Knowledge Bank

Molecular orbital ab initio calculations (with a $3-21G\ast$ basis set) have been carried out for the infrared spectra of 2- and 4-thiouracil and of 2,4-dithiouracil. These calculated spectra compare very well with the experimental spectra from studies of these molecules isolated in an argon matrix at 15 K. Because this agreement is so good, we investigate the results from the calculation to determine just which bond displacements contribute to the intensity of each of the observed fundamentals. Such an ``intensity analysis'' gives a detailed picture of the source of the intensities in the observed spectra, and how these contributions change as a result of substitution of S atoms for O atoms in different positions in the parent uracil molecule.