J. Langer et al., INFRARED-SPECTRA AND FORCE-CONSTANTS OF UREA IN THE GASEOUS-PHASE, Fresenius' journal of analytical chemistry, 352(5), 1995, pp. 489-495
Infrared spectra in the gaseous phase have been recorded for the first
time for urea and the isotopomers urea-D-4, urea-N-15(2) and urea-C-1
3. 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 cons
tants increase by evaporation. This is especially true for the C=O-bon
d with a force constant increased by 10%.