Ab initio study of force constants and intensities of 1,3,5-trioxane

The structure, vibrational force constants and infrared and Raman intensities have been calculated for 1,3,5-trioxane using a 3-21G basis set. These results have been used to identify some possible inaccuracies in experimental diffraction based structures and in vibrational assignments. It is demonstrated that there is a marked contrast between the trends in the vibrational force constants and in the King's effective atomic charges of the axial and equatorial CH bonds in the series cyclohexane, 1,4-dioxane and 1,3,5-trioxane. The axial CH stretching force constant decreases by 0.04 mdyne Å^?1 for each adjacent oxygen atom, whereas that of the equatorial CH bond increases by 0.15 mdyne Å^?1 per oxygen. In trioxane the effective atomic charge of the axial hydrogen is twice that of the equatorial. Atomic polar tensors are calculated in a bond oriented frame, and the effect of the oxygens on CH stretching and bending mode intensities discussed. Some properties are also calculated using the 4-31G basis.