Ab initio/DFT and AIM studies on dual hydrogen-bonded complexes of 2-hydroxypyridine/2-pyridone tautomerism

The second-order M?ller-Plesset (MP2) and density functional theory (DFT) calculations have been carried out to investigate the structures and stabilities of hydrogen (H-) bonded 2-hydroxypyridine (2HP)/2-pyridone (2PY) dimeric forms as well as 2HP-2PY complexes. The results on single-point counterpoise (CP) correction of these complexes were compared against CP-optimized correction. The nature of the intermolecular contacts in the sense of normal H-bond or blue-shifting H-bond was determined on the basis of harmonic vibrational, atom-in-molecule (AIM), and natural bond orbital (NBO) analysis. A blue-shifting C-H...N H-bond was found and NBO analysis revealed a slight decrease in the population of the contacting sigmaC-H* antibonding orbital as the primary reason of the C-H contraction. Good correlations have been established between the interaction energies and the H-bond distances versus other characteristic H-bond parameters.     

The tautomerism of 5-chloro-2-hydroxypyridine. Comments on a paper by Srivastava and Prasad

Infrared and U.V. spectra are presented to show that 5-chloro-2-hydroxypyridine exists mainly in the pyridone form in solution in carbon tetrachloride, chloroform and cyclohexane, with extensive NH⋯OC intermolecular association at least in the former two solvents; in dioxane the pyridone and the pyridinol form are present in roughly equal amounts. As 2-pyridones are capable of forming centrosymmetric bimolecules of zero dipole moment, replacement of a highly polar solvent by a non-polar one does not necessarily displace the tautomeric equilibrium to predominance of the pyridinol tautomer, even though that has a much lower dipole moment than an individual 2-pyridone molecule. Some conclusions of Srivastava and Prasad's are questioned.     

Positron annihilation studies in solid 2-aminopyridine, 3-aminopyridine, 4-aminopyridine and 2-aminopyrimidine

Positron annihilation lifetimes and Doppler-broadened annihilation lines have been measured in solid 2-aminopyridine (2-APY), 3-aminopyridine (3-APY), 4-aminopyridine (4-APY) and 2-aminopyrimidine (2-APYM). The results point to the formation of positronium in the solid pyridines and the yields are discussed in terms of the structures and the electron donation character of the compounds.     

Infrared matrix isolation studies on tautomerism of cytosine and isocytosine methyl-derivatives

IR N₂ matrix isolation spectra have been recorded in the OH, NH and CO stretching region. The spectra were interpreted to determine the structure of investigated compounds: 1-methylcytosine predominates in the aminooxo form I; 3-methylcytosine and 1-methylisocytosine - iminooxo III and IV; 6-methylisocytosine and N(2)-monomethylaminoisocytosine - aminohydroxy forms VII and VIII.     

Distributed data parallel coupled-cluster algorithm: Application to the 2-hydroxypyridine/2-pyridone tautomerism

The recently developed parallel coupled-cluster algorithm of Rendell, Lee, and Lindh [Chem. Phys. Lett., 194 , 84 (1992)] is extended to allow four-indexed quantities containing one or two indices in the virtual orbital space to be stored across the global memory of distributed-memory parallel processors. Quantities such as the double-excitation amplitudes can now be distributed over multiple nodes, with blocks of data retrieved from remote nodes by the use of interrupt handlers. As an application of the new code, we have investigated the potential energy surface of the 2-hydroxypyridine/2-pyridone tautomers. Using large basis sets, the structure of each tautomer and the transition state connecting the two minima has been determined at the SCF level. The relative energy difference and the activation energy were then redetermined using the MP2, CCSD, and CCSD(T) methods. All calculations have been performed on Intel distributed-memory supercomputers. The largest coupled-cluster calculations contained over 2 million double-excitation amplitudes. © John Wiley & Sons, Inc.     

Infrared depletion spectra of 2-aminopyridine2-pyridone, a Watson-Crick mimic of adenine.uracil

The 2-aminopyridine2-pyridone (2AP2PY) dimer is linked by N-H...O=C and N-H...N hydrogen bonds, providing a model for the Watson-Crick hydrogen bond configuration of the adenine.thymine and adenine.uracil nucleobase pairs. Mass-specific infrared spectra of 2AP2PY and its seven N-H deuterated isotopomers have been measured between 2550 and 3650 cm(-1) by IR laser depletion combined with UV two-color resonant two-photon ionization. The 2PY amide N-H stretch is a very intense band spread over the range 2700-3000 cm(-1) due to large anharmonic couplings. It is shifted to lower frequency by 710 cm(-1) or approximately 20% upon H bonding to 2AP. On the 2AP moiety, the "bound" amino N-H stretch gives rise to a sharp band at 3140 cm(-1), which is downshifted by 354 cm(-1) or approximately 10% upon H bonding to 2PY. The amino group "free" N-H stretch and the H-N-H bend overtone are sharp bands at approximately 3530 cm(-1) and 3320 cm(-1). Ab initio structures and harmonic vibrations were calculated at the Hartree-Fock level and with the PW91 and B3LYP density functionals. The PW91/6-311++G(d,p) method provides excellent predictions for the frequencies and IR intensities of all the isotopomers.     

Theoretical studies on tautomerism of imidazole-2-selenone

The tautomerism of all possible forms of imidazole selenone (ISe1–ISe6), induced by proton transfer was studied theoretically in different environments including gas phase, continuum solvent, and microhydrated environment with one explicit water molecule. The calculations were performed at the MP2 and CAM-B3LYP levels of theory, separately. It was found that the imidazole selenone, in the form of ISe3, is the most stable isomer in both gas phase and solvent. The activation energy for conversion of ISe3 to imidazole selenol (ISe6), as the second stable form, is 41.72 and 43.0 kcal/mol in the gas phase and water, respectively. The infrared spectral frequencies as well as the vibrational frequency shifts were reported and assigned to their corresponding vibrational modes. In addition, the variation of dipole moments and charges on the atoms with change of solvent was studied. The energies of HOMO, LUMO, and HOMO–LUMO gap were calculated in both gas phase and solvent. Specific solvent effects with addition of water molecule near the electrophilic centers of tautomers and the transition states of proton transfer, assisted by water molecule, were investigated. It was found that the water molecule can form different hydrogen bonds with the molecule. Aggregation of the isomers with water molecule does not change the order of stability of isomers, but proton transfer reaction assisted by a water molecule needs less energy than when the proton shifts through the intramolecular process.     

Heterocyclic tautomerism. V. 2-Guanidinobenzimidazole

A single-crystal X-ray structure analysis of 2-guanidinobenzimidazole shows the molecule exists in the solid state as tautomer 1 , with an intramolecular hydrogen bond between the benzimidazole-N3 and a guanidino-NH₂. The guanidino group is inclined at an angle of 13.8° to the benzimidazole plane. Other nitrogen atoms and their attached hydrogens are involved in additional intermolecular hydrogen bonds that connect the molecules in a complex three-dimensional network.     

Structural studies of new group 6 complexes of salicylidene-2-aminopyridine

Interaction of salicylidene-2-aminopyridine (Hsap) with [M(CO)₆], M = Cr, Mo and W, in THF under sunlight resulted in formation of dinuclear complexes [Cr₂O₄(sap)], 1, [Mo₂O₄(sap)], 2, and [W₂O₅(sap)₂], 3. Elemental analysis, spectroscopic and magnetic studies of the reported complexes revealed the proposed structures. Magnetic studies of 1 and 2 suggested that the two metal centers have +3 and +6 formal oxidation states, while the tungsten complex 3 has +6 formal oxidation state with d⁰ electron configuration. The thermal properties of the complexes were investigated by thermogravimetry.     

Tautomerism and microsolvation in 2-hydroxypyridine/2-pyridone

The Fourier transform microwave spectra of the hydrated forms of the tautomeric pair 2-pyridinone/2-hydroxypyridine (2PO/2HP) have been investigated in a supersonic expansion. Three hydrated species, 2PO-H?O, 2HP-H?O, and 2PO-(H?O)?, have been observed in the rotational spectrum. Each molecular complex was confidently identified by the features of the 1?N quadrupole hyperfine structure of the rotational transitions. The presence of water affects the tautomeric equilibrium -N═C(OH)- ? -NH-C(═O)-, which is shifted to the enol form for the bare molecules 2PO/2HP but to the keto tautomer for the hydrated forms.     

Carbon–13 NMR studies of tautomerism in some 2-substituted imidazoles and benzimidazoles

The ¹³C NMR spectra of several 2-substituted imidazoles and benzimidazoles have been measured. The substituent was CH₃, COOH and CONHR, where R = H, n-Bu, p-tolyl or m-chlorophenyl. Carbons 4 and 5 in the imidazoles and the carbon pairs 8/9, 4/7 and 5/6 become equivalent by proton transfer from N-1 to N-3, possibly through intermolecular association. The rate of this proton exchange increases with concentration and temperature. It decreases with extension of the 2-substituent (rate CH₃?CONH-phenyl > CONH-p-tolyl ? CONH-m-chlorophenyl ? CONH-n-butyl) due to steric hindrance at the site of the (benz)imidazole nitrogen.     

Imine-enamine tautomerism in dihydroazolopyrimidines. 2. Synthesis and tautomerism of 1,4(3,4)-dihydropyrimido[1,2-a]benzimidazoles

Cyclocondensation of 2-aminobenzimidazole with unsaturated ketones or the hydrochlorides of Mannich bases has yielded aromatic substituted 1,4(3,4)-dihydropyrimido[1,2-a]benzimidazoles. The dependence of the tautomer composition of the products on steric factors and on the electronic character of the substituents introduced has been studied.For Communication 1, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1215–1219, September, 1991.     

UV-induced hydrogen-atom transfer in 3,6-dithiopyridazine and in model compounds 2-thiopyridine and 3-thiopyridazine

Monomeric 3,6-dithiopyridazine (3-mercapto- 6(1H)-pyridazinethione) was studied using the matrix-isolation method combined with quantum chemical calculations. The monomers of 3,6-dithiopyridazine, trapped from the gas phase into a low-temperature Ar matrix, were found to adopt the thione-thiol structure. In agreement with this experimental observation, the thione-thiol form was predicted (at the QCISD level) to be more stable by 13.5 kJ mol(-1) and by 39.6 kJ mol(-1) than the dithiol and the dithione tautomers, respectively. Monomers of 3,6-dithiopyridazine isolated in Ar matrixes were then irradiated with broadband UV (λ > 335 nm) light. Upon such irradiation, the thione-thiol form of the compound converted into the dithiol tautomer. The same phototransformation was observed when monochromatic λ = 385 nm laser light was used for irradiation. This allowed a first observation and spectral characterization of the dithiol form of 3,6-dithiopyridazine. Subsequent irradiation of the UV-generated dithiol tautomer with shorter-wavelength UV (λ > 275 nm) light led to partial repopulation of the thione-thiol form. Spectral signatures of the analogous photoreversibility were also found for the phototautomeric transformation in the model compound 3-thiopyridazine. The reliability of the QCISD predictions of relative energies of thiol and thione tautomeric forms was tested on the archetype example of 2-thiopyridine. For this compound, the comparison of the computed relative energy 10.9 kJ mol(-1) with the experimental estimate 10.0 ± 1.5 kJ mol(-1) (both in favor of the thiol form) was more than satisfactory.     

Structures of 2-hydroxypyridine and its vinyl derivatives

The PMR, Raman, and UV absorption spectra of 2-hydroxypyridine and its vinyl derivatives were investigated. Their electrochemical reduction was studied, and a quantum-chemical analysis of them was performed. It was concluded that the structure of 2-hydroxypyridine in proton-donor solvents is mesomeric, and its wave function, to a first approximation, is a linear combination of the wave functions of the 2-pyridone and zwitterion models. The contribution of the wave functions of each of the models depends substantially on the concentration of the solution and the nature of the solvent. An assumption was made regarding the presence of dynamic conjugation between the vinyl group and the ring in vinyl derivatives of 2-hydroxypyridine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 95–102 January, 1972.     

The constitution of 2-hydroxypyridine in aqueous solution

A computational strategy that replicates the kinetics of the neutral, the acetic acid-catalyzed and the 2-hydroxypyridine-catalyzed aqueous hydration of carbonyl compounds has been employed to examine the tautomeric equilibrium between 2-hydroxypyridine (H) and 2-pyridone (P) in a 2 × 10⁻⁴ M aqueous solution at 298 K. In addition to H and P, the computations have taken into account the symmetrical HH and PP dimers, the HP dimer, and mono and dihydrates of H and P. The calculated [P]/[H] equilibrium constant agrees with an experimental value obtained by measurement of ionization constants, but only if the dimers are excluded. Inclusion of the dimers lowers the calculated equilibrium constant from 862 to 70. This paper is dedicated to the memory of a long time friend and colleague, Fernando Bernardi.     

The reaction of benzil and 2-aminopyridine. A correction

On the basis of X-ray crystallographic analysis, it is shown that the reaction of benzil and 2-aminopyridine affords, in agreement with Sokov's previous work, 2,2-diphenylimidazo-[1,2-a]-pyridin-3-one and not N-(2-pyridyl)-benzilmonoimine as we have previously proposed.     

On the tautomerism and color origin in solid state of N<Subscript>py</Subscript>-protonated-2-aminopyridine: spectroscopic elucidation

A distortion of the aromatic character as a result of the N_py protonation of 2-aminopyridine was established by conventional and linear-polarized IR-spectroscopy, UV- and ¹H-NMR-spectral analysis of model system 2-aminopyridinium tetrachlorocuprate (II) salt. Quantum chemical ab initio calculations are performed at MP2 and B3LYP levels of theory and 6-311 ++G** basis set in order to determine the changes in the geometrical parameters and IR-spectroscopic characteristics as a result of N_py protonation. The observed crystallochromy effect is discussed as well.