No abstract

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.

The H-bond interaction of the cytosine model compound 2-hydroxypyrimidine and its 5-bromo derivative with water is investigated using the combined matrix-isolation FT-IR and theoretical ab initio method. As predicted by the ab initio calculations, both compounds occur dominantly in the hydroxy tautomeric forms. The estimated K T(h/o) values are 60 and 184, respectively. When water is added to the Ar matrix, a noticeable shift of the tautomeric equilibrium towards the oxo form is observed. The theoretical results indicate that the closed N···H−O(H)···H−O and CO···H−O(H)···H−N H-bonded water complexes are the most stable systems for the hydroxy and the oxo tautomers, respectively. The experimental spectra are consistent with this prediction, but additional structures, such as an open N···H−OH complex of the hydroxy tautomer, are also identified. The frequency shift of the stretching mode of doubly H-bonded water in the two closed complexes is larger, and the ratio between the calculated and measured frequencies smaller than expected from the correlation established before for open, singly H-bonded complexes involving similar molecules. Although some cooperativity exists between the two H bonds in each of the closed complexes, this effect is limited because the geometrical structures of both H bonds are noticeably perturbed from the perfect alignment due to the cyclic arrangement of the complex. A possible mechanism of the proton transfer process leading from the hydroxy to the oxo tautomeric form is discussed in terms of proton tunneling and in relation to recent literature data.