High-resolution nuclear-magnetic-resonance spectra of hydrocarbon groupings II. Internal rotation in substituted ethanes and cyclic ethers

Abstract
The hydrogen chemical shifts and the JHH' coupling constants between hydrogen nuclei on adjacent carbon atoms have been measured for the chloroethanes and bromoethanes of formula H3C . CH´2Z, H3C . CH´Z2, ZH2C . CH´Z2, ZH2C . CH´Z2 and Z2HC . CH´Z2 and for the cyclic ethers dioxan, dioxalan and ethylene oxide. Where hydrogen nuclei on adjacent carbons are chemically equivalent, as with molecules of formula ZH2C . CH´2Z and Z 2H C .C H 'Z 2, satellites in the hydrogen resonance spectra caused by 13CH groups have been used to determine the appropriate coupling constants. Those molecules which have distinguishable rotation al isomers have been studied in solvents of different dielectric constant, as well as in the liquid state, in order to vary the relative abundance of the isomers. By this means separate values have been obtained of coupling constants for pairs of hydrogen atoms that are in trans and gauche configurations with respect to each other. It is found that the trans coupling constants (9 to 18 c/s) are markedly greater than the gauche constants (1·2 to 3·5 c/s) and of the same sign. Considerable variations in the experimentally observed mean JHH' coupling constants within the series of chloro- and bromo- ethanes are caused by rotational isomerism.