Rotational barriers in N,N-dimethylbiuret. Experimental and theoretical studies

Abstract

The barriers to internal rotation about the C—NR₂ bonds in N,N-dimethylbiuret are investigated by means of ¹H-n.m.r. line shape analysis. The activation energy for rotation is found to be considerably lower when the hydrogen at the nitrogen molecule is substituted by methyl groups. This result is correctly reflected by ab initio calculations with a minimal GLO basis set, which also indicate that the molecule exists preferably in the cis-configuration, as the unsubstituted biuret. Possible reasons for this, and the difference in the rotational barriers, are discussed on the basis of the quantum chemical results.