| Abstract: | The harmonic molecular force fields for the nucleic acid bases, cytosine, and guanine, that have been previously published by several investigators are tested by the calculation of the relative intensities of the in-plane modes in the ultraviolet resonance Raman (UVRR) effect from the two lowest lying absorption bands using a theoretical approach devised previously.1–3 Since only a fraction of the 2N – 3 in-plane vibrations of a molecule are active in the UVRR, the two criteria that are taken for the adjustment of the force constant are: (1) the closest possible agreement between the observed and calculated frequencies of the 2N – 3 in-plane vibrations, and (2) the closest possible agreement between the calculated and observed intensities of those few vibrations that are strongly active in the ultraviolet resonance Raman effect. In particular it is necessary that the force constants be adjusted to avoid the calculation of intense Raman lines with frequencies that are not observed in the UVRR spectrum. Using this criteria, a new force field has been developed that appears to give better agreement with the observed UVRR intensities than previously published ones. It is suggested that this calculation of the UVRR intensities can be used to refine molecular force fields in combination with other methods such as isotopic replacement currently in use to refine force constants. |
