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Ab Initio Anharmonic Analysis of Vibrational Spectra of Uracil Using the Numerical-Analytic Implementation of Operator Van Vleck Perturbation Theory

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journal contribution
posted on 25.06.2015, 00:00 by Sergey V. Krasnoshchekov, Natalja Vogt, Nikolay F. Stepanov
The numerical-analytic implementation of the operator version of the canonical Van Vleck second-order vibrational perturbation theory (CVPT2) is employed for a purely ab initio prediction and interpretation of the infrared (IR) and Raman anharmonic spectra of a medium-size molecule of the diketo tautomer of uracil (2,4­(1H,3H)-pyrimidinedione), which has high biological importance as one of the four RNA nucleobases. A nonempirical, semidiagonal quartic potential energy surface (PES) expressed in normal coordinates was evaluated at the MP2/cc-pVTZ level of theory. The quality of the PES was improved by replacing the harmonic frequencies with the “best” estimated CCSD­(T)-based values taken from the literature. The theoretical method is enhanced by an accurate treatment of multiple Fermi and Darling–Dennison resonances with evaluation of the corresponding resonance constants W and K (CVPT2+WK method). A prediction of the anharmonic frequencies as well as IR and Raman intensities was used for a detailed interpretation of the experimental spectra of uracil. Very good agreement between predicted and observed vibrational frequencies has been achieved (RMSD ∼4.5 cm–1). The model employed gave a theoretically robust treatment of the multiple resonances in the 1680–1790 cm–1 region. Our new analysis gives the most reliable reassignments of IR and Raman spectra of uracil available to date.