Nuclear Magnetic Resonance Spectroscopy

Asheesh Pandey

NMR = Nuclear Magnetic Resonance

Some (but not all) nuclei, such as 1H, 13C, 19F, 31P have nuclear spin. A spinning charge creates a magnetic moment, so these nuclei can be thought of as tiny magnets.If we place these nuclei in a magnetic field, they can line up with or against the field by spinning clockwise or counter clockwise.

Alignment with the magnetic field (called ) is lower energy than against the magnetic field (called ). How much lower it is depends on the strength of the magnetic field

Physical Principles:

Note that for nuclei that don’t have spin, such as 12C, there is no difference in energy between alignments in a magnetic field since they are not magnets. As such, we can’t do NMR spectroscopy on 12C.

S

A spinning nucleus with it's magnetic field aligned with the magnetic field of a magnet

- spin state,favorable,lower energy

N

S

N

N

S - spin state,unfavorable,higher energy

A spinning nucleus with it's magnetic field aligned against the magnetic field of a magnet

S

N

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• NMR is the most powerful tool available for organic structure determination.

• It is used to study a wide variety of nuclei:– 1H– 13C– 15N– 19F– 31P

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Nuclear Spin• A nucleus with an odd atomic number or an

odd mass number has a nuclear spin.• The spinning charged nucleus generates a

magnetic field.

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External Magnetic Field

When placed in an external field, spinning protons act like bar magnets.

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NMR: Basic Experimental PrinciplesImagine placing a molecule, for example, CH4, in a magnetic field. We can probe the energy difference of the - and - state of the protons by irradiating them with EM radiation of just the right energy.In a magnet of 7.05 Tesla, it takes EM radiation of about 300 MHz (radio waves).So, if we bombard the molecule with 300 MHz radio waves, the protons will absorb that energy and we can measure that absorbance.In a magnet of 11.75 Tesla, it takes EM radiation of about 500 MHz (stronger magnet means greater energy difference between the - and - state of the protons)

But there’s a problem. If two researchers want to compare their data using magnets of different strengths, they have to adjust for that difference. That’s a pain, so, data is instead reported using the “chemical shift” scale as described on the next slide.

E

Bo

E = h x 300 MHz E = h x 500 MHz

7.05 T 11.75 T

proton spin state (lower energy)

proton spin state (higher energy)

Graphical relationship between magnetic field (B o) and frequency (for 1H NMR absorptions

at no magnetic field,there is no difference beteen- and - states.

0 T

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Chemical shift δ is usually expressed in parts per million (ppm) by frequency, because it is calculated from

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Instrumentation:-Instrumentation:-• Types of NMR high resolution spectra

Continuous wave Fourier transform NMR NMR• Imp parts of the NMR spectrometer:- Permanent magnet/electromagnet RF generator RF detector Sample holder Magnetic coils

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diagram:

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• Principle:- based on

• frequency sweep field sweep frequency of RF source frequency is

is varied constant

Bo is constant Bo is varied

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CONSTRUCTION:-CONSTRUCTION:- Magnets:-• permanent:- constant Bo is generated that is 0.7;1.4;2.1o adv:-• construction is simple • cheaper• electromagnet:-Bo can be varied which is done by winding the

electromagnetic coil around the magnet• most expensive components of the nuclear magnetic resonance

spectrometer system Shim Coils• The purpose of shim coils on a spectrometer is to correct minor spatial

inhomogeneities in the Bo magnetic field. • These inhomogeneities could be caused by the magnet design, materials

in the probe, variations in the thickness of the sample tube, sample permeability.

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• A shim coil is designed to create a small magnetic field which will oppose and cancel out an inhomogeneity in the Bo magnetic field.

Superconducting solenoids:-• prepared from superconducting niobium-titanium wire and niobium-tin

wire• operated at lower temp.• kept in liquid He(mostly preferred) or liquid N2 at temp of 4 K• liquid N2 should be changed at 10 days while liquid He shouldd be

changed at 80-130 days• higher Bo can be produced that is upto 21 T.o Advantage:- High stability Low operating cost High sensitivity Small size compared to electromagnets simple

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RADIOFREQUNCY TRANSMITTER• It is a 60 MHz crystal controlled oscillator.• RF signal is fed into a pair of coils mounted at right angles to the path of

field.• The coil that transmit RF field is made into 2 halves in order to allow

insertion of sample holder .• 2 halves are placed in magnetic gap• For high resolution the transmitted frequency must be highly constant.• The basic oscillator is crystal controlled followed by a buffer doubler, the

frequency being doubled by tunning the variable• It is further connected to another buffer doubler tuned to 60 MHz• Then buffer amplifier is provided to avoid circuit loading.

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Signal amplifier and detector• Radiofrequency signal is produced by the resonating nuclei is

detected by means of a coil that surrounds the sample holder• The signal results from the absorption of energy from the

receiver coil, when nuclear transitions are induced and the voltage across receiver coil drops

• This voltage change is very small and it must be amplified before it can be displayed.

The display system• The detected signal is applied to vertical plates of an

oscilloscope to produce NMR spectrum• Spectrum can also be recorded on a chart recorder.

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Sample Probe The sample probe is the name given to that part of the

spectrometer which accepts the sample, sends RF energy into the sample, and detects the signal emanating from the sample.

• It contains the RF coil, sample spinner, temperature controlling circuitry, and gradient coils.

• It is also provided with an air driven turbine for rotating the sample tube at several hundred rpm

• This rotation averages out the effects of in homogeneities in the field and provide better resolution.

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The spectrum obtained either by CW scan or pulse FT at constant magnetic field is shown in series of peaks whose areas are proportional to the number of protons they represent.

Peak areas are measured by an electronic integrator that traces a series of steps with heights proportional to the peak areas.

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• A proton count from the integration is useful to determination or confirm molecular formulas, detect hidden peaks, determine sample purity, and to quantitative analysis.

• Peak positions are measured in frequency units from a reference peak.

• A routine sample for proton NMR on a 300MHz instrument consist of about 2mg of the compound in about 0.4 mL of solvent in a 5-mm o.d.glass tube.

• Under favorable conditions, it is possible to obtain a spectrum of 1μg of a compound of a compound of modest molecular weight in a microtube in a 300-MHz pulsed instrument.

• Microprobe that accept a 2.5mm or 3 mm o.d. tube are convenient and provide high sensitivity.

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The continuous-wave(CW) Instrument

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Fig. illustrates basic elements of a classical 60-MHz NMR spectrometer.

WORKING:-• The sample is dissolved in a solvent containing no interfering proton

(usually CCl4 and CDCl3), and a small amount of TMS is added to serve as an internal reference.

• The sample is a small cylindrical glass tube that is suspended in the gap between the faces of the pole pieces of the magnet.

• The sample is spun around the axis to ensure that all parts of the solution experience a relatively uniform magnetic field.

• Also in a magnetic gap is a coil attached to 60-MHz radiofrequency generator. This coil supplies the electromagnetic energy used to change the spin orientation of the protons.

• Perpendicular to the RF oscillator coil is a detector coil. When no absorption of energy is taking place, the detector coil picks up non of the energy given off by the RF oscillator coil.

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• when sample absorbs energy, however, the reorientation of the nuclear spins induces a radiofrequency signal in the plane of the detector coil, and the instrument responds by recording this as a resonance signal, or peak.

• Rather than changing the frequency of the RF oscillator to allow each of the protons in a molecule to come into a resonance, the typical NMR spectrometer uses a constant frequency RF- signal and varies the magnetic field strength.

• As the magnetic field strength is increased, the precessional frequency of all the protons increase. When the precesional frequency of a given type proton reaches 60MHz, it has resonance.

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• As each chemically distinct type of proton comes into resonance, it is recorded as a peak on the chart.

• The peak at δ = 0 ppm is due to the internal reference compound TMS.• IN THE CLASSICAL NMR EXPERIMENT THE INSTRUMENT SCANS FROM IN THE CLASSICAL NMR EXPERIMENT THE INSTRUMENT SCANS FROM

“LOW FIELD” TO “HIGH FIELD”“LOW FIELD” TO “HIGH FIELD”

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increasing Bo

HIGHFIELD

LOWFIELD

UPFIELDDOWNFIELD

NMR CHART

Pulsed Fourier Transform Spectroscopy

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• FTNMR uses a pulse of RF radiation which causes a nuclei in a magnetic field to flip into higher energy alignment

• Applying such a pulse to a set of nuclear spins simultaneously excites all the nuclei in all local environment.

• All the nuclei will re emit RF radiation at their resonance frequencies which induces a current in a nearby pickup coil, creating an electrical signal oscillating at the NMR frequency. This signal is known as the free induction decay (FID) and contains the sum of the NMR responses from all the excited spins.

• In order to obtain the frequency-domain NMR spectrum (intensity vs. frequency) this time-domain signal (intensity vs. time) must be Fourier transformed. Fortunately the development of FT-NMR coincided with the development of digital computers and Fast Fourier Transform algorithms.

• . This is the principle on which a pulse Fourier transform spectrometer operates. By exposing the sample to a very short (10 to 100 μsec), relatively strong (about 10,000 times that used for a CW spectrometer) burst of RF energy, all of the protons in the sample are excited simultaneously.

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FOURIER TRANSFORMFOURIER TRANSFORMA mathematical technique that resolves a complexFID signal into the individual frequencies that add together to make it.

COMPLEXSIGNAL 1 + 2 + 3 + ......

computer

FourierTransform

FT-NMR

individualfrequencies

TIME DOMAIN FREQUENCY DOMAIN

a mixture of frequenciesdecaying (with time)

converted to

converted to a spectrum

( Details not given here. )

FID NMR SPECTRUM

DOMAINS ARE MATHEMATICALTERMS

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The Composite FID is Transformed into a The Composite FID is Transformed into a classical NMR Spectrum : classical NMR Spectrum :

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CH2 CO

CH3

“frequency domain” spectrum

• Advantage over continuous wave NMR Sample of low conc. can be determined Magnetic nuclei with low natural isotopic abundance can be

determined eg 13C Very rapid pulse repetition can be possible Entire spectrum can be recorded, computerized and

transformed in a few seconds that is every 2 sec For e.g. in 13 min 400 spectra can be recorded. So thus 20 times signal enhancement is seen

Analysis can be possible where magnetogyric ratio is low

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Solvents used in NMRSolvents used in NMR• Properties:- Nonviscous. Should dissolve analyte. Should not absorb within spectral range of analysis. All solvents used in NMR must be aprotic that is they should not possess

proton.• Chloroform-d (CDCl3) is the most common solvent for NMR

measurements • other deuterium labeled compounds, such as deuterium oxide

(D2O), benzene-d6 (C6D6), acetone-d6 (CD3COCD3) and DMSO-d6 (CD3SOCD3) are also available for use as NMRsolvents.

• DMF, DMSO, cyclopropane, dimethyl ether can also be used

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iNTRODUCTION Powerful analytical technique used to characterize organic

molecules by identifying carbon-hydrogen frameworks within molecules.

2 Types :

The source of energy in NMR is radio waves which have long wavelengths, and thus low energy and frequency.

This waves can change the nuclear spins of some elements, including 1H and 13C.

Frequency in the range of 300Mhz-500Mhz.

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13C -NMR1H -NMR

Two Energy StatesThe magnetic fields of

the spinning nuclei will align either with the external field, or against the field.

A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. =>

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E and Magnet Strength• Energy difference is proportional to the

magnetic field strength.• E = h = h B0

2• Gyromagnetic ratio, , is a constant for each

nucleus (26,753 s-1gauss-1 for H).• In a 14,092 gauss field, a 60 MHz photon is

required to flip a proton.• Low energy, radio frequency. =>

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Magnetic Shielding• If all protons absorbed the same amount of

energy in a given magnetic field, not much information could be obtained.

• But protons are surrounded by electrons that shield them from the external field.

• Circulating electrons create an induced magnetic field that opposes the external magnetic field. =>

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Shielded ProtonsMagnetic field strength must be increased for

a shielded proton to flip at the same frequency.

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Protons in a Molecule

Depending on their chemical environment, protons in a molecule are shielded by different amounts.

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NMR Signals

• The number of signals shows how many different kinds of protons are present.

• The location of the signals shows how shielded or deshielded the proton is.

• The intensity of the signal shows the number of protons of that type.

• Signal splitting shows the number of protons on adjacent atoms. =>

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The NMR Spectrometer

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The NMR Graph

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Tetramethylsilane

• TMS is added to the sample.• Since silicon is less electronegative than

carbon, TMS protons are highly shielded. Signal defined as zero.

• Organic protons absorb downfield (to the left) of the TMS signal.

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Si

CH3

CH3

CH3

H3C

Chemical Shift

• Measured in parts per million.• Ratio of shift downfield from TMS (Hz) to total

spectrometer frequency (Hz).• Same value for 60, 100, or 300 MHz machine.• Called the delta scale.

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Delta Scale

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Location of Signals• More electronegative atoms

deshield more and give larger shift values.

• Effect decreases with distance.

• Additional electronegative atoms cause increase in chemical shift. =>

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Typical Values

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O-H and N-H Signals

• Chemical shift depends on concentration.• Hydrogen bonding in concentrated solutions

deshield the protons, so signal is around 3.5 for N-H and 4.5 for O-H.

• Proton exchanges between the molecules broaden the peak. =>

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Carboxylic Acid Proton, 10+

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Number of SignalsEquivalent hydrogens have the same chemical

shift.

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Intensity of Signals• The area under each peak is proportional to

the number of protons.• Shown by integral trace.

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How Many Hydrogens?When the molecular formula is known, each

integral rise can be assigned to a particular number of hydrogens.

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Spin-Spin Splitting

• Nonequivalent protons on adjacent carbons have magnetic fields that may align with or oppose the external field.

• This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed.

• All possibilities exist, so signal is split. =>

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Calculating SHIFT VALUES:Added Chemical Shifts

Substituent Type of Hydrogen -Shift -Shift

C C CH3 0.78 --- CH2 0.75 -0.10 CH --- ---

RC C C

Y

[Y = C or O] CH3 1.08 --- Aryl- CH3 1.40 0.35 CH2 1.45 0.53 CH 1.33 --- Cl- CH3 2.43 0.63 CH2 2.30 0.53 CH 2.55 0.03 Br- CH3 1.80 0.83 CH2 2.18 0.60 CH 2.68 0.25 I- CH3 1.28 1.23 CH2 1.95 0.58 CH 2.75 0.00 OH- CH3 2.50 0.33 CH2 2.30 0.13 CH 2.20 --- RO- (R is saturated) CH3 2.43 0.33 CH2 2.35 0.15 CH 2.00 ---

R–CO

Oor ArO CH3 2.88 0.38

CH2 2.98 0.43 CH 3.43 --- (ester only)

R–C

O

CH3 1.23 0.18 where R is alkyl, aryl, OH, CH2 1.05 0.31 OR', H, CO, or N CH 1.05 ---

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(Hydrogen under consideration)C C HH

H

H

HCl

Base Chemical Shift = 0.87 ppm no substituents = 0.00 one - Cl (CH3) = 0.63 TOTAL = 1.50 ppm

(Hydrogen under consideration)C C HH

H

H

HCl

Base Chemical Shift = 1.20 ppm one - Cl (CH2) = 2.30 no substituents = 0.00 TOTAL = 3.50 ppm

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1,1,2-Tribromoethane

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Nonequivalent protons on adjacent carbons.

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More 1H - 1H CouplingWhat happens when there is more than one proton splitting a neighboring proton? We get more lines. Consider the molecule below where we have two protons on one carbon and one proton on another.

C C

HBHA

HA'HA + HA' HBHA and HA' appear at the same chemical shift because they are

in identical environments They are also split into two lines (called a doublet) because they

feel the magnetic field of HB.

HB is split into three lines because it feels the magnetic

field of HA and HA'

Note that the signal produced by HA + HA' is twice the size

of 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, let’s imagine starting with HB as a single line, then let’s “turn on” the coupling from HA and HA’ one at a time:

HB

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

If uncoupled, HB would appear as a singlet where the dashed line indicates

the chemical shift of the singlet.

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

but notice how the two lines in the middle overlap. Overall, we then have three lines.

C C

HBHA

HA'

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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no. of neighbors relative intensities pattern

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

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. So, if we have four equivalent neighbors, we will have five lines, six equivalent neighbors… well, you can do the math. The lines will not be of equal intensity, rather their intensity will be given by Pascal’s triangle as shown below.

We keep emphasizing that this pattern only holds for when the neighboring protons are equivalent. Why is that? The answer is two slides away.Asheesh Pandey

More About CouplingEarlier we said that protons couple to each other because they feel the magnetic field of the neighboring protons. While this is true, the mechanism by which they feel this field is complicated and is beyond the scope of this class (they don’t just feel it through space, it’s transmitted through the electrons in the bonds). It turns out that when two protons appear at the same chemical shift, they do not split each other. So, in EtBr, we have a CH3 next to a CH2, and each proton of the CH3 group is only coupled to the protons of the CH2 group, not the other CH3 protons because all the CH3 protons come at the same chemical shift.

C C

H

H

H

H

H

BrThe blue protons all come at the same chemical shift and do not split each other

The red protons both come at the same chemical shift and do not split each other

C C

H

H

H

H

H

BrC C

H

H

H

H

H

Br

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Not all Couplings are EqualWhen protons couple to each other, they do so with a certain intensity. This is called the “coupling constant.” Coupling constants can vary from 0 Hz (which means that the protons are not coupled, even though they are neighbors) to 16 Hz. Typically, they are around 7 Hz, but many molecules contain coupling constants that vary significantly from that. So, what happens when a molecule contains a proton which is coupled to two different protons with different coupling constants? We get a different pattern as described in the diagram 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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Doublet: 1 Adjacent Proton

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Triplet: 2 Adjacent Protons

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The N + 1 Rule

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If a signal is split by N equivalent protons, it is split into N + 1 peaks.

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Range of Magnetic Coupling

• Equivalent protons do not split each other.• Protons bonded to the same carbon will split

each other only if they are not equivalent.• Protons on adjacent carbons normally will

couple.• Protons separated by four or more bonds will

not couple. =>

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Splitting for Ethyl Groups

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Splitting for Isopropyl Groups

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Coupling Constants

• Distance between the peaks of multiplet• Measured in Hz• Not dependent on strength of the external

field• Multiplets with the same coupling constants

may come from adjacent groups of protons that split each other. =>

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Values for Coupling Constants

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Complex Splitting

• Signals may be split by adjacent protons, different from each other, with different coupling constants.

• Example: Ha of styrene which is split by an adjacent H trans to it (J = 17 Hz) and an adjacent H cis to it (J = 11 Hz). =>

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C CH

H

Ha

b

c

Splitting Tree

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C CH

H

Ha

b

c

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Spectrum for Styrene

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Some Nonequivalent Protons

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C CH

H

Ha

b

cOH

H

H

H

a

b

c

d

CH3

H Cl

H H

Cl

a b =>

Time Dependence• Molecules are tumbling relative to the

magnetic field, so NMR is an averaged spectrum of all the orientations.

• Axial and equatorial protons on cyclohexane interconvert so rapidly that they give a single signal.

• Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule. =>

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Hydroxyl Proton

• Ultrapure samples of ethanol show splitting.

• Ethanol with a small amount of acidic or basic impurities will not show splitting.

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N-H Proton

• Moderate rate of exchange.• Peak may be broad.

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Identifying the O-H or N-H Peak

• Chemical shift will depend on concentration and solvent.

• To verify that a particular peak is due to O-H or N-H, shake the sample with D2O

• Deuterium will exchange with the O-H or N-H protons.

• On a second NMR spectrum the peak will be absent, or much less intense. =>

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Carbon-13

• 12C has no magnetic spin.• 13C has a magnetic spin, but is only 1% of the

carbon in a sample.• The gyromagnetic ratio of 13C is one-fourth of

that of 1H.• Signals are weak, getting lost in noise.• Hundreds of spectra are taken, averaged.

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Fourier Transform NMR• Nuclei in a magnetic field are given a radio-

frequency pulse close to their resonance frequency.

• The nuclei absorb energy and precess (spin) like little tops.

• A complex signal is produced, then decays as the nuclei lose energy.

• Free induction decay is converted to spectrum. =>

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Hydrogen and Carbon Chemical Shifts

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Combined 13C and 1H Spectra

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Differences in 13C Technique

• Resonance frequency is ~ one-fourth, 15.1 MHz instead of 60 MHz.

• Peak areas are not proportional to number of carbons.

• Carbon atoms with more hydrogens absorb more strongly. =>

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Spin-Spin Splitting• It is unlikely that a 13C would be adjacent to

another 13C, so splitting by carbon is negligible.• 13C will magnetically couple with attached

protons and adjacent protons.• These complex splitting patterns are difficult

to interpret. =>

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Proton Spin Decoupling

• To simplify the spectrum, protons are continuously irradiated with “noise,” so they are rapidly flipping.

• The carbon nuclei see an average of all the possible proton spin states.

• Thus, each different kind of carbon gives a single, unsplit peak. =>

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Off-Resonance Decoupling

• 13C nuclei are split only by the protons attached directly to them.

• The N + 1 rule applies: a carbon with N number of protons gives a signal with N + 1 peaks. =>

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Interpreting 13C NMR

• The number of different signals indicates the number of different kinds of carbon.

• The location (chemical shift) indicates the type of functional group.

• The peak area indicates the numbers of carbons (if integrated).

• The splitting pattern of off-resonance decoupled spectrum indicates the number of protons attached to the carbon. =>

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Two 13C NMR Spectra

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MRI

• Magnetic resonance imaging, noninvasive• “Nuclear” is omitted because of public’s fear

that it would be radioactive.• Only protons in one plane can be in resonance

at one time.• Computer puts together “slices” to get 3D.• Tumors readily detected.

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