Ionic and Tautomeric Composition of Cytosine in Aqueous Solution: Resonance and Non-Resonance Raman Spectroscopy Study

A complex experimental and theoretical study of the structural composition of cytosine in water was performed. Raman and resonance Raman spectra of cytosine in acidic, neutral, and alkaline water solutions (pH = 3, 7, and 10, respectively) were recorded at excitation wavelengths of 514, 266, 218, and 200 nm. The temperature dependence of the frequencies and intensities of the resonance Raman bands was obtained in the temperature interval of 4-80 degrees C. To interpret the experimental Raman and resonance Raman spectra and to determine the structural composition of the water solution of cytosine, the spectra of cytosine and its cation, anion, oxoimine, and hydroxoamine forms were calculated at the B3LYP/6-311++G(d,p) level using the SCRF method. The electronic spectra and geometric parameters of the cytosine and its molecular forms in the excited electronic states close to the excitation wavelengths of the resonance Raman spectra were calculated using the DFT method. The cation exists in the acidic solution together with cytosine and its oxoimine and hydroxoamine tautomeric forms. Cytosine with a small amount of anion dominates in the alkaline medium. The structural composition of the water solution of cytosine is confirmed by the results of quantum-mechanical calculations of the intensity distribution in the resonance Raman spectra.