The ir spectra of solid N-acetyl- (MAH), N,N′-diacetyl- (DAH) and bisN-methyl-N-acetylurea (BMAH) were recorded in the range 4 000–400 cm−1and of N,N′-dimethylurea (DMH) in the range 4 000–100 cm−1. The vibrational assignments of these compounds and of N-methylurea (MMH) are given by normal coordinate analysis on the basis of the GF matrix method. The force field adopted in the calculation is the local symmetry force field (LSFF). The force constants were adjusted by a least-squares method to get a better agreement between the observed and calculated wavenumbers for the well known monosubstituted derivatives. The same force constants without further adjustment were used to calculate the wavenumbers and potential energy distributions ofDMH,DAHandBMAH. One of the main interests in these studies lies in the examination of the amide and imide characteristic vibrations because of their importance in the field of b
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