| Özet: | The vapour phase electronic absorption spectrum of 1,2,4,5-tetrafluorobenzene, in the region of the S1-S0 band system, has been photographed under conditions of high resolution.
Computer simulation of the observed rotational contours has
shown that all the most intense bands are type A and, therefore, that the electronic assignment is A ‘B2uX1Ag. The rotational constants in the A state are consistent with
an out-of-plane angle of the C-F bonds of 11+4”. This is consistent with the result obtained by Okuyama et al., from
the supersonic jet spectrum, that the fluorine atoms are out-of-plane in a “butterfly” configuration. Obser- vation of hot bands of the butterfly vibration, which are frozen out in the jet spectrum, provides strong support for the W-shaped potential function proposed by Okuyama et al. for
this vibration in the A state.
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