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 J HH' coupling constants between hydrogen nuclei on adjacent carbon atoms have been measured for the chloroethanes and bromoethanes of formula H 3 C . CH´ 2 Z, H 3 C . CH´Z 2 , ZH 2 C . CH´Z 2 , ZH 2 C . CH´Z 2 and Z 2 HC . CH´Z 2 and for the cyclic ethers dioxan, dioxalan and ethylene oxide. Where hydrogen nuclei on adjacent carbons are chemically equivalent, as with molecules of formula ZH 2 C . CH´ 2 Z and Z 2H C .C H 'Z 2, satellites in the hydrogen resonance spectra caused by 13 CH 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 J HH' coupling constants within the series of chloro- and bromo- ethanes are caused by rotational isomerism.

Norman Sheppard. 16 May 1921—10 April 2015

“…So Norman started work with his students on an investigation of a wide series of molecules: dichloropropenes (20); substituted ethanes (21); cyclic ethers (21); vinyl compounds (27, 30); 13 C substituted ethane, ethylene and acetylene (33); perfluoroethanes (36); and others. He was instrumental in helping to solve some of the nuances involving relative signs of coupling constants (19,34). Knowing Norman's interest in understanding the vibrational frequencies of hydrocarbons etc., it is not surprising that he wrote extensively on the interpretation of the NMR parameters (32,35).…”