NMR Spectroscopy. Introduction

Transcription

1 Introduction NMR Spectroscopy Over the past fifty years nuclear magnetic resonance spectroscopy, commonly referred to as nmr, has become the most important technique for determining the structure of organic compounds. Although larger amounts of sample are needed than for mass spectroscopy, nmr is non-destructive, and with modern instruments good data may be obtained from samples weighing less than a milligram. 1

2 Principles of NMR Spectroscopy NMR spectroscopy is basically another form of absorption spectroscopy, similar to IR or UV spectroscopy. Under appropriate conditions, a sample can absorb electromagnetic radiation in the radio-frequency region at frequencies governed by the characteristic of the sample. 2

3 Principles of NMR Spectroscopy In NMR, absorption is a function of certain nuclei in the molecule. A plot of the frequencies of the absorption peaks versus peak intensities constitutes an NMR spectrum. 3

4 Principles of NMR Spectroscopy All nuclei carry a charge. In some nuclei this charge spins on the nuclear axis, and this circulation of nuclear charge generates a magnetic dipole along the axis. The most important nuclei for organic structure determination are 1 H (ordinary hydrogen) and 13 C, a stable nonradioactive isotope of ordinary carbon. Although 12 C and 16 O are present in most organic compounds, they do not possess a spin and do not give NMR spectra. 4

5 Principles of NMR Spectroscopy When nuclei with spin are placed between the poles of powerful magnet, they align their magnetic fields with or against the field of the magnet. Nuclei aligned with the applied field have a slightly lower energy than those aligned against the field. By applying energy in the radio frequency range, it is possible to excite nuclei in the lower energy spin state to the higher energy spin state. 5

6 Principles of NMR Spectroscopy The energy gap between the two spin states depends on the strength of the applied magnetic field; the stronger the field, the larger the energy gap. Instruments currently in use have magnetic fields that range from about 1.4 to 14 tesla (T) (by comparison, the earth's magnetic field is only about T). At these field strengths, the energy gap corresponds to a radio frequency of 60 to 600 MHz (megahertz; 1 MHz = 10 6 Hz or 10 6 cycles per second). This energy corresponds to 6-60 x 10-6 kcal/mole. 6

7 Measuring an NMR Spectrum A 1 H-NMR spectrum is usually obtained by dissolving the sample in some inert solvent that does not contain 1 H nuclei. Examples of such solvents are CCl 4, or solvents with the hydrogens replaced by deuterium, such as CDCl 3, and CD 3 COCD 3. A small amount of a reference compound is also added. The solution, in a thin glass tube, is placed in the center of a radio frequency (rf) coil, between the pole faces of a powerful magnet. 7

8 Measuring an NMR Spectrum The nuclei align themselves with or against the field. Continuously increasing amounts of energy is then applied to the nuclei by the rf coil. When this energy corresponds exactly to the energy gap between the lower and higher energy spin states, it is absorbed by the nuclei. At this point, the nuclei are said to be in resonance with the applied frequency hence the term nuclear magnetic resonance. A plot of the energy absorbed by the sample against the applied frequency of the rf coil gives an NMR spectrum. 8

9 The Mechanism of Absorption (a) A top precessing in the earth s gravitational field; (b) the precession of a spinning nucleus resulting from the influence of an applied magnetic field. 9

10 The Mechanism of Absorption The nuclear magnetic resonance process: absorption occurs when ν = ω. Where ν is applied radio frequency and ω is the angular frequency of precession. 10

11 Population densities of nuclear spin states For a proton, if the applied magnetic field strength is 1.41 Tesla, resonance occurs at about 60 MHz. The energy different between the two spin states is about 6 x 10-6 kcal/mole. At room temperature both the states are almost equally populated. There is slight excess of nucleii in the lower energy state. 11

12 Population densities of nuclear spin states 12

13 Measuring an NMR Spectrum In practice, there are two ways by which the resonance frequencies of 1 H nuclei can be determined. Because the magnetic field strength and the size of the energy gap between nuclear spin states are directly related, either the magnetic field strength or the rf can be varied. In earlier NMR spectrometers a constant radio frequency was applied, the strength of the applied magnetic field was varied, and different nuclei resonated at different magnetic field strengths. 13

14 Measuring an NMR Spectrum In modern Fourier transform (FT-NMR) spectrometer, the applied magnetic field is held constant, and the radio frequency is varied. The instrument computer uses a mathematical process called Fourier transformation to sort the signal that is produced into the resonance rfs of the different 1 H nuclei. Whether it is the magnetic field field strength or the rf that is varied, this variable increases from left to right in the recorded spectra. 14

15 The NMR instrument 15

16 Chemical Shifts and Peak Areas Not all 1 H nuclei flip their spins at precisely the same radio frequency because they may differ in chemical (and, more particularly, electronic) environment. The NMR spectrum of p-xylene: The spectrum is very simple and consists of two peaks. The positions of the peaks are measured in (delta) units from the peak of a reference compound, which is tetramethylsilane (TMS), (CH 3 ) 4 Si. 16

17 The reasons for selecting TMS as a reference compound All 12 nuclei of its hydrogens are equivalent, so it shows only one sharp NMR signal, which serves as a reference point. Its hydrogen signals appear at higher field than do most 1 H signals in other organic compounds, thus making it easy to identify the TMS peak. TMS is inert, so it does not react with most organic compounds, and it is low boiling and can be removed easily at the end of a measurement. 17

18 Chemical Shifts and Peak Areas Most organic compounds have peaks downfield (at low field) from TMS and are given positive values. low field high field 18

19 Chemical Shifts and Peak Areas A value of 1.00 means that a peak appears 1 part per million (ppm) downfield from the TMS peak. low field high field 19

20 Chemical Shifts and Peak Areas If the spectrum is measured at 60 MHz (60 x 10 6 Hz), then 1 ppm is 60 Hz (one-millionth of 60 MHz) downfield from TMS. If the spectrum is run at 100 MHz, a value of 1 ppm is 100 Hz downfield from TMS, and so on. low field high field 20

21 Chemical Shifts and Peak Areas The chemical shift of a particular kind of 1 H signal is its value with respect to TMS. It is called a chemical shift because it depends on the chemical environment of the hydrogens. The chemical shift is independent of the instrument on which it is measured. Chemical shift = = distance of peak from TMS, in Hz ppm spectrometer frequency in MHz 21

22 Chemical Shifts and Peak Areas In the spectrum of p-xylene, we see a peak at 2.30 and another at It seems reasonable that these peaks are caused by two different kinds of 1H nuclei in the molecule: the methyl hydrogens and the aromatic ring hydrogens. How can we tell which is which? low field high field 22

23 Chemical Shifts and Peak Areas One way to identify the hydrogens is to integrate the area under each peak. The peak area is directly proportional to the number of 1 H nuclei responsible for the particular peak. All the commercial NMR spectrometers are equipped with electronic integrators that can print an integration line over the peaks. low field high field 23

24 Chemical Shifts and Peak Areas The ratio of heights of the vertical parts of this line is the ratio of peak areas.. The areas of the peaks at 2.30 and 7.10 in p-xylene spectrum give a ratio of 3 : 2 (or 6 : 4). These areas allow us to assign the peak at 2.30 to the methyl hydrogens and the peak at 7.10 to the four aromatic ring hydrogens. low field high field 24

25 Chemical Shifts and Peak Areas A more general way to assign peaks is to compare chemical shifts with those of similar protons in a known reference compound. For example, benzene has six equivalent hydrogens and shows a single peak in its 1 H NMR spectrum at Other aromatic compounds also show a peak in this region. We can conclude that most aromatic ring hydrogens will have chemical shifts at about 7. Similarly, most CH 3 Ar hydrogens appear at

26 Chemical Shift 6 regions of NMR spectrum The chemical shifts of 1 H nuclei in various chemical environments have been determined by 1 H NMR spectra of a large number of compounds with known, relatively simple structures. The chart above gives the chemical shifts for several common types of 1 H nuclei. 26

27 Table of chemical shifts 27

28 Factors that influence chemical shifts The electronegativity of groups in the immediate environment of the 1 H nuclei. Electron withdrawing groups generally cause a downfield chemical shift. Compare, for example, the following chemical shifts: CH 3 CH 2 Cl CHCl 2 ~ 0.9 ~ 3.7 ~ 5.8 Electrons in motion near a 1 H nucleus create a small magnetic field in its microenvironment that tends to shield the nucleus from the externally applied magnetic field. 28

29 Factors that influence chemical shifts Electrons in motion near a 1 H nucleus create a small magnetic field in its microenvironment that tends to shield the nucleus from the externally applied magnetic field. 29

30 Factors that influence chemical shifts CH 3 CH 2 Cl CHCl 2 ~ 0.9 ~ 3.7 ~ 5.8 Chlorine is an electron-withdrawing group. Withdrawal of electron density by the chlorine therefore deshields the nucleus, allowing it to flip its spin at a lower applied external field or lower frequency. The more chlorines, the larger the effect. 30

31 Factors that influence chemical shifts The presence of electrons Hydrogens attached to a carbon that is part of a multiple bond or aromatic ring usually appears downfield from hydrogens attached to saturated carbons. The pi-electrons associated with a benzene ring provide a striking example of this phenomenon. The electron cloud above and below the plane of the ring circulates in reaction to the external field so as to generate an opposing field at the center of the ring and a supporting field at the edge of the ring. 31

32 Factors that influence chemical shifts This kind of spatial variation is called anisotropy, and it is common to nonspherical distributions of electrons. Regions in which the induced field supports or adds to the external field are said to be deshielded, because a slightly weaker external field will bring about resonance for nuclei in such areas. However, regions in which the induced field opposes the external field are termed shielded because an increase in the applied field is needed for resonance. 32

33 Examples of Anisotropy Influences on Chemical Shift The structural constraints of the bridging chain require the middle two methylene groups to lie over the face of the benzene ring, which is a nmr shielding region. 33

34 Spin Spin Splitting Many compounds give spectra that show more complex peaks than just single peaks (singlets) for each type of hydrogen. Based on chemical shifts, the 1H NMR spectrum of diethyl ether is expected to have two lines: one in the region of 0.9 for the six equivalent CH 3 Hydrogens and one at about 3.5 for the four equivalentch 2 hydrogens adjacent to the oxygen atom, with relative areas 6 : 4. In the spectrum, we see absorptions in each of these regions, with the expected total 34 area ratio.

35 Spin Spin Splitting But, we do not see singlets! Instead, the methyl signal is split into three peaks, a triplet, with relative areas 1:2:1; and the methylene signal is split into four peaks, a quartet, with relative areas 1:3:3:1. 35

36 Spin Spin Splitting Spin-spin splittings, tell us quite a bit about molecular structure. Each 1 H nucleus in the molecule acts as a tiny magnet, and each hydrogen feels not only the very large applied magnetic field but also a tiny field due to its neighboring hydrogens. When 1 H nuclei on one carbon is excited, the 1 H nuclei on neighboring carbons can be in either the lower or the higher spin state, with nearly equal probabilities. Due to this, the magnetic field of the nuclei whose peak is observed is perturbed slightly by the tiny fields of its neighboring 1 H nuclei. 36

37 Spin Spin Splitting The splitting pattern can be predicted by the n + 1 rule: if a 1 H nucleus or a set of equivalent 1 H nuclei has n 1 H neighbors with a substantially different chemical shift, its NMR signal will be split into n + 1 peaks. In diethyl ether, each CH 3 hydrogen has two 1 H neighbors (on the CH 2 group). Therefore, the CH 3 signal is split into = 3 peaks. At the same time, each CH 2 hydrogen has three 1 H neighbors (on the CH 3 group). The CH 2 signal is therefore split into = 4 peaks. 37

38 n + 1 rule for signal splitting 38

39 n + 1 rule for signal splitting 39

40 n + 1 rule for signal splitting 40

41 n + 1 rule for signal splitting In the spectrum of 1,1-dichloroethane shown below, it is clear that the three methyl hydrogens (red) are coupled with the single methyne hydrogen (orange) in a manner that causes the former to appear as a doublet and the latter as a quartet. 41

42 n + 1 rule for signal splitting The statistical distribution of spins within each set explains both the n+1 rule and the relative intensities of the lines within a splitting pattern. The action of a single neighbouring proton is easily deduced from the fact that it must have one of two possible spins. Interaction of these two spin states with the nuclei under observation leads to a doublet located at the expected chemical shift. 42

43 n + 1 rule for signal splitting The corresponding action of the three protons of the methyl group requires a more detailed analysis. In the display of this interaction four possible arrays of their spins are shown. The mixed spin states are three times as possible as the all +1/2 or all _ 1/2 collection. Consequently, we expect four signals, two above the chemical shift and two below it. This spin analysis also suggests that the intensity ratio of these signals will be 1:3:3:1. 43

44 n + 1 rule for signal splitting Pascal s Triangle A simple way of estimating the relative intensities of the lines in a first-order coupling pattern is shown below. This array of numbers is known as Pascal's triangle, and is easily extended to predict higher multiplicities. 44

45 Spin Spin Splitting Coupling Constant 1H nuclei that split one another s signals are said to be coupled. The extent of the coupling, or the number of hertz by which the signals split, is called the coupling constant (abbreviated J). 45

46 Spin Spin Splitting 46

47 Spin Spin Splitting Coupling Constant Spin spin splitting falls off with distance. Whereas hydrogens on adjacent carbons may show appreciable splitting ( J = 6 8 Hz), hydrogens farther apart hardly feel each other s presence ( J = 0 1 Hz). Coupling constants can even be used at times to distinguish between cis trans isomers or between positions of substituents on a benzene ring. Chemically equal 1 H nuclei do not split each other. For example, BrCH 2 CH 2 Br shows only a sharp singlet in its 1 H NMR spectrum for all four hydrogens. 47

48 Measuring Coupling Constant Each ppm of chemical shift represents 60Hz. There are 12 grid lines per ppm, each grid line represents 60Hz/12 = 5 Hz. 48

49 Measuring coupling constant The spacing between the component peaks is approximately 1.5 chart divisions. Therefore, J = 1.5 div x 5 Hz/1 div = 7.5 Hz 49

50 Measuring coupling constant Most aliphatic protons in acylic systems, the magnitutes of coupling constants are always near 7.5 Hz. The coupling constants of the groups of protons that split one another are identical within experimental error. This information is useful in interpreting a spectrum that may have several multiplets, each with a different coupling constant. 50

51 Measuring coupling constants on modern FT-NMR spectrometers Coupling constants are determined by printing Hertz values directly on the peaks. The values are subtracted to determine the coupling constants. 51

52 Measuring coupling constants on modern FT-NMR spectrometers 52

53 Measuring coupling constants on modern FT-NMR spectrometers 53

54 Multiplet Skewing (leaning) Multiplet skewing can sometimes be used to link interacting multiplets. There is a tendency for the outermost lines of a multiplet to have nonequivalent heights. If arrows are drawn on both multiplets in the directions of their respective skewing, these arrows will point at each other. 54

55 NMR spectra of alcohols 55

56 NMR spectra of alcohols The chemical shift of OH hydrogen is variable. Its position depending on concentration, solvent, temperature, and presence of water, or acidic or basic impurities. This peak can be found anywhere in the range of ppm. 56

57 NMR spectra of alcohols The variability of this absorption is dependent on the rates of OH proton exchange and the amount of hydrogen bonding in the solution. The OH hydrogen is usually not split by hydrogens on the adjacent carbon (-CH-OH) because rapid exchange decouples this interaction. 57

58 Deuterium exchange for identifying the OH absorption A drop of D 2 O is added to the NMR tube containing the alcohol solution. After shaking the sample and sitting for few minutes, the NMR spectrum is recorded. The disappearance of the OH signal in the spectrum is used to confirm the presence of OH group in the sample. 58

59 Deuterium exchange for identifying amines and carboxylic acids Amino hydrogens exhibit deuterium exchange. Carboxylic hydrogen exhibit deuterium exchange. 59

60 Interpreting NMR spectra 60