The molecular structure of the urea molecule: is the minimum energy structure planar?

Abstract

The structure of the free urea molecule was investigated by both ab initio and semiempirical methods. The minimum-energy structure is found to be non-planar. Although the heavy atom unit is planar, the hybridisation of the nitrogen atoms is sp 3 rather than sp 2, with the hydrogen out-of-plane torsional angles being close to 13° and 150°, respectively. This finding may be of importance with respect to the three-dimensional structure of amino resin networks. A comparison with some related molecules, i.e. formamide, acetamide and tetramethylurea, is provided.