Studies of Rates of Conversion and Populations of Conformations in Saturated Ring Compounds. III. NN′ Dimethyl Piperazine

Abstract

At room temperature the two proton resonance signals observed in NN′-dimethyl piperazine are not of equal half-width. The signal caused by the ring methylene protons broadens as the temperature is lowered and below −40°C the spectrum is typical of an A2B2 case. These observations are indicative of fast interconversion relative to the axial-equatorial chemical shift between two identical chair forms of this molecule at +40°C and slow interconversion below −40°C. The A2B2 spectrum is partially analyzed, showing reasonable values for Je1a1, Je1e2, Je1a2, and the axial-equatorial chemical shift assuming only that the value of Ja1a2, is known. The energy barrier for interconversion is computed from the coalescence temperature and the exchange broadened single peak near the fast exchange limit. It is found to be 13.3±0.3 kcal. A comparison with earlier measurements in similar compounds is made. The N-methyl axial-equatorial motion is discussed.