=C-H stretch: 3010 - 3095 cm-1
=C-H out of plane bending (oop): 1000 - 650 cm-1
C=C stretch: 1660 - 1600 cm-1
Conjugation lowers the frequency of the C=C stretch.
Cis double bonds with the same substituents do not have a C=C peak, because the stretch would have no change in dipole moment. But but the substituents are different then cis peaks have stronger absorptions than trans.
Trans double bonds with the same substituents have an extremely weak C=C peak.
The spectra of cyclohexane:
Notice how there is still a small C=C peak, meaning the C=C is not symmetric. You can see why by looking at a model:
The two images below show the effect of cis-trans isomerisation on the C=C absorption:
No comments:
Post a Comment