h Nmr Spectroscopy

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H NMR SPECTROSCOPY


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H NMR or proton magnetic resonance

Records signals from hydrogen nuclei in

different structural environments

Provides information of the hydrocarbon partthe compound : relative numbers of different

kinds of hydrogen in the compound;nature of

the carbons bonded to them; and which

hydrogen nearby


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Proton NMR vs. 13C NMR

1H is the major isotope of hydrogen (99.985% natural abundance),

while 13C is only a minor isotope (1.1%)

1H NMR is quantitative: the area under the peak tells us the number

of hydrogen nuclei, while 13C NMR may give strong or weak peaksfrom the same number of 13C nuclei

Protons interact magnetically (couple) to reveal the connectivity ofthe structure, while 13C is too rare for coupling between 13C nucleito be seen

1H NMR shifts give a more reliable indication of the local chemistry

than that given by 13C spectra


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1st Possibility


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2nd Possibility


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3rd Possibility


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4th Possibility


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Structural Information

1. Number of Signals

Each group of chemically equivalent protons gives

rise to a signal

Chemically Equivalent Protons

- protons that are in the same environment identical in

every way


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2. Position of Signals based on how far they are from the signal of the reference compound

tells us the kind of proton or protons that are responsible for the signal

and the kinds of neighbouring substituents depends on the chemical shift

Chemical Shift

- a measure of how far the signal produced from the proton is from the

reference signal


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The greater the chemical shift, the higher is the frequency.

Also, chemical shift is independent of the operating frequency

of the NMR spectrometer.


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Right hand side of an

NMR spectrum

Left hand side of an

NMR spectrum

Generalization:


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Distance from the electron-withdrawing group

protons closer to electron-withdrawing group

less shielded

higher frequency

protons farther to electron-withdrawing group

more shielded

lower frequency

Electron withdrawal causes NMR signals to appear at higher

frequencies (at larger chemical shift values)

Electronegativity

More electronegative

Higher frequency

Less electronegative

Lower frequency


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In a similar environment, the signal for methyl protons occurs at a lower

frequency than the signal for methylene protons, which in turn occurs at a

lower frequency than the signal for a methine proton.

3 signals

a protons lowest frequency (farthest)

b protons lower than the frequency of c

c protons highest frequency

B and C have the same distance but b is methyl protons appear

to be lower at frequency than methylene protons.

3 signals

a protons lowest frequency (farthest)

b protons lower than the frequency of c

c protons highest frequency

B and C have the same distance but b is methyl protons

appear to be lower at frequency than methine protons.


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3. Integration of NMR Signals

Tells the relative number of protons responsible for

the signal, not the absolute number

Integration

- area under each signal

- proportional to the number of protons that gives rise to signal

Beers Law amount of energy absorbed or transmitted is proportional to a certain

number of moles present


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1.6 : 7 = 1: 4.4 x 2 = 2 : 9

The numbers do not always correspond

to the exact or absolute number of

protons. Instead, it tells us the relative

number or ratio of the amount of

equivalent protons.


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Diamagnetic Anisotropy

Magnetic field created by pi electrons or rings

Describes an environment where different magnetic fields are found at

different points in space

Pi electrons are held less strongly than sigma electrons they are more

able to move in response to the magnetic field

Magnetic field applied to compound with pi electrons - electrons move in

a circular path - causes an induced magnetic field which affects the

chemical shift of a proton depends on the direction of the induced

magnetic field relative to the direction of the applied magnetic field


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Effect ofDiamagnetic Anisotropy

The unusual chemical shifts

associated with hydrogen

bonded to carbons that

form bonds are due to

diamagnetic anisotropy


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4. Splitting of the signals

caused by (and therefore tells us the number of)

protons bonded to adjacent carbons

N + 1 rule- number of splitting of the signal that occurs

- where N is the number of equivalent protons adjacent to C but not

equivalent to the proton giving rise the signal


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Multiplicity: The number of peaks in a signal

Coupled Protons: They are protons that split each others signalSinglet: 1 peak

Doublet: 2 peaks

Triplet: 3 peaks

Quartet: 1 peak

Quintet: 5 peaks

Multiplet: more complex than the other peaks


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Spin Spin Coupling

Different kinds of protons are close enough for their magnetic fields to

influence on another. The spin of one nucleus influences the spin of

another nucleus.


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The relative intensities of the peaks in a signal reflect the number of ways the

neighbouring protons can be aligned relative to the applied magnetic field.

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Long-range coupling:

occurs when the protons are separated by more than three

bonds and one of the bonds is a double or triple bond. A smallsplitting is observed.


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Spin Spin Coupling

Rules for proton-proton spin-spin coupling.

1. Only non equivalent protons couple. Equivalent protons are in the same environment,

and their signals overlap, so only non equivalent protons can split signals.

2. Protons that are separated by more than three single bonds usually do not couple

because they are not close enough to each other to be influenced by each other magnetic

fields. Pi bonds do not count toward this, but the coupling constants may be too small to

distinguish. Benzene rings have a coupling club in which they only couple with each

other. Nonequivalent benzene ring protons can couple with each other, but the coupling

constants may be too small to be significant. A benzene ring blocks coupling with other

protons outside of the benzene ring. This usually causes complicated splitting patterns.

3. Signals for O-H and N-H protons are usually singlets.


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