Nuclear Magnetic Resonance Spectroscopy
1. Background
Over the past fifty years nuclear magnetic resonance spectroscopy,
commonly referred to as nmr, has become the preeminent technique for
determining the structure of organic compounds. Of all the spectroscopic
methods, it is the only one for which a complete analysis and
interpretation of the entire spectrum is normally expected. Although
larger amounts of sample are needed than for mass spectroscopy, nmr is
non-destructive, and with modern instruments good data may be obtained
from samples weighing less than a milligram.
To be successful in
using nmr as an analytical tool, it is necessary to understand the
physical principles on which the methods are based.
The nuclei of many elemental isotopes have a characteristic spin (
I).
Some nuclei have integral spins (e.g. I = 1, 2, 3 ....), some have
fractional spins (e.g. I = 1/2, 3/2, 5/2 ....), and a few have no spin, I
= 0 (e.g.
12C,
16O,
32S, ....). Isotopes of particular interest and use to organic chemists are
1H,
13C,
19F and
31P,
all of which have I = 1/2. Since the analysis of this spin state is
fairly straightforward, our discussion of nmr will be limited to these
and other I = 1/2 nuclei.
For a table of nuclear spin characteristics Click Here.
|
|---|
The following features lead to the nmr phenomenon:
1. A spinning charge generates a magnetic field, as shown by the animation on the right.
The resulting spin-magnet has a magnetic moment (μ) proportional to the spin. |
 |
2. In the presence of an external magnetic field (B0), two spin states exist, +1/2 and -1/2.
The magnetic moment of the lower energy +1/2 state is aligned with the
external field, but that of the higher energy -1/2 spin state is opposed
to the external field. Note that the arrow representing the external
field points North. |
 |
| 3. The difference in energy between the
two spin states is dependent on the external magnetic field strength,
and is always very small. The following diagram illustrates that the two
spin states have the same energy when the external field is zero, but
diverge as the field increases. At a field equal to Bx a formula for the energy difference is given (remember I = 1/2 and μ is the magnetic moment of the nucleus in the field). |
 |
|---|
Strong magnetic fields are necessary for nmr spectroscopy. The international unit for magnetic flux is the tesla (T). The earth's magnetic field is not constant, but is approximately 10-4
T at ground level. Modern nmr spectrometers use powerful magnets having
fields of 1 to 20 T. Even with these high fields, the energy difference
between the two spin states is less than 0.1 cal/mole. To put this in
perspective, recall that infrared transitions involve 1 to 10 kcal/mole
and electronic transitions are nearly 100 time greater.
For nmr purposes, this small energy difference (ΔE) is usually given as a frequency in units of MHz (106
Hz), ranging from 20 to 900 Mz, depending on the magnetic field
strength and the specific nucleus being studied. Irradiation of a sample
with radio frequency (rf) energy corresponding exactly to the spin
state separation of a specific set of nuclei will cause excitation of
those nuclei in the +1/2 state to the higher -1/2 spin state. Note that
this electromagnetic radiation falls in the radio and television broadcast spectrum. Nmr spectroscopy is therefore the energetically mildest probe used to examine the structure of molecules.
The nucleus of a hydrogen atom (the proton) has a magnetic moment μ = 2.7927, and has been studied more than any other nucleus. The
previous diagram may be changed to display energy differences for the
proton spin states (as frequencies) by mouse clicking anywhere within it. |
| 4. For spin 1/2 nuclei the energy
difference between the two spin states at a given magnetic field
strength will be proportional to their magnetic moments. For the four
common nuclei noted above, the magnetic moments are: 1H μ = 2.7927, 19F μ = 2.6273, 31P μ = 1.1305 & 13C μ = 0.7022. These moments are in nuclear magnetons, which are 5.05078•10-27 JT-1.
The following diagram gives the approximate frequencies that correspond
to the spin state energy separations for each of these nuclei in an
external magnetic field of 2.35 T. The formula in the colored box shows
the direct correlation of frequency (energy difference) with magnetic
moment (h = Planck's constant = 6.626069•10-34 Js). |
 |
|---|
2. Proton NMR Spectroscopy
This important and well-established application of nuclear magnetic
resonance will serve to illustrate some of the novel aspects of this
method. To begin with, the nmr spectrometer must be tuned to a specific
nucleus, in this case the proton. The actual procedure for obtaining the
spectrum varies, but the simplest is referred to as the
continuous wave
(CW) method. A typical CW-spectrometer is shown in the following
diagram. A solution of the sample in a uniform 5 mm glass tube is
oriented between the poles of a powerful magnet, and is spun to average
any magnetic field variations, as well as tube imperfections. Radio
frequency radiation of appropriate energy is broadcast into the sample
from an antenna coil (colored red). A receiver coil surrounds the sample
tube, and emission of absorbed rf energy is monitored by dedicated
electronic devices and a computer. An nmr spectrum is acquired by
varying or sweeping the magnetic field over a small range while
observing the rf signal from the sample. An equally effective technique
is to vary the frequency of the rf radiation while holding the external
field constant.
For a description of the pulse Fourier transform technique, preferred by most spectroscopists over the older CW method, Click Here.
|
|---|
As an example, consider a sample of water in a 2.3487 T external
magnetic field, irradiated by 100 MHz radiation. If the magnetic field
is smoothly increased to 2.3488 T, the hydrogen nuclei of the water
molecules will at some point absorb rf energy and a resonance signal
will appear. An animation showing this may be activated by clicking the
Show Field Sweep
button. The field sweep will be repeated three times, and the resulting
resonance trace is colored red. For visibility, the water proton signal
displayed in the animation is much broader than it would be in an
actual experiment.
Since protons all have the same magnetic moment, we might expect all
hydrogen atoms to give resonance signals at the same field / frequency
values. Fortunately for chemistry applications, this is not true. By
clicking the
Show Different Protons button under the diagram, a
number of representative proton signals will be displayed over the same
magnetic field range. It is not possible, of course, to examine isolated
protons in the spectrometer described above; but from independent
measurement and calculation it has been determined that a naked proton
would resonate at a lower field strength than the nuclei of covalently
bonded hydrogens. With the exception of water, chloroform and sulfuric
acid, which are examined as liquids, all the other compounds are
measured as gases.
Why should the proton nuclei in different compounds behave differently in the nmr experiment ? The
answer to this question lies with the electron(s) surrounding the
proton in covalent compounds and ions. Since electrons are charged
particles, they move in response to the external magnetic field (B
o) so as to generate a secondary field that opposes the much stronger applied field. This secondary field
shields the nucleus from the applied field, so B
o must be increased in order to achieve resonance (absorption of rf energy). As illustrated in the drawing on the right, B
o
must be increased to compensate for the induced shielding field. In the
upper diagram, those compounds that give resonance signals at the
higher field side of the diagram (CH
4, HCl, HBr and HI) have proton nuclei that are more shielded than those on the lower field (left) side of the diagram.
The magnetic field range displayed in the above diagram is very
small compared with the actual field strength (only about 0.0042%). It
is customary to refer to small increments such as this in units of
parts per million
(ppm). The difference between 2.3487 T and 2.3488 T is therefore about
42 ppm. Instead of designating a range of nmr signals in terms of
magnetic field differences (as above), it is more common to use a
frequency scale, even though the spectrometer may operate by sweeping
the magnetic field. Using this terminology, we would find that at 2.34 T
the proton signals shown above extend over a 4,200 Hz range (for a 100
MHz rf frequency, 42 ppm is 4,200 Hz). Most organic compounds exhibit
proton resonances that fall within a 12 ppm range (the shaded area), and
it is therefore necessary to use very sensitive and precise
spectrometers to resolve structurally distinct sets of hydrogen atoms
within this narrow range.
In this respect it might
be noted that the detection of a part-per-million difference is
equivalent to detecting a 1 millimeter difference in distances of 1
kilometer.
Chemical Shift
Unlike infrared and uv-visible spectroscopy, where absorption peaks
are uniquely located by a frequency or wavelength, the location of
different nmr resonance signals is dependent on both the external
magnetic field strength and the rf frequency. Since no two magnets will
have exactly the same field, resonance frequencies will vary accordingly
and an alternative method for characterizing and specifying the
location of nmr signals is needed. This problem is illustrated by the
eleven different compounds shown in the following diagram. Although the
eleven resonance signals are distinct and well separated, an unambiguous
numerical locator cannot be directly assigned to each.
One method of solving this problem is to report the location of an
nmr signal in a spectrum relative to a reference signal from a standard
compound added to the sample. Such a reference standard should be
chemically unreactive, and easily removed from the sample after the
measurement. Also, it should give a single sharp nmr signal that does
not interfere with the resonances normally observed for organic
compounds.
Tetramethylsilane, (CH
3)
4Si, usually referred to as
TMS, meets all these characteristics, and has become the reference compound of choice for proton and carbon nmr.
Since the separation (or dispersion) of nmr signals is magnetic field
dependent, one additional step must be taken in order to provide an
unambiguous location unit.
This is illustrated for the acetone, methylene chloride and benzene signals by clicking on the previous diagram.
To correct these frequency differences for their field dependence, we
divide them by the spectrometer frequency (100 or 500 MHz in the
example),
as shown in a new display by again clicking on the diagram.
The resulting number would be very small, since we are dividing Hz by
MHz, so it is multiplied by a million, as shown by the formula in the
blue shaded box. Note that ν
ref is the resonant frequency of the reference signal and ν
samp is the frequency of the sample signal. This operation gives a locator number called the
Chemical Shift, having units of parts-per-million (ppm), and designated by the symbol
δ
Chemical shifts for all the compounds in the original display will be presented by a third click on the diagram.
The compounds referred to above share two common characteristics:
• The hydrogen atoms in a given molecule are all structurally equivalent, averaged for fast conformational equilibria.
• The compounds are all liquids, save for neopentane which boils at 9 °C and is a liquid in an ice bath.
The first feature assures that each compound gives a single sharp
resonance signal. The second allows the pure (neat) substance to be
poured into a sample tube and examined in a nmr spectrometer. In order
to take the nmr spectra of a solid, it is usually necessary to dissolve
it in a suitable solvent. Early studies used carbon tetrachloride for
this purpose, since it has no hydrogen that could introduce an
interfering signal. Unfortunately, CCl
4 is a poor solvent for many polar compounds and is also toxic. Deuterium labeled compounds, such as deuterium oxide (D
2O), chloroform-d (DCCl
3), benzene-d
6 (C
6D
6), acetone-d
6 (CD
3COCD
3) and DMSO-d
6 (CD
3SOCD
3)
are now widely used as nmr solvents. Since the deuterium isotope of
hydrogen has a different magnetic moment and spin, it is invisible in a
spectrometer tuned to protons.
For the properties of some common nmr solvents Click Here.
|
|---|
From the previous discussion and examples we may deduce that one
factor contributing to chemical shift differences in proton resonance is
the
inductive effect.
If the electron density about a proton nucleus is relatively high, the
induced field due to electron motions will be stronger than if the
electron density is relatively low. The shielding effect in such high
electron density cases will therefore be larger, and a higher external
field (B
o) will be needed for the rf energy to excite the
nuclear spin. Since silicon is less electronegative than carbon, the
electron density about the methyl hydrogens in tetramethylsilane is
expected to be greater than the electron density about the methyl
hydrogens in neopentane (2,2-dimethylpropane), and the characteristic
resonance signal from the silane derivative does indeed lie at a higher
magnetic field. Such nuclei are said to be
shielded. Elements
that are more electronegative than carbon should exert an opposite
effect (reduce the electron density); and, as the data in the following
tables show, methyl groups bonded to such elements display lower field
signals (they are
deshielded). The deshielding effect of electron
withdrawing groups is roughly proportional to their electronegativity,
as shown by the left table. Furthermore, if more than one such group is
present, the deshielding is additive (table on the right), and proton
resonance is shifted even further downfield.
Proton Chemical Shifts of Methyl Derivatives
| Compound |
(CH3)4C |
(CH3)3N |
(CH3)2O |
CH3F |
|---|
| δ | 0.9 | 2.1 | 3.2 | 4.1 |
|---|
| Compound | (CH3)4Si |
(CH3)3P |
(CH3)2S |
CH3Cl |
|---|
| δ | 0.0 | 0.9 | 2.1 | 3.0 |
|---|
|
|
Proton Chemical Shifts (ppm)
| Cpd. / Sub. |
X=Cl |
X=Br |
X=I |
X=OR |
X=SR |
|---|
| CH3X |
3.0 |
2.7 |
2.1 |
3.1 |
2.1 |
|
CH2X2 |
5.3 |
5.0 |
3.9 |
4.4 |
3.7 |
| CHX3 |
7.3 |
6.8 |
4.9 |
5.0 |
|
|
The general distribution of proton chemical shifts associated with
different functional groups is summarized in the following chart. Bear
in mind that these ranges are approximate, and may not encompass all
compounds of a given class. Note also that the ranges specified for OH
and NH protons (colored orange) are wider than those for most CH
protons. This is due to hydrogen bonding variations at different sample
concentrations.
Proton Chemical Shift Ranges*
|
|---|
Low Field
Region |
|
High Field
Region |
|---|
| | * For samples in CDCl3 solution. The δ scale is relative to TMS at δ = 0.
| |
To make use of a calculator that predicts aliphatic proton chemical shifts Click Here. This application was developed at Colby College.
Signal Strength
The magnitude or intensity of nmr resonance signals is displayed
along the vertical axis of a spectrum, and is proportional to the molar
concentration of the sample. Thus, a small or dilute sample will give a
weak signal, and doubling or tripling the sample concentration increases
the signal strength proportionally. If we take the nmr spectrum of
equal molar amounts of benzene and cyclohexane in carbon tetrachloride
solution, the resonance signal from cyclohexane will be twice as intense
as that from benzene because cyclohexane has twice as many hydrogens
per molecule. This is an important relationship when samples
incorporating two or more different sets of hydrogen atoms are examined,
since it allows the ratio of hydrogen atoms in each distinct set to be
determined. To this end it is necessary to measure the relative strength
as well as the chemical shift of the resonance signals that comprise an
nmr spectrum. Two common methods of displaying the integrated
intensities associated with a spectrum are illustrated by the following
examples. In the three spectra in the top row, a horizontal integrator
trace (light green) rises as it crosses each signal by a distance
proportional to the signal strength. Alternatively, an arbitrary number,
selected by the instrument's computer to reflect the signal strength,
is printed below each resonance peak, as shown in the three spectra in
the lower row. From the relative intensities shown here, together with
the previously noted chemical shift correlations, the reader should be
able to assign the signals in these spectra to the set of hydrogens that
generates each.
If you click on one of the spectrum
signals (colored red) or on hydrogen atom(s) in the structural formulas
the spectrum will be enlarged and the relationship will be colored blue.
Hint: When evaluating relative signal strengths, it is useful to
set the smallest integration to unity and convert the other values
proportionally.
Hydroxyl Proton Exchange and the Influence of Hydrogen Bonding
The last two compounds in the lower row are alcohols. The OH proton
signal is seen at 2.37 δ in 2-methyl-3-butyne-2-ol, and at 3.87 δ in
4-hydroxy-4-methyl-2-pentanone, illustrating the wide range over which
this chemical shift may be found. A six-membered ring intramolecular
hydrogen bond in the latter compound is in part responsible for its low
field shift, and will be shown by clicking on the hydroxyl proton. We
can take advantage of
rapid OH exchange
with the deuterium of heavy water to assign hydroxyl proton resonance
signals . As shown in the following equation, this removes the hydroxyl
proton from the sample and its resonance signal in the nmr spectrum
disappears. Experimentally, one simply adds a drop of heavy water to a
chloroform-d solution of the compound and runs the spectrum again. The
result of this exchange is displayed below.
R-O-H + D2O  R-O-D + D-O-H |
 |
Hydrogen bonding shifts the resonance signal of a proton to lower field ( higher frequency ). Numerous experimental observations support this statement, and a few of these will be described here.
| i) The chemical shift of the hydroxyl
hydrogen of an alcohol varies with concentration. Very dilute solutions
of 2-methyl-2-propanol, (CH3)3COH, in carbon
tetrachloride solution display a hydroxyl resonance signal having a
relatively high-field chemical shift (< 1.0 δ ). In concentrated
solution this signal shifts to a lower field, usually near 2.5 δ. |
ii) The more acidic hydroxyl group of phenol
generates a lower-field resonance signal, which shows a similar
concentration dependence to that of alcohols. OH resonance signals for
different percent concentrations of phenol in chloroform-d are shown in
the following diagram (C-H signals are not shown).
|
 |
|---|
| iii) Because of their favored hydrogen-bonded dimeric association,
the hydroxyl proton of carboxylic acids displays a resonance signal
significantly down-field of other functions. For a typical acid it
appears from 10.0 to 13.0 δ and is often broader than other signals. The
spectra shown below for chloroacetic acid (left) and
3,5-dimethylbenzoic acid (right) are examples. |
|
|
| iv) Intramolecular hydrogen bonds, especially those
defining a six-membered ring, generally display a very low-field proton
resonance. The case of 4-hydroxypent-3-ene-2-one (the enol tautomer of
2,4-pentanedione) not only illustrates this characteristic, but also
provides an instructive example of the sensitivity of the nmr experiment
to dynamic change. In the nmr spectrum of the pure liquid, sharp
signals from both the keto and enol tautomers are seen, their mole ratio
being 4 : 21 (keto tautomer signals are colored purple).
Chemical shift assignments for these signals are shown in the shaded box
above the spectrum. The chemical shift of the hydrogen-bonded hydroxyl
proton is δ 14.5, exceptionally downfield. We conclude, therefore, that
the rate at which these tautomers interconvert is slow compared with the
inherent time scale of nmr spectroscopy. |
 |
|---|
Two structurally equivalent structures may be drawn for the
enol tautomer (in magenta brackets). If these enols were slow to
interconvert, we would expect to see two methyl resonance signals
associated with each, one from the allylic methyl and one from the
methyl ketone. Since only one strong methyl signal is observed, we must
conclude that the interconversion of the enols is very fast-so fast that
the nmr experiment detects only a single time-averaged methyl group
(50% α-keto and 50% allyl).
|
Although hydroxyl protons have been the focus of this discussion, it
should be noted that corresponding N-H groups in amines and amides also
exhibit hydrogen bonding nmr shifts, although to a lesser degree.
Furthermore, OH and NH groups can undergo rapid proton exchange with
each other; so if two or more such groups are present in a molecule, the
nmr spectrum will show a single signal at an average chemical shift.
For example, 2-hydroxy-2-methylpropanoic acid, (CH
3)
2C(OH)CO
2H,
displays a strong methyl signal at δ 1.5 and a 1/3 weaker and broader
OH signal at δ 7.3 ppm. Note that the average of the expected carboxylic
acid signal (ca. 12 ) and the alcohol signal (ca. 2 ) is 7. Rapid
exchange of these hydrogens with heavy water, as noted above, would
cause the low field signal to disappear.
For additional information about the influence of hydrogen bonding Click Here.
|
|---|
Ï€-Electron Functions
An examination of the proton chemical shift chart (
above)
makes it clear that the inductive effect of substituents cannot account
for all the differences in proton signals. In particular the low field
resonance of hydrogens bonded to double bond or aromatic ring carbons is
puzzling, as is the very low field signal from aldehyde hydrogens. The
hydrogen atom of a terminal alkyne, in contrast, appears at a relatively
higher field. All these anomalous cases seem to involve hydrogens
bonded to pi-electron systems, and an explanation may be found in the
way these pi-electrons interact with the applied magnetic field.
Pi-electrons are more polarizable than are sigma-bond electrons, as
addition reactions of electrophilic reagents to alkenes testify.
Therefore, we should not be surprised to find that field induced
pi-electron movement produces strong secondary fields that perturb
nearby nuclei. The pi-electrons associated with a benzene ring provide a
striking example of this phenomenon, as shown below. The electron cloud
above and below the plane of the ring circulates in reaction to the
external field so as to generate an opposing field at the center of the
ring and a supporting field at the edge of the ring. This kind of
spatial variation is called
anisotropy, and it is common to
nonspherical distributions of electrons, as are found in all the
functions mentioned above. Regions in which the induced field supports
or adds to the external field are said to be
deshielded, because a
slightly weaker external field will bring about resonance for nuclei in
such areas. However, regions in which the induced field opposes the
external field are termed
shielded because an increase in the applied field is needed for resonance. Shielded regions are designated by a
plus sign, and deshielded regions by a
negative sign.
The anisotropy of some important unsaturated functions will be displayed by clicking on the benzene diagram below.
Note that the anisotropy about the triple bond nicely accounts for the
relatively high field chemical shift of ethynyl hydrogens. The shielding
& deshielding regions about the carbonyl group have been described
in two ways, which alternate in the display.
For additional examples of chemical shift variation near strongly anisotropic groups Click Here.
|
|---|
Sigma bonding electrons also have a less pronounced, but observable,
anisotropic influence on nearby nuclei. This is seen in the small
deshielding shift that occurs in the series C
H3–R,
R–C
H2–R, R
3C
H; as well as the deshielding of equatorial versus axial protons on a fixed cyclohexane ring.
Solvent Effects
Chloroform-d (CDCl
3) is the most common solvent for nmr
measurements, thanks to its good solubilizing character and relative
unreactive nature ( except for 1º and 2º-amines). As noted earlier,
other deuterium labeled compounds, such as deuterium oxide (D
2O), benzene-d6 (C
6D
6), acetone-d6 (CD
3COCD
3) and DMSO-d6 (CD
3SOCD
3)
are also available for use as nmr solvents. Because some of these
solvents have π-electron functions and/or may serve as hydrogen bonding
partners, the chemical shifts of different groups of protons may change
depending on the solvent being used. The following table gives a few
examples, obtained with dilute solutions at 300 MHz.
Some Typical 1H Chemical Shifts (δ values) in Selected Solvents
|
|
CDCl3 | C6D6 | CD3COCD3 | CD3SOCD3 | CD3C≡N | D2O |
|---|
(CH3)3C–O–CH3
C–CH3
O–CH3 |
1.19
3.22 | 1.07
3.04 | 1.13
3.13 | 1.11
3.03 | 1.14
3.13 | 1.21
3.22 |
|---|
(CH3)3C–O–H
C–CH3
O–H |
1.26
1.65 | 1.05
1.55 | 1.18
3.10 | 1.11
4.19 | 1.16
2.18 | ---
--- |
|---|
C6H5CH3
CH3
C6H5 |
2.36
7.15-7.20 | 2.11
7.00-7.10 | 2.32
7.10-7.20 | 2.30
7.10-7.15 | 2.33
7.15-7.30 | ---
--- |
|---|
| (CH3)2C=O
| 2.17 | 1.55 | 2.09 | 2.09 | 2.08 | 2.22 |
|---|
For most of the above resonance signals and solvents the changes are
minor, being on the order of ±0.1 ppm. However, two cases result in more
extreme changes and these have provided useful applications in
structure determination. First, spectra taken in benzene-d
6
generally show small upfield shifts of most C–H signals, but in the case
of acetone this shift is about five times larger than normal. Further
study has shown that carbonyl groups form weak Ï€–Ï€ collision complexes
with benzene rings, that persist long enough to exert a significant
shielding influence on nearby groups. In the case of substituted
cyclohexanones, axial α-methyl groups are shifted upfield by 0.2 to 0.3
ppm; whereas equatorial methyls are slightly deshielded (shift downfield
by about 0.05 ppm). These changes are all relative to the corresponding
chloroform spectra.
The second noteworthy change is seen in the spectrum of tert-butanol in
DMSO, where the hydroxyl proton is shifted 2.5 ppm down-field from
where it is found in dilute chloroform solution. This is due to strong
hydrogen bonding of the alcohol O–H to the sulfoxide oxygen, which not
only de-shields the hydroxyl proton, but secures it from very rapid
exchange reactions that prevent the display of spin-spin splitting.
Similar but weaker hydrogen bonds are formed to the carbonyl oxygen of
acetone and the nitrogen of acetonitrile. A useful application of this
phenomenon is described
elsewhere in this text.
Spin-Spin Interactions
The nmr spectrum of 1,1-dichloroethane (below right) is more
complicated than we might have expected from the previous examples.
Unlike its 1,2-dichloro-isomer (below left), which displays a single
resonance signal from the four structurally equivalent hydrogens, the
two signals from the different hydrogens are split into close groupings
of two or more resonances. This is a common feature in the spectra of
compounds having different sets of hydrogen atoms bonded to adjacent
carbon atoms. The signal splitting in proton spectra is usually small,
ranging from fractions of a Hz to as much as 18 Hz, and is designated as
J (referred to as the coupling constant). In the 1,1-dichloroethane example all the coupling constants are 6.0 Hz,
as illustrated by clicking on the spectrum.
 | |  |
| 1,2-dichloroethane | | 1,1-dichloroethane |
|---|
The splitting patterns found in various spectra are easily
recognized, provided the chemical shifts of the different sets of
hydrogen that generate the signals differ by two or more ppm. The
patterns are symmetrically distributed on both sides of the proton
chemical shift, and the central lines are always stronger than the outer
lines. The most commonly observed patterns have been given descriptive
names, such as
doublet (two equal intensity signals),
triplet (three signals with an intensity ratio of 1:2:1) and
quartet
(a set of four signals with intensities of 1:3:3:1). Four such patterns
are displayed in the following illustration. The line separation is
always constant within a given multiplet, and is called the
coupling constant (J). The magnitude of J, usually given in units of Hz, is magnetic field independent.
The splitting patterns shown above display the ideal or "
First-Order"
arrangement of lines. This is usually observed if the spin-coupled
nuclei have very different chemical shifts (i.e. Δν is large compared to
J). If the coupled nuclei have similar chemical shifts, the splitting
patterns are distorted (second order behavior). In fact, signal
splitting disappears if the chemical shifts are the same. Two examples
that exhibit minor 2nd order distortion are shown below (both are taken
at a frequency of 90 MHz). The ethyl acetate spectrum on the left
displays the typical quartet and triplet of a substituted ethyl group.
The spectrum of 1,3-dichloropropane on the right demonstrates that
equivalent sets of hydrogens may combine their influence on a second,
symmetrically located set.
Even though the chemical shift difference between the A and B protons in
the 1,3-dichloroethane spectrum is fairly large (140 Hz) compared with
the coupling constant (6.2 Hz), some distortion of the splitting
patterns is evident. The line intensities closest to the chemical shift
of the coupled partner are enhanced. Thus the B set triplet lines
closest to A are increased, and the A quintet lines nearest B are
likewise stronger. A smaller distortion of this kind is visible for the A
and C couplings in the ethyl acetate spectrum.
For additional examples of Second Order splitting patterns Click Here.
|
|---|
What causes this signal splitting, and what useful information can be obtained from it ?
If an atom under examination is perturbed or influenced by a nearby
nuclear spin (or set of spins), the observed nucleus responds to such
influences, and its response is manifested in its resonance signal. This
spin-coupling is transmitted through the connecting bonds, and it
functions in both directions. Thus, when the perturbing nucleus becomes
the observed nucleus, it also exhibits signal splitting with the same J.
For spin-coupling to be observed, the sets of interacting nuclei must
be bonded in relatively close proximity (e.g. vicinal and geminal
locations), or be oriented in certain optimal and rigid configurations.
Some spectroscopists place a number before the symbol J to designate the
number of bonds linking the coupled nuclei (colored orange below).
Using this terminology, a vicinal coupling constant is
3J and a geminal constant is
2J.
The following general rules summarize important requirements and characteristics for spin 1/2 nuclei :
1) Nuclei having the same chemical shift (called isochronous) do not exhibit spin-splitting. They may actually be spin-coupled, but the splitting cannot be observed directly.
2) Nuclei separated by three or fewer bonds (e.g. vicinal
and geminal nuclei ) will usually be spin-coupled and will show mutual
spin-splitting of the resonance signals (same J's), provided they have
different chemical shifts. Longer-range coupling may be observed in
molecules having rigid configurations of atoms.
3) The magnitude of the observed spin-splitting depends on many factors and is given by the coupling constant J
(units of Hz). J is the same for both partners in a spin-splitting
interaction and is independent of the external magnetic field strength.
4) The splitting pattern of a given nucleus (or set of
equivalent nuclei) can be predicted by the n+1 rule, where n is
the number of neighboring spin-coupled nuclei with the same (or very
similar) Js. If there are 2 neighboring, spin-coupled, nuclei the
observed signal is a triplet ( 2+1=3 ); if there are three spin-coupled
neighbors the signal is a quartet ( 3+1=4 ). In all cases the central
line(s) of the splitting pattern are stronger than those on the
periphery. The intensity ratio of these lines is given by the numbers in
Pascal's triangle. Thus a doublet has 1:1 or equal intensities, a
triplet has an intensity ratio of 1:2:1, a quartet 1:3:3:1 etc. To see
how the numbers in Pascal's triangle are related to the Fibonacci series
click on the diagram.
 |
 |
|---|
| If a given nucleus is spin-coupled to two or more
sets of neighboring nuclei by different J values, the n+1 rule does not
predict the entire splitting pattern. Instead, the splitting due to one
J set is added to that expected from the other J sets. Bear in mind
that there may be fortuitous coincidence of some lines if a smaller J is
a factor of a larger J. |
 |
|---|
Magnitude of Some Typical Coupling Constants
 < |
|---|
Spin 1/2 nuclei include
1H,
13C,
19F &
31P. The spin-coupling interactions described above may occur between similar or dissimilar nuclei. If, for example, a
19F is spin-coupled to a
1H, both nuclei will appear as doublets having the same J constant.
Spin coupling with nuclei having spin other than 1/2 is more complex and will not be discussed here.
To make use of a calculator that predicts first order splitting patterns Click Here. This application was developed at Colby College.
For additional information about spin-spin coupling Click Here.
|
|---|
Some Examples
Test your ability to interpret
1H nmr spectra by analyzing the seven examples presented below. The seven spectra may be examined in turn by clicking the "
Toggle Spectra" button. Try to associate each spectrum with a plausible structural formula.
Although
the first four cases are relatively simple, keep in mind that the
integration values provide ratios, not absolute numbers. In two cases
additional information from infrared spectroscopy is provided. When you
have made an assignment you may check your answer by clicking on the
spectrum itself. In the sixth example, a similar constitutional isomer
cannot be ruled out by the data given.
For a challenging problem having many spin couplings Click Here.
|
|---|
3. Carbon NMR Spectroscopy
The power and usefulness of
1H nmr spectroscopy as a tool for
structural analysis should be evident from the past discussion.
Unfortunately, when significant portions of a molecule lack C-H bonds,
no information is forthcoming. Examples include polychlorinated
compounds such as chlordane, polycarbonyl compounds such as croconic
acid, and compounds incorporating triple bonds (structures below, orange
colored carbons).
Even when numerous C-H groups are present, an unambiguous
interpretation of a proton nmr spectrum may not be possible. The
following diagram depicts three pairs of isomers (A & B) which
display similar proton nmr spectra. Although a careful determination of
chemical shifts should permit the first pair of compounds (blue box) to
be distinguished, the second and third cases (red & green boxes)
might be difficult to identify by proton nmr alone.
These difficulties would be largely resolved if the carbon atoms of a
molecule could be probed by nmr in the same fashion as the hydrogen
atoms. Since the major isotope of carbon (
12C) has no spin, this option seems unrealistic. Fortunately, 1.1% of elemental carbon is the
13C isotope, which has a spin I = 1/2, so in principle it should be possible to conduct a carbon nmr experiment.
It is worth noting here, that if much higher abundances of 13C
were naturally present in all carbon compounds, proton nmr would
become much more complicated due to large one-bond coupling of 13C and 1H.
Many obstacles needed to be overcome before carbon nmr emerged as a routine tool :
i) As noted, the abundance of 13C in a sample is very low (1.1%), so higher sample concentrations are needed.
ii) The 13C nucleus is over fifty times less sensitive than a proton in the nmr experiment, adding to the previous difficulty.
iii) Hydrogen atoms bonded to a 13C atom split its nmr signal by 130 to 270 Hz, further complicating the nmr spectrum. |
The most important operational technique that has led to successful and routine
13C nmr spectroscopy is the use of high-field
pulse technology coupled with broad-band
heteronuclear decoupling
of all protons. The results of repeated pulse sequences are accumulated
to provide improved signal strength. Also, for reasons that go beyond
the present treatment, the decoupling irradiation enhances the
sensitivity of carbon nuclei bonded to hydrogen.
When acquired in this manner, the carbon nmr spectrum of a compound
displays a single sharp signal for each structurally distinct carbon
atom in a molecule (remember, the proton couplings have been removed).
The spectrum of camphor, shown on the left below, is typical.
Furthermore, a comparison with the
1H nmr spectrum on the right illustrates some of the advantageous characteristics of carbon nmr. The dispersion of
13C
chemical shifts is nearly twenty times greater than that for protons,
and this together with the lack of signal splitting makes it more likely
that every structurally distinct carbon atom will produce a separate
signal. The only clearly identifiable signals in the proton spectrum are
those from the methyl groups. The remaining protons have resonance
signals between 1.0 and 2.8 ppm from TMS, and they overlap badly thanks
to spin-spin splitting.
Unlike proton nmr spectroscopy,
the relative strength of carbon nmr signals are not normally proportional to the number of atoms generating each one.
Because of this, the number of discrete signals and their chemical
shifts are the most important pieces of evidence delivered by a carbon
spectrum. The general distribution of carbon chemical shifts associated
with different functional groups is summarized in the following chart.
Bear in mind that these ranges are approximate, and may not encompass
all compounds of a given class. Note also that the over 200 ppm range of
chemical shifts shown here is much greater than that observed for
proton chemical shifts.
13C Chemical Shift Ranges*
|
|---|
Low Field
Region |
|
High Field
Region |
|---|
| | * For samples in CDCl3 solution. The δ scale is relative to TMS at δ=0.
| |
The isomeric pairs previously cited as giving very similar proton nmr
spectra are now seen to be distinguished by carbon nmr. In the example
on the left below (blue box), cyclohexane and 2,3-dimethyl-2-butene both
give a single sharp resonance signal in the proton nmr spectrum (the
former at δ 1.43 ppm and the latter at 1.64 ppm). However, in its carbon
nmr spectrum cyclohexane displays a single signal at δ 27.1 ppm,
generated by the equivalent ring carbon atoms (colored blue); whereas
the isomeric alkene shows two signals, one at δ 20.4 ppm from the methyl
carbons (colored brown), and the other at 123.5 ppm (typical of the
green colored sp
2 hybrid carbon atoms).
The C
8H
10 isomers in the center (red) box have pairs of
homotopic
carbons and hydrogens, so symmetry should simplify their nmr spectra.
The fulvene (isomer A) has five structurally different groups of carbon
atoms (colored brown, magenta, orange, blue and green respectively) and
should display five
13C nmr signals (one near 20 ppm and the
other four greater than 100 ppm). Although ortho-xylene (isomer B) will
have a proton nmr very similar to isomer A, it should only display four
13C
nmr signals, originating from the four different groups of carbon atoms
(colored brown, blue, orange and green). The methyl carbon signal will
appear at high field (near 20 ppm), and the aromatic ring carbons will
all give signals having δ > 100 ppm. Finally, the last isomeric pair,
quinones A & B in the green box, are easily distinguished by
carbon nmr. Isomer A displays only four carbon nmr signals (δ 15.4,
133.4, 145.8 & 187.9 ppm); whereas, isomer B displays five signals
(δ 15.9, 133.3, 145.8, 187.5 & 188.1 ppm), the additional signal
coming from the non-identity of the two carbonyl carbon atoms (one
colored orange and the other magenta).
A
mass spectrometer creates charged particles (ions) from molecules. It then
analyzes those ions to provide information about the molecular weight of
the compound and its chemical structure. There are many types of mass spectrometers
and sample introduction techniques which allow a wide range of analyses.
This discussion will focus on mass spectrometry as it's used in the powerful
and widely used method of coupling Gas Chromatography (GC) with Mass Spectrometry
(MS).
Pictured above
is a GC/MS instrument used in the organic teaching labs.
A
mixture of compounds to be analysed is initially injected into the GC where
the mixture is vaporized in a heated chamber. The gas mixture travels through
a GC column, where the compounds become separated as they interact with
the column. The chromatogram on the right shows peaks which result from
this separation. Those separated compounds then immediately enter the mass
spectrometer.
CH3OH+.+
2 electrons
