NMR

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NMR

  1. 1. NMR = Nuclear Magnetic Resonance Physical Principles:Some (but not all) nuclei, such as 1H, 13C, 19F, 31P have nuclear spin.A spinning charge creates a magnetic moment, so these nuclei can bethought of as tiny magnets.If we place these nuclei in a magnetic field, they can line up with or againstthe field by spinning clockwise or counter clockwise. N N N - sp in state, S - spin state, favorab le, unfav orable, low er energy high er en erg y S N S SA spin ning n ucleus w ith its m agnetic field A spin ning n ucleus w ith its m agnetic fieldalign ed w ith the m agnetic field of a m ag net aligned a gain st the m agnetic field of a m ag netAlignment with the magnetic field (called ) is lower energy than against themagnetic field (called ). How much lower it is depends on the strength ofthe magnetic fieldNote that for nuclei that don’t have spin, such as 12C, there is no differencein energy between alignments in a magnetic field since they are notmagnets. As such, we can’t do NMR spectroscopy on 12C.
  2. 2. NMR: Basic Experimental PrinciplesImagine placing a molecule, for example, CH4, in a magnetic field.We can probe the energy difference of the - and - state of the protons byirradiating them with EM radiation of just the right energy.In a magnet of 7.05 Tesla, it takes EM radiation of about 300 MHz (radiowaves).So, if we bombard the molecule with 300 MHz radio waves, the protons willabsorb that energy and we can measure that absorbance.In a magnet of 11.75 Tesla, it takes EM radiation of about 500 MHz (strongermagnet means greater energy difference between the - and - state of the at no magnetic field, proton spin stateprotons) there is no difference beteen (higher energy) - and - states. Graphical relationship between magnetic field (B o) and frequency ( E E = h x 300 MHz E = h x 500 MHz 1 for H NMR absorptions proton spin state (lower energy) 0T 7.05 T 11.75 T BoBut there’s a problem. If two researchers want to compare their data usingmagnets of different strengths, they have to adjust for that difference. That’sa pain, so, data is instead reported using the “chemical shift” scale as
  3. 3. The Chemical Shift (Also Called ) ScaleHere’s how it works. We decide on a sample we’ll use to standardize ourinstruments. We take an NMR of that standard and measure its absorbancefrequency. We then measure the frequency of our sample and subtract itsfrequency from that of the standard. We then then divide by the frequencyof the standard. This gives a number called the “chemical shift,” also called , which does not depend on the magnetic field strength. Why not? Let’slook at two examples.Imagine that we have a magnet where our standard absorbs at 300,000,000Hz (300 megahertz), and our sample absorbs at 300,000,300 Hz. Thedifference is 300 Hz, so we take 300/300,000,000 = 1/1,000,000 and callthat 1 part per million (or 1 PPM). Now lets examine the same sample in astronger magnetic field where the reference comes at 500,000,000 Hz, or500 megahertz. The frequency of our sample will increase proportionally,and will come at 500,000,500 Hz. The difference is now 500 Hz, but wedivide by 500,000,000 (500/500,000,000 = 1/1,000,000, = 1 PPM).It’s brilliant.Of course, we don’t do any of this, it’s all done automatically by the NMRmachine.Even more brilliant.
  4. 4. The Chemical Shift of Different ProtonsNMR would not be very valuable if all protons absorbed at the samefrequency. You’d see a signal that indicates the presence of hydrogens inyour sample, but any fool knows there’s hydrogen in organic molecules.What makes it useful is that different protons usually appear at differentchemical shifts ( . So, we can distinguish one kind of proton from another.Why do different protons appear at different ? There are several reasons,one of which is shielding. The electrons in a bond shield the nuclei from themagnetic field. So, if there is more electron density around a proton, it seesa slightly lower magnetic field, less electron density means it sees a highermagnetic field: This represents the electron density of a C-H bond. How much electronZ density is on the proton depends on what else is attached to the carbon. If Z C H is an elelctronegative atom, the carbon becomes electron deficient and pulls some of the electron density away from the H. if Z is an electron donating group, more electron density ends up on the H.How do the electrons shield the magnetic field? By moving. A movingcharge creates a magnetic field, and the field created by the movingelectrons opposes the magnetic field of our NMR machine. It’s not a hugeeffect, but it’s enough to enable us to distinguish between different protonsin our sample.
  5. 5. The Hard Part - Interpreting SpectraLearning how an NMR machine works is straightforward. What is lessstraightforward is learning how to use the data we get from an NMR machine(the spectrum of ethyl acetate is shown below). That’s because each NMRspectrum is a puzzle, and there’s no single fact that you simply have tomemorize to solve these spectra. You have to consider lots of pieces of dataand come up with a structure that fits all the data. What kinds of data do weget from NMR spectra? For 1H NMR, there are three kinds each of which wewill consider each of these separately:what kinds of protons we have. 1) Chemical shift data - tells us 2) Integrals - tells us the ratio of each kind of proton in our sample. 3) 1H - 1H coupling - tells us about protons that are near other protons.
  6. 6. Chemical Shift DataAs previously mentioned, different kinds of protons typically come at differentchemical shifts. Shown below is a chart of where some common kinds ofprotons appear in the scale. Note that most protons appear between 0 and10 ppm. The reference, tetramethylsilane (TMS) appears at 0 ppm, andaldehydes appear near 10 ppm. There is a page in your lab handout withmore precise values for this chart.Note that these are typical values and that there are lots of exceptions! R NH R OH Ph R H Me OH (R) TMS = Me Si Me HO C H3 P h C H3 Me R R R O R R O N R2 O Cl H H OC H 3 C H3 C H3 C H3 R H C H3 TM S 10 9 8 7 6 5 4 3 2 1 0 Downfield region ppm Upfield region of the spectrum of the spectrum
  7. 7. IntegralsIntegrals tell us the ratio of each kind of proton. They are lines, the heightsof which are proportional to the intensity of the signal. Consider ethylacetate. There are three kinds of protons in this molecule, the CH3 next tothe carbonyl, the CH2 next to the O and the CH3 next to the CH2. The ratio ofthe signals arising from each of these kinds of protons should be 3 to 2 to 3,respectively. So, if we look at the height of the integrals they should be 3 to2 to 3. With this information, we can know which is the CH2 signal (it’s thesmallest one), but to distinguish the other two, we have to be able to predicttheir chemical shifts. The chart on the previous page allows us to make thatassignment (the CH3 next to the C=O should appear at ~ 2 PPM, while theother CH3 should be at ~ 1 PPM). 3HS O O CH3 O O H H 3C O O H 3HS 2 HS
  8. 8. - 1H Coupling 1HYou’ll notice in the spectra that we’ve seen that the signals don’t appear assingle lines, sometimes they appear as multiple lines. This is due to 1H - 1Hcoupling (also called spin-spin splitting or J-coupling). Here’s how it works:Imagine we have a molecule which contains a proton (let’s call it HA)attached to a carbon, and that this carbon is attached to another carbonwhich also contains a proton (let’s call it HB). It turns out that HA feels thepresence of HB. Recall that these protons are tiny little magnets, that can beoriented either with or against the magnetic field of the NMR machine. Whenthe field created by HB reinforces the magnetic field of the NMR machine (B0) HA feels a slightly stronger field, but when the field created by HB opposesB0, HA feels a slightly weaker field. So, we see two signals for HA dependingon the alignment of HB. The same is true for HB, it can feel either a slightlystronger or weaker field due to HA’s presence. So, rather than see a singleline for each of these protons, we see two lines for each. For this line, H B is lined up For this line, H B is lined up with the magnetic field aga inst the magnetic field ( adds to the overall (subtracts from the overall magnetic field, so the line magnetic field, so the line comes at higher frequency) comes at lower frequency) HA HB C C HA HB H A is split into two lines because H B is split into two lines because it feels the m agnetic field of H B. it feels the m agnetic field of H A.
  9. 9. More 1H - 1H CouplingWhat happens when there is more than one proton splitting a neighboringproton? We get more lines. Consider the molecule below where we havetwo protons on one carbon and one proton on another. Note that the signal produced by H A + H A is twice the size of that produced by H B HA HB HA C C H A + H A HB H A and H A appear at the same H B is split into three lines chemical shift because they are because it feels the magnetic in identical environments field of H A and H A They are also split into two lines (called a doublet) because they feel the magnetic field of H B.
  10. 10. Why are There Three Lines for HB?H B feels the splitting of both HA and HA’. So, let’s imagine startingwith HB as a single line, then let’s “turn on” the coupling from HA and HA’ oneat a time: HB If uncoupled, H B would appear as a singlet where the dashed line indicates the chemic al shift of the sin glet. Now, lets "turn on" H B - H A coupling. This splits the single line into two lin es HA HB Now, lets "turn on" H B - H A coupling. This HA C C splits each of the two new lines into two lines, but notice how the two lines in the middle overlap. Overall, we then have three lines.Because the two lines in the middle overlap, that line is twice as big as thelines on the outside. More neighboring protons leads to more lines as shownon the next slide.
  11. 11. Splitting Patterns with Multiple Neighboring ProtonsIf a proton has n neighboring protons that are equivalent, that proton will besplit into n+1 lines. So, if we have four equivalent neighbors, we will havefive lines, six equivalent neighbors… well, you can do the math. The lineswill not be of equal intensity, rather their intensity will be given by Pascal’striangle as shown below. intensities no. of neighbors relative pattern example 0 1 singlet (s) H H 1 1 1 doublet (d) C C H H 2 1 2 1 triplet (t) C C H H H 3 1 3 3 1 quartet (q) C C H H H H H 4 1 4 6 4 1 pentet C C C H H H H H 5 1 5 10 10 5 1 sextet H C C C H H H H H 6 1 6 15 20 15 6 1 septet H C C C H H HWe keep emphasizing that this pattern only holds for when the neighboringprotons are equivalent. Why is that? The answer is two slides away.
  12. 12. More About CouplingEarlier we said that protons couple to each other because they feel themagnetic field of the neighboring protons. While this is true, themechanism by which they feel this field is complicated and is beyond thescope of this class (they don’t just feel it through space, it’s transmittedthrough the electrons in the bonds). It turns out that when two protonsappear at the same chemical shift, they do not split each other. So, inEtBr, we have a CH3 next to a CH2, and each proton of the CH3 group isonly coupled to the protons of the CH2 group, not the other CH3 protonsbecause bluethe CH3 protons come at the same chemical shift. come The all protons all come H H The red protons both at the same chemical shift H C C Br at the same chemical shift and do not split each other and do not split each other H H H H H H H C C Br H C C Br H H H H
  13. 13. Not all Couplings are EqualWhen protons couple to each other, they do so with a certain intensity. Thisis called the “coupling constant.” Coupling constants can vary from 0 Hz(which means that the protons are not coupled, even though they areneighbors) to 16 Hz. Typically, they are around 7 Hz, but many moleculescontain coupling constants that vary significantly from that. So, whathappens when a molecule contains a proton which is coupled to twodifferent protons with different coupling constants? We get a differentpattern as described in the diagram below.So, if the protons are not equivalent, they can have different couplingconstants and the resulting pattern will not be a triplet, but a “doublet ofdoublets.” Sometimes, nonequivalent protons can be on the same carbon
  14. 14. Coupling Constants in AlkenesCoupling constants in alkenes can also differ depending on whether theprotons are cis or trans to each other. Note that in a terminal alkene (i.e., analkene at the end of a carbon chain), the cis and trans protons are NOTequivalent. One is on the same side as the substituent, the other is on theopposite side. The coupling of trans protons to each other is typically verylarge, around 16 Hz, while the coupling of cis protons, while still large, is alittle smaller, around 12 Hz. This leads to the pattern shown below, and anexample of a molecule with this splitting pattern is shown on the next slide. H If uncoupled, H would appear as a A A single t where the dashed lin e indicates12Hz coupling 16 Hz Now, lets "turn on" H A - H X coupling. This splits HA the single line into two lines that are 16 Hz appart HM 12 Hz 12 Hz Now, lets "turn on" H A - H M coupling. This splits each of the two new lines in to two lines 16 Hz coupling that are 12 Hz appart for a total of four lines HXThere are other times when protons on the same carbon are nonequivalent,which we’ll see later.
  15. 15. H HO HO CH3A molecule with a terminal alkene H H HO HO CH3 H H H HO H H H HO HO H HA molecule with a nine line splitting pattern Me OH OH Me Nine lines, you just cant Me see two of them because OH they are so small. Me H H Me Me OH H Me OH Me
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