16.1 Introduction to NMR. Spectroscopy

Transcription

1 16.1 Introduction to NMR What is spectroscopy? Spectroscopy NUCLEAR MAGNETIC RESNANCE (NMR) spectroscopy may be the most powerful method of gaining structural information about organic compounds. NMR involves an interaction between electromagnetic radiation (light) and the NUCLEUS of an atom. We will focus on C and H nuclei. WHY? The structure (connectivity) of a molecule affects how the radiation interacts with each nucleus in the molecule. 16-1

2 Protons and neutrons in a nucleus behave as if they are spinning. If the total number of neutrons and protons is an DD number, the atoms will have net nuclear spin. Examples: 16.1 Introduction to NMR Spectroscopy The spinning charge in the nucleus creates a MAGNETIC MMENT. 16-2

3 16.1 Introduction to NMR Spectroscopy Like a bar magnet, a MAGNETIC MMENT exists perpendicular to the axis of nuclear spin. 16-3

4 16.1 Introduction to NMR Spectroscopy If the normally disordered magnetic moments of atoms are exposed to an external magnetic field, their magnetic moments will align. 16-4

5 16.1 Introduction to NMR Spectroscopy The aligned magnetic moments can be either WITH or AGAINST the external magnetic field. The α and β spin states are not equal in energy. WHY? 16-5

6 16.1 Introduction to NMR Spectroscopy When an atom with an α spin state is exposed to radio waves of just the right energy, it can be promoted to a β spin state. 16-6

7 16.1 Introduction to NMR The magnetic moment of the electrons generally reduces the effect of the external field. The amount of radio wave energy necessary for the α β energy transition depends on the electronic environment for the atom. Spectroscopy 16-7

8 16.2 Acquiring a 1 H NMR Spectrum Why do we use a deuterated solvent? 16-8

9 NMRs come in various strengths 60 MHz 300 MHz 900 MHz 7-9

10 Why does NMR Strength Matter? 7-10

11 16.3 Characteristics of a 1 H NMR Spectrum NMR spectra contain a lot of structural information: Number of signals Signal location shift Signal area integration Signal shape splitting pattern 16-11

12 16.4 Number of Signals Protons with different electronic environments will give different signals. Protons in the same electronic environment will give the same signal and are said to be chemically equivalent

13 16.4 Number of Signals Chemical equivalence can be determined by: Hydrogens on the same carbon are chemically equivalent. Rotation Replacement test: 16-13

14 16.4 Number of Signals How many signals should you expect in the 1 H NMR for the following molecules? 16-14

15 Number of Signals Identify the number of signals expected in the 1 H NMR for each of the following compounds: Me 7-15

16 Number of Signals Identify the number of signals expected in the 1 H NMR for each of the following compounds: Geraniol Isolated from roses and used in perfumes H H NH 2 H Dopamine A neurotransmitter that is deficient in Parkinson's disease Isoprene A precursor for natural rubber 7-16

17 16.5 Chemical Shifts Tetramethylsilane (TMS) is used as the standard for NMR chemical shift. In many NMR solvents, 1% TMS is added as an internal standard. The shift for a proton signal is calculated as a comparison to TMS: For benzene on a 300 MHz instrument: 16-17

18 16.5 Chemical Shifts The shift for a proton signal is calculated as a comparison to TMS: For benzene on a 60 MHz instrument: The Hz of the signal is different in different instruments, but the shift relative to TMS (δ) is constant

19 16.5 Chemical Shifts The shift for a proton signal is calculated as a comparison to TMS: The shift relative to TMS (δ) is a dimensionless number because the Hz units cancel out. Units for δ are often given as ppm (parts per million), which simply indicates that signals are reported as a fraction of the operating frequency of the spectrometer. Most 1 H signals appear between 0 and 10 ppm

20 16.5 Chemical Shifts Early NMRs analyzed samples at a constant energy over a range of magnetic field strengths from low field strength (DWNFIELD) to high field strength (UPFIELD)

21 Typical 1 H NMR Shifts 7-21

22 16.5 Chemical Shifts Alkane protons generally give signals around 1 2 ppm. Protons can be shifted downfield when nearby electronegative atoms cause deshielding. HW? 16-22

23 16.5 Chemical Shifts These protons are deshielded because the induced magnetic field of the π electrons reinforces the applied magnetic field

24 16.5 Chemical Shifts Some of the protons in [14]annulene appear at 8 ppm while others appear at 1 ppm. Which are which? 16-24

25 Predicting Chemical Shifts Predict the chemical shift for the signals in the 1 H NMR spectrum of each of the following compounds. 7-25

26 Application: Chemical Shift A 1 H NMR spectrum was acquired for each of the following constitutional isomers. Comparison of the spectra reveals that only one of these spectra exhibits a signal between 6 and 7 ppm. Identify the structure that corresponds with this spectrum 7-26

27 16.6 Integration The INTEGRATIN or area under the peak quantifies the relative number of protons giving rise to a signal. A computer will calculate the area of each peak representing that area with a STEP-CURVE. The curve height represents the integration

28 16.6 Integration The computer operator sets one of the peaks to a whole number to let it represent a number of protons. The computer uses the integration ratios to set the values for the other peaks

29 16.6 Integration Integrations represent numbers of protons, so you must adjust the values to whole numbers. If the integration of the first peak is doubled, the computer will adjust the others according to the ratio

30 16.6 Integration The INTEGRATIN are relative quantities rather than an absolute count of the number of protons. Predict the 1 H shifts and integrations for tert-butyl methyl ether. Symmetry can also affect integrations. Predict the 1 H shifts and integrations for 3-pentanone

31 Integration Practice A compound with the molecular formula C 5 H 10 2 has the following NMR spectrum. Determine the number of protons giving rise to each signal. 7-31

32 Integration Practice A compound with the molecular formula C 10 H 10 has the following NMR spectrum. Determine the number of protons giving rise to each signal. 7-32

33 Application: Integration The 1 H NMR spectrum of a compound with molecular formula C 7 H 15 Cl exhibits two signals with relative integration 2:3. Propose a structure for this compound. 7-33

34 16.7 Multiplicity When a signal is observed in the 1 H NMR, often it is split into multiple peaks. Multiplicity or a splitting patterns results

35 16.7 Multiplicity Multiplicity results from magnetic effects that protons have on each other. Consider protons H a and H b. We already saw that protons align with or against the external magnetic field. H b will be aligned with the magnetic field in some molecules. ther molecules in the sample will have H b aligned against the magnetic field. Some H b atoms have a slight shielding affect on H a and others have a slight deshielding effect

36 16.7 Multiplicity The resulting multiplicity or splitting pattern for H a is a doublet. A doublet generally results when a proton is split by only one other proton on an adjacent carbon

37 16.7 Multiplicity Consider an example where there are two protons on the adjacent carbon. There are three possible affects the H b protons have on H a

38 H a appears as a triplet. WHY? The three peaks in the triplet have an integration ratio of 1:2:1. WHY? 16.7 Multiplicity 16-38

39 16.7 Multiplicity Consider a scenario where H a has three equivalent H b atoms splitting it. Explain how the magnetic fields cause shielding or deshielding

40 16.7 Multiplicity H a appears as a quartet. What should the integration ratios be for the four peaks of the quartet? 16-40

41 16.7 Multiplicity Table 16.3 shows how the multiplicity trend continues. By analyzing the splitting pattern of a signal in the 1 H NMR, you can determine the number of equivalent protons on adjacent carbons

42 16.7 Multiplicity The trend in Table 16.3 also allows us to predict splitting patterns. Explain how the n+1 rule is used

43 16.7 Multiplicity Remember three key rules: 1. Equivalent protons cannot split one another. 2. To split each other, protons must be within three bonds

44 Multiplicity: Practice Determine the multiplicity of each signal in the expected 1 H NMR spectrum of the following compound. 7-44

45 Multiplicity: Practice For each of the following compounds, determine the multiplicity of each signal in the expected 1 H NMR spectrum: 7-45

46 16.7 Multiplicity The degree to which a neighboring proton will shield or deshield its neighbor is called a CUPLING CNSTANT. The coupling constant or J value is the distance between peaks of a splitting pattern measured in units of Hz. When protons split each other, their coupling constants will be equal. J ab = J ba 16-46

47 16.7 Multiplicity The CUPLING CNSTANT will be constant even if an NMR instrument with a stronger or weaker magnetic field is used. Higher field strength instruments will give better resolution between peak because the coupling constant is a smaller percentage of the overall Hz available

48 16.7 Multiplicity Sometimes recognizable splitting patterns will stand out in a spectrum. An isolated ethyl group gives a triplet and a quartet. Note the integrations. The triplet and quartet must have the same coupling constant if they are splitting each other

49 16.7 Multiplicity A peak with an integration equal to 9 suggests the presence of a tert-butyl group. An isolated isopropyl group gives a doublet and a septet. Note the integrations

50 Recognizing Common Groups Below are NMR spectra of several compounds. Identify whether these compounds are likely to contain ethyl, isopropyl, and/or tert-butyl groups. 7-50

51 16.7 Multiplicity The hydroxyl proton and other labile or exchangeable protons undergo rapid exchange with trace amounts of acid. Such exchange blurs the shielding/deshielding effect of the neighboring protons giving a singlet that is often broadened

52 16.10 Analyzing a 1 H NMR Spectrum With a given formula and 1 H NMR spectrum, you can determine a molecule s structure by a four-step process: 1. Calculate the degree or unsaturation or hydrogen deficiency index (HDI). What does the HDI tell you? 2. Consider the number of NMR signals and integration to look for symmetry in the molecule. 3. Analyze each signal, and draw molecular fragments that match the shift, integration, and multiplicity. 4. Assemble the fragments into a complete structure like puzzle pieces

53 Predicting NMR: Practice Draw the expected 1 H NMR spectrum for: H 7-53

54 Distinguishing Between Compounds with NMR How would you use NMR spectroscopy to distinguish between the following compounds? 7-54

55 16.13 Application Medically Speaking MRI (magnetic resonance imaging) instruments are essentially 1 H NMR spectrometers. The body is analyzed rather than a sample in an NMR tube. Different tissues have different concentrations of protons. WHY? The MRI gives a 3D image of different tissues. HW? Are there any known side effects from exposure to either radio waves or a magnetic field? 16-55