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Chemistry 328N
Lecture 6
Infrared spectroscopy
February 4, 2016
Chemistry 328N
D2O in H2O
D2O ice in H2O D2O ice in D2O
Chemistry 328N
Chemical Exchange
Hydrogens on electronegative atoms such as
Oxygen and Nitrogen
– Undergo rapid “exchange” and often give only
a relatively broad singlet due to “averaging”
– These hydrogens also exchange (equilibrate)
with Deuterium in D2O and “disappear” from
the spectrum
Chemistry 328N
Common 1H-nmr Solvents
DCCl3
CCl4
(CD3)2SO
D2O
C6D6
Chemistry 328N7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5
0
1
2
3
4
5
6
4.700.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
1.30
1.40
3.70 3.600
1
2
3
4
5
ClOCH3
H H
a
b c
2680 2675 2670 2665 2660 2655 2650 2645 2640 2635 2630 26250.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
2643.652647.85
2649.932654.132656.232660.43
2662.552666.75
Chemistry 328N
2680 2675 2670 2665 2660 2655 2650 2645 2640 2635 2630 26250.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
2643.652647.85
2649.932654.132656.232660.43
2662.552666.75
ClOCH3
H H
a
b c
Whats with what??
Chemistry 328N
2680 2675 2670 2665 2660 2655 2650 2645 2640 2635 2630 26250.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1.20
2643.652647.85
2649.932654.132656.232660.43
2662.552666.75
ClOCH3
H H
a
b c
bc
ab
Coupling Constants
Chemistry 328N
OCl
CH3
HH
6.0 5.5 5.0 4.5 4.0 3.5 3.00.0
0.5
1.0
1.5
Chemical Shift additivity estimates
Caution…..estimates only!!
Table structure Shift
-CH2-CI 3.3
-CH2-O- 3.6
total 6.9
Subtract -CH2-CH3 1.2
Crude Estimate 5.7
Chemistry 328N
0
Type of H d
1.6-2.6
2.0-3.0
0.9
1.2-1.4
1.4-1.7
2.1-2.3
0.5-6.0
2.2-2.6
3.3-4.0
2.2-2.5
2.3-2.8
0.5-5.0
RC CH
O
O
RCH2 OR
(C H3 ) 4 Si
ArC H3
RCH3
RCCH3
ROH
ArC H2 R
RCH2 R
R3 CH
R2 NH
RCCH2 R
R2 C=CRC HR2
Type of H d
Chemical Shift - 1H-NMR
Chemistry 328N
1H Chemical Shifts
Chemistry 328N
13C Chemical Shifts
Chemistry 328N
Chemistry 328N
13C NMR Spectra
C-H: “decoupled”
c
c
d
d
b
b
a
a
C-H “coupled”
Chemistry 328N
Chemistry 328N
13C-nmr Spectroscopy
Each nonequivalent 13C gives a
different, resolved signal
Chemistry 328N
The DEPT Experiment
In the hydrogen-decoupled mode, information on spin-
spin coupling between 13C and attached hydrogens is
lost
Distortionless Enhancement by Polarization Transfer
(DEPT) is an NMR technique for determining whether 13C signals are from CH3, CH2, CH, or quaternary
carbons
DEPT is an instrumental trick that provides the means
to acquire this information
Chemistry 328N
The DEPT method
DEPT uses a complex series of pulses in
both the 1H and 13C ranges, with the result
that CH3, CH2, and CH signals exhibit
different phases;
– signals for CH3 and CH carbons are recorded as
positive signals (odd numbers of H)
– signals for CH2 carbons are recorded as
negative signals (even numbers of H)
– quaternary carbons give no signals in the DEPT
method (zero H)
OPEN ?? ☺
Chemistry 328N
Broadband decoupled 13C nmr spectrum
From Dept
Chemistry 328N
Even
Even
Odd
Odd
Odd
Chemistry 328N
An Unknown Empirical formula C4H9, MW =114
70 60 50 40 30 20 10 0 -100
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
0.00 TMS
25.60
35.00
35.0
25.6
Positive in DEPT
Disappears in
DEPT
TMS
Chemistry 328N
Whatzit??
Chemistry 328N
13C-NMR Spectroscopy review
Each nonequivalent 13C gives a different signal
Low abundance means weak signals
C-C splitting is insignificant
C-H splitting is big and complex so it is “turned
off” by “decoupling”
Range of Chemical Shifts is large compared to H
Some Coupling info can be recovered by DEPT
Integrals of 13C spectra are not useful except under
very special circumstances
Mnemonic device…. PONE???
Chemistry 328N
Chemistry 328N
Molecular Vibrations For a molecule to absorb IR radiation, the bond
undergoing vibration
– must be polar (change dipole moment)
Covalent bonds that do not meet this criterion are
said to be IR inactive
– the C-C double and triple bonds of symmetrically
substituted alkenes and alkynes, for example, do not
absorb IR radiation because they are not polar bonds
H2 N2 CO2 ??????
Chemistry 328N
THE ENERGY OF ELECTROMAGNETIC WAVES
• (nu-bar) represents wavenumber, the
number of wavelengths in 1 cm
• This is a unit of frequency!
• units are 1/cm or cm-1 (Kaysers)
lu
luluc ==
1
10 microns (micrometers) = 1000 cm-1
ln hc
chhE ===
Chemistry 328N
Harmonic Oscillator Model
μ
K
2π
1
m1
m2
ν =
mm
)m)(m(μ
21
21
=
Eν = h νo (ν + 1/2)
quantized
vibrational
energy
h = Planck’s constant
νo = characteristic frequency
ν = 0, 1, 2, 3,….In real molecules,
oscillation is anharmonic.
Chemistry 328N
Energy quantization of the oscillator
classical and quantum behavior are very different:
energy
classical: all possible values of total
energy are allowed
quantum: only certain ‘energy levels’ allowed
note that although the energy levels are equally spaced for the harmonic oscillator this is not true for all QM systems
note also that it is not possible for the quantum harmonic oscillator to have zero energy – it must have some energy even at 0 K
0 01234
n. . . .
‘quantum number’
Chemistry 328N
The IR Chart
4000 400
2.5 20
0
100 Micrometers
Wavenumber (cm -1 )
Tra
nsm
itta
nce
(%
)
Chemistry 328N
Infrared Spectrum
Chemistry 328N
Functional group absorptions
=>=>
Chemistry 328N
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
sp C—H 3310-3320
sp2 C—H 3000-3100
sp3 C—H 2850-2950
sp2 C—O 1200
sp3 C—O 1025-1200
Infrared Absorption Frequencies
Chemistry 328N
Stretching vibrations (multiple bonds)
Structral unit Frequency, cm-1
Infrared Absorption Frequencies
C C 1620-1680
—C N
—C C— 2100-2200
2240-2280
Chemistry 328N
IR Spectra of chloroformand deuterochloroform
Mode of vibration CHCl3 CDCl3
C-H stretching 3020 2256
C-H bending 1219 912
C-Cl stretching 773 737
C-Cl bending 671 652* Spartan ’02 AM1 minimization
Shift of peak due to replacement of H with D (2x mass)
Incr
eas
ing
abso
rban
ce
Chemistry 328N
IR Group Correlation Tables