield I give you several ways of thinking about this and you should all be able to pretty much estimate that we talked about the effects of Alpha substituent and said that Alpha's substituent have a really big effect being next to on ch next to a halogen extra nitrogen extra oxygen shifts you down in a couple of tenths of a couple of years we talked about beta facts and we said that the smaller beta Faxon and proton NMR shift you down by Oct . 2 2 . 5 ppm C 39 a market is a little bit more subtle and a little bit I won't save less predictable because we're going to see it's actually quite predictable but the factors there more work factors for example gamma effects as well as bait effects tend to be big and there's some really interesting Stuart effects now along with this richness comes tremendous power because this means that carbon atom are also can give you times of rich information about structure and can be really useful for figuring out structures confirming structures disproving structures and at the end show you beautiful example of of some fraudulent work there was disproven by Professor working of skis laboratory and also going ahead and basically having a tool that can get you a want more information than meets the eye so I wanted to show you some of the factors that contribute to these sorts of general ranges and particularly now perturb them and let's start with something pretty simple inductive effects and residents an electron density of course plays a huge role In chemical shift because electrons contribute to the shield would do shielding of the absence of various nuclei so if you have if you have substituent steady increases decrease the electron density in a carbon you're going to shift that carbon upfield or downfield let me show you what I mean will start with a simple bands for now the easiest way 2 in your mind think about chemical shifts is to start with a base value and then perturbed so great waited think about benzene is the normal position for benzene is 128 . 5 ppm and then if you put substituent son you perturbed things in a rational way so may show you what I mean if we put on the foxy on the benzene the oxygen is electron withdrawing through the sigma bond and so the carbon attached to the oxygen shift substantially downfield to you go 100 to 160 ppm another word you shift downfield by 30 ppm more than 30 pct and by putting out oxygen now what's interesting than is the author of carbons end up having by residents extra electron density at them in other words the 2 were tho problems you can push your arrows and you see the electron rich is the same reason why Elektra filet aromatic substitution occurs or so in power when you have a metallic secret so the author of carbons appear and 114 the PM and the foxy benzene you don't get a bigger factor at the matter carbons which makes sense because you're you have inductive effects that are now quite removed so it's very small and resonance doesn't pump up the electron density at the matter carbon you go to the Parra carbon and now you also see an outfield shifting all of the albeit a smaller upfield shifting of 121 pp at 121 .period now I'll come in a moment too empirical and activity relationships but 1 way to think about this is to think about it if you will have on the Oxy Gruber now Cox a group on a benzene that it shifts the Ortho protons upfield by about 14 14 and a half and if you have a myth Oxy group it shifts the power protons upfield by about a piece about 7 and a half and a few of them AXA Group were no Cox a group shifts the matter protons downfield by just a fraction of the and what we'll see in a moment is that you can add up all these effects and then calculate for different aromatics the effects of different substituent Alright let's take a look at some other examples of electrons of inductive and resonance effects on so let's take analogy in another review cycle hexane has a base value In in cycle heck senior al Kane is at 128 .period 127 . 4 ppm let's compare that To cycle Haxtun known in cycle
hacks known we see of very big effect at the beta position and just a little effect just a little inductive effect at the Alpha positions data position is 120 51 ppm and the Alpha positions just just shifted downfield fight by a hair and that makes sense because you look at this and you say OK now you can think of a resonance structure in which your electron deficient right we all know that date that the knowns are Michael acceptors that nuclear files like to attack the debate position are you doing this with models and then rankings class no just so you know about frontier orbitals in an electron density and so you see the effect is actually very substantial right both of these problems symmetrical there 127 you go more than that of more than 20 ppm downfield by decreasing the electric these effects can be absolutely among guests and 1 of the things that I've tried to emphasize when I've talked about these ranges here at user general ranges these in Cobden style and so you already see for example that this was an inductive oxygen here brings us even outside of this very generic range here let me show you just how huge the effects can be so TT nascent talent is a good example Rideout teams are normally like 110 to 150 ppm but if you usually perturb the electron density you can have huge effects so it probably doesn't surprise you too much if I tell you that you now by having a To make vaccines on the NL team can shift it downfield to 169 . 7 but what's really huge is you look at the at the position here the beta position on the and now we're so electron rich this thing is so nucleophilic at this position there's so much electron density at that position that word 45 . 5 ppm you look at a spectrum of that you wouldn't even know international key because you'd say all that's gotta be aliphatic it's going to be somewhere over over here and just pumped up the electorate it's usually the most radical example I I know off the top of my head is this sort of push me pull you system here where you have to electron donating groups and then to electron withdrawing groups to sign groups and so this out you go to 39 . 1 ppm and then discarded here use at 170 and 171 right well that's really downfield but did say it makes makes sense you've got the state electron withdrawing groups but you look 39 . 1 who would have thought that that is a mail he said to so they search through nite trail groups the groups CEO I so since centurions have substantial effects whether the Alpha whether the Bader whether the damn mind you can really see this side give you were gonna walk through this and I'm going to give you some examples so 1st let's talk about how fast how calls substitution if I wouldn't give you a general principle in general L call substitution leads to more downfield shift so freaked again take our benzene example and remember we said that benzene was at 128 . 5 if we put a methyl group on it and make it into toluene now we go to 137 the point is we shift downfield by about 9 ppm by putting putting on a methyl group on the let's take a look at our school systems so will look at propane and will look at the central methylene of the central carbon of propane you put on in Al-Khobar bond a propane and you get ice of butane and now you're down at 25 ppm and notice it's kind of about the same here moved down by about 8 or 9 ppm in and here we again moved down by about 9 ppm so in other words were talking on the order of 10 ppm a alright you put on another alkyl substituent and the effect this interim as dramatic but again you move further down field there were at 28 now what's nice about this it is these ideas are generalisable there is really science here too it misses the point that you can take a little bits of knowledge and generalize and build up in your mind what's going on so let me compare us said to ethanol if we started 58 for ethanol and now we have inventions going 2 ISA probe and all what would you predict your carbon to be aired here 67 would be a very good production because we said OK we take 16 we had 9 we get to 25 you add 9 you get to 67 and that's a very good pass 64 is the answer we are right now imagine we go further and we don't know 2 Turkey you not now what do you think fraternity of 66 you know that it's going to change OK and other gasses ominous estimates 67 partly answer is 69 I knew doing good you don't have that catered to the nearest pp .period located prices right approach the chemical shifts the chemical shift is right to think we have a show here may be really late-night television 2 who made new when webcast of theirs to try it out in a test market alright so In the case of proton Panama we also saw that shifting adding alkyl groups shifted downfield right we set at a methyl group is typically about . 9 parts per million and methylene group we said 1 . 3 to 1 .
5 7 other words you shifting down at 1 . 3 2 1 . 2 1 . 4 I think sits another words were shifting downfield him to about 4 . 5 ppm Amos fine is down at like 1 and a half or 2 ppm these the baseline values in other words we go a few tenths of a ppm more and so it kind of makes sense if you get half of ppm for going adding 1 Alco group and another couple of tenants of a ppm for adding another alkyl groups trying to make sense here with a scale 20 times bigger that we get about 9 ppm at 10 ppm for adding 1 Alco group at about 5 ppm for adding another round of good will with the case a proton NMR beta alkyl groups don't make a huge effect and chemical shift in the case of C 13 alkyl groups that the due date is substitution ends up also leading also leading down to further down the the object being not quite as dramatic but let's take let's take an example and what I'll do is I'll take you to even as an example and we'll look at this problem here and then we're going to put some stituents over here so they'll be data to another words instead of directly on it they'll be won over so if we have 1 methyl groups here so I'll go from butane To to methyl butane this card is those 7 ppm more downfield we go to 32 P P identify if I added another 1 we go another 5 ppm dim field we go this is not not as big but still pretty darned brick OK so that status substitution let's look at gamma substitution now gamma substitution is interesting because Dallas substitution leads to upfield shifting and it's really a steric effects so With Dallas substitution was basically you're doing is now getting interactions that are applied having steric repulsion pushing more electron density onto the carpet and shifting you up field so let me show you show you what I mean so will still keep to this same type of general structure but now will consider this part in here and so if you look at the carbon here and you have hydrogen is banging into where you basically will have hydrogen banging together this is going to lead to more electron density on the continent In other words electrons of the ch fond repelling electrons push electron density of to the carbon and since you don't wanna have Stewart repulsion this occurs most pronouncedly when the molecule cannot avoid direct propulsion let me show you what I mean in other words if we start with butane and we we start over here the leftovers of butane is 30 the UK if I had 1 methyl substituent to write another words if I attach directly to this carbon will be if I attach the methyl substituent here will be if I attach the methyl substituent here would be damned if I had 1 methyl substituent fact carbon shifts from 13 to 12 and it's a bad sign that's not a big deal that's Eugen and part of it is this methyl groups can adopt a rotational isomer Rodenburg where it keeps out of the way but if I add 1 methyl group then no matter what I was going to be staring at other methanol into the face so if I go like that and go to tune to die Mathilde butane now there's no way to avoid that steer wrestling holsters and we shift over tonight so now we've shifted upfield by about 4 P P and relative to our initial so even this very remote substituent can make a big difference you can really see this confirmation only constrained systems like cycle Haq said so if I take cycle hexane and I consider the carbons all the province of Khorasan cycle hexane are the same and the role of 27 . 4 million if I now Adam methyl group damage to this particular problem so remember if I if I attached adherent realfoot if I attach it here it would be data if I attach it here were damn off if I add 1 methyl groups that methyl group keeps out of the way because it keeps Ecuador and so we don't see any change but if I had a 2nd methyl group then 1 methyl group that has the has the axial and there's no way To avoid 1 3 die axial interactions and so now that carbon shifts up to 21 people and that's a pronounced effect and Australia the implications of that in just a moment are so these are all of the sorts of being the effects in the 13 NMR and I will show you the last the key effect and that's heavy Adam effects alright so let's take a look at what happens when you make real various Hello method so again it's very helpful to have a baseline value methane occurs and negative 2 . 3 parts from the Saugatuck and pare the Hello Meth aims to methane itself and so beloved motto hello methane CH 3 at nite here along with things you need to ask you try methane C H X 3 and Tadiran things C X for and will start with XP imploring and so that no great surprises here you put a quarantine on internecine you get ,comma methane and you go downfield it's 25 ppm you add another chlorine you go further down the field its 54 ppm chloroform workers that show up at 77 77 his poor form so you go further down the field you put
yet another scoring on and you go further down the field and say If you're looking at say what am I wasting your time for work this doesn't seem like any sort as well if we move down the periodic table the Adams get bigger and they start to have d orbitals and they start to have the orbitals extend outward and then we see something very surprised you put on a bromine and it's a minutes so I saw so surprising bromine is less Elektra negative and chlorine scholarship you less further down field you add to means you go to methane and you say 20 wanting something that doesn't sound so surprising removing the further down field 3 gets a little bit weird because we start to move up the world and by the time period for word negative 29 and then I Dean gets downright funky so I mean I odometer thing is that negative 21 diode Olmert is negative 54 methane is at negative 140 and Tech dry on thing isn't negative 292 and what's happening is the word are extending so far out with these heavy atoms as you move down the periodic table that the electron clouds are actually enveloping the carbon and shielding it from the applied magnetic field and as you can see in some cases shielding it vary dramatically now the practical implications for this becomes If you go ahead and do a reaction to say make an alkali had died in synthesis and you don't go ahead and lock up field you may miss your carbon and say Oh my God I can't see my copy this a parameter Indiana called sweep with that you would just that sweep with is the spectral window that you look at it if you have a compound that you expect to be very far downfield or very far appeal this is true proton and well if you have a compound that you expect to be very far downfield or very far upfield you can just increase your sweep with field with by whatever number of perks you like you type in the parameter you get a bigger spectral with and then you can see for example your carbon that's attached to the same thing with protons attached idea once took a spectrum of HIV is doing indie protection reaction in Metamora tube and each eye peak was a negative 10 ppm in the probe which was sold 1 the only 1 that if you think but what is so the question is why is wired to bring it more downfield obviously you have the inductive effect pulling away I think by the time you get to 3 you just have that carving completely surrounded by the electron clouds and now you just have electronic clouds sticking out in all directions and for it's just all around these are in part relativistic effects when you're doing animal when you're doing molecular orbital calculations on heavy Adams you actually need to have special terms taking into account relativistic effects of the orbitals far-right so 1 of their take-home messages from this is there are a lot of factors that go into carbon 13 chemical ships and for this reason carbon 13 chemical shifts are very rich and very valuable and being able to predict them can you told lots of useful information on structure that can be extracted that sometimes can give you a problem-solving tool that you might not otherwise have and so what I'd like to do now is to show you some of these problem-solving tools in their applications so we'll talk a little bit now about 13 chemical shift predictions the and were going to talk about 3 different ways of doing this 1 is what is called empirical added tivity relationships that's the simplest the idea there is we're going to go and say OK the effect of an tho substituent is this if it's an oxygen the effect of a matter substituent is this if it's a chlorine the effect of an alpha substituent is this the effective abated substituent is this the effect of a gamma substituent is this and we're going to add up all of those effects if you use can draw and you have 1 of the more advanced versions of it that does chemical shift production that's exactly what it's doing those calculations also for all sorts of systems aliphatic and aromatic systems are shown in your book in your hand in your structure determination the the Orange Book II that you have for the costs you know these supplemental book and you can do it all sorts of system in but they do it to a limited extent in Silverstein and I'll show you that just for aromatic systems unfortunately I the I'm the Kindle program which tries to do chemical shift production thus far the silvery much and development that but they thus far got it wrong there is also a small CD Windows-only only that goes with Europe and with the Orange Book and the course with the supplement the work that has some wondered chemical shift production based on all on Caracol added another way of doing this is based on data bases where you come up and rather than simply say I we're going to have alpha effects invaded the effects and affects you training set and then from the training set for that set of reference compounds you find parameters that fit them and then you can extrapolate to 1st interpolate to encompass all of the compounds in the sector and then you can extrapolate to encompass other other compounds and so the generalized databases and also specific ones for example for various during a chemical problems in the 3rd Way of doing this is going to be by electronic structure calculations you were not going to do this in the course but I'll show you show you an example of the and basically since carbon 13 chemical shifts come from electron density if you can calculate the molecular orbitals an appropriate level of theory to figure out how the electrons are distributed your molecule then you can figure out the electron density around any carbon and hence at shielding from the applied field are let's take a look at these various methods in an all-stock deal with a with a simple page right out of silver In practice has a the Orange Book fracture has a similar table that doesn't look good helps to
put an end alright said there should be should be on your hand out and as I said this is just 1 of the many tables in French but a tool is so much more useful when you actually have you still know how to use your talk right so this is this is a table of empirical added Tiffany relationships were substituent sits on a benzene and literally what was done to make this table was as we said Bentsen's 128 . 5 so somebody took a spectrum of and said the carbon that's directly attached to the OH final it shifted downfield by 26 . 6 ppm of 21 and Isiah at 26 . 6 the CIA the carbon that's worth tho is shifted upfield by 12 . 7 the carbon that's matter is shifted downfield by 1 . 6 and the carbon that's power is shifted downfield bye but by 7 . 4 3 PTO Alright let's take a look at the following compound and then I'll give you 2 spectra draft emotionally or how it will use its so let us imagine for a moment that we were trying to to distinguish the following compound from other isomer so let us suppose for a moment that we were trying to distinguish say this compound 2 forward I chloroform all From this isomer 3 4 died often that would be a tough 1 to do based on coupling pack because in the proton because if you look at the proton animal you would say we would expect to have 1 proton that is splendid buying also couplings so we would expect it to be a double-edged and its chemical and it's Jay value would be somewhere on the order of about 7 or 8 we would see another Proton what would you expect the this Proton here to appear as as a double that of doublets with J values of what matter couplings 3 so we'd expect this to be a doubling of goblets with a big James about 7 Irish and small Jeffrey and this 1 we would expect to be 3 as a double it and you would expect this exact same pattern of coupling here a doubling of doublets with the big James Small Drive in Dublin with a big J and a double with a small child so we wouldn't be able to tell them apart but let's look at this this compound here and use these principles if we want to calculate the chemical shifts for let's just take others do 3 of the carbon steel so if we were to wanna calculate the chemical shift here we would start with the base value of 128 . 5 and then we would say OK we have and worth so while toxicity and we look in our lookup table and we'd say we are aware tho hydroxy and we subtract 12 . 7 minus 12 . 7 now we would then look at the effects we see from this angle showing movies calculations over itself 128 . 5 as our base value then we have 8 were tho hydroxy and so we take away 12 . 7 now a going to look at the effects of scoring for this compound and so we have 2 matter chlorine so we'll go do I look up table and will look at the effect of a chlorine so draw lines here and so now we have to medical remains so we have a medical Over here and that's going to have an effect of plus 1 . 0 0 and we're going to have a decision making the numbers lineup it from going to run tally and we have another medical arena and that affect is plus 1 . 0 so we tally all of those up In we predict 117 . 8 and now I can do so that's for this carbon here now I can this next carbon so you help me out with this calculation for for this purpose so what are we used as our base value 128 . 5 and then what's what are we do for the oxygen and 1 so we add that the oxygen is matter always reach so we had lost 1 . 6 OK and then what we do next james . 2 4 more worth tho chorus so we do old Colorado is you said plus point to plus 2 . 2 0 and then somebody else what we do last subtract 2 for apparent and so we tally all of that up and we predict 128 . 3 alright I'm not going to do any more we could do it for all all 6 of them but let's look to the next stage in your hand out and I grabbed from the old rich library of spectra the protons in carbon spectra of the 2 columns and we're just got a look at the carbon spectrum and the thing that jumped out at me In the carbon spectrum for a difference that was really cool I was in the cotton
spectrum of the 1st company member the Qantas's small and the ch is a big dude doing real relaxation differences and relaxation time and the nuclear over how surface and like jumped out at me immediately was that here we have 2 of the ch is close to 130 any 1 of them so too basically down field of 120 and 1 of field so what about about 115 and what jumped out at me here was the dramatic difference were here we have 1 at about 150 about 130 and then 2 of them as yet the range between 110 and of course we could do this for the quarks the dramatic if there were are substantial differences as well you can see quite so all I didn't preparing for today's discussion was I just calculated for the 2 compounds and so we did 118 . 5 ends it was the right thing .period but when I do I did my Phil I don't know how I managed to to tally things after prolonged here played 109 let's see 128 129 hundreds 30 118 . weighed 118 points promises was the 1
let's wait a 2nd whatever calculus was at 128 . 5 and 100 in 117 . OK so I get to know what I was writing down here 117 . 8 and 128 . 3 In and over here and I think what it I wanted I end up doing 130 . 7 and then over here 115 . 2 4 In the 117 . 5 and I think here 131 . 7 alright so the point of this is even tho it might be very hard to tell what looking at the proton
NMR to assign which isomer we have by looking at the carbon atom all you can say Oh wait a 2nd OK the 1st isomer isomers that has the 130 . 728 . 3 and 117 matches this spectrum and we can do this with the courts as well so this spectrum corresponds to the 1st isomer this 2nd isomer that has the 115 . 7 117 . 5 and 131 matches this spectrum and so sold manager for a moment that you're doing an elector fell like aromatic combination of phenol you want to determine whether you got the product you expect you know you have a a dyke Clairol phenol you don't know
the real geochemistry you can very quickly check yourself and the good news is these types of relationships generally good too let's say plus plus or minus let's say 3 to 5 ppm on the average so the point is that you can with very little difficulty tell things apart alright there are many many more sophisticated ways of doing this as I said you can do this for aliphatic compounds as well With aliphatic compounds you've gotta add up Alpha effects beta effects gamma facts there is even a little bit of Delta and epsilon affects you have to to look at the whole molecule the steric effects on you can do this easily by pencil and paper but it's even easier to do it with computer software and as I said even can draw on more sophisticated versions incorporates this there are other empirical added attended relationships that are out there off and on but in addition to that there are databases that have training sets so for example this is just 1 poster right felt polled that's a database approach and what they were looking at was how well their system that was trained on a training set would predict the sea 13 and Amara of tax solved and in general their values were good too good too Our plus or minus 1 ppm on the average and so for example they weren't able to distinguish here's another 1 that would be hard to Dubai in a mark they were able to use the database approach and here we have to 2 taught emerged and so on they if you synthesize this molecule you might not know which taught Amir you have whether you have the molecule with the carbon yellow or the molecule with the hydroxy and it's not so easy that you can take an IR spectrum because by that point you end up with a very strong stretches associated with the city and bonds of the period taught America and so the IRA spectrum is not clearly going to scream carbon he'll let you see 13 and mark the difference between what you would expect here and what you had would expect here for the carbon yield is small enough you can't just look and say obviously I have the period down or obviously I have the other compound on but in fact they can go ahead and calculate to within a couple of I'm ppm the army so they bridle to experimentally measure the value and then say OK what's the deviation and here they're getting an average deviation of about 2 ppm In other words some of the some of the shifts will be off by 3 ppm some of the shifts will be off by 1 ppm but on the average the B-2 to within about 2 alright so that's an example of a database approach and in general the methods good to about 3 key here many times you encounter specific problems you are working on a research project that involves a specific class of molecules for example the wreck knobs the group here and the key she group at Harvard are very interested in 1 3 dials 1 3 dials are important classes of natural products they come from Pires synthetic pathways that give rise to molecules of many different structures and the big issue becomes determining stirred chemistry direct the group
noticed after the fact they were working with 1 3 all 2nd nights and the question was do they have a say in die all or an inside guy all right so as soon die all it would be a 1 3 dialog with the two-hour wages are like so and an aunt died all would be 1 in which they are like so In the beautiful thing is if you're a clever graduate student were clever professor and you keep your eyes open and look at you and your data you say Hey we've got apparent here in that tells us something and then pattern means something and when they went back and looked at all of their C 13 NMR spectra of the 1 3 all set nights that they had prepared they saw something very interesting the methyl groups of the 1 3 die all asset nights when the . when the dials were seen appeared at about into 2 sets of dire stereo peaks 1 peak at about 20 ppm the other peak at about 30 ppm and when they had the same guy all the 2nd nite both of the methyl groups appeared at about 25 know the words you good now take a guy all of unknown Styria chemistry make Mediaset nite and just by looking at the sea 13 more spectrum say all that's seemed ideal where that's in died dial it's hard to do that otherwise that's extremely valuable why does this occur effect can affect you get when you make this die all you end up with the chair and in that chair 1 of your Mathilde is axial and 1 of your math is Ecuador Orioles and remember the axial 1 gets Stewart compression so it is shifted upfield the equatorial 1 doesn't get that secure compression so it shifted downfield on the other hand when you make the aren't all a 2nd nite then you can go ahead and you're both your methyl groups a sort of in the middle neither of them really has you get in a twist confirmation neither of them has unusual effects and they end up in the middle so just by a quick see 13 Adamov with this bit of knowledge in you can go ahead and sign the scary mystery the key she group at Harvard has taken this type of approach with their also and this is a really really cold use of of synthesis so they have dials where you have alternating hydroxy and methyl groups Caesar compounds that occur in a variety of different natural products and said they wanted to be able to tell the story of chemistry of these patterns so they went ahead and they said Art we've got lots of natural products with this type of unit where you have a dialog on methyl and alcohol and ethyl alcohol and methyl and so what they did was make a training set of all the possible stereo isomers so they may be ones in which they will also send which 1 is in dire which you to orient died in different positions and so forth and they found that there were characteristic that each carbon in their training sets the use this molecule is the training set each molecule In the training set had an average chemical shift of a certain position and so then they took the average value when they said if you compare it to the average value you have a fingerprint where certain ones are shifted upfield certain ones are shifted downfield and each of the possible stereo isomers has their own path so that means you can then take an unknown compound based on this training set and the database and take an unknown compound and say got the pattern matches this our last example I wanna talk about this was this was really cool said this is this is all calculating based on known information we started with the simplest thing you just took phenol and said OK what's the effect of an author substitute what's the effect of a matter substitute what's the effect of a Paris substituent took chlorine Decorah benzene did the same and say you could predict another of these dials are also examples where you have a training session but for really funky and unusual compounds that have very unusual readings and strain rings there may not be a good model out there and there was a really cool compound that was synthesized in "quotation mark by a single researcher in San Diego to publish this on his own in Ongar bond with no co-authors and no university affiliation his name was James J. with Klee stylistic and he published a synthesis that looked really cool because the molecule he said he synthesized was this Texas all won that 1 that fell Baron I think would would appreciate as a challenging target and like a student who maybe didn't quite get names and send 1 of is that the has had some stuff that was funny about it in addition to him being the sole author there were steps that just didn't seem to make sense of the buys in the community was something wasn't right here nobody wants to tell their entire lab hang guys let's reproduce James J. Claire's experiments and see if it really works at several graduate students and dissertations worth of work that's not exactly a great job to be doing so professor at nasty used electronic structure calculations 2 see how well he could 1st predict the chemical shifts of a known molecule so he took unknown compound are ellipses said Terrence and did these electronic structure calculations and plotted a graph of the actual chemical shifts verses that the observed chemical ship and as you could see most of the chemical shift calculated that the calculated ones and the the observed ones matched within a couple of tenths of a PC so in other words here we have a match of plus or minus 2 ppm on the average aren't so confident confident of his methodology he then took the structure of Texas cycle and all and I'll put this in quotes here as you'll see in a moment and found that terrible matched the vertical scale on this graph is from 0 to 5 parts per million you look at the scale of this craft it's from 0 to 25 parts per billion the deviations on the average are within about 7 ppm it's of many of the shifts are way off so that's the average now he then went ahead and thought this guy got the structural and he had a hypothesis of what so not only did he I think that the structure the synthesis was a fraud he thought that the structure wasn't even right because it didn't matter so he reinterpreted the publicist structure of the molecule the 1 from the Journal of natural products where they had isolated this molecule and thought it was this structure reinterpreted the data In considering biosynthetic factors came up with what he believed the correct structure to be wise and there are the structure matched any did 2 different kind formers of it and there are for this structure matches within plus or minus 2 ppm average by 2 ppm average I mean or amassed deviation root mean square deviations so on the average with you sell the answer became not only did James J. Leclair Morris cheap forge a synthesis of the molecule body for instance that there little wrong molecule and it was an impossible services anyway it was pretty cool in the most group was really has been a very
unusual character of the other thing that was interesting and it's all out there is if you look at his spectrum of Texas cycle and all Texas cycle also turf unit has methyl single etc. which obtained tall Peter if you look at is spectrum of his synthetic Texas like when all the methyl groups missing see 13 satellite those poolside peaks spaced 125 ppm 125 herds around there at . 5 per cent of the height which for 20 centimeter high peak you can actually see the the spectrum so he fabricated this this is this is and spectrum out of whole cloth wasn't part of his Ph.D. if you go going during the period stood here legitimately and then later on as an independent scientists working at the Zenobia Research Institute your own research institute published absolute nonsense in the literature of I was I don't know how you interesting question is very controversial story
Single electron transfer
Chemische Bindung
Optische Aktivität
Fülle <Speise>
Elektron <Legierung>
Impfung <Chemie>
Bukett <Wein>
Advanced glycosylation end products
Hydroxybuttersäure <gamma->
Methylbutan <2->
Induktiver Effekt
Funktionelle Gruppe
Weibliche Tote
Aktivierung <Physiologie>
Setzen <Verfahrenstechnik>
Gangart <Erzlagerstätte>
Oxoglutarsäure <2->
Sterische Hinderung
Gang <Geologie>
Monomolekulare Reaktion
Chemische Forschung
Konkrement <Innere Medizin>
Chemische Verbindungen
Chemische Struktur
Chemische Verschiebung
Pommes frites
Anomalie <Medizin>
Systemische Therapie <Pharmakologie>
Aktivität <Konzentration>
Potenz <Homöopathie>
Primärer Sektor
Organischer Kationentransporter
Chemischer Prozess


Formale Metadaten

Titel Lecture 16. The Importance of 13C Chemical Shifts in Structure and Stereochemistry Determination
Alternativer Titel Lecture 16. 13C Chemical Shifts in Structure & Stereochemistry
Serientitel Chemistry 203: Organic Spectroscopy
Teil 16
Anzahl der Teile 29
Autor Nowick, James
Lizenz CC-Namensnennung - Weitergabe unter gleichen Bedingungen 3.0 USA:
Sie dürfen das Werk bzw. den Inhalt zu jedem legalen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen und das Werk bzw. diesen Inhalt auch in veränderter Form nur unter den Bedingungen dieser Lizenz weitergeben.
DOI 10.5446/19256
Herausgeber University of California Irvine (UCI)
Erscheinungsjahr 2012
Sprache Englisch

Inhaltliche Metadaten

Fachgebiet Chemie
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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