Nuclear Magnetic Resonance Spectroscopy. Conformational Equilibria and Rates of Conformational Interconversion of Halogenated Ethanes

The relative ground-state energies of the rotational isomers of some halogenated ethanes and the rates and barriers to interconversions of these isomers have been determined by nuclear magnetic resonance spectroscopy. In the ethanes studied in this research, those with a single fluorine were found to have the ^(19)F resonance of the least populated conformation at highest field, while that of the most populated conformation was found at the lowest field. All three barriers for interconversion could be determined for the ethanes substituted so as to give three distinguishable conformations. For these cases it was found that the highest barrier separates the two least populated conformations and the lowest barrier separates the most stable conformations. The substances for which complete nmr line-shape analysis have given the equilibrium constants, rates, and barriers for internal rotation include 1,1,2-tribromo-1,2-dichloro-2-fluoroethane, 1,1- dibromo-1,2,2-trichloro-2-fluoroethane, 1-bromo-1,1,2,2-tetrachloro-2-fluoroethane, 1,1,2,2-tetrabromo-1-chloro-2-fluoroethane, 1,2-dibromo-1,1,2-trichloro-2-fluoroethane, 1,1,2-tribromo-1-chloro-2-fluoroethane, 1-bromo-l,1,2-trichloro-2-fluoroethanaen,d 1,2-dibromo-1,1,24richloro- 3,3,3-trifluoropropane. For those ethanes carrying five halogens other than fluorine, the barriers (to rotation) range from 13.2 to 14.8 kcal/mol, while for those with four halogens other than fluorine, the barriers range from 9.0 to 10.2 kcal/mol. The ground-state energy differences for all of the ethanes studied did not exceed 0.3 kcal/mol