Knowledge Bank

A Urey-Bradley force field has been fitted to the observed vibrational frequencies of pyridine and four of its more symmetrical deuterium isotopes. The resonance modification to the basic UBFF have been tried and the results are compared with those determined using the basic UBFF. The vibrational assignments of these molecules are discussed in terms of the calculated normal coordinates and the potential-energy distributions in Urey-Bradley space.

Califano and $Crawford1$ have recently demonstrated that a basic Urey-Bradley Force Field reproduces Ingold’s assignments of the planar $B2u$ vibration fundamentals of benzene but does not give acceptable agreement with the Mair-Hornig assignment. We propose an extension to the basic UBFF (Kekul\’e model) which removes these objections to the Mair-Hornig assignment, and the normal modes for this model are discussed and compared with those obtained for the Ingold assignment. This model gives a slightly better frequency fit for certain vibrational modes and although the Ingold assignment cannot be excluded on the basis of our results for benzene, a slight preference of the Mair-Hornig assignment is indicated. The benzene assignment has been resolved by a study of the chlorobenzenes. We have shown that an unmodified UBFF cannot explain satisfactorily the observed spectra of \emph{sym}. trichlorabenzene, but, when the basic UBFF is modified with the Kekul\’e model the calculated frequencies agree well with the observed spectra.