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A vibrational analysis of the inelastic neutron scattering spectrum (INS) of pyrimidine has been carried out combined with quantum mechanical calculations at the RHF, MP2, and B3LYP levels using the 6-31G* basis set in all the cases. The INS spectrum was first calculated from the atomic displacement matrix in order to determine which level of theory gives the best agreement between observed and calculated INS intensities. In a second approach, force constants from a calculation at the MP2/6-31G* level were transformed to a symmetrized set (C 2 v ) and all the diagonal and some off-diagonal force constants were fitted. Good agreement between observed and calculated wavenumbers and spectral INS intensities was achieved which demonstrates the validity of our force field.

This paper introduces a new approach to the problem of describing molecular vibrations in redundant valence coordinates. A consistent definition for canonical force field, which is different of that by Kuczera and Pupyshev et al., is outlined. This new definition is also generalized to other matrices like kinetic-energy matrices and transformation matrices between different sets of vibrational coordinates. The importance of such canonical matrices follows from the fact that they are uniquely determined and allow us to consider the redundant coordinates as though they were independent.

This article develops a general thermodynamic treatment to predict the direction of shift in a chemical equilibrium when it is subjected to a stress. This treatment gives an inequality that relates the change in the perturbed variable and the change that the equilibrium shift produces in the conjugated variable. To illustrate the generality of this approach, it has been applied to predict the direction of shift caused by changes of pressure, volume, and amount of substance. In this last case, the well-known unexpected shift in the ammonia synthesis equilibrium upon addition of nitrogen is easily explained. From the above referred inequality and the stability criteria of thermodynamics some conclusions have been obtained about the direction of shift in terms of extensive and extensive variables. This article is suitable for physical chemistry courses.

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