Organic Chemistry Semester 1 LABORATORY MANUAL - Moravian ...

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Nuclear Magnetic Resonance Appendix E18 Part V. Techniques and Theory Theory of Spin-Spin Splitting in Proton NMR Spectra: (See Also: CGWW pp. 258-268 and Padías pp. 87-90.) a. Structural requirements for Spin-Spin Splitting (1.) Splitting is caused by the effects of the magnetic field produced by one magnetic nucleus (in this case a proton) on the magnetic environments of other nuclei in the molecule. (2.) Protons are only affected by other magnetically non-equivalent protons (protons with different δ values). Thus, compounds such as Figure 9: (1.), (3.), (5.), & (6.) in which all of the protons are indistinguishable (equivalent) yield one peak that is not split, while spectra from compounds such as Figure 9: (2.) & (4.) with non-equivalent groups of protons often exhibit splitting. (3.) Although long-range splitting is possible in some situations, the major splitting (referred to as first order splitting) occurs only among protons that are attached to adjacent carbon atoms. Thus, Figure 9: compound (7.) yields two singlets (unsplit peaks) since its non-equivalent protons are separated by a CCl 2 group. Thus there are no protons on adjacent carbon atoms. However, the spectra of very similar compounds Figure: (2.), (4.), (8.) & (9.) show splitting because their non-equivalent protons are on adjacent carbon atoms. NOTE: Because protons on oxygen and nitrogen atoms tend to be acidic and are thus transferred from one molecule to another while the NMR spectrum is being collected, usually they do not split adjacent protons nor are they split by adjacent protons. b. The multiplicity of Spin-Spin Splitting. (How many peaks are produced) In general: Protons split by n equivalent adjacent nuclei yield (2nI + 1) peaks, where I is the nuclear spin quantum number of the adjacent nucleus. For protons I = 1 2 . So: For Splitting by Protons Peak Multiplicity = n + 1 Where n = # of equivalent protons on adjacent carbon atoms. c. Relative Intensities of peaks within a multiplet are given by the coefficients of the binomial expansion (X + Y) n , where n = the number of adjacent equivalent protons. These coefficients can be determined using Pascal’s Triangle. Pascal's Triangle n 1 1 1 1 2 1 2 1 3 1 3 3 1 4 1 4 6 4 1 5 1 5 10 10 5 1
Part V. Techniques and Theory Appendix E19 Nuclear Magnetic Resonance d. Examples: (1.) Consider Figure 3: compound (8.) on E8 • Why are there 4 peaks for only 2 protons J AX J AX Cl Cl Cl Cl C C C Cl Cl H H A X A X • Explanation: B o Proton A experiences an effective magnetic field, B eff , which is made up of B o (the external field), B el (the field produced by proton A's local environment) and B n (the field produced by proton X). H X H X antiparallel (shielding) B n B n parallel (deshielding) However, B n has 2 probable orientations with respect to B o . ∴ ~ 1 2 of the H A's experience the B n of H X parallel to B o (deshielding) and ~ 1 2 of H A's experience the B n of H X anti-parallel to B o (shielding). So half of the H A ’s absorb at a slightly lower frequency and the other half at a slightly higher frequency. (2.) Consider Figure 3: Compound (9.) on E8 • Why are there 3 peaks for proton A X X Cl Cl C Cl C H C Cl H H A X Cl A
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MORAVIAN COLLEGE CHEMISTRY DEPARTME
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Table of Contents (continued) 2 PAR
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I. Introduction 4 Fall 2010 Assignm
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I. Introduction 6 Fall 2010 Assignm
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Part I. Introduction 8 Fall 2010 La
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Part I. Introduction 10 Fall 2010 D
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Part II. Lab Rules and Regulations
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Part II. Lab Rules and Regulations
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III Laboratory Records & Report For
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Part V. Experiments 22 Fall 2010 Pa
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Part V. Experiments 24 Fall 2010 Ex
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Part V. Experiments 26 Fall 2010 H
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Page 64 and 65: Nuclear Magnetic Resonance Appendix
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Organic Chemistry Semester 1 LABORATORY MANUAL - Moravian ...

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