Infrared spectra and force constants of urea in the gaseous phase

Infrared spectra in the gaseous phase have been recorded for the first time for urea and the isotopomers urea-D₄, urea-¹⁵N₂ and urea-¹³C. On the base of these data a normal coordinate calculation has been performed, resulting in an optimized force field of urea, showing that, due to the very small intermolecular forces, the internal force constants increase by evaporation. This is especially true for the C=O-bond with a force constant increased by 10%.Dedicated to Professor Dr. H. Kriegsmann on the occasion of his 70th birthday, to whom since 1957 scientific contacts with the second author developed to a good friendship