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The primary challenge of spectroscopic techniques in selective detection and characterization of tautomeric structures of DNA and RNA bases, and moreover, the accurate interpretation and explanation of the experimental results are the main motives of theoretical studies. Surface‐enhanced Raman spectroscopy (SERS) can be a powerful approach to distinguish cytosine in the presence of its tautomers. For this respect, herein, the theoretical simulation of the SERS spectra of cytosine and its three most stable tautomers adsorbed on silver clusters is carried out. The calculations of SERS spectra is based on the excited‐state energy gradient approximation as a well‐established approach that gives us the knowledge about the properties of excited states and their effect on the pattern of spectra. The good consistency of this theoretical simulation in solution and also the gas phase with the experimental results is achieved and represents the capacity of the current theoretical investigations to interpret and supplement the experimental findings. [ABSTRACT FROM AUTHOR]

In the present study, 21 cytosine tautomers were investigated so that some tautomers were reported for the first time in the gas phase and aqueous solution. C 3 tautomer was the most stable tautomer in gas phase but C 1 was the most stable structure in aqueous solution. The potential energy surface of all trajectories was determined for 21 tautomers and 22 transition states. Also, interactions of cytosine tautomers with Na + , K + , Mg 2+ , Ca 2+ and Zn 2+ metal ions were studied in gas phase and aqueous solution. Three types of interactions among metal ions and (N1 and O10), (N3 and O10) and (N3 and N9) of cytosine tautomers were investigated. The study of interaction energies of all complexes showed the stability of complexes in which interactions among Mg 2+ and Zn 2+ with tautomers were stronger than interactions among Ca 2+ , Na + and K + with tautomers, respectively. Some interactions of metal ions with cytosine tautomers made the most stable tautomers. So, the stability of rare tutomeric forms had a significant effect on stabilization of anomalous DNA (deoxyribonucleic acid) double helix and spontaneous mutations. Also, one of the most important causes of mutations in DNA (deoxyribonucleic acid) was the reaction of OH radical with nucleotide bases. So, interactions of OH radical with cytosine and its tautomers were investigated in gas phase and aqueous solution.Communicated by Ramaswamy H. Sarma.