Lecture 9. Chemical Shift. 1H NMR Chemical Shifts.
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are sold chemicals shift is the idea very quickly that was introduced which I solid James sort of a light bulb go on that the frequency at which a proton resonates is going to be proportional to the applied magnetic field so for example Tetra methyl cyanide Lane at a 17 thousand 500 Dallas magnetic undergoes procession the protons undergo processions were flipped their spin at 300 cycles 300 million cycles per 2nd you fully tape that's a molecule of Mass and put it into 117 thousand 500 doused magnet then amassed undergoes procession and flips it spin the 500 million words but what happens is OK so if we Maryland have just sort of a plain-vanilla methyl group so not TMS not a methyl group on so can't get enough of propranolol called the methyl group is going to undergo procession at approximately 4 300 million 300 later on I'll be saying it's closer to 300 million 270 but we'll just use 300 million 300 for round numbers at the 70 thousand doused magnet and at 317 thousand doused magnet it's going to undergo procession at 500 million 500 hertz so rather than saying all at a certain magnet with 300 hurts downfield of TMS and a different magnet where 500 megahertz downfield to him as we can just normalized and state in both of these cases we are 1 ppm of downfield downfield means higher frequency but then TMS and so that normalization allows us to compare the frequencies of protons regardless of the matter that here and of course if we go ahead with the math is really simple here so if I tell you that the methyl group in methane firewall undergoes of resonance 600 herds downfield all of the s and I ask you how many Hertz would it be on the 117 thousand doused magnet how many 1 thousand exactly but in both cases it would be how many people here 2 so when you look at the x-axis of animal spectrum to remember I said we transformed are time axis in the FID into a frequency access you know no 1 ppm the span from 0 to 1 4 1 2 2 4 2 to 3 corresponds to 300 dots on a 300 hurts 300 megahertz and Amara spectrum it corresponds to 500 hertz on a 500 megahertz spectrum and conversely since coupling constant is independent in frequency in we'll get to that later on and verses is of of the applied magnetic field that triplet of say a methyl group in ethanol is going to look tighter it's going to look more closely together on the 500 megahertz and Amara spectrometer because that trip would is still going to be 7 plus 7 is 14 hurts wide but 14 hurts wide on a 300 megahertz spectrum 14 370 ppm where's 14 hurts wide on a 500 megahertz spectrometer is 14 500 ppm so instead here stand to 107 ppm and if I'm doing the math right my my head and here will spend a little less than 2 one-hundredths of a ppm said the peaks will be tighter and more dispersed and fields spectrum all right chemical shift depends on the electronic environment that the protons or and this was what the physicists were so upset white they gave it is contemptuous if you have an element that polls electron density away from the proton so for example so the sulfur is a little bit electron withdrawing it's a little bit Elektra negative relative departments so you poll electron density then the hydrogen is which are shielded by the electron cloud around them the electrons oppose the applied magnetic field have less electron density and so the feels stronger magnetic field and hence resonated a higher frequency to TMS the silicon so that electron donating it shows up upfield lower frequency here in methane all it shows up downfield at higher frequencies and there really is a nice relationship you can see this in the case of the halogen itself I take methyl iodide it shows up the methyl group obviously at 2 . 1 0 ppm if I take methyl bromide if the 2 . 7 0 ppm if I take methyl chloride it said 3 . 0 5 ppm and ppm here and if I take methyl fluoride it set forth .period at 3 0 ppm and if you look at the the Electra negativity the polling Elektra negativity of agenda Of course as you go down the periodic table you become less Elektra negative and so by the time you could you start you start with flooring the Electra negativity is 4 . 0 the Electra negativity of chlorine is 3 . 0 that of bromine is 2 . 8 and added by his 2 . 4 so you can almost see here it is almost a direct proportionality early near relationship the Moritz polling electrons away from the carbon the more you're going ahead and the shield some more electric "quotation mark elected negative substituent is more electron brought withdrawing yeah but more of the shielded now what's
cool and what's significant is that these effects really end up being reasonably added insult to see if you can spot the trend and make some predictions in your head we start with methane and the chemical shift by the way Delta is a term that often used to mean chemical shift in ppm it was an older scale house that was used in the sixties the 2 scales were competing in opposite Delta was started 0 4 TN masts and by the time you got to like in all the you'd be attacked the town scale it was completely reversed started at 10 40 AM Mass by the time we got to like Canal the Hyde CHU 0 ends in fact yeah I don't talk about this anymore recently as former student from a suspect class came to my office with a paper for his research and was asking me about the scale it's like Wow I haven't seen that a long time you call that the 19 sixties anyway Delta ppm 0 . 2 3 4 methane if we just look at the chlorinated hydrocarbons chlorinated methane and we have 1 chlorine we already saw 3 . 0 fine so in other words we shift down to 0 and then some ppm go to die Koroma's fame In it shouldn't surprise you that you go on a belt another 2 ppm and you're running out of electron densities you don't pull away quite as much with the 2nd but again you jump from about 3 . 0 5 to 5 . 3 2 sets another 2 women some ppm you've got a coffin were score for sure 7 . 2 7 a 7 . 2 sacks right in the middle and now again you go about 2 more years and so you can start to use these ideas in your head to say Oh I can have a reference value for 1 peak and then perturbed and just as I was saying what I always spectroscopy it's worth having a base of knowledge in Europe there is a huge amount of information in Silverstein's there's a huge amount of information Ian on in practice but just like you have a vocabulary and sometimes you go to the dictionary you have a vocabulary of Ohio are you have the vocabulary of animals so let me give you the way I think about my are about Anna Morris from France so this sort of reference frame I keep in my head and I can do a hell of a lot with the numbers that I'm going to give you in just the next year so the number I like to keep in my mind for sort of plain-vanilla Mathilde group is .period million people that's why I said when I use 1 of the 1st example of is an oversimplification .period curling ppm is a methyl group that's not near any electron withdrawing or electron donating methyl group than any of its a plain-vanilla methylene group did not hear any electron withdrawing or any electron donating group of about 1 . 3 to 1 . 5 ppm unless the buying group again not near anything in particular about 1 . 5 to 2 . 0 3 so in other words the difference between a methanol and a methylene groups let's call at about . 4 ppm the difference between a methylene and group let's call it about . 5 people why is methane solo so it's a very electron rich environment part of the reason you wind up the shielding here is that the steric crowding is actually pushing electron density away from carbon because you'd say Oh I would think of let's say you take ice of butane you'd say I always heard than a methyl group is electron donated so why is the Mefin at why is the Mefin advice of butane actually shifted downfield and 1 way to think of it is that the electrons are basically pushing into each other and pushing away here so methane methane and we're going to talk about how you rigorously calculate what's called on called impure collaborative body relationships and most of the empirical added tivity relationships use methane as the starting point they use . 2 3 as the starting point where I because we don't normally take spectra of methane by reference frame In my mind's eye I really becomes the story values and you can build a hell of a lot from that and that's what I'm going to show you how far I saw a little knowledge may be a dangerous thing but a little knowledge is also a very valuable so we already have a little knowledge that quarrel Maffei said 3 . 0 5 PM near let's consider the methylene group In chloral effort so where do you expect the methylene group to show up 3 .period OK how do you get 3 . 0 9 the problem all of them is about they had from during rush by show yeah so add .period speaking of the we were relieved .period 4 5 OK don't worry I screw up simple arithmetic on my feet all time and you would be .period close to write it's actually 3 . 4 per cent it's a little knowledge is not a dangerous thing let's take take chloride let's try that same logic with that 3 . 9 and the actual is 4 . 1 4 and guess what it's good enough for reading the spectrum because now you look at a peak and you say all that takes about 4 .
0 ppm that's probably not a methyl group Lexis Nexis something electron withdrawing it's probably something that were already further down the field I want to give you a couple of other basic values and then we'll have some fun with them so all of these examples that we're looking at our 2 an electron withdrawing group and we can see that in general alpha means on the carbon directly attached and we can see that being Alford to an electron withdrawing group shifts you have 2 or 3 ppm downfield with respect to the tastes value so for example I things I'll keep in mind I like to keep it light a wire keep in mind but I happened to keep could say I'm going to keep mouthfuls In I happened to keep methyl in mind because I see a lot of methylene next to an oxygen so methylene in ether group is approximately 3 . 6 ppm that kind of makes sense right oxygen is a little more electron withdrawing and glorious a little further down the honestly if you send 3 and a half nobody would fall but from that then you can go ahead and say Oh if it were methyl group will be closer to 3 parts per million 83 . 2 parts per million if it were a must find group we'll be a little further down the field we'd be maybe at 4 . 1 before the and so did you have a baseline of knowledge it shouldn't surprise you that if you have more electron withdrawing its going to shift you even further down field and so if you have a methylene next to an Astor you go a little bit further down field and I don't know maybe because I've seen far too many samples of mine with a little bit about philosophy Tate left over in after running a column I'll always think of methylene next to Ernesta group is being a little further down the yield of 4 . 1 all right so this is Alford to an electron withdraw their we all know about the inductive effect and so if you have a big enough To an electron withdrawing group you would expect that have some effect but not nearly to be as big as Alpha to an electron withdrawing group another words if we have Texas saying h the inductive effective your electron withdrawing group acts is going the poll electron density away from the help for carbon from hydrogen and that in turn is going to pull away from the data carbon and hydrogen on it and we're going to see a smaller effect sold what I hear from mines is about 0 . 2 to 0 . 5 ppm more downfield in other words than say resting values you would have debated the original value and in general what I mean by electron withdrawing groups something pretty generous here here legitimate oxygen like saying nitrogen anything that's Elektra negative also a carbon deal ,comma maybe a little bit will be at last to even things like abandoned so that's that's worth keeping in mind OK so what does that tell us if you take a molecule like ethanol and forget about the OH right now what would you expect the CH 2 an ethanol around 3 . 6 and what would you expect for the CH for the 1 . 3 1 .period 4 somewhere somewhere around In other words in other words you would expect since normal plain-vanilla Mathilde would be it .period 9 and you have an electron drawing graffiti it's going to be all over the 1 . 2 1 . 3 1 .period force I well a piece of information I keep because I like to to keep it handy in mind and but I don't know why here Europe I always like to keep a methyl group again maybe it's the ethyl acetate problem maybe it's the fact that I'm used to seeing people :colon methyl group next to a carbon typically abound 2 people if you wanna get fussy you can go ahead and sell it's closer to 2 . 1 ppm but again for keeping numbers in your head I've just thrown out of a very small amount of data to yield that you can do a hell of a lot and so put 2 in your head you can go ahead and file to things that you also have will talk more about this in a moment but if you want to you can also talk about a methyl group that has been seen as about 2 pp also work for any sort of beans alike often offer cycle and it'll be a little bit of a cheap because it's really closer to 1 . 7 but if you fit all 3 of these into your head as 2 parts per million again you'll have that baseline knowledge again if you wanna prefer 1 . 7 if you've got a good memory for Anderlecht Mathilde take 1 . 7 all let's take a moment to see how little knowledge really is a very powerful so let us take the molecule Ethel turned away In let's apply let's apply the knowledge that we just talked about the information basically basically and the this blackboard and what I said before for those baseline values tell me take a moment to think about the chemical shift of each type of proton yeah the
it's a let's start with starboard that methylene were referred I don't know the 3 . 6 to 4 . 1 1 4 . 1 my reference reference value and again a few estimated 3 . 6 you wouldn't be doing badly on that electron withdrawing group and to enhance ppm more downfield in the 3 ppm more downfield in the reference value somewhere somewhere like that but if you happen to have that value I gave you forever at velocity had basically there's something of methylene next to for the next 1 oxygen faster than before .period this guy over here the methyl group 1 . 4 1 . 5 does matter To pushes it because it's so into Baidoa to an oxygen and so on so 1 . 1 . 4 reverses right of the . 9 plus point . 5 new . 5 somewhere around there .period . 4 1 . 3 what about this guy here 2 . 0 3 . 6 in the of the 2 that's 2 ppm as the value of 4 so this is 2 ppm and so free I guess my thinking on this is if we say I'm at Filene's another year four-tenths of a ppm they are surprised to . 4 on the whole often these 2 1st 2nd this methyl groups when old this methylene group in this 1 of only 1 5 1 6 other votes on this 1 . 8 0 OK what we're going to see a 2nd so we have or what about this methylene group 1 . 3 other boats 1 1 . 4 year figuring maybe down maybe Will right let's let's pull the spectrum and say so 1 of the things you'll find is the Sigma-Aldrich library the Sigma-Aldrich Catalog www SIA l . com has lots and lots of animal respected and I will poll lots of them for the cost I think we need to send since some of those over here so you can actually look at real spectra test test your knowledge of things and you can find cool example are so this is the spectrum we have peaked at 4 . 1 side staring into the lights serves welded here years 1 looks like but why I can't see him absolutely blind here 2 . 3 1 . 6 1 challenge for 1 . 3 In . and so we can we can really calibrate ourselves that 4 . 1 value instead of 1 but that's basically what I told you it would be the point 9 is dead on the EU methyl here is at 1 . 3 so that's right about where we expected to see at the methylene that's Alford to the carbon was at 2 . 3 so that standpoint to 0 . 5 ppm downfield that reference value of 2 ppm the methylene that's Bader was 1 . 6 0 it's about . 2 . 3 ppm downfield about reference value of 1 . 4 1 . 3 to 1 . 5 and the next methylene is at about 1 . 4 the gamma naphthalene so right within the range were not returned a whole heck of a lot from beaches perturbed by the gamma just step back here all 4 1 and all right 1 of the reasons why I wanted to do this is there is no replacement for being able to In 2 ways to being able to read the spectrum and be able to know where different things come on and having that knowledge will take you far there are many ways of calculating the were precisely chemical shifts patch gives in great detail and just beautiful procedures and their offer for calculating chemical shifts that involve Alpha effects demo beta effects damn effects adding everything up and coming up with good values generally the best of these will take you within on the average for a molecule within about 3 to 5 ppm another were so that's carbon for Proton within a few tense of a ppm on the average 10 draw does this extremely well for any of you who have fancy versions of 10 draw I can draw headed for the
non fancy version doesn't have this you all have available for free Kevin Durant and it's a licensed version from the department there they have implemented many of the features of Kevin broad they have their own additive tivity procedures that are very similar to the estimations that
we're doing here I don't like there's as much they have a few what factors I didn't introduce this in the course last year because there were enough errors in the program and in fact today's
example I was sufficiently botched that that that they actually got it got it wrong and I've been in communication with the company but will take take the same example of Ethel contain away so you all have this available in your your own toolbox so here's a rather disappointing drawing of Ethel 10 away In the fighting you can drag this why wholehearted today I can drag this right over here so that is a simulator that's doing essential this is 1 of those so it does have chloroform and they're they're a bunch of silly silly settings on this thing so for example it basically member high Center the multiplied get narrower at higher frequency because the ppm is is because of ppm is more hurts so let's take a look so here's the thing if you look at this so I can click on those hydrogen and its estimation procedure is a little bit different it says we're going to use to forum methylene and then we're going to add 1 ppm for being next to a carbonyl is another correction it comes up with 2 . 3 and if we Click on I think is a all you have to do it done for each of us if I do do this 1 needs estimating it at 1 . 5 and of course you don't have that after click on it you can just highlighted this is 1 . 3 this is . 9 this is to 4 . 0 1 per cent this is 1 . 4 so in is essentially doing exactly the same thing that we've done in the same for the C 13 and shifts for example ,comma that's next to the oxygen that's a handy tool as is as his press which I want to show you 1 more way love doing
estimates and another way of another way of doing estimates is based on Friday so I want to show you this molecule and there's also another point that will come out of this so let's take this 3 methyl to Pentre known as an example and also from French and I just photocopied this just to help show you the patch is great for a bunch of things were going to get 2 molecules like pureed deans and paroles and fire fiends and they're really nice tables of coupling constants in there where they have J values and that's going to be relevant as you start to attack some of the homework problems that have Puritans and 5 themes in them so there's something really nice nice reference tables in there alright it's I want to show you consent send mine over so this is just somebody having having tabulated different types of molecules and you can say OK let's look at acetone and acetone is kind of like this Mathilde the tone let's love and yet tool and that's kind of like this part if we look out here and let's lot add ice apropos Mathilde the tone and that's kind of like this part so in other words you can go ahead and say alright we're going to go ahead and make our estimates based on this for this this for this and this protest and if you look at this table the 1st time this year that this is a page 162 when 163 from your scratched for the 1st time you see sailed soul confusing OK what is this if we have a Mathilde key tone With a methyl group on it so that's acetone we say to . 0 9 for the method so if you were trying to estimated saying 2 . 0 9 :colon 2 . 1 since nobody's going to estimate that exactly I if we have a Pro Bowl tone sewer Mathilde towns With a probable group on it now the Terminal C H 3 is at . 9 3 and the methylene here is at 1 . 5 6 and so you could say OK he will call this . 9 3 will just call . 9 and will call this 1 . 5 6 will just call this 1 . 6 and in notice these the same numbers that we were estimating based on that very limited data sets that I gave you and then if we continue across the table here we have other substituent so here we have our Mathilde key towns With an ISA probable group on it and so you say OK The must sign of an ISA probable group is 2 . 5 4 war so these are actual values taken from actual compounds tabulated by Real People sounds like a boring project In 1 . 0 8 and again these are the same principles we discussed methyl key tone is set to ppm methionine brings you down a little bit further we might have estimated 2 . 9 we find its 2 . 5 for methyl group that's data a town instead of being a . 9 it's a couple of tenths of a ppm downfield more 1 . 0 8 so again I'll just tabulate these numbers here will call that 1 . 1 2 . 5 more equal than 2 . 4 0 OK so now the question comes up How we don't and so we go for the real thing and again I've downloaded this from the www SIA L . com website also linked to Europe your costs materials I liked so let's see how we're doing I see peaked at 2 . 4 ppm a single added 2 . 1 and multiply at 1 . 7 a multiplied at 1 point for a double at at 1 . 1 and a triplet . 9 I just started the traffic that's easy the feeling pretty good there you go ahead you said what else what else is kind of easy we have this double-edged here at 1 . 1 that's exactly where we expect we have our 3rd methyl group here at 2 . 1 that's where we expect we're doing pretty well on Our miscellaneous to for the Watts what's happening here In the case of New World so it's not this to hide the bodies were place you do so they are OK so 1st of all I guess the question is is this a carol sing-along and that OK and this is this is 1 of the points of of why I put this out here so we have a Apparel Center In the molecule if you will have a Cairo center in the molecule every methylene groups Will the dire stereotype the 2 1 hydrogen is here are dire stereotyping their topologically difference it doesn't matter how fast you rotate in rotation about single bonds with vary very rare exception that I will tell you about is always fast at room temperature slow rotation is almost never be answered if you're dealing with with only single bonds being this is a question of topology and you would have no trouble seeing this affair on a To say all 1 proton is up 1 proton is down we have a Carroll center in the molecule of course we have 50 per cent of 1 50 per cent of the other what doesn't matter because in this molecule this hydrogen says I'm on the same side as the metal this 1 says opposite that's as simple way of as you said mentioning replacing 1 with the deuterium and saying all Veneman 1 barrister from another we're going to come work but the simple so this simple level of explanation I'm going to give right now is if you have a serious center in the molecule every methylene group is topologically dire stereotypes dire stereo topic protons are
not the same To put it more technical terms they are not chemically equivalent again we're going to come to this later sometimes they will be coincident which means they will shop at the same position and behave as if they're the same particularly if they're very far from mysterious center but topologically every methylene groups in a molecule on the matter how long that changes is dire stereotype topic every Isa poeple group if you put a nice apropos group in a molecule with Asturias center the 2 methyl groups are dire stereotactic they're not chemically equivalent they often show up different chemical shift and as we will see later and they support each other it's protons that are at the same exploited each other if you have 1 doesn't matter if you have a made deep ties and it is the Rasmei speaker lies this Proton here and this Proton here show up at the same chemical ship In this Proton here and this Proton here show up at the the same chemical shift because in 1 case 1 looks at the stereo sentences unempowered Proton and that's a nest area center so I have this relationship and then in the other molecules the other Proton says I'm a pro-West Proton and that serious center is an Austria center and so you have this same topological relationship of those opposite proponents of mysterious it was on 1 absolutely they could either be separated the and what we would call first-order on your 1st like this where they could be close to each other forming a bigger multiplex where they can be completely coincident and that's not visibly splitting and in general the further you are from the mysterious centered on less different environment they and so the more likely they are to fall in that category is not splitting each other not easily but a great great question and actually I mean the answer is the answer becomes guests by conformational analysis because what you need to consider becomes the 3 the 3 different road emerged and then the proximity of each of those 2 dire stereo topic protons to the carbon yield which is creating a magnetic and I saw her case to the answer becomes yes under special circumstances and in the case of making dire streamer derivatives like Moshe arrested the years 1 can do it in a systematic fashion and the acknowledged the group is doing this and systematic ways with other sorts of groups and again being able to do it in a systematic fashion means that you can then determine if you have a molecule and make the choral derivative you can determine the absolute still campus which is extremely important when you're developing new reaction I I wanna finish by adding to our little baseline of knowledge and gonna throw out some numbers so what I talked about before was the stuff that I really think is scored to figuring out so much let me throw out some others alcohols move around depending on hydrogen bonding let's say 1 to 5 ppm carboxyl against since I'm talking now about various protons on oxygen generally 10 to 13 ppm sometimes not seen due to exchange with water In chloroform if I wanna see my carboxyl like acids used the MSO working the sample dry Ryan may be more concentrated aromatic alcohols ARO H 4 Knowlson the light again about 7 ppm and these are all going to be approximate numbers aromatics in general I think everyone knows that aromatic protons appeared downfield so if you have sort of a R H meaning like an Eirena benzene Syafii in period Dean of benzene itself C 6 8 6 is at 7 . 3 here we're talking generally 7 to 8 but these ranges are loops electron withdrawing groups will bring you further electron donating groups will bring further down the electron donating groups will bring you up field I can show you aromatic protons that occur at below 6 4 ppm numbers I can show you aromatic protons that period at 9 ppm In the case of all of these you're getting magnetic and I saw the futures ring current and nice model for what's going on is a classical models if you apply a magnetic field to a solenoid solenoid generates and you can think of the pie electrons of benzene as a solenoid the solenoid generates a current In the ring of electrons that opposes the applied magnetic field that generates flux lines that go down and come around In point up over here so this Proton feels a stronger magnetic field Marseille feels stronger magnetic field and show it shows up down the field that same type of argument can be used were vinyl protons you can treat the pie electrons here as also being like a ring current generally were talking let's say generally 5 to 6 and again I can show you ones that lie outside that range in the case of an elder time where you have an electron withdrawing carving were talking maybe not only to the pedia the same principles here which I talked about really apply at a distance over here so all of these face is a little like bit out then select an elephant to carbon you'll go on hold further downfield than where you would expect a regular methyl group on benzene around a double bond were just know so I'm saying in other words a regular Rafael would be .period knowing we go about it keeps him further down field I like the 1 on ball in this whole equation and again you can draw ring current explanation for it is alkaline and I think that that's going to kind of RAF common protons and I want to give you 1 last summary during current you can think of is going like this in the case about Koreans which actually reinforces which actually opposes the applied magnetic field so I'll claims or about 2 . 5 foreign just as I like to be able to read an eye spectrum I like to be able to read in a March 4 and where I read Metamora spectra I generally walk from about 0 to about 10 ppm of course you may have things that are upfield of 0 you may have things that are down sales and I generally think of this region as Al behind this region of deliberately drawing this as very loose rangers because you can find aromatics that fall outside but this range here as aromatics this range here as means this range over here as next to an electron withdrawing group Alpha to an electron withdrawing troops nitrogen somewhat less downfield shift things sell more outfield this range here and Alford carbon Neal a like Indians alike and remember we're talking Metaline methylene Mathilde kind of all over here for my fellow lines kind of over here for methylene and kind of over here for Mattel so this is how I look at Minamata spectrum and try to try to read it fight next time we will pick up atop a little bit about carbon in Amara and then we're going to move on to discuss I spin spin coupling in and other factors that are involved I guess next time will will get both of those
Single electron transfer
Transdermales therapeutisches System
Single electron transfer
Single electron transfer
Transdermales therapeutisches System
|Titel||Lecture 9. Chemical Shift. 1H NMR Chemical Shifts.|
|Serientitel||Chemistry 203: Organic Spectroscopy|
|Anzahl der Teile||29|
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|Herausgeber||University of California Irvine (UCI)|
|Abstract||UCI Chem 203 Organic Spectroscopy (Fall 2011) Lec 09. Organic Spectroscopy -- Chemical Shift -- 1H NMR Chemical Shifts Instructor: James Nowick, Ph.D. Description: 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.|