Matrix-Isolation FT-IR Studies and ab Initio Calculations of Hydrogen-Bonded Complexes of Molecules Modeling Cytosine or Isocytosine Tautomers. 6. Experimental Observation of a Water-Induced Tautomeric Shift for 2-Hydroxypyrimidine and 5-Bromo-2-hydroxypyrimidine

Abstract

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.

Investigations of the excited-state properties of isocytosine: An ab initio approach

“…The ground‐state relative total energies of the water complexed tautomers given in Table I show that complexation slightly favors the stability of the IC3 tautomer; however, ICE would also be present. Shift of tautomeric equilibria toward the oxo form under complexation with water has also been detected experimentally for the model compounds of isocytosine 10. Ground‐ and excited‐state optimized bond lengths, bond angles, and dihedral angles of the IC3, ICE, and IC1 moieties in the IC3W, ICEW, and IC1W forms are also presented in Tables II–IV, respectively, while the hydrogen bond lengths and some dihedral angles are shown in Figure 3.…”

“…Theoretical studies show that while the enol form is the most stable in the gas phase, under aqueous solvation or complexation with a water molecule, the N 3 H form turns out to be the most stable 7b, e. Recently, the shift of tautomeric equilibrium toward the keto form under the addition of a water molecule has been shown experimentally on the model compounds of cytosine and isocytosine, namely 2‐hydroxypyrimidine and 5‐bromo‐2‐hydroxypyrimidine 10. The present work was undertaken with a view to investigate the excited states and was aimed at obtaining adequate answers to the following questions: (i) Is tautomerism also possible in the excited state?…”

Theoretical Calculations and Matrix-Isolation FT-IR Studies of Hydrogen-Bonded Complexes of Molecules Modeling Cytosine or Isocytosine Tautomers. 7. 2-Hydroxypyridine/2-Oxopyridine Complexes with H2O

“…This effect was analyzed previously for the 2-hydroxypyrimidine-H 2 O complexes. 1 Another important result emerging from the theoretical calculations is that the H-bond energies (ZPE and BSSE corrected) for the two closed complexes are -27.77 (MP2) or -33.50 kJ‚mol -1 (DFT) for 2HP-H 2 O and -36.11 (MP2) or -36.41 kJ‚mol -1 (DFT) for 2OP-H 2 O. These results indicate that 2OP interacts more strongly with a single water molecule than 2HP.…”

“…This approach has allowed a detailed description of the tautomeric and H-bonding behavior of cytosines. [1][2][3][4][5][6] In some of these previous studies, only a single tautomer of the studied base was present in low-temperature Ar matrixes, [2][3][4][5] while in other cases the presence of several tautomeric forms was detected in the matrices. 1,6 In continuing the line of study, which has included investigations of 2-OH-pyrimidine‚H 2 O 1 and 1-CH 3 -cytosine‚H 2 O, 6 we consider in the present work the H-bond interaction of 2-OHpyridine and its tautomer 2-oxopyridine with water in Ar matrices.…”

“…The first supporting argument is that the ν b OH mode of the closed H-bonded complex is expected to be a rather broad absorption, as was the case for 2-pyrimidone-H 2 O. 1 The frequency 3270 cm -1 yields a relative shift of about 0.12, which is situated far above the correlation curve of ∆ν/ν versus PA CdO for the CdO‚‚‚H-OH systems. 41 The shift of -228 cm -1 for the base N-H mode is not unusual for such a strong complex, since shifts of -120 f -180 cm -1 have been observed for open N-H‚‚‚OH 2 complexes of uracils in Ar.…”