spectroscopy ch101
TRANSCRIPT
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Modern Techniques in Structural
Elucidation of Compounds
(UV Vis, IR, NMR)
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Spectroscopy is the study of the interactionbetween matter and electromagneticradiation
Ultraviolet light and visible light have just theright energy to cause an electronic transition
the promotion of an electron from one orbital to
another of higher energy.
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UV-Visible Spectroscopy
Depending on the energy needed for the electronictransition, a molecule will absorb either ultraviolet or
visible light.
Ultraviolet light is electromagnetic radiation withwavelengths ranging from 180 to 400 nm(nanometers);
visible light has wavelengths ranging from 400 to780 nm.
1 nm = 10-9 m or 10
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normal electronic configuration of a molecule is known as its
ground state
When a molecule absorbs light of an appropriate wavelengthand an electron is promoted to a higher energy molecular
orbital, the molecule is then in an excited state.
an electronic transition is the promotion of an electron to ahigher energy MO
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HOMO-LUMO
C C
HOMO
LUMO
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only organic compounds with electrons can
produce UV Vis spectra.diethyl ether does not have a UV spectrum, even
though it has lone-pair electrons
UV-Visible Spectroscopy
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Chromophore: part of a molecule that absorbs UV orvisible light.
The carbonyl group is the chromophore of acetone.
If same chromophore, then approximately the samemax :the wavelength corresponding to the highest
point (maximum absorbance) of the absorption
band.
UV-Visible Spectroscopy
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Either glass or quartz cells can be used forvisible spectra, but quartz cells must be used
for UV spectra because glass absorbs UV light.
UV-Visible Spectroscopy
cuvette
sourceslit
detector
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UV-Vis and conjugated double bonds
UV/Vis spectroscopy provides information aboutcompounds with conjugated double bonds.
Effect of Conjugation on max.
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Effect of Conjugation on max
Conjugation raises the energy of the
HOMO and lowers the energy of the LUMO
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HOMO-LUMO
C C
HOMO
LUMO
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both the max and the increase as the number ofconjugated double bonds increases.
Effect of Conjugation
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If a compound has enough conjugated double bonds,it will absorb visible light (max > 400 nm) and thecompound will be colored.
Auxochrome : a substituent that when attached to achromophore, alters the max and the intensity of the
absorption, usually increasing both.
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Auxochrome
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Uses of UV/VIS Spectroscopy
one of the reactants or one of the products absorbsUV or visible light at a wavelength at which the other
reactants and products have little or no absorbance.
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pKa determination using
UV/Vis spectroscopy
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Infrared Spectroscopy
The covalent bonds in molecules are constantlyvibrating.
A bond vibrates with both stretching and bending
motions. A stretch is a vibration occurring along the line of the
bond that changes the bond length.
A bend is a vibration that does not occur along theline of the bond, but changes the bond angle.
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Stretching Vibrations
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Bending Vibrations
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Infrared Spectroscopy
Each stretching and bending vibration of a bond in amolecule occurs with a characteristic frequency.
When a compound is bombarded with radiation of a
frequency that exactly matches the frequency of oneof its vibrations, the molecule will absorb energy.
By experimentally determining the wavenumbers ofthe energy absorbed by a particular compound, we
can ascertain what kinds of bonds it has.
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An IR spectrum can be divided into two areas.
Functional group region: 4000 to 1400 cm-1
: most ofthe functional groups show absorption bands.
Fingerprint region: 1400 to 600 cm-1 : it ischaracteristic of the compound as a whole, just as a
fingerprint is characteristic of an individual.
Even if two different molecules have the samefunctional groups, their IR spectra will not be
identical, since the functional groups are not in
exactly the same environment; this difference is
reflected in the pattern of absorption bands in the
fingerprint regions.
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Characteristic IR Absorption Bands
Organic chemists generally do not try toidentify all the absorption bands in an IR
spectrum.
They will look at some characteristic bands sothat they will be able to tell something about
the structure of a compound that gives a
particular IR spectrum
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Characteristic IR Absorption Bands
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The Position of Absorption Bands
The amount of energy required to stretch abond depends on the strength of the bond
and the masses of the bonded atoms
The equation shows that stronger bonds andlighter atoms give rise to higher
frequencies/wavenumbers
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The Position of Absorption Bands
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Characteristic IR Absorption Bands
Effect of Bond Order
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Resonance and InductiveElectronic Effects
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C-H Absorption Bands
The strength of a C-H bond depends on the
hybridization of the carbonThe greater the s character of the carbon, the
stronger the bond it forms
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Summary of IR Absorptions
Alcohol and amine peaks are broad at around 3300-3500
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NMR Spectroscopy
Nuclear Magnetic Resonance
Electrons are charged, spinning particles withtwo allowed spin states: +1/2 and -1/2.
Certain nuclei also have allowed spin states of+1/2 and -1/2 and and this property allows
them to be studied by NMR. Examples of such
nuclei are 1H, 13C, 15N, 19F, 31P.
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Nuclear Magnetic Resonance Spectroscopy
NMR is one of the most powerful tool availablefor organic structure determination.
It is used to study a wide variety of nuclei: 1H
13C
15N
19F
31P
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Nuclear Spin
A nucleus with an odd atomic number or anodd mass number has a nuclear spin.
The spinning charged nucleus generates amagnetic field.
=>
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External Magnetic Field
When placed in an external field, spinning protonsact like bar magnets.
=>
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Two Energy States
The magnetic fields of the spinning nuclei will align
either with the external field, or againstthe field.A photon with the right amount of energy can be
absorbed and cause the spinning proton to flip.
Energy difference isproportional to themagnetic field strength.
E B0
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Magnetic Shielding
If all protons absorbed the same amount of energyin a given magnetic field, not much information
could be obtained.
But protons are surrounded by electrons thatshield them from the external field.
Circulating electrons create an induced magneticfield that opposes the external magnetic field.
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Protons in electron-dense environments sense asmaller effective magnetic field. They, therefore,will require a lower frequency to come into
resonance (more shielded). Protons in electron-poor environments sense a
larger effective magnetic field and, therefore, willrequire a higher frequency to come into
resonance (less shielded).
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Protons in a Molecule
Depending on their chemical environment, protons
in a molecule are shielded by different amounts.
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NMR Signals
Thenumber of signals shows how many different kinds ofprotons are present.
The location of the signals shows how shielded or deshieldedthe proton is.
The intensity of the signal shows the number of protons of
that type. Signalsplitting shows the number of protons on adjacent
atoms.
Si
CH3
CH3
CH3
H3C TMS = 0 ppm
Th N b f Si l i h
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The Number of Signals in the1H NMR Spectrum
Protons in the same environment are calledchemically equivalent protons
Each set of chemically equivalent protons in acompound gives rise to a signal in the 1H NMR
spectrum of that compound.
C i h i ll i l
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Counting chemically equivalent
protons
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chlorocyclobutane
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Chemical Shift
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Chemical Shift
The positions of the signals in an NMRspectrum are defined according to how far
they are from the signal of the reference
compound.
The position at which a signal occurs in anNMR spectrum is called the chemical shift.
measure of how far the signal is from thereference TMS signal.
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Th R l ti P iti f NMR
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The Relative Positions of NMRSignals
the right-hand side of an NMR spectrum is the low-frequencyside, where protons in electron-dense environments (more
shielded) show a signal.
The left-hand side is the high-frequency side, where less
shielded protons show a signal.
Th R l ti P iti f NMR
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The Relative Positions of NMRSignals
Electron withdrawal causes NMR signals to appear at
higher frequencies
Characteristic Val es of
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Characteristic Values ofChemical Shifts
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Integration of NMR Signals
the area under each signal is proportional tothe number of protons that gives rise to the
signal.
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Splitting of the Signals
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Splitting of the Signals
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=
>
If a signal is split by Nequivalent protons,
it is split into N+ 1 peaks.
The N+ 1 Rule