Basics of Nmr Spectroscopy

7/30/2019 Basics of Nmr Spectroscopy

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BASICS OF NMR SPECTROSCOPY


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Principle

The NMR phenomenon is based on the fact that nuclei of atoms have

magnetic properties that can be utilize to yield chemical information's.

many nuclei can be studied with NMR spectroscopy but most commonly it

is used for hydrogen and carbon atoms.


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Many atomic nuclei have a property called spin. The nuclei behave as they were

spinning. In fact any atomic nucleus that possesses either odd mass, odd atomic

number, or both has a quantized spin angular momentum. e.g. atoms such as 1H, 13C,

19F, 31P, 17O have nuclear spin.

A spinning charge creates a magnetic moment, so these nuclei can be thought of as

tiny magnets.

Nuclei of ordinary Oxygen 16O and carbon 12C do not posses that spinningproperty.


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Since a nucleus is a charged particle in motion, it will develop a magnetic field.

1H and 13C have nuclear spins of 1/2 and so they behave in a similar fashion to asimple, tiny bar magnet.

In the absence of a magnetic field, these are randomly oriented but when a field

is applied they line up parallel to the applied field, either spin aligned or spinopposed.

The more highly populated state is the lower energy spin state spin alignedsituation.

Two schematic representations of these arrangements are shown below:


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In NMR, EM radiation is used to "flip" the alignment of nuclear spins from the low

energy spin aligned state to the higher energy spin opposed state. The energy

required for this transition depends on the strength of the applied magnetic field

(see below) but in is small and corresponds to the radio frequency range of the EM

spectrum.

As this diagram shows, the energy required for the spin-flip depends on the magnetic field

strength at the nucleus. With no applied field, there is no energy difference between the spin

states, but as the field increases so does the separation of energies of the spin states and

therefore so does the frequency required to cause the spin-flip, referred to as resonance.


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Reason and Significance of Chemical Shifts

Why do different protons appear at different d? There areseveral reasons, one of which is shielding. The electrons in abond shield the nuclei from the magnetic field. So, if there ismore electron density around a proton, it sees a slightly lower

magnetic field, less electron density means it sees a highermagnetic field.

The various factors that influence the field include:

Inductive effect by electronegative groups

Magnetic anisotropy

Hydrogen bonding


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Magnetic anisotropy

In applied magnetic field, the valance / electrons are caused to circulate.

This circulation is called local diamagnetic current. It generates a counter

magnetic field which opposes or supports the applied magnetic field.


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Interpreting Spectra

What kinds of data do we get from NMR spectra?

For 1H NMR, there are three kinds each of which we will consider

each of these separately:

Chemical shift data - tells us what kinds of protons we have.

Integrals - tells us the ratio of each kind of proton in our sample.

1H - 1H coupling - tells us about protons that are near otherprotons. The spectrum of ethyl acetate is shown below


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Chemical Shift Data

As mentioned, different kinds of protons typically come at different chemical

shifts. Shown below is a chart of where some common kinds of protons appearin the d scale. Note that most protons appear between 0 and 10 ppm. Thereference, tetramethylsilane (TMS) appears at 0 ppm, and aldehydes appearnear 10 ppm.

ppm

TMS

CH3CH3

RONR2

CH3OCH3

RO

HR

R R

HH

RO

Ph CH3

HR

Cl

CH3

Ph

OH

OH

R

NHR

Upfield region

of the spectrum

Downfieldregion

of the spectrum

TMS = Me Si

Me

Me

Me

012345678910

CH3HO(R)


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integrals

Integrals tell us the ratio of each kind of proton. They are lines, the heightsof which are proportional to the intensity of the signal. Consider ethyl acetate.There are three kinds of protons in this molecule, the CH3 next to the

carbonyl, the CH2 next to the O and the CH3 next to the CH2. The ratio ofthe signals arising from each of these kinds of protons should be 3 to 2 to 3,respectively. So, if we look at the height of the integrals they should be 3 to 2to 3. With this information, we can know which is the CH2 signal (its thesmallest one), but to distinguish the other two, we have to be able to predicttheir chemical shifts. The chart on the previous page allows us to make thatassignment (the CH3 next to the C=O should appear at ~ 2 PPM, while the

other CH3 should be at ~ 1 PPM

H3C O

O

3H'S

3H'S

O CH3

O

O

O H H

2 H'S


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1H - 1H Coupling

Signals, sometimes appear as multiple lines. This is due to 1H - 1Hcoupling (also called spin-spin splitting or J-coupling). Consider the belowmolecule. It turns out that HA feels the presence of HB. Recall that theseprotons are tiny little magnets, that can be oriented either with or againstthe magnetic field of the NMR machine. When the field created by HBreinforces the magnetic field of the NMR machine (B0 ) HA feels a slightlystronger field, but when the field created by HB opposes B0, HA feels aslightly weaker field. So, we see two signals for HA depending on thealignment of HB. The same is true for HB, it can feel either a slightly

stronger or weaker field due to HAs presence. So, rather than see a singleline for each of these protons, we see two lines for each.

C C

HBHA

HA HBHAis split into two lines becauseit feels the magnetic field of HB.

HBis split into two lines becauseit feels the magnetic field of HA.

For this line, HBis lined upwiththe magnetic field

(adds to the overallmagnetic field, so the line

comes at higher frequency)

For this line, HBis lined upagainstthe magnetic field(subtracts from the overallmagnetic field, so the line

comes at lower frequency)


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More 1H - 1H Coupling

What happens when there is more than one proton splitting a neighboring

proton? We get more lines. Consider the molecule below where we have twoprotons on one carbon and one proton on another.

C C

HBHA

HA'HA + HA' HB

HA and H A' appear at the samechemical shift because they are

in identical environmentsThey are also split into tw o lines(called a doublet) because they

feel the magnetic f ield of H B.

HB is split into three linesbecause it feels the magnetic

f ield of HA and HA'

Note that the signal producedby HA + HA' is tw ice the sizeof that produced by HB


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Why are There Three Lines for HB?

HB feels the splitting of both HA and HA. So, lets imagine starting with HB as a

single line, then lets turn on the coupling from HA and HA one at a time.

HB

Now, let's "turn on"HB - HA coupling. This splitsthe single line into two lines

If uncoupled, HB would appear a s asinglet where the dashed line indicates

the chemical shift of the singlet.

Now, let's "turn on"HB - HA' coupling. Thissplits each of the two new lines into two lines,

but not ice how the two lines in the middleoverlap. Overall, we then have three lines.

C C

HBHA

HA'


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Because the two lines in the middle overlap, that line is twice as big as the lines

on the outside. More neighboring protons leads to more lines as shown on the

next slide.


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Splitting Patterns with Multiple Neighboring Protons

If a proton has n neighboring protons that are equivalent, that proton will be split into

n+1 lines. The lines will not be of equal intensity, rather their intensity will be given by

Pascals triangle as shown below.

no. of neighbors re lative intensities patter n

1

1 1

1 2 1

1 3 3 1

1 4 6 4 1

1 5 10 10 5 1

1 6 15 20 15 6 1

0

1

2

3

4

5

6

singlet (s)

doublet (d)

triplet (t)

quartet (q)

pentet

sextet

septet

example

H

C C

H

H

C C

H

H

H

C C

H

H

H

H

C CC

H

H

H

H

H

C CC

H

H

HH

H

H

C CC

H

H

H

H

H

H


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More About Coupling

Earlier we said that protons couple to each other because they feel the magneticfield of the neighboring protons. While this is true, the mechanism by which theyfeel this field is complicated. They dont just feel it through space, itstransmitted through the electrons in the bonds. It turns out that when two protonsappear at the same chemical shift, they do not split each other. So, in EtBr, we havea CH3 next to a CH2, and each proton of the CH3 group is only coupled to theprotons of the CH2 group, not the other CH3 protons because all the CH3 protonscome at the same chemical shift.

C CH

H

H

H

HBr

The blueprotons all comeat the same chemical shiftand do not split each other

The redprotons both comeat the same chemical shiftand do not split each other


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Not all Couplings are Equal

Intensity by which protons couple to each other is called the coupling constant.

Coupling constants can vary from 0 Hz to 16 Hz. Typically, they are around 7 Hz,

but many molecules contain coupling constants that vary significantly from that. Sowhen a molecule contains a proton which is coupled to two different protons with

different coupling constants it give different pattern, described below.

So, if the protons are not equivalent, they can have different coupling constants

and the resulting pattern will not be a triplet, but a doublet of doublets.

Sometimes, nonequivalent protons can be on the same carbon as described on the

next slide.


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Magnitude of Some Typical Coupling Constants


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NMR spectra of some molecules


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Advanced techniques in NMR

Presently verity of advanced techniques are used to get more clear

spectral analysis of compounds. Some of these techniques are.

1. DEPT (Distortion-less Enhancement by Polarization Transfer)

2. 2DNMR (Two dimensional NMR)

3. COSY (Correlation Spectroscopy)

4.

NOESY (Nuclear Overhauser effect Spectroscopy)

5. HETCOR (Hetronuclear Correlation Spectroscopy)


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Basics of Nmr Spectroscopy