Preparation and structure elucidation by NMR of the charge delocalised β,β-carotene dication

The β,β-carotene dication (λmax 985 nm at room temperature) prepared by treatment of β,β-carotene with BF3–etherate had considerable stability. The reaction, monitored by EPR studies at −25°C, occurred by free radical intermediates. EIMS further revealed a C40H56BF3 intermediate. The detailed structure of the charge delocalised β,β-carotene dication was established by COSY, HSQC, HMBC and 1D and 2D ROESY NMR techniques (600 MHz, CDCl3, −25°C) leading to complete assignments of 1H and 13C chemical shifts and 3JH,H coupling constants. Considerable downfield shifts were observed relative to β,β-carotene. The results are consistent with charge delocalisation mainly in the C-5–C-9 and C-5′–C-9′ regions and with retro shifted double bonds in the central C-13–C-13′ region. The effect of the two delocalised charges on chemical shift (charge distribution) and bond distance (3JH,H) is discussed.