THE SHAPE OF UREA

The shape of the urea molecule has been studied by analysing the micro wave spectra of several isotopic species, yielding r(s) coordinates of all atoms except that of C, the latter being derived from first-momen t equations, and by ab initio molecular-orbital calculations at the MP 2/6-311(++)G(d,p) level. The derived bond lengths and angles are: r(CO ), 1.22(1) Angstrom; r(CN), 1.37(8) Angstrom;r(NH(5)), 0.99(8) Angstro m; r(NH(6)), 1.02(1) Angstrom; angle NCN, 114.7 degrees; sum of pyrami dal angles around N, 350.6 degrees. The conformer of lowest energy is predicted to be nearly planar viith C-2 symmetry, a second minimum for a shape of C-s symmetry being higher in energy by 421 cm(-1). However , these are separated by a barrier estimated to be no higher than abou t 130 cm(-1). Thus the two shapes are likely to be parts of the potent ial energy surface domain that is associated with the most stable shap e of urea, i.e. one in which the zero-point vibration covers both C-2 and C-s geometries, the most probable (r(p)) geometry being C-2. Compu ted ab initio frequencies and their eigenvectors (harmonic approximati on) imply that the lowest frequency vibration is a C-N torsion, in con flict with King's assignment of NH2 wag. However, the substantial anha rmonicity of the large-amplitude motion (LAM) associated with the inte rconversion between the C-s and C-2 forms seems likely to perturb the upsilon = 1 level of this LAM so that its 1 <-- 0 transition becomes l owest in frequency, so removing the apparent conflict in assignment. ( C) 1997 Elsevier Science B.V.