ORGANIC CHEMISTRY: NMR spectroscopy

NMR: Nuclear Magnetic Resonance

Nuclear Magnetic Resonance [NMR]

This is radiation emitted by spinning protons in certain elements. Protons are made to spin under certain magnetic frequencies. The excited spinning protons fall back to ground state in a relaxation process by emitting a radio-frequency that can be detected and recorded to determine the position or chemical environment of the nucleus that emitted the radiation. The spectroscopy of the spinning nuclei is called NUCLEAR MAGNETIC RESONANCE or simply NMR.

Elements usually used for NMR spectroscopy include Carbon, Hydrogen, Nitrogen, Oxygen, Sulfur and Fluorine and Phosphorus. These have ½ nuclear spins or odd multiples of ½ spin.

TERMINOLOGY

Multiplicity: Number of signal peaks from a proton in a molecule.

Coupling:  Interaction between two elements from spinning of the nuclei

Coupling constant: Number representing magnitude of coupling of two element nuclei spins.

SHIFT: How exposed or shielded a nucleus has been to the external field. This gives the chemical shift number value for the signal position of the element. The peak position on x-axis in NMR spectrum is called a CHEMICAL SHIFT in hertz. [0 to 12].

The position of a peak signal is shifted UPFIELD to higher wavenumbers in the spectrum, if a proton is more exposed to the external magnetic field due to de-shielding chemical environment such as

1. Presence of electronegative atom or group.
2. Presence of de-shielding hybridization such as sp or sp² carbon

Exposed or de-shielded nuclei appear as signals in HIGHER wavenumbers up-shift or up-field in the spectrum.

The position of a peak signal is shifted DOWNFIELD to lower wavenumbers in the spectrum, if a proton is less exposed to the external magnetic field by a shielding electron cloud in the chemical environment such as

1. Presence of bulky less electronegative atom or group.
2. Presence of BULKY sp³ hybridized carbon bonds, atoms or groups.
3. Presence of pi bond electron cloud

Shielded nuclei appear as signals with lower wavenumbers down-shift or down-field in the spectrum.

De-shielding of electron cloud can be caused by

1. highly electronegative such as HALOGENS, OXYGEN
2. high s-character hybrid molecular orbitals such as sp, sp² presence in the molecule.
3. Positive effective charge on the molecule.

SIGNAL INTEGRATION

SIGNAL INTEGRATION: number of protons per signal peak in NMR spectrum.

Integral sign used and can be measured from the computer or manually from the relative RATIOS of the vertical lengths or relative ratios of the AREAS of each integral.

E.g. A peak signal integral from one two protons will be twice the peak integral of a signal caused by only one proton in the same molecule.

The height intensity or intensity of a peak signal relates to the number of protons causing the signal to be produced.

E.g. Singlet peak from a single proton is shorter peak compared to the height intensity of a singlet peak caused by two, three, four, five or six protons.

TYPES OF SIGNAL PEAKS IN PROTON NMR

Proton or protons would give a single peak signal (SINGLET) if

1. because the chemical environment is the same for each proton in each compound. Meaning the protons are the same or EQUIVALENT or HOMOTOPIC.
2. They are isolated by more than 3 or 4 bonds away
3. Isolated attached to a different element other than carbon.

EXAMPLE

Protons in the structures below will show as single peak or SINGLET in the NMR spectrum because they have no adjacent protons of different chemical environment that will split their peak signals. However, each singlet would appear at a different chemical shift and integrals will equal the number of protons for the singlet peak.

CH₄ , CH₃-C(Br)=O , -A-CH₂-A- , H₂C=O ,  -COOH ,  CH₃-BrC=CBr-CH₃ , -A-C(H)=O ,

, A-NH₂

The following below have more than one singlet peaks

SIGNAL COUPLING

Proton in a different chemical environment has a different chemical shift and therefore a different spin frequency relaxation magnetic effect.

Magnetic effect from spin relaxation of a proton in a different chemical environment is different can interact with the magnetic effect of a proton spin on adjacent carbon of different chemical environment. This interaction called SPIN Coupling causes the splitting of the total magnetic effect of the proton spin relaxation into multiples of signal peaks in the NMR spectrum. Number of peak signals corresponds to the number of protons on the adjacent carbon of different chemical environment.

There is NO splitting of signals of protons of the same chemical environment.; all proton spin relaxation occurs in one relaxation mode or direction. Corresponding to only ONE peak signal.

Splitting of proton signals usually occur if the two protons are separated by 3 bonds. This is the number

of bond separation that gives clear and meaningful splitting of signal speaks in proton NMR spectrum.

A is same as B = NO coupling or splitting of peaks = single peak = singlet peak only

A is not B = COUPLING = SPLITTING of signal peak into Doublet of doublets

MULTIPLICITY

 

NUMBER OF PEAKS EXPECTED FROM A PROTON IN A MOLECULE = MULTIPLICITY = N+ 1 [ where N = Number of nonequivalent protons on adjacent carbons that can split the peak signal. ]

Nonequivalent protons are protons on adjacent carbon with a different chemical environment.

A proton will show multiple peaks or MULTIPLETS when its signal peak is split by more than two nearby protons.

Multiplets can be a QUARTET, PENTET, SEXTET, HEPTET or more.

E.g.  All these structures [ A-CH₃ ,  A-CH₂-CH₂-A, CH₄  , CH₃-CH₃ , Benzene , cyclohexane ] would give a SINGLET peak  at room temperature because all the protons are equivalent or homotopic and therefore N=0 . Multiplicity = 1 = singlet.

Molecules with more than one singlet peaks have protons located at positions where their signals cannot be split by other proton signals.

Coupling can occur if protons are separated by carbons of different hybridizations [sp, sp2 or sp3]

H-(Cl)C=C(Br)-H

Signal splitting due coupling through double bond. Cis / trans coupling will have different coupling constants.

Proton bonded to a DIFFERENT element can split with proton bonded to carbon but exchange of these protons between the molecules and water impurities will remove the coupling splitting and cause the hetero protons to appear as singlets on low resolution instruments. A resolution spectrum will easily show the coupling splitting of heteroatom-H coupling with C-H protons. E.g.