Approximate solution of the mode–mode coupling integral: Application to cytosine and its deuterated derivative

Ab initio Hartree–Fock (HF), density functional theory (DFT) and second-order Møller–Plesset (MP2) methods were used to perform harmonic and anharmonic calculations for the biomolecule cytosine and its deuterated derivative. The anharmonic vibrational spectra were computed using the vibrational self-consistent field (VSCF) and correlation-corrected vibrational self-consistent field (CC-VSCF) methods. Calculated anharmonic frequencies have been compared with the argon matrix spectra reported in literature. The results were analyzed with focus on the properties of anharmonic couplings between pair of modes. A simple and easy to use formula for calculation of mode–mode coupling magnitudes has been derived. The key element in present approach is the approximation that only interactions between pairs of normal modes have been taken into account, while interactions of triples or more are neglected. FTIR and Raman spectra of solid state cytosine have been recorded in the regions 400–4000 cm−1 and 60–4000 cm−1, respectively. Vibrational analysis and assignments are based on calculated potential energy distribution (PED) values.