NMR Spectroscopy. Chapter 19

Transcription

1 NMR Spectroscopy Chapter 19

2 Nuclear Magnetic Resonance spectroscopy is a powerful analytical technique used to characterize organic molecules by identifying carbon-hydrogen frameworks within molecules. Two common types of NMR spectroscopy 1 HNMRis used to determine the type and number of H atoms in a molecule 13 CNMRis used to determine the type of carbon atoms in the molecule. The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency. When low-energy radio waves interact with a molecule, they can change the nuclear spins of some elements, including 1 Hand 13 C.

3 Nuclear Magnetic Resonance Spectroscopy Schematic of an NMR spectrometer 3

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5 When a charged particle such as a proton spins on its axis, it creates a magnetic field.thus, the nucleus can be considered to be a tiny bar magnet. Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of a magnetic field B 0, they are oriented with or against this applied field. More nuclei are oriented with the applied field because this arrangement is lower in energy. The energy difference between these two states is very small (<0.1 cal).

6 Two variables characterize NMR: an applied magnetic field B 0, the strength of which is measured in tesla (T) the frequency of radiation used for resonance, measured in hertz (Hz), or megahertz (MHz) (1 MHz = 10 6 Hz). 6

7 Nuclei that respond to an external magnetic field include: 1 H, 13 C, 19 F and 31 P. (These nuclei are NMR-active nuclei)

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10 Example: Many proton NMR instruments employ a magnet that provides a field strength of 4.69T. At what frequency would the hydrogen nucleus absorb in such field?

11 Example: Many proton NMR instruments employ a magnet that provides a field strength of 4.69T. At what frequency would the hydrogen nucleus absorb in such field? E=hv v(frequency)= B o 2 v = (2.68x10 8 T -1 s -1 )(4.69 T) 2 = 2.00 x 10 8 s -1 = 200 MHz

12 The energy difference between the two spin states ( - and -spins) depends on the strength of the magnetic field The greater the operating frequency and the stronger the magnet the better is the resolution of the NMR spectrum 300 MHz NMR = $300K 400 MHz NMR = $400K 500 MHz NMR = $500K 600 MHz NMR = $600K 900 MHz NMR = $900K 12

13 Magnetic resonance imaging (MRI) Nobel Laureate: Peter Mansfield Routine method within medical diagnostics Worldwide, more than 60 million investigations with MRI are performed/year MRI has replaced several invasive modes of examination and thereby reduced the 13 risk and discomfort for many patients. Examination with MRI is especially valuable for detailed imaging of the brain and the spinal cord.

14 MRI Magnetic resonance imaging, noninvasive Nuclear is omitted because of public s fear that it would be radioactive. Only protons in one plane can be in resonance at one time. Computer puts together slices to get 3D. Tumors readily detected. 14

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16 The common scale for chemical shifts = = distance downfield from TMS (Hz) operating frequency of the spectrometer (MHz) Measured in parts per million (ppm). Same value for 60, 100, or 300 MHz machine. Called the delta scale.

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20 The chemical shift is independent of the operating frequency of the spectrometer

21 The reference point of an NMR spectrum is defined by the position of TMS (zero ppm) H 3 C CH 3 Si CH 3 CH 3 Tetramethylsilane TMS is added to the sample. Since silicon is less electronegative than carbon, TMS protons are highly shielded. Signal defined as zero. Organic protons absorb downfield (to the left) of the TMS signal. 21

22 Delta Scale 22 =>

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25 1 H NMR CHEMICAL SHIFTS Range: 0-20 ppm (parts per million) 25

26 Typical Values 26 =>

27 1 H NMR Number of Signals The number of NMR signals equals the number of different types of protons in a compound. Protons in different environments give different NMR signals. Equivalent protons give the same NMR signal. To determine equivalent protons in cycloalkanes and alkenes, always draw all bonds to hydrogen. 27

28 1 H NMR Number of Signals The number of 1 H NMR signals of some representative organic compounds 28

29 13 Range: ppm C Chemical Shifts Just as in 1 H NMR spectroscopy, chemical shifts in 13 C NMR depend on the electron density around the carbon nucleus Decreased electron density causes the signal to move downfield (desheilding) Increased electron density causes the signal to move upfield (sheilding) Because of the wide range of chemical shifts, it is rare to have two 13 C peaks coincidentally overlap A group of 3 peaks at 77 comes from the common NMR solvent deuteriochloroform and can be ignored 29

30 1 H NMR Number of Signals In comparing two H atoms on a ring or double bond, two protons are equivalent only if they are cis (or trans) to the same groups. 30

31 1 H NMR Number of Signals Proton equivalency in cycloalkanes can be determined similarly. 31

32 1 H NMR Number of Signals CH 3 CH 2 CH 2 -NO 2

33 1 H NMR Number of Signals CH 3 CCl 3

34 1 H NMR Number of Signals BrCH 2 CH 2 CH 2 Br

35 1 H NMR Number of Signals

36 1 H NMR Number of Signals

37 Nuclear Magnetic Resonance Spectroscopy 1 H NMR Enantiotopic and Diastereotopic Protons signal

38 Nuclear Magnetic Resonance Spectroscopy 1 H NMR Enantiotopic and Diastereotopic Protons. 2 signals 38

39 1 H NMR Position of Signals Since the electron experiences a lower magnetic field strength, it needs a lower frequency to achieve resonance. Lower frequency is to the right in an NMR spectrum, toward a lower chemical shift, so shielding shifts the absorption upfield. 39 The less shielded the nucleus becomes, the more of the applied magnetic field (B 0 ) it feels. This deshielded nucleus experiences a higher magnetic field strength, to it needs a higher frequency to achieve resonance. Higher frequency is to the left in an NMR spectrum, toward higher chemical shift so deshielding shifts an absorption downfield. Protons near electronegative atoms are deshielded, so they absorb downfield.

40 1 H NMR Position of Signals How chemical shift is affected by electron density around a nucleus 40

41 1 H NMR Position of Signals 41

42 1 H NMR Chemical ShiftValues The chemical shift of a C H bond increases with increasing alkyl substitution. 42

43 1 H NMR Chemical ShiftValues In a magnetic field, the six electrons in benzene circulate around the ring creating a ring current. The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons. The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance.thus they are deshielded and absorb downfield. 43

44 1 H NMR Chemical ShiftValues In a magnetic field, the loosely held electrons of the double bond create a magnetic field that reinforces the applied field in the vicinity of the protons. The protons now feel a stronger magnetic field, and require a higher frequency for resonance. Thus the protons are deshielded and the absorption is downfield. 44

45 1 H NMR Chemical ShiftValues In a magnetic field, the electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (B 0 ). Thus, the proton feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is shielded and the absorption is upfield. 45

46 1 H NMR Chemical Shift Values 46

47 1 H NMR Intensity of Signals The area under an NMR signal is proportional to the number of absorbing protons. 47

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49 1 H NMR Spin-Spin Splitting Consider the spectrum below: 49

50 1 H NMR Spin-Spin Splitting Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons. Let us consider how the doublet due to the CH 2 group on BrCH 2 CHBr 2 occurs: When placed in an applied electric field, (B 0 ), the adjacent proton (CHBr 2 ) can be aligned with ( )oragainst( )B 0. Thus, the absorbing CH 2 protons feel two slightly different magnetic fields one slightly larger than B 0, and one slightly smaller than B 0. Since the absorbing protons feel two different magnetic fields, they absorb at two different frequencies in the NMR spectrum, thus splitting a single absorption into a doublet. 50

51 1 H NMR Spin-Spin Splitting The frequency difference, measured in Hz between two peaks of the doublet is called the coupling constant, J. 51

52 1 H NMR Spin-Spin Splitting 52

53 1 H NMR Spin-Spin Splitting Three general rules describe the splitting patterns commonly seen in the 1 H NMR spectra of organic compounds. 1. Equivalent protons do not split each other s signals. 2. A set of n nonequivalent protons splits the signal of a nearby proton into n + 1 peaks. 3. Splitting is observed for nonequivalent protons on the same carbon or adjacent carbons. If H a and H b are not equivalent, splitting is observed when: 53

54 1 H NMR Spin-Spin Splitting Splitting is not generally observed between protons separated by more than three bonds. 54

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56 1 H NMR Spin-Spin Splitting Whenever two (or three) different sets of adjacent protons are equivalent to each other, use the n + 1 rule to determine the splitting pattern. The 1H NMR spectrum of 2-bromopropane, [(CH 3 ) 2 CHBr] 56

57 1 H NMR Spin-Spin Splitting Protons on carbon-carbon double bonds often give characteristic splitting patterns. A disubstituted double bond can have two geminal protons, two cis protons, or two trans protons. When these protons are different, each proton splits the NMR signal of the other so that each proton appears as a doublet. The magnitude of the coupling constant J for these doublets depends on the arrangement of hydrogen atoms. 57

58 1 H NMR Spin-Spin Splitting 1 H NMR spectra for the alkenyl protons of (E)- and (Z)-3-chloropropenoic acid 58

59 1 H NMR Spin-Spin Splitting The 1 H NMR spectrum of vinyl acetate (CH 2 =CHOCOCH 3 ) 59

60 1 H NMR Spin-Spin Splitting Splitting diagrams for the alkenyl protons in vinyl acetate are shown below. Note that each pattern is different in appearance because the magnitude of the coupling constants forming them is different. Splitting diagram for the alkenyl protons in vinyl acetate (CH 2 =CHOCHOCH 3 ) 60

61 1 H NMR OH Protons Under usual conditions, an OH proton does not split the NMR signal of adjacent protons. The signal due to an OH proton is not split by adjacent protons. The 1 H spectrum of ethanol (CH 3 CH 2 OH) 61

62 1 H NMR OH Protons Ethanol (CH 3 CH 2 OH) has three different types of protons, so there are three signals in its NMR spectrum. The H a signal is split by the two H b protons into three peaks (a triplet). The H b signal is split only by the three H a protons into four peaks, a quartet.the adjacent OH proton does not split the signal due to H b. H c is a singlet because OH protons are not split by adjacent protons. Protons on electronegative atoms rapidly exchange between molecules in the presence of trace amounts of acid or base. Thus, the CH 2 group of ethanol never feels the presence of the OH proton, because the OH proton is rapidly moving from one molecule to another. This phenomenon usually occurs with NH and OH protons. 62

63 1 H NMR Protons on Benzene Rings Benzene has six equivalent deshielded protons and exhibits a single peak in its 1 H NMR spectrum at 7.27 ppm. Monosubstituted benzenes contain five deshielded protons that are no longer equivalent, and the appearance of these signals is highly variable, depending on the identity of Z. The ppm region of the 1H NMR spectrum of three benzene derivatives 63

64 1 H NMR Structure Determination 64

65 1 H NMR Structure Determination 65

66 1 H NMR Structure Determination 66

67 1 H NMR Structure Determination 67

68 13 CNMR 13 C Spectra are easier to analyze than 1 H spectra because the signals are not split. Each type of carbon atom appears as a single peak. 68

69 13 CNMR The lack of splitting in a 13 C spectrum is a consequence of the low natural abundance of 13 C. Recall that splitting occurs when two NMR active nuclei like two protons are close to each other. Because of the low natural abundance of 13 C nuclei (1.1%), the chance of two 13 C nuclei being bonded to each other is very small (0.01%), and so no carbon-carbon splitting is observed. A 13 C NMR signal can also be split by nearby protons. This 1 H- 13 C splitting is usually eliminated from the spectrum by using an instrumental technique that decouples the proton-carbon interactions, so that every peak in a 13 C NMR spectrum appears as a singlet. The two features of a 13 C NMR spectrum that provide the most structural information are the number of signals observed and the chemical shifts of those signals. 69

70 13 C NMR Number of Signals The number of signals in a 13 C spectrum gives the number of different types of carbon atoms in a molecule. Because 13 C NMR signals are not split, the number of signals equals the number of lines in the 13 C spectrum. In contrast to the 1 H NMR situation, peak intensity is not proportional to the number of absorbing carbons, so 13 C NMR signals are not integrated. 70

71 13 C NMR Position of Signals In contrast to the small range of chemical shifts in 1 H NMR (1-10 ppm usually), 13 C NMR absorptions occur over a much broader range (0-220 ppm). The chemical shifts of carbon atoms in 13 C NMR depend on the same effects as the chemical shifts of protons in 1 H NMR. 71

72 13 C NMR Number of Signals Representative 13 C NMR spectra 72

73 13 C NMR Number of Signals 73

74 TRY PROBLEMS