Chapter 11 Spectroscopy(??)

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Chapter 11 Spectroscopy(??)
11.1 Introduction (A) The instrumental Methods
of structure determination to organic
compounds (B) Property of electromagnetic
radiation (C) Relationship between the
molecular structure and
absorption spectrum 11.2 Infrared spectroscopy
(A) Infrared Spectroscopy (IR) (B) Modes of
molecular vibrations (C) Factors to affect
frequency of molecular stretching
vibration and its
location in an IR spectrum
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(D) Treatment of the sample (E) Interpreting
of IR spectra 11.3 Nuclear Magnetic Resonance
(NMR) Spectroscopy 11.4 Shielding and
deshielding of protons 11.5 The chemical
shift 11.5.1 The chemical shift 11.5.2 The
relation of molecular structure with
1H chemical shift 11.5.3 Interpretting 1H NMR
spectra Diastereotopic hydrogens
Enantiotopic hydrogens
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11.6 Signal splitting spin-spin coupling
Coupling constant nJab Splitting role
(n 1)role Reciprocity of coupling
constants Splitting patterns 11.7 13C
NMR Spectroscopy 11.7.1 Introduction
The function of 13C NMR
The great advantage of 13C
spectroscopy Decoupling
techniques 11.7.2 13C chemical shifts
The factors that most affect 13C
chemical shifts
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(A) The principal instrumental methods of
structure determination to organic compounds
1. Carbon skeleton 2. The Environments of
the hydrogens atta- ched to carbon or
other atoms
Nuclear Magnetic Resonance Spectroscopy ?????
NMR
Infrared Spetroscopy ????
IR
Key functional groups
Ultraviolet-Visible Spectroscopy ??????
Electron distribution in the Mole. With
Conjugated p systems
UV-VIS
Molecular weight and Formula of the mole.
and the structure units in it.
Mass Spectrometry ??
MS
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(B) Property of electromagnetic radiation
The properties of both particles and waves
Ch.P153
E h ? hc/ ? h- Planck' s constant 6.63
10-34 J. s ?- Frequency( Hz), ? c s ?-
Wavelength (nm) c The speed of light 3 10 8
m. s-1 s-The wave counts /cm
The greater the frequency, the shorter the
wavelength The shorter the wavelength, the
higher the energy.
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(C) Relationship between the structure of
Mole. and absorption spectrum
h? ?E E2 - E1
Electronic energy-UV-VIS Vibrational energy-
IR The spin states of an atomic nucleus-NMR
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11.2 Infrared spectroscopy (IR)
Function of Infrared Spectroscopy
Identifying the presence of certain
functional groups in the molecules.
Electromagnetic spectrum
Ch.P154
Electromagnetic radiation
The compound absorb IR energy to cause the
change of vibrational energy states of the
covalent bond in the mole.
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B. Modes of molecular vibrations
Biatomic molecule
Triatomic molecule
Asymmetric stretching
Symmetric stretching
A stretching vibration
An out-of-plane bending vibration
An in-plane bending vibration
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(C) Factors affecting frequency of molecular
stretching vibration and its location in an IR
spectrum
1. The masses of the bonded atoms
Light atoms vibrate at higher frequencies
2. The relative stiffness of the bond (????)
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(D) Treatment of the sample
Neat sample pure liquid between two NaCl
disks Solid mixed with KBr and pressed into
a thin wafer Solution CCl4, chloroform as solvent
(E) Interpreting of IR spectra
1600-4000 cm-1 The region in which vibrations
of functional
groups
1300-625 cm-1 Fingerprint region
Characteristic absorptions of the compounds
Ch.P519
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Table 1 Infrared Absorption Frequencies of Some
Common Structural Units
Stretching vibrations
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Bending vibrations of diagnostic value
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CH3(CH2)6CH3
2960cm-1 2930cm-1 1467cm-1 1380cm-1
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Toluene
Ar-H (stretch)
CH ( bending)
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2-Methyl-2-propanol
OH (stretch)
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(E)-2-??
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(Z)-3-??
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2-??-1-??
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1-??
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2-??
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11.3 Nuclear Magnetic Resonance (NMR)
Spectroscopy
Essential condition of NMR
The spinning atomic nuclei 1H, 13C (I 1/2)
Process of Nuclear Magnetic Resonance
The different Nuclei, the different ?E
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Schematic operation of an NMR spectrometer
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11.4 Shielding and deshielding of protons
At the proton, the induced magnetic field opposes
The external magnetic field
Heffect H0 - Hin
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11.5.1 The chemical shift
The shift of absorption signal of proton by
shielding and deshileding effect.
ppm part per million ?????
Reference compound (CH3)4Si, TMS
The chemical shift is expressed in hertz
The chemical shift is different with different
external magnetic field strengths.
Independent of the field strength
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11.5.2 Relation of molecular structure with
1H chemical shifts
Different environments of protons in the mole.,
different degrees of shielding in the experiment,
different chemical shifts.
Increased shielding of methyl protons Decreasing
electronegativity of attached atom
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The induced magnetic field of thepelectrons of
an arene reinforces the applied fields.
Ch.P164
Aromatic protons are in the deshielding region.
???????
11.5.3 Intepreting proton NMR spectra (?????)
Information about the mole. Structure by 1H NMR
spectra

• The number of signals-the kind of proton
• 2. Intergration of the area under each peak-
• relative ratios of different protons
• 3. Splitting of each signal-number of vicinal
• protons to that one.

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CH3OCH2CN
Chemical nonequivalent
H1H2 23
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How many signals on NMR spectra?
Diastereotopic ?????
-CH2Br The same d
Enantiotopic hydrogen
Enantiomers
Geminal protons
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P424 Tab.13.3
Ch. P165 Table 7-4
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1H NMR d can be in the range of 1-10 ppm










P423
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11.6 Signal splitting spin-spin coupling
CHCl2CH2Cl
The phenomenon of signal splitting is spin-spin
splitting.
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Signal splitting
The protons on neighboring atoms
Magnetic fields
Singnal from the proton of Ha in the absence of Hb
Magnetic moments of Hb split the signal from Ha
into two peaks of equal intensity, a 11 doublet.
The interaction of the nuclear Spins of
neighboring atoms is called spin-spin coupling.
3JHH 7.5Hz
nJab-coupling constant(Hz)
Distance
Vicinal coupling
Independence of the field strength.
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Signal splitting is not observed for protons

1. Chemically equivalent protons

CH3CH3
2. Enantiotopic protons
3. Separated by more than three sbonds
Two groups of protons coupled to each other
have the same coupling constant J.
When protons have the same chemical shifts, no
signal splitting
Singlet Doublet Triplet Quartet
11 121 1331
Multiple Broad complex OH
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Splitting role -(n 1)role
Multiplicity of signal for Ha n 1
n quivalent protons on neighboring atoms
split a signal into n 1 peaks.
(signal in the absence of protons Hb)
(proton of Hb split the signal
into 1331quartet)
P425, Ch.P165
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CH3CH2Br
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Reciprocity of coupling constants
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Integration 223
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Problem 1. Describe the appearance of
the 1H NMR spectrum
4.2(m,1H,CHCl),
1H NMR(CDCl3) d
1.52 (d, J 3.5Hz, 6H, CH3)
Signals may overlap
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Splitting patterns
A. Doublet
Nonequivalent vicinal protons
First order spectrum (n1)
? ?/ J gt 6
? ?/ J 6
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B. Two bond coupling
Geminal protons
doublet
Diastereotopic
2J 2Hz
Different chemical shifts
doublet
1-Chloro-1-cyanoethene
C. Complex Splitting
Doublet of doublet dd
J trans gt Jcis
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The function of 13C NMR
13C NMR spectrum directly provide the
information about carbon skeleton of a molecule.
P430 13.12
ClCH2CH2CH2CH2CH3
1H NMR 4 Signals
13C NMR 5 Signals
The natural abundance 1 H
99.985 13C 1.1 12C
99
Carbon isotopes
Pulse FT techniques (?? ???????)
Routine tool
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The great advantage of 13C spectroscopy
(1) The wide range of chemical shifts
1H NMR spectrum 012 ppm 13C NMR spectrum 0200
ppm
R-
d 1H spectra 3 ppm 13C spectra
8090 ppm
(2) Signals are less likely to overlap.
Peaks arising from all the carbon atoms.
13C spectra were further simplified
Spin-spin coupling between the carbon
nuclei could be not seen.
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Decoupling techniques
Ch.P168
1) Broad band decoupling (????)
2) Proton off-resonance decoupling (?????)
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Proton off-resonance decoupling
Broad band decoupling
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3) DEPT(Distortionless Enchancement by
Polarization Transfer, ?????? ????)technique
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11.7.2 13C chemical shifts
In most 13C spectra, the areas under signals
are not proportional to the atoms causing the
signals.
The 13C chemical shifts are important data
to determine the structures of molecules.
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P430 Fig. 13.17
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The factors that most affect 13C chemical
shifts
1. The hybridization of carbon
d(ppm) sp2 gt sp gt sp3
2. The electronegativity of the group
attached to carbon
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Problems to Chapter 11
P434 13.19(a), (e)CH3CH2CH2COCOOCH3 13.30(a),(c
) 13.31(b) 13.34 13.35 13.36 13.39(c) 13.40
13.41 13.43 13.49 13.50 13.51 13.52