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Nuclear Magnetic Resonance Information Gained: Different chemical environments of nuclei being analyzed ( 1 H nuclei): chemical shift The number of nuclei.

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Presentation on theme: "Nuclear Magnetic Resonance Information Gained: Different chemical environments of nuclei being analyzed ( 1 H nuclei): chemical shift The number of nuclei."— Presentation transcript:

1 Nuclear Magnetic Resonance Information Gained: Different chemical environments of nuclei being analyzed ( 1 H nuclei): chemical shift The number of nuclei with different chemical environments: number of signals in spectrum The numbers of protons with the same chemical environment: integration Determine how many protons are bonded to the same carbon: integration Determine the number of protons that are adjacent to one another: splitting patterns Determine which protons are adjacent to one another: coupling constants Lecture 6

2 Chemical Shifts Shielded protons appear more upfield (lower ppm value). Deshielded protons appear more downfield (higher ppm value). Correlation Chart

3 Chemical Shifts TMS - Tetramethylsilane (Me 4 Si) is the internal reference used in many examples. TMS’s chemical shift is set at zero since most peaks appear more downfield from it. (Note: You will use your NMR solvent, CDCl 3, as your reference peak. Why use CDCl 3 to make up your sample for NMR?) The Delta (  ) Scale An arbitrary scale 1  = 1 part per million (ppm) of the spectrometer operating frequency. For example, if using an 80 MHz instrument to run a 1 H NMR spectrum, 1  would be 1 ppm of 80,000,000 Hz, or 80 MHz. Since the radiofrequency absorption of a nuclei depends on the magnetic field strength, chemical shift in Hz would vary from instrument to instrument. The stronger the field, the greater the  E (magnetic transition). Thus, report the nuclei absorption in relative terms (  ) as opposed to absolute terms (Hz). This way, the chemical shifts will be the same for nuclei of a sample despite what instrument you use - leads to correlation charts!

4 1 H NMR Spectrum of Ethanol ppm CH 3 CH 2 O H TMS Three signals - three different types of H’s a a b b c c downfield upfield

5 Equivalent & Non-Equivalent Hydrogens As seen in the 1 H NMR spectrum of ethanol, the number of signals equals the number of different types of protons in a compound. General rules: Protons attached to the same sp 3 carbon are equivalent (if there are no chiral centers in the molecule; if there are, could be equivalent or non-equivalent). If there is symmetry in the molecule, protons that are symmetrical will have the same signal, the same chemical shift. Note: Protons attached to the same sp 2 carbon (in alkenes) need to be evaluated for equivalency.

6 Equivalent & Non-Equivalent Hydrogens Consider the following molecules. Determine which protons are equivalent and non-equivalent. Predict the number of signals that would appear in the 1 H NMR spectra of these compounds.

7 1 H NMR Spectrum of Ethanol: Spin-Spin Splitting ppm (  ) CH 3 CH 2 O H TMS a - triplet b - quartet c - singlet a a b b c c downfield upfield

8 Spin-Spin Splitting CH 3 CH 2 O H a - triplet b - quartet c - singlet a b c General rules: Neighboring, non-equivalent protons split each other’s signals Equivalent protons do not split each other’s signals Use the n + 1 rule to predict the splitting pattern of a proton’s signal n + 1 rule The signal of a proton with n equivalent neighboring protons is split into a multiplet of n + 1 peaks. In ethanol, a neighbors b; they split each other’s peaks. Note that b neighbors c and no splitting occurs between the two; b is only affected by a. In general, protons that reside on heteroatoms (O, N) do not get involved with spin-spin splitting with neighboring protons. Thus, c appears as a singlet.

9 Spin-spin Splitting


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