The molecular structure of allenes and ketenes. Part 20. Excited states of 3-halogeno- and other 3-substituted phenylallenes: circular dichroism and absorption spectra, and CNDO/S calculations

Abstract

Excited states of phenylallenes C₆H₅CHCCHR with energies ⩽ 52 000 cm^–1 have been investigated by means of circular dichroism [R = Cl (4), R = Br (5), and R = l (6)] and u.v. absorption spectroscopy [R = MeO (1), R = CO₂H (2), R = CN (3), and (4)—(6)]. Semiempirical CNDO/S-Cl calculations including singly and doubly excited configurations for (1)—(4) have been used to support and specify the empirical inferences about excited states. Furthermore, the theoretical results have provided band assignments for certain electronic bands of phenylallenes with acceptor groups [(2) and (3)]. Substituent effects on the lowest-energy intense absorption bands [excited state S₄(π,π*)] are discussed in more detail for a series of phenylallenes [(1)—(6) and R = H (7), R = Me (8), and R = Bu^t(9)]. It is suggested that in producing the marked shift of the S₄(π,π*) band of the 3-halogeno-1-phenylallenes (4)—(6) relative to (7), hyperconjugation across the allenic CCC system plays an important role.