Nuclear Magnetic Resonance (NMR) Spectroscopy

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 =>

Introduction to NMR Spectroscopy
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.

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).

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 (hn) that matches the energy difference (DE) 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.

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

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.

NMR Spectrometer

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.

1H NMR—The Spectrum 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 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 d 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: Chemical shift The chemical shift of the x axis gives the position of an NMR signal, measured in ppm, according to the following equation: By reporting the NMR absorption as a fraction of the NMR operating frequency, we get units, ppm, that are independent of the spectrometer.

Features of 1H NMR spectrum
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.

1H NMR—Number of Signals 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. To determine equivalent protons in cycloalkanes and alkenes, always draw all bonds to hydrogen.

1H NMR—Number of Signals

1H NMR—Number of Signals 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.

1H NMR—Number of Signals Proton equivalency in cycloalkanes can be determined similarly.

1H NMR—Position of Signals 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 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. =>

1H NMR—Position of Signals

1H NMR—Chemical Shift Values Protons in a given environment absorb in a predictable region in an NMR spectrum.

1H NMR—Chemical Shift Values The chemical shift of a C—H bond increases with increasing alkyl substitution.

Calculating 1H NMR—Chemical Shift Values 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: 31

Calculating 1H NMR—Chemical Shift Values 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 (b) substituents. 32

Calculating 1H NMR—Chemical Shift Values 34

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

1H NMR—Chemical Shift Values)

1H NMR of Methyl Acetate

2,3-Dimethyl-2-Butene

1H NMR—Intensity of Signals 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

Methyl a,a-Dimethylpropionate

1H NMR—Spin-Spin Splitting Consider the spectrum below:

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…..)

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

1H NMR—Spin-Spin Splitting
If Ha and Hb are not equivalent, splitting is observed when: Splitting is not generally observed between protons separated by more than three  bonds. 54

The Origin of 1H NMR—Spin-Spin Splitting
Spin-spin splitting occurs only between nonequivalent protons on the same carbon or adjacent carbons. 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.

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

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(ab). one slightly smaller than B0(ab) and one the same strength as B0 (ab).

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, ab and ab—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.

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.

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. 63

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: Jab = Jbc 64

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: Jab = Jbc 65

1H NMR—Structure Determination 66

MODERN INSTRUMENTATION
PULSED FOURIER TRANSFORM TECHNOLOGY FT-NMR requires a computer

PULSED EXCITATION (n1 ..... nn) n2 n1 n3 N S BROADBAND RF PULSE
contains a range of frequencies n3 (n nn) S All types of hydrogen are excited simultaneously with the single RF pulse.

n1 n2 n3 FREE INDUCTION DECAY ( relaxation )
n1, n2, n3 have different half lives

COMPOSITE FID “time domain“ spectrum n1 + n2 + n time

FOURIER TRANSFORM n1 + n2 + n3 + ......
A mathematical technique that resolves a complex FID signal into the individual frequencies that add together to make it. ( Details not given here. ) converted to DOMAINS ARE MATHEMATICAL TERMS TIME DOMAIN FREQUENCY DOMAIN FID NMR SPECTRUM FT-NMR computer COMPLEX SIGNAL n1 + n2 + n Fourier Transform individual frequencies a mixture of frequencies decaying (with time) converted to a spectrum

The Composite FID is Transformed into a classical NMR Spectrum :
“frequency domain” spectrum

CONTINUOUS WAVE (CW) METHOD
THE OLDER, CLASSICAL METHOD The magnetic field is “scanned” from a low field strength to a higher field strength while a constant beam of radiofrequency (continuous wave) is supplied at a fixed frequency (say 100 MHz). Using this method, it requires several minutes to plot an NMR spectrum. SLOW, HIGH NOISE LEVEL

PULSED FOURIER TRANSFORM (FT) METHOD
FAST LOW NOISE THE NEWER COMPUTER-BASED METHOD Most protons relax (decay) from their excited states very quickly (within a second). The excitation pulse, the data collection (FID), and the computer-driven Fourier Transform (FT) take only a few seconds. The pulse and data collection cycles may be repeated every few seconds. Many repetitions can be performed in a very short time, leading to improved signal …..

IMPROVED SIGNAL-TO-NOISE RATIO
By adding the signals from many pulses together, the signal strength may be increased above the noise level. signal enhanced signal noise 1st pulse 2nd pulse add many pulses noise is random and cancels out nth pulse etc.

THE COUPLING CONSTANT

THE COUPLING CONSTANT J J J J J
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.

100 MHz 6 5 4 3 2 1 200 MHz 3 2 1 FIELD COMPARISON 200 Hz 100 Hz
Coupling constants are constant - they do not change at different field strengths 7.5 Hz J = 7.5 Hz 6 5 4 3 2 1 200 MHz 400 Hz Separation is larger 200 Hz 7.5 Hz The shift is dependant on the field J = 7.5 Hz 3 2 1 ppm

100 MHz 200 Hz 100 Hz J = 7.5 Hz J = 7.5 Hz 6 5 4 3 2 1 200 MHz Separation is larger 400 Hz Note the compression of multiplets in the 200 MHz spectrum when it is plotted on the same scale as the 100 MHz spectrum instead of on a chart which is twice as wide. 200 Hz J = 7.5 Hz 6 5 4 3 2 1 ppm

50 MHz J = 7.5 Hz Why buy a higher field instrument? 3 2 1 Spectra are simplified! 100 MHz J = 7.5 Hz Overlapping multiplets are separated. 3 2 1 200 MHz J = 7.5 Hz Second-order effects are minimized. 3 2 1

NOTATION FOR COUPLING CONSTANTS
The most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms. This is sometimes called vicinal coupling. It is designated 3J since three bonds intervene between the two hydrogens. 3J Another type of coupling that can also occur in special cases is 2J or geminal coupling ( most often 2J = 0 ) Geminal coupling does not occur when the two hydrogens are equivalent due to rotations around the other two bonds. 2J

LONG RANGE COUPLINGS C C C
Couplings larger than 2J or 3J also exist, but operate only in special situations. H C H C C 4J , for instance, occurs mainly when the hydrogens are forced to adopt this “W” conformation (as in bicyclic compounds). Couplings larger than 3J (e.g., 4J, 5J, etc) are usually called “long-range coupling.”

PROTONS ON C=C DOUBLE BONDS
COUPLING CONSTANTS PROTONS ON C=C DOUBLE BONDS 3J-cis = 8-10 Hz 3J-trans = Hz protons on the same carbon 2J-geminal = 0-2 Hz For protons on saturated aliphatic chains 3J ~ 8 Hz

SOME REPRESENTATIVE COUPLING CONSTANTS
6 to 8 Hz three bond 3J vicinal 11 to 18 Hz three bond 3J trans cis 6 to 15 Hz three bond 3J geminal 0 to 5 Hz two bond 2J Hax,Hax = 8 to 14 Hax,Heq = 0 to 7 three bond 3J Heq,Heq = 0 to 5

cis 6 to 12 Hz three bond 3J trans 4 to 8 Hz 4 to 10 Hz three bond 3J
four bond 4J 0 to 3 Hz four bond 4J Couplings that occur at distances greater than three bonds are called long-range couplings and they are usually small (<3 Hz) and frequently nonexistent (0 Hz).

Analysis of Vinyl Acetate
H 3 O HC HA HB 3J-trans > 3J-cis > 2J-gem HC HB HA 3JAC 3JBC 3JBC cis trans trans 3JAC 2JAB 2JAB cis gem gem

2,4-DINITROANISOLE 8.72 ppm ppm ppm

HYDROXYL AND AMINO PROTONS

Hydroxyl and Amino Protons
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. Carboxylic acid protons generally appear far downfield near 11 to 12 ppm.

NMR Spectrum of Ethanol
3 2 1

C O H H SPIN-SPIN DECOUPLING BY EXCHANGE
In alcohols coupling between the O-H hydrogen and those on adjacent carbon atoms is usually not seen. This is due to rapid exchange of OH hydrogens between the various alcohol molecules in the solution. C O H H In ultrapure alcohols, however, coupling will sometimes be seen. R-O-Ha + R’-O-Hb R-O-Hb R’-O-Ha The exchange happens so quickly that the C-H group sees many different hydrogens on the O-H during the time the spectrum is being determined (average spin = 0)

NMR Spectrum of 2-Chloropropanoic Acid
COOH 3 1 1 ~12 ppm offset = 4.00 ppm

PURE ETHANOL

ETHANOL HO-CH2-CH3 400 MHz Old sample Rapid exchange catalyzed
by impurities hydrogen on OH is decoupled HO-CH2-CH3 triplet broad singlet quartet

ETHANOL 400 MHz expansion expansion doublet of triplet quartets
Ultrapure sample (new) Slow or no exchange 400 MHz triplet

CARBON-13 NMR

SALIENT FACTS ABOUT 13C NMR
12C is not NMR-active I = 0 however…. 13C does have spin, I = 1/2 (odd mass) 13C signals are 6000 times weaker than 1H because: 1. Natural abundance of 13C is small (1.08% of all C) 2. Magnetic moment of 13C is small PULSED FT-NMR IS REQUIRED The chemical shift range is larger than for protons ppm

1H 13C SALIENT FACTS ABOUT 13C NMR
For a given field strength 13C has its resonance at a different (lower) frequency than 1H. 1H Divide the hydrogen frequency by 4 (approximately) for carbon-13 1.41 T MHz 2.35 T MHz 13C 7.05 T MHz 1.41 T MHz 2.35 T MHz 7.05 T MHz

SALIENT FACTS ABOUT 13C NMR
(cont) Because of its low natural abundance (0.0108) there is a low probability of finding two 13C atoms next to each other in a single molecule. not probable 13C - 13C coupling NO! Spectra are determined by many molecules contributing to the spectrum, each having only one 13C atom. However, 13C does couple to hydrogen atoms (I = 1/2) very common 13C - 1H coupling YES!

COUPLING TO ATTACHED PROTONS

COUPLING TO ATTACHED PROTONS
13 C 13 C 13 C 13 H H H H n+1 = 4 n+1 = 3 n+1 = 2 n+1 = 1 Methyl carbon Methylene carbon Methine carbon Quaternary carbon The effect of attached protons on 13C resonances ( n+1 rule applies ) (J’s are large ~ Hz)

ETHYL PHENYLACETATE 13C coupled to the hydrogens

DECOUPLED SPECTRA

DECOUPLING THE PROTON SPINS
PROTON-DECOUPLED SPECTRA A common method used in determining a carbon-13 NMR spectrum is to irradiate all of the hydrogen nuclei in the molecule at the same time the carbon resonances 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. RF source 1 RF source 2 1H-13C pulse tuned to carbon-13 “the decoupler” continuously saturates hydrogens 13C signal (FID) measured

In this method the hydrogen nuclei are “saturated”,
a situation where there are as many downward as there are upward transitions, all occuring rapidly. During the time the carbon-13 spectrum is being determined, the hydrogen nuclei cycle rapidly between their two spin states (+1/2 and -1/2) and the carbon nuclei see an average coupling (i.e., zero) to the hydrogens. The hydrogens are said to be decoupled from the carbon-13 nuclei. You no longer see multiplets for the 13C resonances. Each carbon gives a singlet, and the spectrum is easier to interpret.

ETHYL PHENYLACETATE 13C coupled to the hydrogens 13C decoupled
in some cases the peaks of the multiplets will overlap 13C coupled to the hydrogens this is an easier spectrum to interpret 13C decoupled from the hydrogens

SOME INSTRUMENTS SHOW THE MULTIPLICITIES
OF THE PEAKS ON THE DECOUPLED SPECTRA s = singlet t = triplet d = doublet q = quartet CODE : q t s d This method gives the best of both worlds.

CHEMICAL SHIFTS OF 13C ATOMS

APPROXIMATE 13C CHEMICAL SHIFT RANGES FOR
SELECTED TYPES OF CARBON (ppm) R-CH C C R2CH C=C R3CH C N C-I C-Br O O C-Cl R-C-OR R-C-OH O C-N R-C-NH2 O O C-O R-C-H R-C-R

Correlation chart for 13C Chemical Shifts (ppm)
200 150 100 50 RANGE R-CH3 8 - 30 Saturated carbon - sp3 R-CH2-R no electronegativity effects R3CH / R4C C-O Saturated carbon - sp3 C-Cl electronegativity effects C-Br Alkyne carbons - sp C C Unsaturated carbon - sp2 C=C Aromatic ring carbons Acids Amides Esters Anhydrides C=O C=O Aldehydes Ketones 200 150 100 50 Correlation chart for 13C Chemical Shifts (ppm)

13C Correlation Chart for Carbonyl and Nitrile Functional Groups
nitriles acid anhydrides acid chlorides amides esters carboxylic acids aldehydes a,b-unsaturated ketones ketones 220 200 180 160 140 120 100 ppm 13C Correlation Chart for Carbonyl and Nitrile Functional Groups

SPECTRA

1-PROPANOL HO-CH2-CH2-CH3 c b a
PROTON DECOUPLED 200 150 100 50 Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz)

Nuclear Magnetic Resonance (NMR) Spectroscopy

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