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The i.r. spectra of solid and in CH3CN dissolved normal urea and thiourea, their 15N and 2H isotopic species, and solid selenourea-D4 were recorded. The normal coordinate analysis of 13 in-plane vibrations of these molecules, including the in-plane vibrations of urea-18O assigned by Laulicht [1] has been made using Wilson's GF matrix method and the Urey Bradley force field. The Urey—Bradley force field reproduces well the heavy istope frequency shifts. Several sets of force constants have been calculated and their validity is discussed in terms of other bond characteristics. The values of KCN force constants increase in this order of urea, thiourea and selenourea. The frequency shifts and the differences in the force constants of ureas and thioureas in the solid phase and in solution show that hydrogen bonding strongly affects the potential energy distribution, specially for the CO stretching mode.

Thermodynamics protonation constants of 2-ethyl-, 3-ethyl-, 4-benzyl-, 3-ethyl-4-methyl-,and 6-ethyl-2-methyl-substituted pyridine-N-oxides in aqueous media have been measured spectrophotometrically in the range 20–60°C. The effects of alkyl and benzyl substituents on base strength are explained in terms of inductive and resonance effects, and the resulting thermodynamic quantities as sums of environmental and internal contributions. The Hammett sigma values of a number of alkyl and benzyl substituted pyridine-N-oxides are also reported.

Force constant and the potential energy distribution have been calculated for the two geometries ( C 2 and C 2 h symmetry). Agreement with the observed frequencies, including the 2 H and 15 N isotopic species, has been obtained using the same set of force constants for both forms.