1559T_ch10_175-19810/30/0518:09Pa9e175 ⊕ EQA 10 Using Nuclear Magnetic Resonance Spectroscopy to Deduce Structure In this chapter we address the question that faces anyone trying to identify the molecular structure of a stance.nan what ns to answer the bvious qucstio really know that all those molecules are what we say they are?In the"oen days."tedious indirect iden magnetic resonance (NMR).is described in this chapter. Outline of the Chapter 1o28egpaee rsomething different. 10-3 Hydrogen Nuclear Magnetic Resonance An example of fairly simple physics put to very good use.A general overview of NMR spectroscopy. 10-4 The Hydrogen Chemical Shift The first two pieces of information available from an NMR spectrum 10-5 Chemical Shift Equivalence How to tell when you have it. e of information available from an NMR spectrum 10-7 Spin-Spin Splitting on that gives rise to the fourth and,perhaps,most useful kind of information present in 10-8 Spin-Spin Splitting:Complications e complications get a bit more complicate 175
10 Using Nuclear Magnetic Resonance Spectroscopy to Deduce Structure In this chapter we address the question that faces anyone trying to identify the molecular structure of a substance, namely, “What is it?” To put it another way, this chapter begins to answer the obvious question that you may already have been thinking after the first nine chapters of this book, namely, “How does anyone really know that all those molecules are what we say they are?” In the “olden days,” tedious indirect identification methods had to be used, and some are described in the text. Nowadays these questions are answered through the use of spectroscopy, a technique that serves as the “eyes” of an organic chemist with respect to the structures of molecules. The most important and widely used type of spectroscopy, nuclear magnetic resonance (NMR), is described in this chapter. Outline of the Chapter 10-1 Physical and Chemical Tests The “classical” (translate as “tedious”) methods of structure elucidation. 10-2 Defining Spectroscopy Or, now for something just a little bit different. 10-3 Hydrogen Nuclear Magnetic Resonance An example of fairly simple physics put to very good use. A general overview of NMR spectroscopy. 10-4 The Hydrogen Chemical Shift The first two pieces of information available from an NMR spectrum. 10-5 Chemical Shift Equivalence How to tell when you have it. 10-6 Integration The third piece of information available from an NMR spectrum. 10-7 Spin–Spin Splitting A complication that gives rise to the fourth and, perhaps, most useful kind of information present in an NMR spectrum. 10-8 Spin–Spin Splitting: Complications The complications get a bit more complicated. 175 1559T_ch10_175-198 10/30/05 18:09 Page 175
15597.ch10175-19810/30/0518:09Pag0176 176.chapter 10 USING NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY TO DEDUCE STRUCTURE 1o9atbam1aNdearMa2na8anaoR Keys to the Chapter 10-2.Defining Spectroscopy yis mainly physics.In spi e of that.the material in this section is really very basic and quite copy dete cts the absorption of energy by molecules.The stake e that th in terms of structural features of an unknow chapter de scribes th e physical phenom ciated with nuclear magnetic re nance(NMR) n"Don't he fri a molecule from scopic data in terms of molecular structureis actually one of the easiest skills toacqure in this couses almost fun 10-3.Hydogen Nuclear Magnetic Reson ance 如 r magnet comp 盟加a ss,the nuclear magnet docs have field. state.For nuclear magnets,this is commonly described as a spin flip (a spin state to B spin state for a protor the nucle ogen aton mount of cn pends on the ance ene abso rtions by magnetic nucei at certain values of magnetic field strent and (in the form of radio waves)that constitutes the physical basis for nuclear magnetic resonance spectroscopy. 10-4.The Hydrogen Chemical Shift The The f our important pieces of stn environments display separate e res nce lines in a high resolutionH NMRs ectrun the number nce sig nals in the.on 、sth mber o s of hydr nal is characteristic of certain kinds of chemical environn ts,e.git can imply pro imity to The phenom on r fromss strongly shielded (deshilded)hydrogens.whereas signals to the right are representative of more
10-9 Carbon-13 Nuclear Magnetic Resonance The utility of another magnetic nucleus in NMR. Keys to the Chapter 10-2. Defining Spectroscopy Spectroscopy is mainly physics. In spite of that, the material in this section is really very basic and quite understandable: Spectroscopy detects the absorption of energy by molecules. The stake that the organic chemist has in this is also simple. Determining the structure of a molecule requires that the chemist be able to interpret spectroscopically observed energy absorptions in terms of structural features of an unknown molecule. This chapter describes the physical phenomena associated with nuclear magnetic resonance (NMR). It then describes their logical implications as applied to identifying structural features in a molecule from a nuclear magnetic resonance “spectrum.” Don’t be frightened. The end result—the ability to interpret spectroscopic data in terms of molecular structure—is actually one of the easiest skills to acquire in this course. It’s almost fun! 10-3. Hydrogen Nuclear Magnetic Resonance In the upcoming text sections the physical basis for the utility of nuclear magnetic resonance in organic chemistry is presented. This text section describes a couple of concepts that are likely to be unfamiliar to you. The first is the idea that a magnet can align with an external magnetic field in more than one way. Most of us are familiar with bar magnet compasses, which orient themselves along the Earth’s magnetic field in one direction only. Nuclear magnets are actually similar, except for the fact that the reorientation energies are so small that energy quantization becomes significant. Like the compass, the nuclear magnet does have one energetically preferred alignment (the spin for a proton). Less favored alignments (spin states) are very close in energy to the preferred one on a nuclear scale, however. So, unlike the macroscopic compass, the microscopic nuclei can commonly be observed in less favorable, higher energy orientations in a magnetic field. The second new idea is that of resonance. This is the term that describes the absorption of exactly the correct amount of quantized energy to cause a species in a lower energy state to move to a higher energy state. For nuclear magnets, this is commonly described as a spin flip ( spin state to spin state for a proton, the nucleus of the hydrogen atom). The amount of energy involved depends on the identity of the nucleus and the size of the external magnet. The text section describes these relationships in detail. It is the observation of resonance energy absorptions by magnetic nuclei at certain values of magnetic field strength and energy input (in the form of radio waves) that constitutes the physical basis for nuclear magnetic resonance spectroscopy. 10-4. The Hydrogen Chemical Shift The normal NMR spectrum can provide four important pieces of structural information about an unknown molecule. The first two pieces of information are derived from the fact that hydrogens in different chemical environments display separate resonance lines in a high resolution 1 H NMR spectrum. Thus, first, by counting the number of resonance signals in the spectrum, one knows the number of sets of hydrogen atoms in different chemical environments contained in the molecule. Second, the actual position of each resonance signal is characteristic of certain kinds of chemical environments, e.g., it can imply proximity to a specific type of functional group, or attachment to a certain type of atom. The phenomenon responsible for this is the shielding of a magnetic nucleus under observation by the nearby electrons in the molecule. How this comes about physically is described in detail. On a typical NMR spectrum, resonance signals to the left of the chart result from less strongly shielded (deshielded) hydrogens, whereas signals to the right are representative of more 176 • Chapter 10 USING NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY TO DEDUCE STRUCTURE 1559T_ch10_175-198 10/30/05 18:09 Page 176
1559T_ch10_175-19810/30/0518:09Pa9e177 EQA Keys to the Chopter·177 etic fields for Deshiclded hydroge Aod 沈 0 Chemical shift den right to left,with the usual hydrogen spectrum covering a range from to 10 ppm.Table 10-2 shows typical General regions of the nur spectrum E D B 00g5 10 Chemical shif(ppm) Region Chemical shint Hydrogen type 0-15 ppm -type hydrogen 1.5-3.0pm to carbon-containing 3.045ppm hydrogenson cabons attached toelectroegive atoms .0 ppn alkene-type hydroge四 nzene-type hydrogen -10.0Ppm ydrogens of aldehyde group NMR of this 10-5.Chemical Shift Equivalence
strongly shielded ones. Deshielded hydrogens require lower external magnetic fields for resonance, whereas shielded ones require higher fields. So we have NMR spectra that have the following qualitative relationships: As described in the text, a resonance signal’s position is measured as a field-independent chemical shift, which has units of parts per million (ppm) of the total applied field, often called units. These read from right to left, with the usual hydrogen spectrum covering a range from 0 to 10 ppm. Table 10-2 shows typical chemical shifts for common types of hydrogens. There is a lot here, but for most purposes all you really need to know are the types of hydrogens that resonate in several general regions of the NMR spectrum. Region Chemical shift Hydrogen type A 0–1.5 ppm alkane-type hydrogens B 1.5–3.0 ppm hydrogens on carbons next to carbon-containing functional groups C 3.0–4.5 ppm hydrogens on carbons attached to electronegative atoms D 4.5–6.0 ppm alkene-type hydrogens E 6.0–9.5 ppm benzene-type hydrogens F 9.5–10.0 ppm hydrogens of aldehyde group With this as a basis, you are ready to start interpreting NMR spectra. For the initial problems of this type, simply count the number of signals in the spectrum and note the position of each one. Then see if you can come up with a structure that displays the correct number of signals in approximately the observed places, using the material in the text section. If you can, you have probably picked a sensible structure for the unknown molecule. 10-5. Chemical Shift Equivalence This section presents detailed procedures for determining which hydrogens in a molecule have identical chemical shifts due to chemical equivalence. The simplest examples of this are the four hydrogens of methane or Deshielded hydrogens Shielded hydrogens Keys to the Chapter • 177 1559T_ch10_175-198 10/30/05 18:09 Page 177
1559rch10175-19910/30/0518:09Pag0178 178.chapter 10 USING NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY TO DEDUCE STRUCTURE Inte piece of information available from NMR:the relative number or each sparate NMR signal.The is measured by the NME r of che for the correct interpretation of NMR spectra. 10-7.Spin-Spin Splitting ets,the NMR with or against the xtemal magnetic field,so their fields can add to or subtract from the field generated by the Ihe result is a slight change in the resonance ine position for the the theor presented in the text (althoug it is not really complicated at all),is secondary to und tanding g of the phenomenon in terms of molecular structure.Thus it is often sufficient to rely on two There are two important qualifications to rule 2: hydrogen whose signal is being considered (because of rule 1). A little careful observation of the text examples will help t used to the consequences of spin-spin spttingin its most common forms.Table 10-5 and Figures106and 2 nicely llustrate these sitations. 10-8. Spin-Spin Splitt ng:Co mplications trum for the other by a distance much greater than the coupling constant within each of their pattems (ie..) coupng constants values)as with any hydrogen must beid e,even fulfilled.the spectrum won't look exactly as you might expect.Fortunately.a lot of the time these conditions ar approached, to worry a whole lot about them. 10-9.Carbon-13 Nuclear Magnetic Resonance s an extension NMR opy that is nov widely used.There are two reasons.First.modem contain useful information that is very easy to interpret,especially under conditions of broad-band hydroger decoupling.which"wipes out"the spin-spin splitting by neighboring hydrogens.The results are spectra that carbo n or group of ch mically equivalent ca
the six hydrogens of ethane. There are some minor complications that might require a review of the material toward the end of Chapter 5, but on the whole these procedures are not difficult. 10-6. Integration Integration provides the third major piece of information available from NMR: the relative number of hydrogens responsible for each separate NMR signal. The integration is measured electronically by the NMR spectrometer and plotted directly on the spectrum. It tells you whether a given NMR signal is due to a single hydrogen or some number of chemical-shift-equivalent hydrogens in the molecule. The integration is critical for the correct interpretation of NMR spectra. 10-7. Spin–Spin Splitting Because many atomic nuclei are magnets, the NMR signal of a nucleus under observation can, in principle, be affected by the presence of other nearby magnetic nuclei. These neighboring nuclear magnets can align with or against the external magnetic field, so their fields can add to or subtract from the field generated by the NMR machine. The result is a slight change in the resonance line position for the nucleus under observation and is called spin–spin coupling or spin–spin splitting. Again, for spectral interpretation purposes, the theory, presented in the text (although it is not really complicated at all), is secondary to understanding the meaning of the phenomenon in terms of molecular structure. Thus it is often sufficient to rely on two simple rules: 1. Spin–spin splitting is not observed between chemical-shift-equivalent hydrogens. 2. The signal of a hydrogen with N neighboring hydrogens will be split into N 1 lines (“N 1 rule”). There are two important qualifications to rule 2: a. N 1 lines is a minimum. There may be more (Section 10-8). b. In determining N, you count only neighbors whose chemical shifts are different from that of the hydrogen whose signal is being considered (because of rule 1). A little careful observation of the text examples will help you get used to the consequences of spin–spin splitting in its most common forms. Table 10-5 and Figures 10-16, 21, and 22 nicely illustrate these situations. 10-8. Spin–Spin Splitting: Complications The splitting rules outlined in the previous section are idealized for two conditions that are only rarely met in any NMR spectrum. For these rules to hold exactly: First, all the signals must be separated from one another by a distance much greater than the coupling constant within each of their patterns (i.e., J ). Second, all the coupling constants (J values) associated with any hydrogen must be identical in size, even if that hydrogen is coupled to more than one group of neighboring hydrogens. If either of these conditions is not fulfilled, the spectrum won’t look exactly as you might expect. Fortunately, a lot of the time these conditions are approached, especially for hydrogens near electronegative atoms or functional groups. You will therefore have to be aware of the possible effects of “non-first-order” situations; but, for the most part, you won’t have to worry a whole lot about them. 10-9. Carbon-13 Nuclear Magnetic Resonance This is an extension of NMR spectroscopy that is now widely used. There are two reasons. First, modern NMR instruments make these spectra much easier to obtain than originally was the case. Second, the spectra contain useful information that is very easy to interpret, especially under conditions of broad-band hydrogen decoupling, which “wipes out” the spin–spin splitting by neighboring hydrogens. The results are spectra that contain only singlets, that is, a single line for each carbon or group of chemically equivalent carbons. Given such a spectrum, you can quickly determine whether or not it corresponds to a proposed structure simply by counting the lines in the spectrum. Of course, more extensive information is also available from the 13C 178 • Chapter 10 USING NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY TO DEDUCE STRUCTURE 1559T_ch10_175-198 10/30/05 18:09 Page 178
1559T_ch10_175-19810/30/0518:09Pa9e179 ⊕ EQA Solutions to Problems.179 Solutions to Problems 21.To do 一o装owwo论】 03x10 22.The conversion formulas are=1/and v=c/A(Section 10-2). (a)λ=11050cm=9.5×10-4cm=9.5m (b)510nm=5.1×10-5cm:v=(3×100 cm s-l(5.1×10-5cm)=5.9×104s (⊙)6.154m=6.15×104cm=1/6.15×10-4cm)=1.63×103cm-1 (dv=cm=(3×10cms2.25×103cm=6.75×103s1 (aA=1/750=1.33×10-3cm=1.33×10nm(1cm=10-2m,and1nm=10-9m,or 1cm=10nm.s0△E=(2.86×101.33×10=2.15 kcal mol (e)=350 nm (given).so AE=(2.86x 10)/350 =82 keal mol- (d)A=3×1078.8×10=3.4×102cm=3.4×10°m,s0△E=(2.86×103.4×10%= 8.4×10-6 kcal mol (e)λ=7×10-2nm,s0△E=(2.86×10y7×10-=4.1×10 kcal mol- 24.Only the value ofvis needed to calculate AE.Use AE=28.600/A.together with=c/v. 3X0298×103m=3310m0E=6×10y (a)A=(6×100g (b)AE=4.76 x 10-5 keal mol-1 25.(a℉ H6=21,150G 37c1 84.6 22.68.827.340←MH (b)Like (a),but with an additional signal at 90 MHz (H). (e)This will show all the signals present in both (a)and (b).In addition,signals forBr andBr will be present (at 22.5 and 24.3 MHz
spectrum if desired, in the form of the carbon chemical shifts (Table 10-6), the proton splittings of the “undecoupled” spectrum (e.g., Figure 10-30), or the “DEPT” spectra (Figure 10-33). Solutions to Problems 21. To do this, you need to tell the difference between frequencies, , in units of s1 , and wavenumbers, ˜, in units of cm1 . Section 10-2 shows how they are related: c/ and ˜ 1/ , so c˜, or ˜ /c. For AM radio ( 106 s 1 ), ˜ 106 /(3 1010) ≈ 3 105 cm1 ; and for FM and TV ( 108 s 1 ), ˜ 108 /(3 1010) ≈ 3 103 cm1 . All these are well to the right end of the chart, very low in energy relative to most of the forms of electromagnetic radiation on the chart. 22. The conversion formulas are 1/˜ and c/ (Section 10-2). (a) 1/(1050 cm1 ) 9.5 104 cm 9.5 m (b) 510 nm 5.1 105 cm; (3 1010 cm s1 )/(5.1 105 cm) 5.9 1014 s 1 (c) 6.15 m 6.15 104 cm; ˜ 1/(6.15 104 cm) 1.63 103 cm1 (d) c˜ (3 1010 cm s1 )(2.25 103 cm1 ) 6.75 1013 s 1 23. Use E 28,600/ (Section 10-2), and use the equations 1/˜ and c/. Be sure to convert the units of to nm before calculating E, though! (a) 1/750 1.33 103 cm 1.33 104 nm (1 cm 102 m, and 1 nm 109 m, or 1 cm 107 nm), so E (2.86 104 )/(1.33 104 ) 2.15 kcal mol1 (b) 1/2900 3.45 104 cm 3.45 103 nm, so E (2.86 104 )/(3.45 103 ) 8.29 kcal mol1 (c) 350 nm (given), so E (2.86 104 )/350 82 kcal mol1 (d) 3 1010/(8.8 107 ) 3.4 102 cm 3.4 109 nm, so E (2.86 104 )/(3.4 109 ) 8.4 106 kcal mol1 (e) 7 102 nm, so E (2.86 104 )/(7 102 ) 4.1 105 kcal mol1 24. Only the value of is needed to calculate E. Use E 28,600/ , together with c/. (a) (3 1010 cm s1 )/(9 107 s 1 ) 333 cm 3.33 109 nm, so E (2.86 104 )/ (3.33 109 ) 8.59 106 kcal mol1 (b) E 4.76 105 kcal mol1 25. (a) (b) Like (a), but with an additional signal at 90 MHz (1 H). (c) This will show all the signals present in both (a) and (b). In addition, signals for 79Br and 81Br will be present (at 22.5 and 24.3 MHz, respectively). At 84,600 gauss the positions of all lines will be at frequencies 4 greater than at 21,150 gauss. For example, a 1 H signal will be at 360 MHz. CFCl3 84.6 19F Solutions to Problems • 179 1559T_ch10_175-198 10/30/05 18:09 Page 179