The Number of Absorptions Protons have different chemical shifts when they are in different chemical environments Types of protons: – Homotopic Protons Chemically and electronically equivalent Have identical chemical shifts (δ) Same product is formed regardless of which H is replaced by

Heterotopic Protons: 1)Constitutionally Heterotopic Identical atoms in non-equivalent chemical environments Have different chemical shifts Give constitutional isomers upon replacement by X 2

2)Enantiotopic protons Not identical, but same electronically Have identical chemical shifts (when in an achiral solvent) Replacement by X leads to different enantiomers 3

3)Diastereotopic protons – Have different chemical shifts – Replacement by X leads to different diastereomers 4

The Number of Absorptions The two most common situations for diastereotopic protons are: The NMR Spectrum: Chemical Shift and Integral

Integration The relative peak area (not height!) depends on the number of contributing protons The NMR Spectrum: Chemical Shift and Integral

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Peaks, Peeks, Piques! 8

The n+1 Rule n + 1 rule: protons that have n equivalent neighboring protons show n + 1 peaks in their NMR spectrum – Called “Splitting” or “Spin-Spin Splitting” – Arises from the effect that one set of protons has on neighboring protons Allows you to figure out how many equivalent protons are adjacent to your proton(s) in question – If your absorbance is: 1 peak – No H’s on adjacent carbons – Singlet 9

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2 peaks – One H on adjacent carbon – Doublet 3 peaks – Two adjacent protons – Triplet Hence, splitting provides connectivity information 11

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Splitting Patterns The NMR Spectrum: Spin-Spin Splitting

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Spin-Spin Splitting Splitting is always mutual: If H a splits H b, then H b must split H a H a and H b are then said to be coupled Splitting is not observed between chemically equivalent protons The NMR Spectrum: Spin-Spin Splitting

NMR Spectrum for Iodomethane 16

With saturated carbons, splitting is normally not observed between protons on nonadjacent carbons 17

Coupling Constants The spacing between adjacent peaks of a splitting pattern is the coupling constant (J) The value of J is reported in Hz Two coupled protons must have the same value of J The coupling constant does not vary with the operating frequency of the spectrometer The NMR Spectrum: Spin-Spin Splitting

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Why Splitting Occurs Consider all of the spin permutations for the methylene group (-CH 2 -) protons in CH 3 CH 2 B r 20

NMR Information The three elements of an NMR spectrum (chemical shift, integration, splitting) can piece a molecular structure together The information in an NMR spectrum is oftentimes reported in text form For ethyl bromide: The NMR Spectrum: Spin-Spin Splitting

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NMR Spectra of Alkanes and Cycloalkanes Alkanes absorb in a narrow region (  0.7 – 1.7) One exception is that of cyclopropanes which are typically around  0.0 – 0.5 (or lower) Characteristic Functional-Group NMR Absorptions

Dynamic Systems Cyclohexane possess two diastereotopic sets of protons (axial and equatorial) However, only one singlet appears at  1.4 Cyclohexane undergoes rapid conformational interconversion On the NMR timescale this interconversion appears as a blur 25

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NMR Spectroscopy of Dynamic Systems

13 C NMR Spectroscopy 28

13 C NMR Spectroscopy 13 C NMR covers a 200 ppm range Relative abundance of 13 C is low – NMR machine must add together thousands of spectra to get adequate intensity 13 C- 13 C coupling is not generally observed

Chemical Shift in 13 C NMR 30

Chemical Shift in 13 C NMR Carbon NMR

Chemical Shift in 13 C NMR The chemical shift is also influenced by the number of attached carbons Typically  (quaternary) >  (tertiary) >  (secondary) >  (primary) Carbon NMR

Chemical Shift in 13 C NMR Also, in general:  (C=O) >  (sp) >  (sp 2 ) >  (sp 3 ) 33

Additional Information from 13 C NMR 13 C NMR reveals whether a molecule possess symmetry Carbon NMR

13 C NMR spectra are not normally integrated Generally, carbons that bear no hydrogens are usually smaller (quaternary carbons, α- carbons of tertiary alcohols, carbonyl carbons) 35

DEPT 13 C NMR Distortionless enhancement with polarization transfer The DEPT technique yields separate spectra for methyl, methylene, and methine carbons Each line in these spectra corresponds to a line in the complete 13 C NMR spectrum Quaternary carbons do not appear in any of the DEPT spectra Carbon NMR

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DEPT 13 C NMR Carbon NMR

Combined Spectroscopy Some suggestions for solving structures 1. Find the molecular mass if possible (MS) 2. Use elemental analysis to calculate the molecular formula 3. Identify functional groups or fragments (IR, NMR) 4. Determine if there is symmetry ( 13 C NMR, but possibly from 1 H NMR too) Solving Structure Problems with Spectroscopy

Combined Spectroscopy 5. Write out partial structures and all possible complete structures that are consistent with the spectra and molecular formula 6. Rationalize all spectra for consistency with the proposed structure Solving Structure Problems with Spectroscopy

FT-NMR Modern instruments employ pulse-Fourier- transform NMR (FT-NMR) In FT-NMR all proton spins are excited instantaneously with a broad band rf pulse The spectrum is obtained by analyzing the emission of rf energy as the spins reestablish equilibrium This permits the collection of a large number of scans (50 – 20,000) in a relatively short time The NMR Spectrometer

FT-NMR A computer stores, analyzes, and mathematically sums the scans Random electronic noise sums to zero when averaged over many scans This dramatically enhances the signal-to-noise This technique is particularly important to 13 C NMR considering the low natural abundance of the 13 C nucleus The NMR Spectrometer

Other Useful Forms of NMR Solid-state NMR: Can study the properties of solid substances (coal, drugs, polymers) Phosphorous NMR ( 31 P): Can be used to study biological processes (including intact cells or even whole organisms) NMR tomography or magnetic resonance imaging (MRI): Physicians can image organs without X-rays or other harmful radiation Other Uses of NMR

Magnetic Resonance Imaging Other Uses of NMR