Proton Nuclear Magnetic Resonance Problem Session A Laboratory Report

match each compound with its 3H-NMR spectrum. Draw the structure above the corresponding spectrum and clearly assign the chemical shifts to the appropriate protons on the molecule

NMR LAB: Proton Nuclear Magnetic Resonance (1H NMR) Problem Session A Group Laboratory Exercise

OBJECTIVE

To understand the important role of nuclear magnetic resonance spectroscopy in the study of the structures of organic compounds

To develop an understanding of the significance of the number, positions, intensities, and splitting of signals in nuclear magnetic resonance spectra

To be able to assign structures to simple molecules on the basis of nuclear magnetic resonance spectra

INTRODUCTION

Spectroscopic methods are one of the more useful ways of structure determination in Organic

Chemistry. Usually, one will do a combination of several methods in order to elucidate the structure.

Three types that one commonly encounters in organic chemistry are Nuclear Magnetic Resonance

(NMR), Infrared (IR), and Mass Spectral (MS) analysis. As the name implies Nuclear Magnetic Resonance Spectroscopy involves applying an external magnetic field to bring the nucleus of certain atoms (1H or 13C) into resonance (equilibrium between a ground and an excited state). In each case, all the nuclei being either hydrogen or carbon will resonate at the same effective field strength, but depending on the environment, the applied field strength may differ. For example, in a video game, it may take one hit to liquidate the villain (effective); however, depending on the environment (buildings, woods, etc.), one may have to apply more “shots” to produce this one effective hit. In the plot produced by the spectrometer, the applied field (or a percentage of it) is plotted against the absorption (the intensity).There are four important outcomes that we shall consider for 1HNMR.

The Number of Signals Present

The number of signals will give an indication of the number of different types of hydrogens that are present. Protons in the same environment are chemically equivalent and will resonate at the same applied strength. Those in a different environment will require a different applied field and therefore will appear at a different position in the spectrum.

Examples:

  • CH – 1 signal, all H’s are in the same environment.
  • CH3CH3 – 1 signal, all H’s are in the same environment.
  • CH3CH2CH3 – 2 signals, the two sets of CH3 hydrogens are in the same environment, but are different from the CH2 hydrogens.
  • CH3-C=O OCH3 – 2 signals, the two CH3 groups are in different environments. One is attached to the C=O, and the other is attached to oxygen.

The Position of the Signal: Chemical Shift

The shift (chemical shift) is the exact place on the chart where the nucleus absorbs. Shifts are measured in parts per million (ppm), i.e., a percentage of the applied magnetic field. A typical 1H NMR scale is from 0 to 14 ppm, and in most cases, an internal standard (tetramethylsilane, TMS) is added and is used as the zero reference point. The chemical shift is dependent on the environment (what neighboring nuclei are present) of the hydrogen(s) in question. The nucleus of the hydrogen atom is shielded by the electron that is present, and the neighboring nuclei can either reinforce this shielding or oppose this shielding (deshielding). If the shielding is reinforced (as in the case of a neighboring hydrocarbon residue), a higher applied magnetic field is needed and the resonance will appear at lower ppm (upfield). If an electronegative element (halogen, oxygen), or an electron withdrawing group (C=O, Ar, C=C) is adjacent, then the shielding is lessened (deshielding occurs) and the resonance will appear at higher ppm (downfield). See the chart below how chemical shift appear in different ppm for difference resonance.

 

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Splitting of the Signal into Several Peaks: Spin-Spin Splitting

This is a powerful tool in deducing the 1HNMR spectra. The neighboring hydrogen nuclei (on an atom adjacent to the one in question) will cause the signal to split. Two things one should consider when examining spin-spin splitting are: Chemically equivalent hydrogen nuclei do not cause splitting to occur, whether they are on the same carbon or a different one.

 

Figure 1:

 

All three H’s are equilvalent; therefore, no splitting

 

 

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Figure 2: All 6 H’s are chemically equilvalent; therefore, no splitting

The signal for a hydrogen nuclei that has n equivalent neighboring H’s (on adjacent atoms) is split

into n + 1 peaks.

# of neighbors peaks

  1. 1 (singlet)
  2. 2 (doublet)
  3. 3 (triplet) 3 4 (quartet)

n n + 1 (multiplet)

 

 

 

 

 

1,2-dichloro-isomer (below left), which displays a single resonance signal from the four structurally equivalent hydrogens, the two signals from the different hydrogens are split into close groupings of two or more resonances. This is a common feature in the spectra of compounds having different sets of hydrogen atoms bonded to adjacent carbon atoms.

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The spectrum of 1,3-dichloropropane on the right demonstrates that equivalent sets of hydrogens may combine their influence on a second, symmetrically located set.

Even though the chemical shift difference between the A and B protons in the 1,3-dichloroethane spectrum is fairly large (140 Hz) compared with the coupling constant (6.2 Hz), some distortion of the splitting patterns is evident. The line intensities closest to the chemical shift of the coupled partner are enhanced. Thus the B set triplet lines closest to A are increased, and the A quintet lines ne

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Practice Problems

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