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Ordinarily the barrier to rotation about a carbon-carbon double bond is quite high but in compound P double bond between two rings was observed by NMR to have a rotational energy barrier of only about 20 cal.\mol., showing that it has lot of single bond charcter. The reason for this is

(1) Double bond having partial triple bond charcter because of resonance

(2) Doule bond undergo flipping

(3) Double bond having very high single bond charcter because of aromaticity gained in both three and five membered rings.

(4) +I effect of nC3H7 groups makes double bond having partial single bond character.