Matrix-isolation FT-IR studies and ab initio calculations of hydrogen-bonded complexes of molecules modeling cytosine or isocytosine tautomers. 4. H-bonded complexes of 1-methyl-2-pyrimidone and N,N-1-trimethylcytosine with water

We continue our combined experimental and theoretical ab initio studies of hydrogen-bonded complexes involving molecules modeling cytosines by investigating the H-bond interaction of 1-methyl-2-pyrimidone and N,N-1-trimethylcytosine with water. The objective of the present work is theoretical and spectral characterization of the base-water complexes in the argon matrix, where the water molecule can become hydrogen bonded to the base through two H bonds with hydrogens of both bonds coming from the water molecule. For the 1-methyl-2-pyrimidone base, the ab initio calculations predict the H-bonded complex at the N₃ site to be slightly more stable than the alternative complex at the C₂=O site. Both H-bonded complexes have a second, weaker H bond between the other water OH group and the C₂=O or N₃ base site. The predicted vibrational spectra of the two complexes do not match well with the observed experimental FT-IR spectrum. Possible reasons for the discrepancies between the experiment and the theory attributed to the presence of argon are discussed. While the theory predicts two closed complexes, each containing a double hydrogen bond, from the experimental spectrum, two open complexes at the C₂=O and N₃ interaction sites are identified. H-bonding of water with N,N,1-trimethylcytosine can occur at either the N₃, C₂=O, or (CH₃)₂N site. The stability difference between the first two complexes is relatively small, and the second H bond in both complexes is considerably weaker than in the pyrimidone complexes. As results from the calculations, the H-bond interaction at the methylated amino group is much weaker than the H bonds formed with N₃ and C₂=O, and this finds conformation in the experimental matrix spectra, where the spectral signature of this complex is absent. The relative frequency shifts and the ratios between the calculated and measured frequency values for the stretching mode of the bonded water for all the complexes of both basis are discussed in relation to the earlier established correlations for similar H-bonded complexes. Water-rich matrices contain also 1:2 complexes B⋯HO(H)⋯HOH. It is found that the central H bond in these linear structures is characterized by the cooperativity factor of 1.3-1.4.