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1 The world leader in serving science A Practical Introduction to Nuclear Magnetic Resonance Spectroscopy Basic Theory.

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Presentation on theme: "1 The world leader in serving science A Practical Introduction to Nuclear Magnetic Resonance Spectroscopy Basic Theory."— Presentation transcript:

1 1 The world leader in serving science A Practical Introduction to Nuclear Magnetic Resonance Spectroscopy Basic Theory

2 2 Key Learning Why NMR Spectroscopy Introduction to NMR Chemical Shift Shielding Demystified Chemical Shift and PPM Scale Integration Multiplet Analysis Summary

3 3 3 Why NMR Spectroscopy? picoSpin45 ethyl acetate spectrum Arguably the most powerful analytical method in organic chemistry Provides clues to neighboring functional groups and their location Yields quantitative concentration ratios (+/- 1%) Reveals the structural arrangement of a molecule for a given nuclei Fast, non-destructive analysis An excellent complimentary technique to Infra-red and Mass spec.

4 4 What NMR Signals Tell Us The number of signals shows how many different kinds of protons (Hydrogen) are present. The location (chemical shift) of the signals shows how shielded or deshielded the proton is. The intensity of the signal shows the number of protons of that type. Signal splitting shows the number of protons on adjacent atoms.

5 5 Chemical Shift Chemical Shift is measured as a shift along the x axis relative to a standard (in most cases Tetramethylsilane or TMS). TMS always has a chemical shift of 0.0 Hz TMS

6 6 This shift is caused by the magnetic environment experienced by the nuclei (H for picoSpin) which changes based on the electron density surrounding the nuclei being measured. Blue indicates areas of low electron density. Red indicates areas of high electron density. Ethanol CH 3 CH 2 OH Image From: Dr. Sheila Woodgate, University of Auckland, 2002, Chemical Shift

7 7 Blue indicates areas of low B e strength. Red indicates areas of high B e strength. Ethanol CH 3 CH 2 OH Chemical Shift Because electrons are moving and have charge, they induce a magnetic field B e. This field opposes the applied field B 0 (the magnetic field from the instrument’s magnet). This is called “shielding” because the electrons are shielding the nucleus from the effect of B 0.

8 8 Chemical Shift e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- nucleus e - cloud (shields the nucleus from B 0 ) B0B0 Because the chemical shift is dependent upon the total magnetic field strength experienced by the nuclei, different electron densities surrounding a given nuclei will cause it’s signal to appear at different positions along the x axis.

9 9 Chemical Shift Highly electronegative atoms (like oxygen, halogens, etc.) pull electron density away from the nucleus “deshielding” it from the magnetic field of the instrument. H e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- smaller e - cloud (deshielded by the oxygen atom) B0B0 The electron density surrounding the nucleus is dependent upon the electronegativity and atoms bonded to it, therefore… Chemical shift tells us about the functional groups surrounding a given NMR signal. O

10 10 Shielding Demystified e - cloud (shields the nucleus from B 0 ) nucleus Magnetic Field from Instrument += Down-Field Deshielded Up-Field Shielded Magnetic field from the instrument Opposing field from e - cloud Total magnetic field experienced by the nucleus smaller e - cloud (less shielding) Nuclei with less electron density surrounding them are said to be de- shielded, and appear further “downfield” or to the left of the spectrum. B0B0 BeBe BeBe += Magnetic field from the instrument Opposing field from e - cloud Total magnetic field experienced by the nucleus

11 11 Examples: 2 ppm = 90 Hz chemical shift / 45 MHz magnet 1 ppm = 400 Hz chemical shift / 400 MHz magnet This example eludes to the advantage of stronger magnet systems: chemical shift dispersion Chemical Shift and the ppm Scale As we’ve seen, chemical shift is dependent on the field strength. Instrument’s with different strength magnets (45 MHz magnet vs. a 400 MHz magnet) produce different chemical shifts for the same signal. To make the chemical shifts of NMR spectra magnet strength independent, we use the ppm, or parts-per-million scale. chemical shift in ppm = chemical shift in Hz / Frequency of the magnet in MHz

12 12 1 H Chemical Shifts of Protons Accompanying Common Functional Groups. Note: Anything that can change the magnetic environment around a given nuclei (solvent, concentration, temperature, dielectric strength, etc.) can influence chemical shift. Protons that are susceptible to exchange and/or hydrogen bonding (OH groups for example) tend to show these shifts the most.

13 13 Integration NMR is inherently quantitative. Two protons will produce a signal with twice the area of a signal produced by a single proton. If the sample contains only a single component, the area of the peaks can be used in conjunction with the chemical shift and multiplet information to determine or verify the chemical structure of the sample as shown in the next slide.

14 14 Ethyl Alcohol Integration

15 15 If the sample contains more than one component, and the chemical structures of those components are known, the integration values can be compared to determine the relative concentrations of the components in the sample. Integration

16 16 Both of these signal groups represents two protons. Since the relative integration of these two signals is one to one, it indicates that the two components of the sample ( ethanol and ethyl acetate) are present in a one to one mole ratio. Integration

17 17 Both of these signal groups represents two protons. Since the relative integration of these two signals is two to one, it indicates that there are twice as many moles of ethanol in this sample as there are moles of ethyl acetate. Note: The entire spectrum doesn’t need to be resolved to accomplish the quantitation. Integration

18 18 Multiplet Analysis Multiplet splitting is a splitting of a NMR signal into multiple peaks due to interactions between the quantum spins of neighboring nuclei. or Signal of Interest Neighboring nuclear spin 1 neighbor 2 neighbors couples to

19 19 number of peaks in the multiplet number of neighboring protons within three bonds of the proton responsible for the signal = + 1 The n+1 rule Count all protons within 3 bonds to determine the number of neighbors Multiplet Analysis

20 20 Ethyl Alcohol These protons have two neighboring protons (from the CH 2 group) within three bonds. These protons have three neighboring protons from the terminal CH 3 group. Note: splitting is not normally observed through hetero-atoms such as oxygen. There are no neighboring protons on the same side of the oxygen atom. Multiplet Analysis

21 21 ConfigurationPeak Ratios A1Singlet AB1:1Doublet AB 2 1:2:1Triplet AB 3 1:3:3:1Quartet AB 4 1:4:6:4:1 Pentet or Quintet AB 5 1:5:10:10:5:1Sextet AB 6 1:6:15:20:15:6:1Septet number of peaks in the multiplet number of neighboring protons within three bonds of the proton responsible for the signal = + 1 The n+1 rule Multiplet Analysis

22 22 Summary NMR is an incredibly important analytical technique that provides both quantitative and qualitative information quickly and non destructively. NMR Spectra are interpreted using three main properties Signals – the number of signals tells us about the number of different proton types Chemical Shift – displacement along the x axis, indicates electron withdrawing potential of neighboring functional groups. Integration – quantifies relative amounts by measuring area under peaks Multiplet Splitting – signal splitting : Doublets, triplets, quartets, etc.reveals neighboring nuclei and structural conformation Follow the “n+1 rule”


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