1 Nuclear Magnetic Resonance (NMR) Spectroscopy. Nuclear Spin A nucleus with an odd atomic number or...

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Nuclear Magnetic Resonance (NMR) Spectroscopy

Nuclear Spin

• A nucleus with an odd atomic number or an odd mass number has a nuclear spin. Nuclei of such kinds are 1H, 13C, 19F, 31P etc.

• The spinning charged nucleus generates a magnetic field.

=>

External Magnetic Field

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

=>

Two Energy States

The 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. =>

E and Magnet Strength

• Energy difference is proportional to the magnetic field strength. In a 14,092 gauss field, a 60 MHz photon is required to flip a proton.

• Low energy, radio frequency. =>

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•Nuclear magnetic resonance spectroscopy is a powerful analytical technique used to characterize organic molecules by identifying carbon-hydrogen frameworks within molecules.

•Two common types of NMR spectroscopy are used to characterize organic structure: 1H NMR is used to determine the type and number of H atoms in a molecule; 13C NMR is used to determine the type of carbon atoms in the molecule.

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

•When low-energy radio waves interact with a molecule, they can change the nuclear spins of some elements, including 1H and 13C.

Introduction to NMR Spectroscopy

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• When a charged particle such as a proton spins on its axis, it creates a magnetic field. Thus, the nucleus can be considered to be a tiny bar magnet.

• Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of a magnetic field B0, they are oriented with or against this applied field. More nuclei are oriented with the applied field because this arrangement is lower in energy.

• The energy difference between these two states is very small (<0.1 cal).


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• In a magnetic field, there are now two energy states for a proton: a lower energy state with the nucleus aligned in the same direction as B0, and a higher energy state in which the nucleus aligned against B0.

•When an external energy source (h) that matches the energy difference (E) between these two states is applied, energy is absorbed, causing the nucleus to “spin flip” from one orientation to another.

•The energy difference between these two nuclear spin states corresponds to the low frequency RF region of the electromagnetic spectrum.

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•Thus, two variables characterize NMR: an applied magnetic field B0, the strength of which is measured in tesla (T), and the frequency of radiation used for resonance, measured in hertz (Hz), or megahertz (MHz)—(1 MHz = 106 Hz).


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•The frequency needed for resonance and the applied magnetic field strength are proportionally related:

•NMR spectrometers are referred to as 300 MHz instruments, 500 MHz instruments, and so forth, depending on the frequency of the RF radiation used for resonance.

•These spectrometers use very powerful magnets to create a small but measurable energy difference between two possible spin states.

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NMR Spectrometer

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Nuclear Magnetic Resonance Spectroscopy

• Protons in different environments absorb at slightly different frequencies, so they are distinguishable by NMR.

• The frequency at which a particular proton absorbs is determined by its electronic environment.

• The size of the magnetic field generated by the electrons around a proton determines where it absorbs.

• Modern NMR spectrometers use a constant magnetic field strength B0, and then a narrow range of frequencies is applied to achieve the resonance of all protons.

• Only nuclei that contain odd mass numbers (such as 1H, 13C, 19F and 31P) or odd atomic numbers (such as 2H and 14N) give rise to NMR signals.

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• An NMR spectrum is a plot of the intensity of a peak against its chemical shift, measured in parts per million (ppm).

1H NMR—The Spectrum

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•NMR absorptions generally appear as sharp peaks.

• Increasing chemical shift is plotted from left to right.

•Most protons absorb between 0-10 ppm. •The terms “upfield” and “downfield” describe

the relative location of peaks. Upfield means to the right. Downfield means to the left.

•NMR absorptions are measured relative to the position of a reference peak at 0 ppm on the scale due to tetramethylsilane (TMS). TMS is a volatile inert compound that gives a single peak upfield from typical NMR absorptions.

1H NMR—The Spectrum

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• The chemical shift of the x axis gives the position of an NMR signal, measured in ppm, according to the following equation:

1H NMR—The Spectrum: Chemical shift

• By reporting the NMR absorption as a fraction of the NMR operating frequency, we get units, ppm, that are independent of the spectrometer.

• Four different features of a 1H NMR spectrum provide information about a compound’s structure:

– Number of signals– Position of signals– Intensity of signals.– Spin-spin splitting of signals.

Features of 1H NMR spectrum

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• The number of NMR signals equals the number of different types of protons in a compound.

• Protons in different environments give different NMR signals.

• Equivalent protons give the same NMR signal.

1H NMR—Number of Signals

• To determine equivalent protons in cycloalkanes and alkenes, always draw all bonds to hydrogen.

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• In comparing two H atoms on a ring or double bond, two protons are equivalent only if they are cis (or trans) to the same groups.


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•Proton equivalency in cycloalkanes can be determined similarly.


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• In the vicinity of the nucleus, the magnetic field generated by the circulating electron decreases the external magnetic field that the proton “feels”.

• Since the electron experiences a lower magnetic field strength, it needs a lower frequency to achieve resonance. Lower frequency is to the right in an NMR spectrum, toward a lower chemical shift, so shielding shifts the absorption upfield.

1H NMR—Position of Signals

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•The less shielded the nucleus becomes, the more of the applied magnetic field (B0) it feels.

•This deshielded nucleus experiences a higher magnetic field strength, to it needs a higher frequency to achieve resonance.

•Higher frequency is to the left in an NMR spectrum, toward higher chemical shift—so deshielding shifts an absorption downfield.

•Protons near electronegative atoms are deshielded, so they absorb downfield.


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. =>

Shielded Protons

Magnetic field strength must be increased for a shielded proton to flip at the same frequency.

=>

Protons in a Molecule

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

=>

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• Protons in a given environment absorb in a predictable region in an NMR spectrum.

1H NMR—Chemical Shift Values

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• The chemical shift of a C—H bond increases with increasing alkyl substitution.


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• The chemical shift of a C—H can be calculated with a high degree of precision if a chemical shift additivity

table is used.• The additivity tables starts with a base chemical shift value

depending on the structural type of hydrogen under consideration:

Calculating 1H NMR—Chemical Shift Values

CH3 CH2

CH

Methylene Methine

0.87 ppm 1.20 ppm 1.20 ppmBase Chemical Shift

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• The presence of nearby atoms or groups will effect the base chemical shift by a specific amount:

• The carbon atom bonded to the hydrogen(s) under consideration are described as alpha () carbons.

• Atoms or groups bonded to the same carbon as the hydrogen(s) under consideration are described as alpha () substituents.

• Atoms or groups on carbons one bond removed from the a carbon are called beta () carbons.

• Atoms or groups bonded to the carbon are described as beta () substituents.

Calculating 1H NMR—Chemical Shift Values

(Hydrogen under consideration)C C H

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

O

or 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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Nuclear Magnetic Resonance SpectroscopyCalculating 1H NMR—Chemical 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

O

or 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 ---

(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


one - Cl (CH2) = 2.30

no substituents = 0.00

TOTAL = 3.50 ppm

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• In a magnetic field, the six electrons in benzene circulate around the ring creating a ring current.

• The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons.

• The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance. Thus they are deshielded and absorb downfield.

1H NMR—Chemical Shift Values of benzene

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• In a magnetic field, the loosely held electrons of the double bond create a magnetic field that reinforces the applied field in the vicinity of the protons.

• The protons now feel a stronger magnetic field, and require a higher frequency for resonance. Thus the protons are deshielded and the absorption is downfield.

1H NMR—Chemical Shift Values of C=C structure

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• In a magnetic field, the electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (B0).

• Thus, the proton feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is shielded and the absorption is upfield.

1H NMR—Chemical Shift Values of Carbon-carbon triple-bond structure

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1H NMR—Chemical Shift Values)

1H NMR of Methyl Acetate

C

O

R O

H3C C O

Base Chemical Shift = 0.87 ppm one = 2.88 ppm TOTAL = 3.75 ppmO

CH3

C

O

R

Base Chemical Shift = 0.87 ppm one = 1.23 ppm TOTAL = 2.10 ppm

2,3-Dimethyl-2-Butene

(Hydrogen under consideration)


one (CH3) = 0.78 ppm

TOTAL = 1.65 ppm

H2C CH

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• The area under an NMR signal is proportional to the number of absorbing protons.

• An NMR spectrometer automatically integrates the area under the peaks, and prints out a stepped curve (integral) on the spectrum.

• The height of each step is proportional to the area under the peak, which in turn is proportional to the number of absorbing protons.

• Modern NMR spectrometers automatically calculate and plot the value of each integral in arbitrary units.

• The ratio of integrals to one another gives the ratio of absorbing protons in a spectrum. Note that this gives a ratio, and not the absolute number, of absorbing protons.

1H NMR—Intensity of Signals

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1H NMR—Intensity of Signals

Methyl -Dimethylpropionate

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• Consider the spectrum below:

1H NMR—Spin-Spin Splitting

Ethyl Bromide

Spin-Spin Splitting in 1H NMR Spectra

• Peaks are often split into multiple peaks due to magnetic interactions between nonequivalent protons on adjacent carbons, The process is called spin-spin splitting

• The splitting is into one more peak than the number of H’s on the adjacent carbon(s), This is the “n+1 rule”

• The relative intensities are in proportion of a binomial distribution given by Pascal’s Triangle

• The set of peaks is a multiplet (2 = doublet, 3 = triplet, 4 = quartet, 5=pentet, 6=hextet, 7=heptet…..)

1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 11 6 15 20 15 6 1

singlet

doublet

tripletquartet

pentet

hextet

heptet

Rules for Spin-Spin Splitting• Equivalent protons do not split each other

• Protons that are farther than two carbon atoms apart do not split each other

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Splitting is not generally observed between protons separated by more than three bonds.

If Ha and Hb are not equivalent, splitting is observed when:

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• Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons.

The Origin of 1H NMR—Spin-Spin Splitting

Let us consider how the doublet due to the CH2 group on BrCH2CHBr2 occurs:• When placed in an applied field, (B0), the adjacent proton

(CHBr2) can be aligned with () or against () B0. The

likelihood of either case is about 50% (i.e., 1,000,006 vs 1,000,000).

• Thus, the absorbing CH2 protons feel two slightly different magnetic fields—one slightly larger than B0, and one slightly smaller than B0.

• Since the absorbing protons feel two different magnetic fields, they absorb at two different frequencies in the NMR spectrum, thus splitting a single absorption into a doublet, where the two peaks of the doublet have equal intensity.

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The frequency difference, measured in Hz, between two peaks of the doublet is called the coupling constant, J.

J

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Let us now consider how a triplet arises:

• When placed in an applied magnetic field (B0), the adjacent protons Ha and Hb can each be aligned with () or against () B0.

• Thus, the absorbing proton feels three slightly different magnetic fields—one slightly larger than B0(ab). one slightly smaller than B0(ab) and one the same strength as B0 (ab).

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• Because the absorbing proton feels three different magnetic fields, it absorbs at three different frequencies in the NMR spectrum, thus splitting a single absorption into a triplet.

• Because there are two different ways to align one proton with B0, and one proton against B0—that is, ab and ab—the middle peak of the triplet is twice as intense as the two outer peaks, making the ratio of the areas under the three peaks 1:2:1.

• Two adjacent protons split an NMR signal into a triplet.

• When two protons split each other, they are said to be coupled.

• The spacing between peaks in a split NMR signal, measured by the J value, is equal for coupled protons.

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Whenever two (or three) different sets of adjacent protons are equivalent to each other, use the n+1 rule to determine the splitting pattern.

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Nuclear Magnetic Resonance Spectroscopy 1H NMR—Spin-Spin Splitting

Whenever two (or three) different sets of adjacent protons are equivalent to each other, use the n+1 rule to determine the splitting pattern.

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Nuclear Magnetic Resonance Spectroscopy 1H NMR—Spin-Spin Splitting

Whenever two (or three) different sets of adjacent protons are not equivalent to each other, use the n + 1 rule to determine the splitting pattern only if the coupling constants (J) are identical:

a a

b

c

Free rotation around C-C bonds averages coupling constant to J = 7Hz

Jab = Jbc

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Nuclear Magnetic Resonance Spectroscopy1H NMR—Spin-Spin Splitting

Whenever two (or three) different sets of adjacent protons are not equivalent to each other, use the n + 1 rule to determine the splitting pattern only if the coupling constants (J) are identical:

a

b

c

c

Jab = Jbc

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Nuclear Magnetic Resonance Spectroscopy1H NMR—Structure Determination

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1H NMR—Structure Determination

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Nuclear Magnetic Resonance Spectroscopy1H NMR—Structure Determination

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1H NMR—Structure Determination

MODERN INSTRUMENTATIONMODERN INSTRUMENTATION

PULSED FOURIER TRANSFORM TECHNOLOGY

requires a computerFT-NMR

PULSED EXCITATIONPULSED EXCITATION

CH2 C

O

CH3BROADBANDRF PULSE

All types of hydrogen are excitedsimultaneously with the single RF pulse.

contains a range of frequencies

N

S

n1

n2

n3

(n1 ..... nn)

CH2 C

O

CH3

FREE INDUCTION DECAYFREE INDUCTION DECAY( relaxation )

n1

n2

n3

n1, n2, n3 have different half lives

COMPOSITE FIDCOMPOSITE FID

“time domain“ spectrum

n1 + n2 + n3 + ......

time

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

COMPLEXSIGNAL n1 + n2 + n3 + ......

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

The Composite FID is The Composite FID is Transformed into a classical Transformed into a classical

NMR Spectrum : NMR Spectrum :

“frequency domain” spectrum

CH2 C

O

CH3

CONTINUOUS WAVE (CW) METHODCONTINUOUS WAVE (CW) METHOD

The magnetic field is “scanned” from a low field strength to a higher field strength while a constantbeam of radiofrequency (continuous wave) is supplied at a fixed frequency (say 100 MHz).

Using this method, it requires several minutes to plotan NMR spectrum.

THE OLDER, CLASSICAL METHOD

SLOW, HIGH NOISE LEVEL

PULSED FOURIER TRANSFORM PULSED FOURIER TRANSFORM (FT) METHOD(FT) METHOD

THE NEWER COMPUTER-BASED METHOD

The excitation pulse, the data collection (FID), and the computer-driven Fourier Transform (FT) take only a few seconds.

Most protons relax (decay) from their excited states very quickly (within a second).

The pulse and data collection cycles may be repeatedevery few seconds.

Many repetitions can be performed in avery short time, leading to improved signal …..

FASTLOW NOISE

By adding the signals from many pulses together, thesignal strength may be increased above the noise level.

IMPROVED IMPROVED SIGNAL-TO-NOISESIGNAL-TO-NOISE RATIO RATIO

noisesignal

add manypulses

noise is randomand cancels outetc.

1st pulse

2nd pulse

nth pulse

enhancedsignal

THE COUPLING CONSTANTTHE COUPLING CONSTANT

J J

J

J J

THE COUPLING CONSTANTTHE COUPLING CONSTANT

The coupling constant is the distance J (measured in Hz) between the peaks in a multiplet.

J is a measure of the amount of interaction between the two sets of hydrogens creating the multiplet.

C

H

H

C H

H

H

J

100 MHz

200 MHz

123456

123

100 Hz

200 Hz

200 Hz

400 Hz

J = 7.5 Hz

J = 7.5 Hz

7.5 Hz

7.5 Hz

Coupling constants areconstant - they do not change at differentfield strengths

The shift isdependanton the field

ppm

FIELD COMPARISON

Separationis larger

100 MHz

200 MHz

123456

123

100 Hz

200 Hz

J = 7.5 Hz

J =7.5 Hz

ppm4

200 Hz

400 Hz

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J = 7.5 Hz

Note the compression ofmultiplets in the 200 MHzspectrum when it is plotted on the same scale as the 100 MHz spectruminstead of on a chart whichis twice as wide.

Separationis larger

123

123

100 MHz

200 MHz

Why buy a higherfield instrument?

Spectra aresimplified!

Overlapping multiplets areseparated.

Second-ordereffects are minimized.

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50 MHz

J = 7.5 Hz

J = 7.5 Hz

J = 7.5 Hz

NOTATION FOR COUPLING CONSTANTSNOTATION FOR COUPLING CONSTANTSThe most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms.

C C

HH This is sometimes called vicinal coupling.It is designated 3J since three bondsintervene between the two hydrogens.

Another type of coupling that can also occur in special cases is

C H

H2J or geminal coupling

Geminal coupling does not occur whenthe two hydrogens are equivalent due torotations around the other two bonds.

( most often 2J = 0 )

3J

2J

Couplings larger than 2J or 3J also exist, but operate only in special situations.

Couplings larger than 3J (e.g., 4J, 5J, etc) are usually called “long-range coupling.”

CC

CH H

4J , for instance, occurs mainlywhen the hydrogens are forcedto adopt this “W” conformation(as in bicyclic compounds).

LONG RANGE COUPLINGSLONG RANGE COUPLINGS

• 3J-cis = 8-10 Hz • 3J-trans = 16-18 Hz

• protons on the same carbon 2J-geminal = 0-2 Hz H

H

HH

H

H

PROTONS ON C=C DOUBLE BONDSPROTONS ON C=C DOUBLE BONDSCOUPLING CONSTANTS

For protons on saturated aliphatic chains 3J 8 Hz

C C

H H

C CH

H

C CHH

CH

H

6 to 8 Hz

11 to 18 Hz

6 to 15 Hz

0 to 5 Hz

three bond 3J

two bond 2J

three bond 3J

three bond 3J

SOME REPRESENTATIVE COUPLING CONSTANTSSOME REPRESENTATIVE COUPLING CONSTANTS

Hax

Hax

Heq

Heq

Hax,Hax = 8 to 14

Hax,Heq = 0 to 7

Heq,Heq = 0 to 5

three bond 3J

trans

cis

geminal

vicinal

CH

C H4 to 10 Hz

H C C CH

0 to 3 Hz four bond 4J

three bond 3J

C CC H

H0 to 3 Hz four bond 4J

H

H

cis

trans

6 to 12 Hz

4 to 8 Hzthree bond 3J

Couplings that occur at distances greater than three bonds arecalled long-range couplings and they are usually small (<3 Hz) and frequently nonexistent (0 Hz).

Analysis of Vinyl AcetateAnalysis of Vinyl Acetate

HC HB HA

CCHH33 CC

OO

OOCC

HHCC

CCHHAA

HHBB

3JBC

3JAC

3JAC3JBC

2JAB2JAB

trans trans

cis

cis

gem gem

3J-trans > 3J-cis > 2J-gem

2,4-DINITROANISOLE2,4-DINITROANISOLE

8.72 ppm 8.43 ppm 7.25 ppm

HYDROXYL AND AMINO HYDROXYL AND AMINO PROTONSPROTONS

Hydroxyl and Amino Protons

Carboxylic acid protons generally appear fardownfield near 11 to 12 ppm.

Hydroxyl and amino protons can appear almost anywhere in the spectrum (H-bonding).

These absorptions are usually broader than other proton peaks and can often be identified because of this fact.

NMR Spectrum of Ethanol

CH3CH2 OH

2 1

3

SPIN-SPIN DECOUPLING BY EXCHANGESPIN-SPIN DECOUPLING BY EXCHANGE

In alcohols coupling between the O-H hydrogen andthose on adjacent carbon atoms is usually not seen.

C O

H H

This is due to rapid exchange ofOH hydrogens between the various alcohol molecules in the solution.

R-O-Ha + R’-O-Hb R-O-Hb + R’-O-Ha

The exchange happens so quickly that the C-H groupsees many different hydrogens on the O-H during thetime the spectrum is being determined (average spin = 0)

In ultrapure alcohols, however,coupling will sometimes be seen.

NMR Spectrum of 2-Chloropropanoic Acid

offset = 4.00 ppm

COOH

C

O

OHCH

Cl

CH31

1

3

~12 ppm

PURE ETHANOLPURE ETHANOL

ETHANOLETHANOLOld sampleRapid exchange catalyzedby impurities

quartet

triplet

broadsinglet

HO-CH2-CH3

hydrogen on OHis decoupled

400 MHz

ETHANOLETHANOLUltrapure sample (new)Slow or no exchange

tripletdoublet ofquartets

triplet400 MHz

expansion expansion

CARBON-13 NMRCARBON-13 NMR

12C is not NMR-active I = 0

however…. 13C does have spin, I = 1/2 (odd mass)

1. Natural abundance of 13C is small (1.08% of all C)

2. Magnetic moment of 13C is small

13C signals are 6000 times weaker than 1H because:

SALIENT FACTS ABOUT SALIENT FACTS ABOUT 1313C NMRC NMR

PULSED FT-NMR IS REQUIRED

The chemical shift range is larger than for protons

0 - 200 ppm

SALIENT FACTS ABOUT SALIENT FACTS ABOUT 1313C NMRC NMR

For a given field strength 13C has its resonance at adifferent (lower) frequency than 1H.

1H

13C1.41 T 60 MHz

2.35 T 100 MHz7.05 T 300 MHz

1.41 T 15.1 MHz2.35 T 25.0 MHz7.05 T 75.0 MHz

Divide the hydrogenfrequency by 4 (approximately)

for carbon-13

Because of its low natural abundance (0.0108) thereis a low probability of finding two 13C atoms next toeach other in a single molecule.

However, 13C does couple to hydrogen atoms (I = 1/2)

13C - 13C coupling NO!

13C - 1H coupling YES!

Spectra are determined by many molecules contributingto the spectrum, each having only one 13C atom.

SALIENT FACTS ABOUT SALIENT FACTS ABOUT 1313C NMRC NMR (cont)

not probable

very common

COUPLING TO ATTACHED PROTONS

The effect of attached protons on 13C resonances

n+1 = 4 n+1 = 3 n+1 = 2 n+1 = 1

C13

3 protons 2 protons 1 proton 0 protons

H

H

H

C13

H

H

C13

H C13

Methylcarbon

Methylenecarbon

Methinecarbon

Quaternarycarbon

( n+1 rule applies )

COUPLING TO ATTACHED PROTONSCOUPLING TO ATTACHED PROTONS

(J’s are large ~ 100 - 200 Hz)

ETHYL PHENYLACETATEETHYL PHENYLACETATE

13C coupledto the hydrogens

DECOUPLED SPECTRA

DECOUPLING THE PROTON SPINSDECOUPLING THE PROTON SPINSPROTON-DECOUPLED SPECTRA

A common method used in determining a carbon-13NMR spectrum is to irradiate all of the hydrogen nuclei in the molecule at the same time the carbonresonances are being measured.

This requires a second radiofrequency (RF) source (the decoupler) tuned to the frequency of the hydrogen nuclei, while the primary RF source is tuned to the 13C frequency.

1H-13CRF source 2 RF source 1

continuouslysaturateshydrogens

pulse tuned tocarbon-13

13C signal (FID) measured

“the decoupler”

In this method the hydrogen nuclei are “saturated”,a situation where there are as many downward asthere are upward transitions, all occuring rapidly.

During the time the carbon-13 spectrum is beingdetermined, the hydrogen nuclei cycle rapidly betweentheir two spin states (+1/2 and -1/2) and the carbon nucleisee an average coupling (i.e., zero) to the hydrogens.

The hydrogens are said to be decoupled from thecarbon-13 nuclei.

You no longer see multiplets for the 13C resonances.Each carbon gives a singlet, and the spectrum is easier to interpret.

ETHYL PHENYLACETATEETHYL PHENYLACETATE

13C coupledto the hydrogens

13C decoupledfrom the hydrogens

in some casesthe peaks of the multiplets willoverlap

this is aneasier spectrumto interpret

q

tt

s s

d d

d

SOME INSTRUMENTS SHOW THE MULTIPLICITIES SOME INSTRUMENTS SHOW THE MULTIPLICITIES OF THE PEAKS ON THE DECOUPLED SPECTRAOF THE PEAKS ON THE DECOUPLED SPECTRA

s = singlet t = tripletd = doublet q = quartetCODE :

This method gives the best of both worlds.

CHEMICAL SHIFTS OF 13C ATOMS

R-CH3 8 - 30

R2CH215 - 55

R3CH 20 - 60

C-I 0 - 40

C-Br 25 - 65

C-N 30 - 65

C-Cl 35 - 80

C-O 40 - 80

C C 65 - 90

C=C 100 - 150

C N 110 - 140

110 - 175

R-C-OR

O

R-C-OH

O

155 - 185

R-C-NH2

O

155 - 185

R-C-H

O

R-C-R

O

185 - 220

APPROXIMATE APPROXIMATE 1313C CHEMICAL SHIFT RANGES FOR C CHEMICAL SHIFT RANGES FOR SELECTED TYPES OF CARBON (ppm)SELECTED TYPES OF CARBON (ppm)

AldehydesKetones

Acids AmidesEsters Anhydrides

Aromatic ringcarbons

Unsaturated carbon - sp2

Alkyne carbons - sp

Saturated carbon - sp3

electronegativity effects

Saturated carbon - sp3

no electronegativity effects

C=O

C=O

C=CC C

200 150 100 50 0

200 150 100 50 0

8 - 30

15 - 55

20 - 60

40 - 80

35 - 80

25 - 65

65 - 90

100 - 150

110 - 175

155 - 185

185 - 220

Correlation chart for 13C Chemical Shifts (ppm)

C-O

C-Cl

C-Br

R3CH R4C

R-CH2-R

R-CH3

RANGE

/

nitriles

acid anhydrides

acid chlorides

amides

esters

carboxylic acids

aldehydes

,-unsaturated ketones

ketones

220 200 180 160 140 120 100 ppm

13C Correlation Chart for Carbonyl and Nitrile Functional Groups

SPECTRA

Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz)

200 150 100 50 0

1-PROPANOL1-PROPANOL

PROTONDECOUPLED

HO-CH2-CH2-CH3c b a

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1 Nuclear Magnetic Resonance (NMR) Spectroscopy. Nuclear Spin A nucleus with an odd atomic number or...

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