Transcript ppt only

Chemistry 125: Lecture 58
March 4, 2011
This
Normal Modes:
Mixing and Independence
Practical IR
NMR Spectroscopy
Precession
For copyright
notice see final
page of this file
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.
Timing has been disabled on this slide so you can step back
C
Straight
Chain
Hydrocarbons
8
and forth with the arrow keys to study vibrational modes.
“Breathing”
gives no net
E(t)
E(t)helps
helpspush
push
dipole
change
- no IR peak
48Hs
H in
up
and
andout
down
Half of
C-H
C-H
C-CH3 CH2
CH2
CH2
C4H10’s ten stretch
stretch
scissors umbrella wag
rock
C-H stretch
+ C-C stretch
Octane
normal modes
have no “handle” C8H18 26 atoms  72 normal modes (not all IR active)
Timing has been disabled on this slide so you can step back
C
Straight
Chain
Hydrocarbons
8
and forth with the arrow keys to study vibrational modes.
Why not in
4-octyne?
(symmetric compound
has no handle)
2120
C
C
C
C H
sp
C
630
3315
Functional
Octane
Group
CH
Identification 8 18
C H
4-Octyne
1-Octyne
C8 Straight Chain Hydrocarbons
1655
C
C
828
710
dipole
change
(weak)
Functional
Group
Identification
Octane
trans-4-Octene
967
cis-4-Octene
2-Methyl-2-Hexene
Timing has been disabled on this slide so you can step back
and forth with the arrow keys to study vibrational modes.
IR Active
out-of-plane
C-H bending
(paired H atoms move
in the same direction)
828 cm-1
Twist reduces  overlap
harder
967 cm-1
Folding preserves  overlap
easier
710 cm-1
The Jewel inOthe Crown of
C
Infrared Spectroscopy
CH3-C=O(X)
strong & independent
C=O strengthened
by resonance
CH3C
O+:
Cl
1746
CH3C
1727
+
O:
-OCH
O
nO *C-Cl nO *C-OMe
O
H
1681
CH3C
CH3C
CH3C
3
1715
O+
:
1806
C=O weakened
by resonance
CH3
NH2
nN *C=O
CH3-C=O(X)
But strong
peak is at
higher
frequency
than ketone
C=O C=C
coupling
OCH3C
1683?
1618
+
CH=CH2
strong & independent
out-of-phase
in-phase 1618
mostly
?
C=C
1720 C=O
C=O
C=C
1683?
1618
C=O strengthened
by resonance
1806
O
CH3C
1746
1720
Double!
?
O
CH3C
Cl
1727
C=O weakened
by resonance
O
CH3C
OCH3
nO *C-Cl nO *C-OMe
1715
O
CH3C
H
1681
O
CH3C
CH3
NH2
nN *C=O
“Mostly C=C” is strong because
small C=O vibration helps it
C=O
Calculated
Positions
for
C=C
Difference
(new spectrum - old)
1718 &1623 grow
1696 shrinks
2.5 hours
irradiation
at 308 nm
syn periplanar
combination of two lower
? frequency transitions?
Calculated
Positions
for s-trans
“Mostly C=C” is very weak
because small C=O
vibration fights its
dipole change
anti periplanar
1696 (a different species)
1718
1623
?
IR Spectrum of
Methyl Vinyl Ketone
in Ar at 13K
Sankaran & Lee, J. Phys. Chem. A
2002, 106, 1190-1195
C=O C=C Coupling in MVK
In-Phase Normal Mode (1618 cm-1)
C=C stretch
= 9
C=O stretch
Actual
Exaggerated
Amplitude
C=O C=C Coupling in MVK
Out-of-Phase Normal Mode (1720 cm-1)
C=C stretch
= -1/6
C=O stretch
Cf. Frames
25-26 of
Lecture 8
(Erwin-Goldilocks on
Vibrational Amplitude)
Actual
Exaggerated
Amplitude
IR in the “Real” World
of a
*
Polymorphic
Multibillion-Dollar
Pharmaceutical
*different crystal forms
crystal packing “isomers”
(different solubility, bioavailability)
!
Form A
Form A
O
H
Cl +
H
N
O
O
F
?
H
O
H
O
H
H
NH2+
C-H stretch
"Fingerprint"
Form B
Form B
O
Cl - H +
H
N
O
O
F
NH2+
C-H stretch
"Fingerprint"
Form C
Form C
O
Cl - H +
H
N
O
O
F
NH2+
C-H (truncated)
"Fingerprint"
675
674
665
1604
1562
1574
1248
1240
1598
1221
1230
1182
1183
Patent Dispute:
592
Can one detect
5% of protected B (675)
in the presence of 95%
of unprotected A (665)?
1194
Form C
Form B
1598 1240
1574 1230
Form A
1562 1194 665 592
Paroxetine Hydrochloride
Spectroscopy for
Structure and Dynamics
Electronic (Visible/UV)
Vibrational (Infrared)
NMR (Radio)
Precession
Magnetism
Do precession problems on 125 webpage.
young chemist
Michael Faraday
with the kind permission of Alfred Bader
(discovered benzene
In illuminating gas)
Faraday 1831
Discovers Magnetic Induction
and Invents Fields
Magnetism from
electric current
Electricity from
changing magnetism
http://micro.magnet.fsu.edu/electromag/java/faraday/
Thirty years later Maxwell built these into a
comprehensive theory of light and electromagnetism.
Precession
“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.
Feynman, I, 20-6
above center
force to right
90° Phase Lag  “falls around”
accumulating
rightward
velocity
Where on the rim is the rightward velocity maximum?
Where it has been pushed to the right for the longest time.
Right
FORCE
from string
and gravity
via spokes
below center
force to left
Left
VELOCITY
back
top
front
bottom
RIM POSITION
back
66,000 MHz
CT Public
Radio 90.5
eH1
F19
MHz
Radio Frequency
Precession
of
99.98%
“Spin = 1/2” Nuclei
in Magnetic Field
We know these nuclei spin,
because
are magnetic,
ofthey
~23.5
kGauss
and they precess when
an -1
-5
MHz x 3 x 10 = cm
applied field tries to twist them.
cm-1 x 2.9 =
°K
100 MHz = 0.003 cm-1 = 0.01 °K
two quantized angles for S = 1/2
P31
WCBS 0.88
C13
1%
O17
6%
equilibrium Up:Down ratio (@RT):
eE/T = e0.01/300 = 1.00003
excess of 3 in 200,000 !
90° RF Pulse and the “Rotating Frame”
Fast precession
(~100 MHz)
Slow
precession
(~0.1 MHz)
Applied Magnetic Field
Until “relaxation”
100 MHz RF
reestablishes
in
lab frame
equilibrium.
Precessing proton gives rise
to constant vertical field
and rotating horizontal field.
Will rotating horizontal field
generate 100 MHz RF signal?
No, because there are many
precessing protons with
all possible phases.
Horizontal fields cancel.
Consider a “rotating frame”
in which the observer orbits
at 100 MHz - protons seem to
stand still as if no applied field.
Pulse a very weak magnetic field
fixed in this rotating frame
(just long enough to rotate all
nuclear spin axes by 90°).
Subsequent precession generates
100 MHz RF signal in lab frame.
A 90° pulse makes
spinning nuclei (1H, 13C)
“broadcast” a frequency
that reports their
local magnetic field.
End of Lecture 58
March 4, 2011
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J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0