A vibrational analysis has been performed for nitrogen trichloride based on a frequency fit of the 14N and 15N species. Force constants for several modifications of the Urey–Bradley force field (UBFF) are compared to the six constants of the general valence force field. Modification of the UBFF to include angle–angle and bond–bond cross terms results in an excellent agreement with the GVFF constants. The chlorine–chlorine repulsion force constant obtained agrees satisfactorily with the calculated Lennard-Jones nonbonded interaction between two argon atoms.