8/11/2019 Chir Optical

1/11

B. Through chiroptical methods- qualitative: optical activity

(refraction phenomenon, circularly polarized light)

gives , which gives []- quantitative

I. ORD (optical rotatory dispersion)

2. Relative configuration continued

- measures the specific rotation as a function of the wavelength

- in the UV-Vis region, 200-800 nm

[ ] [ ]TT M

100

= M: molar mass

- instead of the specific rotation [], often the molrotation [] is used

Molrotation: rotation on a per mole basis,

for the comparison of [] for different compounds.

[]

, nm

8/11/2019 Chir Optical

2/11

8/11/2019 Chir Optical

3/11

- two more issues on ORD spectra

2. Relative configuration continued

O

O

H

O

O

HO

- negative plain dispersion + positive CE

[]

,nm

I

II

Djerassi, Optical rotatory dispersion, McGraw-Hill, NY, 1960

skeleton of II causes the

negative background curve

- importance of second cross-over point

700

[]

,nm

500 O

S

SPh

3

C

cross-over close to the

Na D-line

8/11/2019 Chir Optical

4/11

O H

COOH

I

O H

COOH

I

O H

COOH

I

- two more issues on ORD spectra

2. Relative configuration continued

, nm

Djerassi, Optical rotatory dispersion, McGraw-Hill, NY, 1960

- importance of second cross-over point: example

[]

10-2

I II

[]D

I

IIIII

III

(-) (+) (+)[]D

but all plain dispersion curves positive!

8/11/2019 Chir Optical

5/11

-magnitude of is normalized as specific ellipticity []:

- instead of the specific ellipticity [], often the molar ellipticity [] is used

B. Through chiroptical methodsII. CD (circular dichroism) spectroscopy

2. Relative configuration continued

- measures the absorption as a function of the wavelength

- in the UV-Vis region, 200-800 nm

[ ] [ ]TT M

=100

- but: anisotropic absorption of circularly polarized light

= L - R (by definition)

leads to elliptically polarized light, which has an ellipticity

, nm

?

[ ] clT

=

8/11/2019 Chir Optical

6/11

Insert Polarized light

http://www.enzim.hu/~szia/cddemo/edemo0.htm

This is what happens to circularly polarized light in an absorbing sample:

8/11/2019 Chir Optical

7/11

back

Insert Polarized light

http://www.enzim.hu/~szia/cddemo/edemo0.htm

This is how circular dichroism comes about (formation of elliptically polarized light):

Here the right circularly polarized wave is absorbed to a greater extent.

8/11/2019 Chir Optical

8/11

B. Through chiroptical methodsII. CD (circular dichroism) spectroscopy

2. Relative configuration continued

- the sign of defines the sign of the CD

- the sign of the CD is the same as that of the ORD at the CE

[]

,nm

positive CE

negative CE

- CD spectrum (for an absorbing compound)

UV-Vis signal inserted for clarification

- Cotton-effect (CE) curveO

8/11/2019 Chir Optical

9/11

[]

or

- two more issues on CD/ORD spectra

2. Relative configuration continued

- CD maximum coincides with ORD cross-over

superposition of both spectra (not normally done):

700

[]

,nm

[]

or

O

O

H

,nm300 700

- there might not be a CD spectrum, but there will always be an ORD!

[]

8/11/2019 Chir Optical

10/11

8/11/2019 Chir Optical

11/11

- and the final issue on CD spectra

2. Relative configuration continued

- sensitivity in comparison to UV-Vis spectra

very sensitive to

stereochemical issues

very insensitive

O

O

tBu

tBu

3 diastereomers:

O

O

tBu

tBu

O

O

tBu

tBu

O

O

tBu

tBu