Physical methods for the determination of cis-trans isomers
- NMR ( 1H, 13C both)
- Vibrational (IR-Raman) spectra
- UV- visible spectra
- Refractive index
- Boiling points
- Dipole moments
- Less often mass spectra
- Also X-ray, electron diffraction method, as well as microwave spectra, is used for.
N.B: Density, refractive index, b.p, m.p not very reliable
UV-visible spectra; not very reliable
Normally stabilization of the ground ( trans-stilbene is more stable than its cis-counterpart: π–overlapping in coplanar trans-stilbene makes it more stable, o-hydrogens interact to force out (430) the benzene ring of the plane in cis-stilbene. No such π- overlapping so less stable) state shifts λ max to
- shorter λ whereas stabilization of the excited state shifts λ max to longer wavelength.
- Still, the more stable trans isomer absorbs at the longer wavelength
The singlet excited state is a diradical and since the Franck-Condon principle demands that it have the same geometry as the ground state, stabilization of the radical on the side of the twisted benzene ring by benzylic resonance is sterically impeded, thus explaining the much higher energy level of the singlet excited state of the cis isomer.
Vibrational (IR-Raman) spectra
Can be used in some cases with confidence e.g. trans-1,2-dichloroethylene, (E) –CHCl = CHCl no IR during vibration –i.e., no IR absorption(dipole moment zero and no change in μ during vibration— so no IR) but there is a strong Raman absorption ( due to change in polarizability) at 1577 cm-1).
Cis- dichloroethylene, (Z) –CHCl = CHCl shows strong
IR C =C stretching vibration at 1590cm-1
NMR is by far the most useful & versatile technique for distinguishing cis and trans isomer.
Both 1H /& 13C chemical shifts & coupling constants are useful.
The formula is available for calculation of chemical shifts F.9.39 P. 570.
In alkenes of the type RCH = CHR” or RCH = CFR’; RCF=CFR’; H – H, H-F, F-F coupling constants can be used to assign configuration.
- Mass spectrometry is not a major tool for distinguishing cis-trans isomers
- Chromatographic methods do not usually serve to identify cis-trans isomers.