Proton Nuclear Magnetic Resonance Spectroscopy

Full text

(1)

Proton Nuclear Magnetic Resonance Spectroscopy

In this laboratory exercise we will learn how to use the Chemistry Department's Nuclear Magnetic Resonance (NMR) spectrometer and how to interpret the spectra obtained using this spectrometer. NMR is one of the most powerful techniques available to the organic chemist for molecular structure determination. Therefore, knowing how to obtain and interpret NMR spectra is of critical importance.

The proton at the heart of the Hydrogen atom, like the electron, exhibits behavior reminiscent of a spinning top. And, like the electron, its “spin” is quantized; limited to the states of “Up” and

“Down”. In the presence of a strong external magnetic field (Ho), the energy of the two spin states splits; the stronger the field the greater the splitting.

A photon whose frequency ( = c/) is such that its energy (Ephoton) matches the energy difference (E) between the spin states can be absorbed by the proton and flip the proton's spin:

 E = Ephoton = hc / 

where h is Planck's constant (6.626 x 10-34 J sec), c is the speed of light (300 x 108 m/sec) and  is the photon's wavelength. For magnetic fields typically employed in modern NMR

spectrometers the spin state splitting is such that these photons will lie in the Radio Frequency region of the electromagnetic spectrum. Absorbance of these photons can then be detected by the spectrometer.

(2)

P a g e | 2

Because the splitting energy depends of the size of the NMR spectrometer's magnet field, the frequency of absorbance will also depend on the machine's magnet "size." Therefore, signal absorbance is generally reported in terms of a machine independent Chemical Shift (). The chemical shift of a signal is defined as the signal's frequency "downfield" from a reference compound's signal (Tetramethylsilane, THS, (CH3)4Si), reported in Hertz (Hz) relative to the machine's magnet "size," reported in MegaHertz (MHz):

 =

So, if a proton's signal is 2130 Hz downfield from the TMS signal produced by a 300 MHz NMR, it would have a chemical shift value of:

 = = 7.1 ppm

Chemical shift values have units of parts per milliom (ppm).

Now, this would all be rather uninteresting if all the Hydrogen atoms in a molecule had nuclei that absorbed at exactly the same frequency; we would observe a single absorbance peak in the spectrum. However, locally, within a molecule, each Hydrogen atom will be in a different magnetic environment. This is because the electron cloud surrounding a nucleus can act like a small wire loop within which an electric current induced by the external magnetic field

establishes a local magnetic field which acts in opposition to the externally established field.

This local field "shields" the nucleus from the external field. Slight molecular differences then lead to slight differences in the shielding experienced by different protons within a molecule and this causes their associated NMR signals to exhibit different chemical shifts.

For instance, the molecule CH3CCl2CH2Cl has protons in two different "magnetic"

environments, CH3- and -CH2-, and so will produce a spectrum with two signals of slightly different chemical shift. The CH3- signal occurs at 2.23 ppm and that of -CH2- at 4.00 ppm.

(3)

Introduction to Organic Chemistry, 2nd Ed.

Andrew Streitwiesser & Clayton H. Heathcock

Notice also that the CH3- signal is "larger," has a greater area, than the -CH2- signal. This is because it is due to the absorbance of three protons, versus two protons for the -CH2- signal.

Typical chemical shift values for common proton types are provided below. You should become familiar with these values.

(4)

P a g e | 4

Finally, neighboring protons can influence the

"magnetic" environment of each other via a mechanism called spin-spin coupling. Consider two neighboring protons A and B. A will observe that B can occupy its two possible spin states. Whether B is spin Up or Down will influence the spin states of A; the coupling is through the chemical bonds connecting the Hydrogen atoms. One case will cause A to absorb at a slightly higher frequency and the other a slightly lower frequency. This will lead to a splitting of A’s absorbance into a “doublet”.

If A couples to two Hydrogen atoms, then the splitting will occur again and a “triplet” will be observed; etc.

The relative intensities of simple multiplets are:

Multiplet Intensities Doublet 1:1 Triplet 1:2:1 Quartet 1:3:3:1 Quintet 1:4:6:4:1 Sextet 1:5:10:10:5:1 Septet 1:6:15:20:15:6:1

For example, the molecule CH3CH2Cl should exhibit two NMR signals. The first is due to the CH3- protons and will be split into a triplet due to the neighboring two protons. The other signal, due to the -CH2- protons, should be split into a quartet. As seen in the spectrum below, this is in fact the case:

(5)

Introduction to Organic Chemistry, 2nd Ed.

Andrew Streitwiesser & Clayton H. Heathcock

So, spectral information concerning Chemical Shift and Coupling will provide important clues about the molecular environment of each Hydrogen atom in a molecule. This information is extracted from the spectrum via:

 The number of NMR signals tells us how many different "types" of protons occur within the molecule.

 The relative intensity of an NMR signal tells us the relative number of protons of that

"type" which occur within the molecule.

 The chemical shift of an NMR signal tells us the nature of the environment the protons find themselves in within the molecule.

 The splitting pattern of the NMR signal tells us about the neighboring protons within the molecule.

Let us consider the following spectrum from Streitwiesser and Heathcock:

(6)

P a g e | 6

A separate analysis of the compound producing this spectrum indicates it has the chemical formula C2H4Br2. The spectrum exhibits two signals, indicating the molecule possesses two types of protons. The integrated areas of the signals are in a 3:1 ratio, indicating one signal (~2.25 ppm) is due to 3 protons and the other (~5.75 ppm) is due to one proton. Thus, we have signals due to CH3- and -CH- moieties. The signal due to the CH3- moiety is split into a doublet, indicating it neighbors a -CH- moiety. The signal due to the -CH- moiety is split into a quartet, indicating it neighbors a -CH3 moiety. Thus, the molecular structure that produced this spectrum should be: CH3-CHBr2. In this way, NMR spectra can be used to elucidate molecular structure.

The Department's NMR is a Bruker 400 MHz machne, similar to the one pictured below.

Sample Insertion Pt.

Liquid N2 Fill Port

Liquid N2 Dewar Surrounding the Electromagnet

(http://www.chem.mq.edu.au/~vislab/galleries/equipment/imagepages/400NMR.html)

The magnetic field in which the sample is inserted is generated by a 400 MHz superconducting solenoid. The solenoid is cooled in liquid He jacketed with a liquid Nitrogen dewar such that a sufficiently low temperature can be achieved to allow the windings of

the solenoid to exhibit superconductivity. This electromagnet is capable of producing very large magnetic fields, thereby increasing the resolution of the instrument significantly. A solution of the sample is placed in a small glass tube and is inserted into the magnet. It is then pulsed with RF radiation and the resulting signal is Fourier

Transformed into the type of spectrum we have been examining.

(7)

In this lab exercise, you will obtain and examine the spectra of some simple compounds. You will then be asked to correlate the spectral peaks with the Hydrogen atoms in the molecules of these compounds. You should become familiar with how to interpret NMR spectra in terms of molecular structure.

(8)

P a g e | 8

Procedure

Your laboratory instructor will demonstrate how to:

 prepare a sample for NMR analysis.

 take an NMR spectrum.

Special Deuterated solvents are available with which solutions of your compounds can be prepared. The deuterated solvent with then interfere with the proton NMR spectrum minimally.

These solvents typically come with the TMS reference already added.

You will be expected to master the use of the Department's spectrometer such that you can independently take spectra without assistance.

Each person in the laboratory class should take the spectrum of one of the following compounds:

Ethyl Amine t-Butyl Chloride

Ethyl Benzoate p-Bromo Toluene

2-Bromo Butane 1,2-Dichloro Ethane

Then, as a class, you should collectively assign all the peaks in each spectrum to the Hydrogen atoms of the molecules comprising the compounds.

Figure

Updating...

References

Updating...