Chapter 8-Spectroscopy.pptx

Spectroscopy
“seeing the unseeable”
Using electromagnetic radiation as a probe to obtain information about atoms
and molecules that are too small to see.
Electromagnetic radiation is propagated at the speed of light through a vacuum as
an oscillating wave.
Electromagnetic relationships:
λυ = c λ µ 1/υ
E = hυ E µ υ
E = hc/λ E µ 1/λ
where: λ = wave length
υ = frequency
c = speed of light
E = kinetic energy
h = Planck’s constant

Spectroscopy & Structure
Determination
The major steps involved in determining the structure of an unknown
compound are:
•Isolate and purify unknown compound
•Determine the elements present (empirical formula)
•Determine the molecular formula
•Identify the functional groups present
•Assemble the formula of the molecule with the correct constitution
and stereochemistry
Structural analysis of organic compounds is based upon the study of spectra
of the compounds , even if available in small amounts.
These are spectroscopic methods of analysis.
The important among them are
•Ultraviolet (UV)
•Infrared (IR)
•Nuclear Magnetic Resonance(NMR)
•Mass spectroscopy

Mass Spectroscopy
In order to determine the molecular formula of a compound, the
molecular mass of that compound is required. Benzene (C6H6) and
acetylene (C2H2) both have the empirical formula CH, but different
molecular masses and molecular formulas.
Mass spectrometry is used to determine the molecular mass of an
organic compound.
A small sample of the compound is vaporised under very low pressure
and high temperature and the vapour is irradiated with a beam of high
energy electrons (4000 – 6000 kJ mol-1).
This causes electrons to be ejected from molecules in the sample,
leaving them as positively charged cations – the molecular
ion or parent ion.

Electromagnetic Spectrum
The arrangement of all types of electromagnetic radiations in order of their
increasing wavelengths or decreasing frequencies is called electromagnetic
spectrum.

Region Wavelength
(nm)
Far ultraviolet 10-200
Near ultraviolet 200-380
Visible 380-780
Near infrared 780-3000
Middle infrared 3000-30,000
Far infrared 30,000-300,000
Microwave 300,000-1,000,000,000
Electromagnetic Spectrum

Types Of Spectroscopy
•Emission Spectroscopy :-used in case of atoms , less used
since the organic compounds generally break under the
conditions required to record an emission spectrum.
•Absorption Spectroscopy :- Organic compounds absorb
radiation in different regions of the electromagnetic spectrum.
The energy of radiation is proportional to its frequency
(E = hu) and the frequency and wavelength of light are related
by the speed of light (l n = c). The absorption of
electromagnetic radiation can be detected and used to identify
features of the molecule and this is termed absorption
spectroscopy.
i. UV spectroscopy
ii. IR spectroscopy
iii. NMR spectroscopy
iv. Microwave spectroscopy
v. Electron-spin resonance spectroscopy

UV Spectroscopy
Absorption Laws
Lambert’s Law : When a beam of monochromatic light
passes through a homogeneous medium , the rate of decrease
of intensity of radiation with the thickness of absorbing
medium is proportional to the intensity of the incident
radiation.
Mathematically
-dI/dx = kI
I= Intensity of the radiation after passing through a particular
thickness x
-dI/dx = rate of decrease of intensity of the radiation with the
thickness of the absorbing medium
K = constant of proportionality of absorption constant

Beer’s Law:- The absorption of light by a substance of
particular wavelength is proportional to the number of
molecules in the path of light.
Mathematically
log(I0/I) = εcl
I0=Intensity of incident light
I=Intensity of transmitted light
C=concentration of absorbing substance in moles /litre
l=path length or thickness of cell containing the sample in
centimetres
ε=proportionality constant known as molar absorptivity or
molar extinction constant

Principle Of UV spectroscopy
Radiation in the ultraviolet (UV) and visible region of the
spectrum has the correct energy to excite electrons in one
orbital into an orbital of higher energy. The electrons that are
most easily promoted are those in conjugated p-bonds. A
conjugated molecule is one in which there is an alternation
between single and multiple bonds in at least part of the
molecule, for example: aromatic compounds, 1,3-dienes
Organic compounds which contain conjugated multiple
bonds strongly absorb ultraviolet-visible radiation.
The UV and visible spectra of organic molecules consist of
bands rather than peaks since there will be a large number of
possible transitions requiring only slightly different energies
which will require absorption of a large number of

Types of Electronic Transitions
i. - * ii. n- * iii. - *
iv. n- *
Probability of electronic transitions
i. Allowed transitions : The transitions which usually take
place at εmax ranging between 104 to 106 are called allowed
transitions. These are generally due to - * transitions.
ii. Forbidden transitions : the electronic transitions for
ranging between 10-1000 are known as forbidden
transitions . for example , n- * transition in case of
saturated aldehydes and ketones.

The UV absorption Process
•  * and   * transitions: high-energy,
accessible in vacuum UV (max <150 nm). Not usually
observed in molecular UV-Vis.
•n  * and   * transitions: non-bonding
electrons (lone pairs), wavelength (max) in the 150-250
nm region.
•n  * and   * transitions: most common
transitions observed in organic molecular UV-Vis,
observed in compounds with lone pairs and multiple
bonds with max = 200-600 nm.
•Any of these require that incoming photons match in
energy the gap corrresponding to a transition from
ground to excited state.
•Energies correspond to a 1-photon of 300 nm light are
ca. 95 kcal/mol

Absorption spectroscopy carried
out in this region is sometimes
called "electronic spectroscopy".
Of the six transitions outlined,
only the two lowest energy ones
(left-most, colored blue) are
achieved by the energies available
in the 200 to 800 nm spectrum
When sample molecules are exposed to light having an energy that matches a
possible electronic transition within the molecule, some of the light energy
will be absorbed as the electron is promoted to a higher energy orbital. An
optical spectrometer records the wavelengths at which absorption occurs,
together with the degree of absorption at each wavelength. The resulting
spectrum is presented as a graph of absorbance (A) versus wavelength
The corrected absorption value is called "molar absorptivity", and is particularly
useful when comparing the spectra of different compounds and determining
the relative strength of light absorbing functions (chromophores). Molar
absorptivity (ε) is defined as:
Molar Absorptivity, ε = A / c l
(where A= absorbance, c = sample concentration in moles/liter & l = length of
light path through the sample in cm.)

Chromophore Concept
In general, the greater the length of a conjugated system in a molecule,
the nearer the λmax comes to the visible region.
Thus, the characteristic energy of a transition and hence the
wavelength of absorption is a property of a group of atoms rather than
the electrons themselves. When such absorption occurs, two types of
groups can influence the resulting absorption spectrum of the
molecule: chromophores and auxochromes.
A chromophore (literally color-bearing) group is a functional group, not
conjugated with another group, which exhibits a characteristic
absorption spectrum in the ultraviolet or visible region.
If any of the simple chromophores is conjugated with another (of the
same type or different type) a multiple chromophore is formed having
a new absorption band which is more intense and at a longer
wavelength that the strong bands of the simple chromophores.
This displacement of an absorption maximum towards a longer
wavelength (i.e. from blue to red) is termed a bathochromic shift.
The displacement of an absorption maximum from the red to
ultraviolet is termed a hypsochromic shift.

Nature of Shift Descriptive term
To Longer Wavelength Bathochromic
To Shorter Wavelength Hypsochromic
To Greater Absorbance Hyperchromic
To Lower Absorbance Hypochromic
Terminology for Absorption Shifts

Auxochromes
The color of a molecule may be intensified by groups called auxochromes
which generally do not absorb significantly in the 200-800nm region, but will
affect the spectrum of the chromophore to which it is attached.
The most important auxochromic groups are OH, NH2, CH3 and NO2 and
their properties are acidic (phenolic) or basic.
The actual effect of an auxochrome on a chromophore depends on the
polarity of the auxochrome, e.g. groups like CH3
-, CH3CH2
-and Cl- have very
little effect, usually a small red shift of 5-10nm. Other groups such as -NH2
and-NO2 are very
popular and completely alter the spectra of chromophores such as benzene.
In general it should be possible to predict the effect of non-polar or weakly
polar auxochromes, but the effect of strongly polar auxochromes is difficult
to predict. In addition, the availability of non-bonding electrons which may
enter into transitions also contributes greatly to the effect of an auxochrome.

An Electronic Spectrum
Absorbance
Wavelength, , generally in nanometers (nm)
0.0
400 800
1.0
200
UV Visible
maxwith certain
extinction 
Make solution of
concentration low
enough that A≤ 1
(Ensures Linear Beer’s
law behavior)
Even though a dual
beam goes through a
solvent blank, choose
solvents that are UV
transparent.
Can extract the  value
if conc. (M) and b (cm)
are known
UV bands are much
broader than the
photonic transition
event. This is because
vibration levels are
superimposed on UV.

Solvents for UV (showing high
energy cutoffs)
Water 205
CH3CN 210
C6H12 210
Ether 210
EtOH 210
Hexane 210
MeOH 210
Dioxane 220
THF 220
CH2Cl2 235
CHCl3 245
CCl4 265
Benzene 280
Acetone 300
Various buffers for HPLC,
check before using.

Choice of Solvent
A suitable solvent for ultraviolet spectroscopy should meet
the following requirements :
• It should not itself absorb radiations in the region under
investigation
• It should be less polar so that it has minimum interaction
with the solute particles
The most commonly used solvent is 95% ethanol . It is
cheap ; has good dissolving power and does not absorb
radiations above 210 mm. Some other solvents used are n-
hexane , cyclohexane , methanol , water and ether . n-
hexane and other hydrocarbons are sometimes preferred to
polar solvents because they have minimum interactions
with the solute particles.

Polyenes, and Unsaturated
Carbonyl groups;
an Empirical triumph
Predict max for π* in extended conjugation systems to within
ca. 2-3 nm.
Homoannular, base 253 nm
heteroannular, base 214 nm
Acyclic, base 217 nm
Attached group increment, nm
Extend conjugation +30
Addn exocyclic DB +5
Alkyl +5
O-Acyl 0
S-alkyl +30
O-alkyl +6
NR2 +60
Cl, Br +5

Similar for Enones
O
x

b
b O O
X=H 207
X=R 215
X=OH 193
X=OR 193
215 202 227 239
Base Values, add these increments…
Extnd C=C +30
Add exocyclic C=C +5
Homoannular diene +39
alkyl +10 +12 +18 +18
OH +35 +30 +50
OAcyl +6 +6 +6 +6
O-alkyl +35 +30 +17 +31
NR2
S-alkyl
Cl/Br +15/+25 +12/+30
 b g d,+
With solvent
correction of…..
Water +8
EtOH 0
CHCl3 -1
Dioxane -5
Et2O -7
Hydrcrbn -11

Some Worked Examples
O
Base value 217 2
x alkyl subst. 10
exo DB 5
total 232
Obs. 237
Base value 214 3
x alkyl subst. 30
exo DB 5
total 234
Obs. 235
Base value 215 2
ß alkyl subst. 24
total 239
Obs. 237

IR Spectroscopy
Electromagnetic radiation in the infrared (IR) region of the
spectrum has the correct energy to cause bonds in a molecule to
stretch and bend. Individual functional groups have a
characteristic absorption in the IR region. The absence of an
absorption in the IR spectrum of a compound can be important.
For example, if an oxygen-containing compound shows no
absorption in the C=O region (1680-1750 cm-1) or in the O-H
region (2500 - 3650 cm-1) of the IR spectrum, the compound is
likely to be an ether.
λ = 2.5 to 17 μm
υ = 4000 to 600 cm-1
These frequencies match the frequencies of covalent bond
stretching and bending vibrations. Infrared spectroscopy can be
used to find out about covalent bonds in molecules.

I. Introduction
C. The IR Spectroscopic Process
1. The quantum mechanical energy levels observed in IR spectroscopy
are those of molecular vibration
2. We perceive this vibration as heat
3. When we say a covalent bond between two atoms is of a certain
length, we are citing an average because the bond behaves as if it were
a vibrating spring connecting the two atoms
4. For a simple diatomic molecule, this model is easy to visualize:

Infrared Spectroscopy of Organic
Molecules
• IR region lower energy than visible light (below red – produces
heating as with a heat lamp)
• 2.5  106 m to 2.5  105 m region used by organic chemists for
structural analysis
• IR energy in a spectrum is usually measured as wavenumber (cm-
1), the inverse of wavelength and proportional to frequency

IR source  sample  prism  detector
graph of % transmission vs. frequency
=> IR spectrum
4000 3000 2000 1500 1000 500
v (cm-1)
100
%T
0
IR is used to tell:
1. what type of bonds are present
2. some structural information

5. There are two types of bond vibration:
• Stretch – Vibration or oscillation along the line of the bond
• Bend – Vibration or oscillation not along the line of the bond
H
H
C
C
rock twist
in plane out of plane
H
H
C
scissor
asymmetric
H
H
C
C
H
H
C
C
H
H
C
C
symmetric
H
H
C
wag

6.As a covalent bond oscillates – due to the oscillation of the dipole of the
molecule – a varying electromagnetic field is produced
7.The greater the dipole moment change through the vibration, the more
intense the EM field that is generated
The region of an infrared spectrum below about 1500 cm-1 is termed the
fingerprint region. Many absorptions in this region result from vibrations
of the molecule as a whole and no two compounds have exactly the same
absorption in the fingerprint region.

D. The IR Spectrum
1. Each stretching and bending vibration occurs with a characteristic
frequency as the atoms and charges involved are different for different
bonds
The y-axis on an IR
spectrum is in units of %
transmittance
In regions where the EM
field of an osc. bond
interacts with IR light of
the same n –
transmittance is low
(light is absorbed)
In regions where
no osc. bond is
interacting with
IR light,
transmittance
nears 100%
Infrared Spectroscopy

IR Spectroscopy
D. The IR Spectrum
2. The x-axis of the IR spectrum is in units of wavenumbers, n, which is the
number of waves per centimeter in units of cm-1 (Remember E = hn or E =
hc/)

IR Spectroscopy
D. The IR Spectrum
3. This unit is used rather than wavelength (microns) because wavenumbers are
directly proportional to the energy of transition being observed –
chemists like this, physicists hate it
High frequencies and high wavenumbers equate higher energy
is quicker to understand than
Short wavelengths equate higher energy
4. This unit is used rather than frequency as the numbers are more “real” than
the exponential units of frequency
5. IR spectra are observed for what is called the mid-infrared: 400-4000 cm-1
6. The peaks are Gaussian distributions of the average energy of a transition

IR Spectroscopy
D. The IR Spectrum
7. In general:
Lighter atoms will allow the oscillation to be faster – higher energy
This is especially true of bonds to hydrogen – C-H, N-H and O-H
Stronger bonds will have higher energy oscillations
Triple bonds > double bonds > single bonds in energy
Energy/n of oscillation

E. The IR Spectrum – The detection of different bonds
7. As opposed to chromatography or other spectroscopic methods, the
area of a IR band (or peak) is not directly proportional to concentration
of the functional group producing the peak
8. The intensity of an IR band is affected by two primary factors:
Whether the vibration is one of stretching or bending
Electronegativity difference of the atoms involved in the bond
• For both effects, the greater the change in dipole moment in a
given vibration or bend, the larger the peak.
• The greater the difference in electronegativity between the atoms
involved in bonding, the larger the dipole moment
• Typically, stretching will change dipole moment more than
bending

Regions of the Infrared
Spectrum
• 4000-2500 cm-1 N-H,
C-H, O-H
(stretching)
– 3300-3600 N-H, O-H
– 3000 C-H
• 2500-2000 cm-1 CC
and C  N
(stretching)
• 2000-1500 cm-1
double bonds
(stretching)
– C=O 1680-1750
– C=C 1640-1680 cm-1
• Below 1500 cm-1
“fingerprint” region

Infrared Spectra of Hydrocarbons
• C-H, C-C, C=C, C  C have characteristic peaks
– absence helps rule out C=C or C  C

IR spectra of ALKANES
C—H bond “saturated”
(sp3) 2850-2960 cm-1
+ 1350-1470 cm-1
-CH2- + 1430-1470
-CH3 + “ and 1375
-CH(CH3)2 + “ and 1370, 1385
-C(CH3)3 + “ and 1370(s), 1395 (m)

IR of ALKENES
=C—H bond, “unsaturated” vinyl
(sp2) 3020-3080 cm-1
+ 675-1000
RCH=CH2 + 910-920 & 990-1000
R2C=CH2 + 880-900
cis-RCH=CHR + 675-730 (v)
trans-RCH=CHR + 965-975
C=C bond 1640-1680 cm-1 (v)

IR spectra BENZENEs
=C—H bond, “unsaturated” “aryl”
(sp2) 3000-3100 cm-1
+ 690-840
mono-substituted + 690-710, 730-770
ortho-disubstituted + 735-770
meta-disubstituted + 690-710, 750-810(m)
para-disubstituted + 810-840(m)
C=C bond 1500, 1600 cm-1

IR spectra ALCOHOLS & ETHERS
C—O bond 1050-1275 (b) cm-1
1o ROH 1050
2o ROH 1100
3o ROH 1150
ethers 1060-1150
O—H bond 3200-3640 (b)

IR: Aromatic Compounds
• Weak C–H stretch at 3030 cm1
• Weak absorptions 1660 - 2000 cm1 range
• Medium-intensity absorptions 1450 to 1600
cm1
• spectrum of phenylacetylene,

IR: Carbonyl Compounds
• Strong, sharp C=O peak 1670 to 1780 cm1
• Exact absorption characteristic of type of carbonyl compound
– 1730 cm1 in saturated aldehydes
– 1705 cm1 in aldehydes next to double bond or aromatic ring

Effects on IR bands
1. Conjugation – by resonance, conjugation lowers the energy of a double or triple
bond. The effect of this is readily observed in the IR spectrum:
• Conjugation will lower the observed IR band for a carbonyl from 20-40 cm-1
provided conjugation gives a strong resonance contributor
• Inductive effects are usually small, unless coupled with a resonance
contributor (note –CH3 and –Cl above)
O
O
1684 cm-1
1715 cm-1
C=O C=O
C
H3C
O
X X = NH2 CH3 Cl NO2
1677 1687 1692 1700 cm-1
H2N C CH3
O
Strong resonance contributor
vs.
N
O
O
C
CH3
O
Poor resonance contributor
(cannot resonate with C=O)

Effects on IR bands
1. Steric effects – usually not important in IR spectroscopy, unless they reduce the
strength of a bond (usually ) by interfering with proper orbital overlap:
• Here the methyl group in the structure at the right causes the carbonyl
group to be slightly out of plane, interfering with resonance
2. Strain effects – changes in bond angle forced by the constraints of a ring will
cause a slight change in hybridization, and therefore, bond strength
• As bond angle decreases, carbon becomes more electronegative, as well
as less sp2 hybridized (bond angle < 120°)
O
C=O: 1686 cm-1
O
C=O: 1693 cm-1
CH3
O O O O O
1815 cm-1
1775 cm-1
1750 cm-1
1715 cm-1
1705 cm-1

Effects on IR bands
1. Hydrogen bonding
• Hydrogen bonding causes a broadening in the band due to the creation of
a continuum of bond energies associated with it
• In the solution phase these effects are readily apparent; in the gas phase
where these effects disappear or in lieu of steric effects, the band appears
as sharp as all other IR bands:
Gas phase spectrum of
1-butanol
Steric hindrance to H-bonding
in a di-tert-butylphenol
• H-bonding can interact with other functional groups to lower frequencies
OH
C=O; 1701 cm-1
O
O
H

Nuclear magnetic resonance (NMR) spectroscopy
A spectroscopic technique that gives us information about the number and
types of atoms in a molecule, for example, about the number and types of
hydrogen atoms using 1H-NMR spectroscopy.
carbon atoms using 13C-NMR spectroscopy.
phosphorus atoms using 31P-NMR spectroscopy.
Within a collection of 1H and 13C atoms, nuclear spins are completely
random in orientation.
When placed in a strong external magnetic field of strength B0, however,
interaction between nuclear spins and the applied magnetic field is
quantized. The result is that only certain orientations of nuclear magnetic
moments are allowed.

51
• 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).
Introduction to NMR Spectroscopy

Nuclear Spins in B0
 for 1H and 13C, only two orientations are allowed.

53
Nuclear Magnetic Resonance Spectroscopy
• 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.

54
• 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.

55
• 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).

56
• 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.

57
• 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.

58
• 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

59
• 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.

60
• 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.
• Four different features of a 1H NMR spectrum provide
information about a compound’s structure:
a. Number of signals
b. Position of signals
c. Intensity of signals.
d. Spin-spin splitting of signals.

61
• 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.

62

63
• 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.

64
• Proton equivalency in cycloalkanes can be determined similarly.

65
• 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

66
• 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.

67

68

69

70
• Protons in a given environment absorb in a predictable region in an
NMR spectrum.
1H NMR—Chemical Shift Values

71
• The chemical shift of a C—H bond increases with increasing alkyl substitution.

72
• 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 C
H2
C
H
Methylene Methine
0.87 ppm 1.20 ppm 1.20 ppm
Base Chemical Shift

73
• 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 (b) carbons.
• Atoms or groups bonded to the b carbon are described as alpha ()
substituents.
(Hydrogen under consideration)
C C H
b 

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C C H
H
H
H
H
Cl
b 
Base Chemical Shift = 0.87 ppm
no substituents = 0.00
one b -Cl (CH3) = 0.63
TOTAL = 1.50 ppm
C C H
H
H
H
H
Cl
b

one  -Cl (CH2) = 2.30
nob 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.

76
• 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.

77
• 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.

78

79
1H NMR—Chemical Shift Values)

1H NMR of Methyl Acetate
C
O
R O
H3C C O
one  = 2.88 ppm
TOTAL = 3.75 ppm
O
CH3
C
O
R
one  = 1.23 ppm
TOTAL = 2.10 ppm

2,3-Dimethyl-2-Butene
one  (CH3) = 0.78 ppm
TOTAL = 1.65 ppm
H2C C
H

82
• 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

83
1H NMR—Intensity of Signals

Methyl ,-Dimethylpropionate

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 1
1 6 15 20 15 6 1
singlet
doublet
triplet
quartet
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

88
1H NMR—Spin-Spin Splitting
Splitting is not generally observed between protons separated by
more than three  bonds.
If Ha and Hb are not equivalent, splitting is observed when:

89
• 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.

90
The frequency difference, measured in Hz, between two peaks of
the doublet is called the coupling constant, J.
J

91
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).

92
• 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.

93

96
Whenever two (or three) different sets of adjacent protons are equivalent
to each other, use the n + 1 rule to determine the splitting pattern.

97
Whenever two (or three) different sets of adjacent protons are equivalent
to each other, use the n + 1 rule to determine the splitting pattern.

98
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

99
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

100
1H NMR—Structure Determination

101

102

103

Coupling constant (J):
The separation on an NMR spectrum (in hertz) between adjacent peaks in a
multiplet.
A quantitative measure of the spin-spin coupling with adjacent nuclei.
8-11 Hz
8-14 Hz 0-5 Hz 0-5 Hz
6-8 Hz
11-18 Hz 5-10 Hz 0-5 Hz
C
C
Ha
C C
Hb
Ha
C
Hb
C
Ha
Hb
Ha
Hb
Ha
Hb Hb
Ha
Hb
Ha
C C
Ha Hb

Coupling Constants
 An important factor in vicinal coupling is the angle 
between the C-H sigma bonds and whether or not it is fixed.
 Coupling is a maximum when  is 0° and 180°; it is a
minimum when  is 90°.

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