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Ring size effect on internal double bonds

For internal double bonds, absorption frequency decreases as ring size decreases. This reaches a minimum at 90 degrees, then the frequency rises again:


This is because the C=C stretch in rings is strongly coupled to the neighboring C-C stretch, which happens more as their vectors are in line.


As the angle approaches 90 degrees, the vector sum approaches 0, hence the degree of coupling approaches 0 and frequency approaches a minimum. Below 90 degrees, the coupling can once again take place.

If an endo double bond is part of a ring fusion, frequency moves to the right an amount equilivant to one carbon being removed from the ring:


Absorption frequency increases significantly when one or more alkyl groups are attached to the double bond. This effect is more pronounced for smaller rings, especially cyclopropene.


For this to be useful, ring size is typically determined first via another method,otherwise the frequency can represent too many possibilities.

10 comments:

  1. What do you mean by : "frequency moves to the right an amount equilivant to one carbon being removed from the ring." Please elaborate!!

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    Replies
    1. "frequency moves to the right an amount equivalent to one carbon being removed from the ring."
      it mean frequency decrease from cyclohexene to cyclopentene
      1646 cm-1 to 1611 cm-1

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  3. Here What is the meaning of degree of coupling ??

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  4. Why does the vibrational frequency of c=c in cyclopropene increases when an alkyl group is attached to it

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  5. Why does the vibrational frequency of c=c in cyclopropene increases when an alkyl group is attached to it

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    Replies
    1. Because R group is electron donating group it's increase the triple bond character n vibrational frequency of triple bond is high that why e assume that double bond to have a triple bond character once it's recive electron from R

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  6. By pushing electron density, it increases the c=c bond strength and hence the frequency.

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  7. why exo double cyclopropene get higher frequency compared with other compounds?

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  8. How does vector couple in cyclopropene ring?

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