The use of resonant Raman intensities in refining molecular force fields for Wilson G–F calculations and obtaining excited state molecular geometries

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Abstract
In this paper equations are developed for the quantitative refinement of molecular force fields for Wilson G–F calculations through the use of molecular orbital (MO) calculations of the bond order change between the resonantexcited electronic state and the ground state together with the observed intensity of the resonantRaman spectrum (RRS). The method should permit an unambiguous choice between two or more force fields which give equally good fit of the observed frequencies but which give different potential energy distributions among the internal coordinates. We have used this method to study methyl uracil, a planar ring molecule of low symmetry, where partitioning of the matrices is not possible. A least‐squares calculation of the difference between the MO bond order changes and the bond order changes calculated from the force field together with the measured intensities in the RRS allows a choice of the correct sign for the excited state changes in the normal coordinates of modes which are active in the RRS. This least‐squares calculation allows us to improve a previous choice of signs taken for uracil and obtain calculated excited state geometry in better agreement with bond order changes obtained from MO calculations. From our force field we have not only calculated all the in‐plane vibrational frequencies but also the relative intensities for the RRS in both the 260 nm and 200 nm excited states. The theory correctly predicts that only 8 of the 21 in‐plane molecules will be observed in the RRS.