The Configurational Stability of Primary Grignard Reagents

The n.m.r. spectrum of the methylene hydrogens of 3,3-dimethylbutylmagnesium chloride in diethyl ether that the trans and gauche coupling constants, J_t and J_g, have the same respective values for each conformation (I-III), the separation of the outer lines for I should be ( J_t + J_g), while the corresponding separation for a rapidly interconverting mixture of II and III would be 1/2(J_t + 3J_g). Therefore, if the averaging of coupling constants observed in the high-temperature spectra is a consequence of changes in conformational populations, the separation of the outer lines would be expected to change appreciably with temperature.