Temperature Dependence of JFF in 2-Fluoro-α-Chloro-α,α-Difluorotoluene and 2-Fluoro-α,α-Dichloro-α-Fluorotoluene

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Abstract
The F‐F coupling constants J FF between the fluorine atom at the 2 position and the fluorines in the CFCl2 and CF2Cl groups were measured and found to have an appreciable temperature dependence in 2‐fluoro‐α,α‐dichloro‐α‐fluorotoluene and 2‐fluoro‐α‐chloro‐α,α‐difluorotoluene. The effect of temperature on the fluorine chemical shift of the CFCl2 and CF2Cl groups was also studied in these compounds. The observed temperature dependences of J FF are consistent with the existence of several conformers differing in the rotational angle of the CFCl2 and CF2Cl groups. The relative stabilities of the rotamers depend upon the repulsive nonbonded interactions of the atoms in the CFCl2 and CF2Cl groups with the 2‐F and 6‐H atoms. The potential functions for rotation of the CFCl2 and CF2Cl groups were estimated by means of both the simple van der Waals repulsive potential and the modified Buckingham potential. The approximate dependence of J FF upon the dihedral angle between the C‐F bond in the CFX2 group and that of the 2‐fluorine atom was obtained from an analysis of the high‐ and low‐temperature limits for J FF in several compounds, including results reported earlier for some benzotrifluorides. It is found that J FF increases sharply at F‐F internuclear separations of 2.5 to 2 Å, which supports the view that a nonbonded interaction makes a major ``through‐space'' contribution to coupling of the fluorines.