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If a double bond is introduced between two double bonds of a conjugated polyene to form a cumulene the λ maximum shifts by 34 mμ to longer wavelength. The conjugation is much smaller in polyacetylenes than in polyenes. The formulation of a cumulene structure for the ground state of long polyacetylenes seems to be incorrect.
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Helmut Krauch (1958) studied this question.
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