chemistry 125: lecture 58 march 4, 2011 normal modes: mixing and independence practical ir nmr...
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Chemistry 125: Lecture 58March 4, 2011
Normal Modes:Mixing and Independence
Practical IR NMR Spectroscopy
Precession This
For copyright notice see final page of this file
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Butane C4H10
3 x (4 + 10) = 42 degrees of freedom
- 3 (translation) - 3 (rotation) = 36 vibrations
C4 : 3 stretch, 2 bend, 1 twist
10 C-H : 10 stretch, 20 bend or twist
Mixed (according to frequency-match / coupling) into 36 normal modes.
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C8 Straight Chain Hydrocarbons
OctaneC8H18
C-H stretch
C-CH3 umbrella+ C-C stretch
CH2
rockCH2 wag
CH2 scissors
26 atoms 72 normal modes (not all IR active)
C-H stretch
“Breathing” gives no net dipole change - no IR peak
Half ofC4H10’s tenC-H stretch
normal modes have no “handle”
E(t) helps push 8 H in and out
E(t) helps push 4 Hs up and down
Timing has been disabled on this slide so you can step back and forth with the arrow keys to study vibrational modes.
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C8 Straight Chain Hydrocarbons
OctaneC8H18
4-Octyne 1-Octyne
3315630
2120
Why not in 4-octyne?
CC Hsp
CC H
CC
(symmetric compound has no handle)
Functional Group
Identification
Timing has been disabled on this slide so you can step back and forth with the arrow keys to study vibrational modes.
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C8 Straight Chain Hydrocarbons
Octane trans-4-Octene cis-4-Octene
2-Methyl-2-Hexene
828
710
967
1655
CC
dipolechange(weak)
Functional Group
Identification
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Twist reduces overlap Folding preserves overlap
967 cm-1 710 cm-1
harder easier
828 cm-1
IR Active out-of-plane
C-H bending(paired H atoms move in the same direction)
Timing has been disabled on this slide so you can step back and forth with the arrow keys to study vibrational modes.
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The Jewel in the Crown of The Jewel in the Crown of Infrared SpectroscopyInfrared SpectroscopyC
O
C
O
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1681
CH3CO
NH2
CH3-C=O(X) strong & independent
1727
CH3CO
H
1715
CH3CO
CH3
CH3CO
OCH3
17461806
CH3CO
Cl+
-+
-
+
-
nN *C=OnO *C-Cl nO *C-OMe
: :
:
C=O weakened by resonance
C=O strengthened by resonance
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1683?1618
CH3CO
CH=CH2
1681
CH3CO
NH2
CH3-C=O(X) strong & independent
1727
CH3CO
H
1715
CH3CO
CH3
CH3CO
OCH3
17461806
CH3CO
ClnN *C=OnO *C-Cl nO *C-OMe
+
-
C=O weakened by resonance
C=O strengthened by resonance
C=O C=C coupling
out-of-phasemostlyC=O
Double !
in-phasemostlyC=CBut strong
peak is at higher
frequency than ketone
C=O C=C
1720
1683?1618
1618
1720 ?
?
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Sankaran & Lee, J. Phys. Chem. A 2002, 106, 1190-1195
1718
1696
1623
Difference(new spectrum - old)
1718 &1623 grow1696 shrinks
1718
1696
1623
2.5 hours irradiationat 308 nm
IR Spectrum of Methyl Vinyl Ketone
in Ar at 13K
anti periplanar
syn periplanar
“Mostly C=C” is very weak because small C=O vibration fights its
dipole change
“Mostly C=C” is strong because small C=O vibration
helps it
CalculatedPositionsfor s-trans
Calculated Positions
for
(a different species)
C=O C=C
?
?
combination of two lower frequency
transitions?
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ExaggeratedAmplitudeActual
Amplitude
C=O C=C Coupling in MVKIn-Phase Normal Mode (1618 cm-1)
C=C stretchC=O stretch
= 9
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Cf. Frames 25-26 ofLecture 8
(Erwin-Goldilocks on
Vibrational Amplitude)
ExaggeratedAmplitudeActual
Amplitude
C=O C=C Coupling in MVKOut-of-Phase Normal Mode (1720 cm-1)
C=C stretchC=O stretch
= -1/6
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IR in the “Real” Worldof a
PolymorphicMultibillion-Dollar
Pharmaceutical*different crystal forms
*
crystal packing “isomers”(different solubility, bioavailability)
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Form AN O O
O
H
H+
F
Cl -
HO
HO
H
H"Fingerprint"
C-H stretch
Form A
N O O
O
H
H+
F
Cl -
NH2+
?
!
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Form B
"Fingerprint"C-H stretch
Form B
N O O
O
H
H+
F
Cl -
NH2+
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Form C
"Fingerprint"C-H
Form C
N O O
O
H
H+
F
Cl -
NH2+
(truncated)
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Form C
Paroxetine Hydrochloride
Form A
1598
674
1183
1240
1562
665
592
1194
1604
1221
1248
Form B
1574
675
12301182
1598 1240 1574 1230 1562 1194 665 592
Patent Dispute: Can one detect
5% of protected B (675) in the presence of 95% of unprotected A (665)?
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Spectroscopy forStructure and Dynamics
Electronic (Visible/UV)
Vibrational (Infrared)
NMR (Radio)
Do precession problems on 125 webpage.
MagnetismPrecession
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young chemistMichael Faraday(discovered benzeneIn illuminating gas)
with
the
kind
per
mis
sion
of
Alf
red
Bad
er
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Faraday 1831Discovers Magnetic Induction
and Invents Fields
Magnetism fromelectric current
http://micro.magnet.fsu.edu/electromag/java/faraday/
Electricity from changing magnetism
Thirty years later Maxwell built these into a comprehensive theory of light and electromagnetism.
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“many simple things can be deduced mathematically more rapidly than they can be really understood in a fundamental or simple sense… the precession of a top looks like some kind of a miracle involving right angles and circles, and twists and right-hand screws. What we should do is to understand it in a more physical way.
Precession
Feynman, I, 20-6
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90° Phase Lag “falls around”
FORCEfrom string and gravity via spokes
Right
RIM POSITION
back backtop bottomfront
LeftVELOCITY
Where on the rim is the rightward velocity maximum?
Where it has been pushed to the right for the longest time.
above center force to right
below center force to left
accumulating rightward velocity
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Radio Frequency Precession of
“Spin = 1/2” Nuclei in Magnetic Field of ~23.5 kGauss MHz
MHz x 3 x 10-5 = cm-1
cm-1 x 2.9 = °K
H1
F19
P31
C13
O17
1%
99.98%
6%
66,000 MHz e-
CT Public Radio 90.5
100 MHz = 0.003 cm-1 = 0.01 °K
equilibrium Up:Down ratio (@RT):
eE/T = e0.01/300 = 1.00003WCBS 0.88
excess of 3 in 200,000 !
two quantized angles for S = 1/2
We know these nuclei spin, because they are magnetic, and they precess when an applied field tries to twist them.
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90° RF Pulse and the “Rotating Frame”
Applied Magnetic Field
Precessing proton gives riseto constant vertical field
Will rotating horizontal field generate 100 MHz RF signal?No, because there are many
precessing protons with all possible phases.
Consider a “rotating frame” in which the observer orbits at 100 MHz - protons seem to
stand still as if no applied field.
(just long enough to rotate all nuclear spin axes by 90°).
Fast precession(~100 MHz)
Slow precession
(~0.1 MHz)
Horizontal fields cancel.
Subsequent precession generates100 MHz RF signal in lab frame.
100 MHz RFin lab frame
Until “relaxation”reestablishes equilibrium.
and rotating horizontal field.
Pulse a very weak magnetic field fixed in this rotating frame
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A 90° pulse makesspinning nuclei (1H, 13C) “broadcast” a frequency
that reports theirlocal magnetic field.
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End of Lecture 58March 4, 2011
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