Signs and mechanisms of 13C, 19F spin–spin coupling constants in benzotrifluoride and its derivatives

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Abstract
The magnitudes of nJ(C, F) for n = 3, 4, 5 bonds are remeasured for benzotrifluoride. The signs of 2J(C, F) and 3J(C, F) are determined for 4-chloro-3,5-dinitrobenzotrifluoride and that of 5J(C, F) for 2-amino-3,5-dibromobenzotrifluoride. 4J(C, F) is not observable in these compounds. The contact contributions to nJ(C, F) are calculated for benzotrifluoride by INDO MO FPT. The long-range coupling constants are discussed in terms of the mechanisms which transmit nuclear spin state information. It seems that 5J(C, F) is a σ–π coupling, 4J(C, F) is a composite of a and σ–π components of opposite sign, and that 3J(C, F) has a Karplus behaviour modified by a σ–π contribution.

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