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• Nonaxial (no rotation)- C1, Cs, Ci
• Cyclic (rotational)-Cn, Cnv, Cnh, Sn
• Dihedral ( C⊥ 2)
- Dn, Dnd, Dnh
• Polyhedral- T, Th, Td, O, Oh, I, Ih
• Linear- C∞v, D ∞h
Types of Point Groups
http://symmetry.jacobs-university.de/
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Outline
• Dipole Moment/Polarity• Dipole and Symmetry• Dipole and Crystals• Chirality• Circular Dichroism• Optical Activity and Symmetry• Dynamic Molecules• Applications of CD
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Polarity/Dipole MomentPolarity- a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment.
Dipole moment- magnitude of charges and the distance of separation between the charges.
A molecule will have a dipole moment (that is, it will be polar) if the bond dipole moments do not cancel each other out.
O C O C OMolecular Dipole Moment
Polar
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Dipole Moment and SymmetryA molecule will have a dipole moment (that is, it will be polar) if the bond dipole moments do not cancel each other out.
Molecular dipole is dependent on symmetry!
1) Any molecule with an inversion center (i) cannot have a dipole (or be polar).
2) Any molecule with a C2 to C⊥ n cannot have a dipole (or be polar).
3) Any molecule with a sh cannot have a dipole (or be polar).
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1) Any molecule with an inversion center (i) cannot have a dipole (or be polar).
Dipole Moment and Symmetry
2) Any molecule with a C2 to C⊥ n cannot have a dipole (or be polar).
i
3) Any molecule with a sh cannot have a dipole (or be polar).
x
z
yCan have a dipole in z or –z unless there is a C⊥ 2 or a sh
Only C1, Cs, Cn, C∞v and Cnv can have a molecular dipole and be polar.
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Dipole Moment and Symmetry
Only C1, Cs, Cn, C∞v and Cnv can have a molecular dipole and be polar.
1) Any molecule with an inversion center (i) cannot have a dipole (or be polar).
2) Any molecule with a C2 to C⊥ n cannot have a dipole (or be polar).
3) Any molecule with a sh cannot have a dipole (or be polar).
C CH H C CH
H H
H
NH
HH
H
OH
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Dipole Moment and SymmetryOnly C1, Cs, Cn, and Cnv can have a molecular dipole and be polar.
Symmetry does not tell you the direction or the magnitude of the dipole moment.
x
z
y
x
z
y
Direction-
C1 could be any direction (no axis)
Cs could be direction except to ⊥ sh
Cn and Cnv must be z or -z
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Dipole Moment and SymmetryOnly C1, Cs, Cn, and Cnv can have a molecular dipole and be polar.
Symmetry does not tell you the direction or the magnitude of the dipole moment.
Direction-
Cs could be any direction (no axis)
Cs could be direction except to ⊥ sh
Cn and Cnv must be z or -z
Magnitude-
Depends on the atoms, lone pairs and bond dipoles
NH3
NF3?
Bond Dipole
Lone pair
Molecular Dipole
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Dipole Moment and SymmetryWhy symmetry and dipole moment matter?
1) Solubility
2) Miscibility
3) Boiling/melting points
4) pKa
5) Vibrational Transitions
6) Crystal Structure/Property
NHO
Acetanilide p-chloroacetanilide
NHO
Cl
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Dipole Moment and SymmetryAcetanilide-
Form pairs related by an inversion center.Cancelation of dipoles.
NHONHO
Cl
p-chloroacetanilide- Head to tail alignment.Aligned dipoles.
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Dipole Moment and Symmetry
Non-polar crystal Polar crystal
Symmetry Through the Eyes of a Chemist
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Dipole Moment and Symmetry
Non-polar crystal Polar crystal
Symmetry Through the Eyes of a Chemist
1) Solid/gas reactions
2) Melting temp
3) Hardness
4) Conductivity
5) Optical Polarity
6) Pyroelectricity-must have dipole
7) Piezoelectricity
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1) Any molecule with an inversion center (i) cannot have a dipole (or be polar).
Dipole Moment and Symmetry
2) Any molecule with a C2 to C⊥ n cannot have a dipole (or be polar).
i
3) Any molecule with a sh cannot have a dipole (or be polar).
x
z
yCan have a dipole in z or –z unless there is a C⊥ 2 or a sh
Only C1, Cs, Cn, C∞v and Cnv can have a molecular dipole and be polar.
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Optical Activity and Symmetry
• Chirality• Circular Dichroism• Optical Activity and
Symmetry• Dynamic Molecules• Applications of CD
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• DNA is right handed
• Amino acids are L
• Carbohydrates are D
• Alpha Helix is right handed
• Origin/Evolution of life
• Drug delivery/processing
• Olfactory receptors
Why Chirality Matters
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(+) –rotates the plane polarized light clockwise (when viewing towards the light source) (-) –rotates the plane polarized light counterclockwise
R / S system- requires no reference molecule (Cahn–Ingold–Prelog priority rules)
D/L system- referenced vs glyceraldehyde.
(+)/(−) system- related to the direction to which it rotates plane polorized light.
Naming Conventions
R / S, D/L and (+)/(−) are not related.
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Optical Activity
hn
Sample
• Transmittance:T = P/P0
• Absorbance: A = -log T = log P0/P
hn
P0
Sample
(power in)
P
(power out)
We don’t measure absorbance. We measure transmittance.
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The Beer-Lambert Law (l specific):
A = absorbance (unitless, A = log10 P0/P)
e = molar absorptivity (L mol-1 cm-1)
l = path length of the sample (cm)
c = concentration (mol/L or M)
Beer’s Law
P0
Concentration Absorbance
A = e c l
Path length Absorbance
Molar Abs. Absorbance
Sample
(power in)
P
(power out)
l in cm
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Polarization of Light
Side View End View
Vertically Polarized
Horizontally Polarized
http://www.enzim.hu/~szia/cddemo/edemo0.htm
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Adding Polarized LightIn phase (peak at the same time) + same amplitude
Vertical + Horizontal = 45° diagonal
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Adding Polarized Light
Green peaks when red baselines.Sum (blue) is always 1.
¼ l (90°) out of phase + same amplitude
Vertical + Horizontal = Circular
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Adding Polarized LightIn phase (peak at the same time) + same amplitude
left circular + right circular = vertical
Green peaks when red peaks.
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The Beer-Lambert Law:
A = absorbance (unitless, A = log10 P0/P)
e = molar absorptivity (L mol-1 cm-1)
l = path length of the sample (cm)
c = concentration (mol/L or M)
A = e c l e is the same for D and LIf C are equal:
50:50 D to L100% D100% L
Then A is the same.
Assuming the light is unpolarized!
Absorption Spectroscopy
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P0
(power in)
P
(power out)
left-circularly polarized
right-circularly polarized
Circular Absorption
Absorbs moreSmaller P
Absorbs LessLarger P
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Circular Absorption
Absorbance: A(left) = log P0(left) /P(left)
A(right) = log P0(right)/P(right)
P0
(power in)
P
(power out)
P0(left)
P(left)
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Circular DichroismCD measures the difference between the absorption of left and right handed cirularly-polarized light:
De is typically <100 M-1 cm-1
e is typically > 10,000 cm-1
= 3298 Δε.
CD spectra reported in ellipticity () or De
e in L/mol cm (liters mol-1 centimeters-1) in degrees cm2/dmol-1 (degrees centimeters2 mol-1)
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Polarizer
Prism
Source
Circular Dichroism
Process:1) Unpolarized white light2) Monochrometer3) Plane polarizer4) left-right modulator5) left (then right) through sample6) measure P for right (then left) through sample
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CD Spectrometer
AVIV 202 CD spectrometerInstitute of Molecular Biophysics
170-875 nm
-10oC to 110oC
titrator attachment
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Optical Activity and SymmetryMolecules are optically active if it contains at least one chiral center.
Many molecules have no chiral centers and yet are optically active.
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Optical Activity and SymmetryWhich molecules are expected to be optically active?
Molecules with no improper axis of rotation (Sn) are optically active.Note S1 = σ and S2 = i.
C1, Cn, and Dn
• Nonaxial (no rotation)
- C1, Cs, Ci
• Cyclic (rotational)
-Cn, Cnv, Cnh, Sn
• Dihedral ( C⊥ 2)
- Dn, Dnd, Dnh
• Polyhedral
- T, Th, Td, O, Oh, I, Ih
• Linear
- C∞v, D ∞h
Chiral molecules lack an improper axis of rotation (Sn), a center of symmetry (i) or a mirror plane (σ)!
Also T, O, and I
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Optical Activity and SymmetryChiral molecules lack an improper axis of rotation (Sn), a center of symmetry (i) or a mirror plane (σ)! C1, Cn, and Dn
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Optical Activity and SymmetryChiral molecules lack an improper axis of rotation (Sn), a center of symmetry (i) or a mirror plane (σ)! C1, Cn, and Dn
Also T, O, and I
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Optical Activity and Dynamic Molecules
10^10 per second
CD spectra-average of both isomers (at room temperature).
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UV-Vis or CD
source
hn ?
Optical Activity and Symmetry
350 400 450 500 550 6000.0
0.5
1.0
1.5
2.0
Ab
sorb
ance
(a.
u.)
Wavelength (nm)
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• Determination of secondary structure of proteins• Investigations of protein-protein interactions• Investigation of the effect of drug binding• Protein structure in a membrane• Stereoselective synthesis• Dynamic processes
- protein folding- reaction dynamics• DNA denaturation
Applications
GCN4-p1 coiled–coil