Ab initio study of the tautomerism of 2,5-substituted diazoles

Detailed investigation of the tautomerism of oxadiazoles, thiadiazoles, and selenadiazoles containing hydroxy, mercapto, or selenyl groups in position 2 and 5 of the five-membered ring was carried out at MP2, MP4, and CCSD(T) levels of theory. The relative stabilities of the tautomers of the heterocycles were investigated in gas phase. It is found that the dioxo form is the preferred tautomer if the exocyclic group is hydroxy. If the exocyclic group is mercapto or selenyl group the most stable tautomer is the mixed mercapto/thione or selenyl/selone form. The barriers of intramolecular proton transfer are too large and tautomerization should not occur.     

A solid state and solution NMR study of the tautomerism in hydroxyquinoline carboxylic acids

Some hydroxyquinoline carboxylic acids and their conjugate acids and bases were characterized by 13C and 15N NMR spectroscopy in solution and in the solid state. Differences in 13C and, in particular, 15N chemical shift patterns allow to distinguish between individual tautomers and confirm the presence of zwitterionic species in the solid state. Solution NMR spectra in dimethyl sulfoxide (DMSO) show effects resulting as a consequence of dynamic exchange and suggest the presence of an equilibrium mixture of hydroxyquinoline carboxylic acid and zwitterionic hydroxyquinolinium carboxylate tautomers.     

NMR studies of the tautomerism of cyclo-tris(4-R-2,6-pyridylformamidine) in solution and in the solid state

Two novel cyclo-tris(4-R-2,6-pyridylformamidine)s (R = H, CH(3)) have been studied by solution- and solid-state NMR. Both compounds show fast exchange of NH protons in dimethyl sulfoxide at room temperature. The E-syn configuration of the formamidine group with the NH protons in the outer ring position could be proved by low-temperature (1)H NMR experiments. The influence of deuteration of the NH group on the exchange rate has been demonstrated qualitatively. Proton exchange at the formamidine groups results in both a symmetric (S) and an asymmetric (A) isomer which could be spectroscopically identified and characterized at 193 K. Whereas two degenerate forms exist for S, six degenerate A forms can be distinguished. Prototropic tautomerism at one formamidine group results in exchange from S into A, whereas A is transformed to a degenerate A form or to S. It could be shown that some transitions between substructures are impossible by a single -NH-CH=N-/-N=CH-NH- exchange. The S isomer with three equivalent formamidine groups is the preferred isomer in solution as indicated by the S/A ratio at 193 K. From this result we conclude that in polyformamidines, ordered sequences of formamidine tautomers are also formed at low temperatures. Prototropic exchange was not observed in the solid state, neither by (13)C nor by (15)N solid-state NMR. For one trimer (R = CH(3)), three molecules dimethyl sulfoxide per trimer molecule are within the lattice as could be proved by (13)C CPMAS NMR.     

The tautomerism of 1H-pyrazole-3(5)-(N-tert-butyl)carboxamide in the solid state and in solution

The X-ray crystal structure of 1H-pyrazole-3-(N-tert-butyl)-carboxamide was determined. In the solid state, the 13C and 15N CP/MAS NMR spectra correspond to this tautomer. In solution, both tautomers are present in a ratio that depends on the temperature (at 293 K, 90% 3-substituted/10% 5-substituted). Some unusual 1H, 1H couplings involving the NH proton were observed. DFT (GIAO) calculations were carried out.     

Ab initio study of 2,4-substituted azolidines. II. Amino-imino tautomerism of 2-aminothiazolidine-4-one and 4-aminothiazolidine-2-one in water solution

The relative stabilities of the tautomers of 2-aminothiazolidine-4-one and 4-aminothiazolidine-2-one were calculated at the MP2/6-31+G(d,p) level by considering their mono- and trihydrated complexes. Single-point calculations at the MP4/6-31+G(d,p)//MP2/6-31+G(d,p) level of theory were performed to obtain more accurate energies. The values of proton transfer barriers in the isolated, mono- and trihydrated tautomers of 2-aminothiazolidine-4-one (2AT) and 4-aminothiazolidine-2-one (4AT) were calculated for two different mechanisms of tautomerisation. In the absence of water, the process of proton transfer should not occur. Addition of water molecules decreases the barrier making the process faster, as the participation of two water molecules in a proton transfer reaction is more favorable than the participation of only one water molecule. To estimate the effect of the medium (water) on the relative stabilities of the tautomers of the studied compounds we applied the polarizable continuum model (PCM). (13)C NMR chemical shieldings were calculated using the GIAO approach at MP2/6-31+G(d,p) optimized geometries. HF and the DFT B3LYP functional with 6-31+G(d,p) basis set were employed. The quantum chemical results for the chemical shifts in gas phase and in polar solvents (water and DMSO) were compared with experimental data. TD DFT B3LYP/aug-cc-pVTZ calculations were performed to predict the absorption maxima of tautomers A and B of 2AT and 4AT.     

Ab initio molecular dynamics study of the keto-enol tautomerism of acetone in solution.

We have studied the keto-enol interconversion of acetone to understand the mechanism of tautomerism relevant to numerous organic and biochemical processes. Applying the ab initio metadynamics method, we simulated the keto-enol isomerism both in the gas phase and in the presence of water. For the gas-phase intramolecular mechanism we show that no other hydrogen-transfer reactions can compete with the simple keto-enol tautomerism. We obtain an intermolecular mechanism and remarkable participation of water when acetone is solvated by neutral water. The simulations reveal that C deprotonation is the kinetic bottleneck of the keto-enol transformation, in agreement with experimental observations. The most interesting finding is the formation of short H-bonded chains of water molecules that provide the route for proton transfer from the carbon to the oxygen atom of acetone. The mechanistic picture that emerged from the present study involves proton migration and emphasizes the importance of active solvent participation in tautomeric interconversion.     

Ab initio study on tautomerism of 2-thiouracil in the gas phase and in solution

Ab initio geometry optimizations were carried out at the HF/3-21G and HF/6-31+G^** levels for the six tautomeric forms of 2-thiouracil (2TU, 2TU1, 2TU2, 2TU3, 2TU4, 2TU5) in the gas phase and in solution. To obtain a more definitive estimate of the relative stabilities for 2-thiouracil tautomers in the gas phase, single-point MP2/6-31+G^** calculations were performed on the HF/6-31+G^** optimized geometries. The tautomeric equilibria in 1,4-dioxane (=2.21), acetonitrile (=38), and in water (=78.54) were studied using the self-consistent reaction field (SCRF) theory. The calculated relative free energies indicated that 2TU is the energetically preferred tautomer in the gas phase and in solution. The stability order of 2-thiouracil tautomers depends on the level of theory and the dielectric constant of the solvent. The obtained results are compared with the available experimental data.     

Ab initio and infrared spectral study of 4′-substituted phenylthiolbenzoates

The geometry of the 4′-cyano-(4′-CNPTB) and the 4′-methoxy-(4′-MePTB) phenylthiolbenzoates were obtained by ab initio calculations employing 6–31G basis set at Hartee-Fock level of theory. The results predict an extended form of the molecules and torsional angle between the phenyl rings at 90.85(6)⁰ and 90.87(3)⁰, respectively. On the basis of vibrational analysis the frequency assignment was carried out. The calculated frequencies were compared with the experimental IR spectral data in carbon tetrachloride, carbon disulfide solutions and in solid state.     

Ab initio molecular orbital study of the tautomerism of 4-hydroxy-2-pyridinone

Ab initio calculations were performed on 2,4-pyridinediol, 4-hydroxy-2-pyridinone, and 2hydroxy-4-pyridinone at the HF /3-21G level with full geometry optimization. Two conformations of the hydroxyl group were considered for each tautomer. Corrections for polarization functions, electron correlation, and zero point energy were made by comparison with previous calculations on 2- and 4-pyridinone tautomerism which included these contributions. The most stable structure is 4-hydroxy-2-pyridinone, in agreement with experiment. Relative to 4-hydroxy-2-pyridinone, the energies of the other tautomers are estimated to be 1.9 kcal/mol for 2,4-pyridinediol and 8.9 kcal/mol for 2-hydroxy-4-pyridinone. These are in accord with the experimental values 0.3 ± 1.9 and 10.6 ± 1.9 kcal/mol, respectively, deduced from equilibration studies of the tautomeric methyl derivatives.     

Structure and dynamics of ND3BF3 in the solid and gas phases: a combined NMR,neutron diffraction,and Ab initio study

The decrease in D-->A bond lengths, previously reported for some Lewis acid/base complexes, in going from the gas to the solid phases is investigated by obtaining an accurate crystal structure of solid ND(3)BF(3) by powder neutron diffraction. The B-N internuclear distance is 1.554(3) A, 0.118 A shorter than the most recent gas-phase microwave value and 0.121 A shorter than the single molecule geometry optimized (1.672 A, CISD/6-311++G(d,p)) bond length. The crystal structure also shows N-D.F-B hydrogen bonds. The effects of this change in structure and of intermolecular hydrogen-bonding on nuclear magnetic shielding (i.e., chemical shifts) and the nuclear quadrupolar coupling constants (QCC) are investigated by ab initio molecular orbital and density functional theory calculations. These calculations show that the nitrogen ((15)N and (14)N) and boron ((11)B and (10)B) chemical shifts should be rather insensitive to changes in r(BN) and that the concomitant changes in molecular structure. Calculations on hydrogen-bonded clusters, based on the crystal structure, indicate that H-bonding should also have very little effect on the chemical shifts. On the other hand, the (11)B and (14)N QCCs show large changes because of both effects. An analysis of the (10)B[(19)F] line shape in solid ND(3)(10)BF(3) yields a (11)B QCC of +/-0.130 MHz. This is reasonably close an earlier value of +/-0.080 MHz and the value of +/-0.050 MHz calculated for a [NH(3)BF(3)](4) cluster. The gas-phase value is 1.20 MHz. Temperature-dependent deuterium T(1) measurements yield an activation energy for rotation of the ND(3) group in solid ND(3)BF(3) of 9.5 +/- 0.1 kJ/mol. Simulations of the temperature-dependent T(1) anisotropy gave an E(a) of 9.5 +/- 0.2 kJ/mol and a preexponential factor, A, of 3.0 +/- 0.1 x 10(12) s(-)(1). Our calculated value for a [NH(3)BF(3)](4) cluster is 16.4 kJ/mol. Both are much higher than the previous value of 3.9 kJ/mol, from solid-state proton T(1) measurements.     

Ab initio calculations for adsorbed state of OH group on Ni(110)

Ab initio quantum-chemical calculations of OH radical adsorption on Ni(110) have been carried out in cluster approximations. It is shown that unlike energy calculations of adsorbed complexes, their vibrational frequencies must be calculated with taking into account 3d orbitals of Ni.

---

OH Ni(110) . , , 3d-.

     

A new practical synthesis of 3-amino-substituted 5-aminopyrazoles and their tautomerism

It was found that 3-amino-substituted 5-aminopyrazoles could be effectively prepared via hydrolytic decarboxylation of the corresponding 3,5-diaminopyrazole-4-carboxylates under microwave irradiation. The reactions required short time (4?min) and were successfully reproduced in a larger scale and under conventional heating mimicking the microwave heating pattern. X-ray crystallography identified two different types of tautomers in crystals of related 5-aminopyrazoles with p-toluidyl and p-anisidyl moieties at the position 3, respectively.     

Biogenetically inspired synthesis of marine C6N4 2-aminoimidazole alkaloids: Ab initio calculations, tautomerism, and reactivity

[reaction: see text] A simple synthesis of the fused tetrahydro-imidazopyridine 13 was accomplished via selective addition of protected guanidine to N-carbomethoxy-1,2-dihydropyridine in the presence of bromine. Base-mediated semicleavage of the aminal gave 4-substituted 2-aminoimidazole 14. With this new method, natural marine metabolite 3-amino-1-(2-aminoimidazol-4-yl)-prop-1-ene (1) and derivatives may now be prepared from pyridine. Ab initio calculations of the energies of tautomers I-IV and deuteration experiments have provided insight into their reactivity.     

Structure and stability of BN microclusters: Ab initio calculations for (BN)n (n = 2–4)

Unrestricted Hartree–Fock calcuations coupled with second-order Møller–Plesset correlation correction were performed to study the structures and energetics of microlusters. For (BN)₂, linear and rhombus forms are almost isoenergetic, whereas cyclic forms are preferable for (BN)₃ and (BN)₄ clusters. As a general trend, linear isomers prefer the triplet spin state, whereas cyclic isomers prefer the singlet spin state. Total charge density plots show a strong dominace of the B? N bond, indicating that the extent of its polar character becomes stronger with the increase in the cluster size. The loss of a BN monomer is shown to be the most likely fragmentation channel for both neutral and single-ionized clusters. We find that neutral (BN)_n clusters have the same structural configurations as those of their corresponding C_2n counterparts. This similarity follows the isoelectronic principle and is of importance due to recent interest in the investigations of BN fullerene analogs. © 1994 John Wiley & Sons, Inc.     

An Ab initio study of tautomerism between formhydroxamic acid and formhydroximic acid

Ab initio MO calculations including electron correlation with the 4-31G and 4-31G ** basis sets were performed in order to study the formhydroxamic acid-formhydroximic acid tautomerism. The geometries, relative energies, and activation energy of the tautomer and transition state were determined. Based on total-energy calculations at the MP 4/4-31G **//RHF /4-31G ** plus the scaled zero-point vibration energy level, the energy of formhydroxamic acid is determined to be lower than that of formhydroximic acid by 40.7 kJ/mol. The activation energy of the formhydroxamic acid-formhydroximic acid tautomerism via a 1,3-intramolecular hydrogen shift is 151.4 kJ/mol. © 1992 John Wiley & Sons, Inc.     

Ab initio study of cyclic siloxanes (H2SiO)n: n = 3, 4, 5

The geometry optimizations for several conformations of tri-, tetra-, and pentacyclosiloxane (H₂SiO)_n (n = 3, 4, and 5) were carried out, and the relative stabilities were compared at the Hartree-Fock (HF) and second order perturbation theory (MP2) levels of theory using the 6–31G* and 6–311G(d, p) basis sets. At the highest levels of theory, the only minimum for n = 4 (D₄) occurs at the highly symmetric D_4h structure. In contrast, several, nearly isoenergetic, minima are found on the D₅ surface. These have C₁, C₂, C_s, and D_5h symmetries. While the C₁ structure has the lowest MP2/6–311G(d, p) energy, this species is predicted to be highly fluxional, and the distribution of isomers is dependent on temperature. © 1996 by John Wiley & Sons, Inc.     

Thermodynamic modelling of the partially ordered solid solution Hf5−xNbxGe4 supported by ab initio calculations

The site preferences in the solid solution phase Hf_5−xNb_xGe₄ (0 < x < 3.8) were modelled with the “Compound Energy Formalism” model. The ground state energies of the end members calculated from ab initio density functional theory were taken as input parameters for the model. These ground state energies were computed for models with either fixed atomic positions or fully relaxed structures. The results of the former computation were used for the construction of a simplified model. Site fractions modelled with both, original and simplified models fit well to the experimental values, confirming the almost stepwise substitution mechanism in this compound. Non-linear trends of the lattice parameters found experimentally could be reproduced very well.     

Structure determination of beta-Pb2ZnF6 by coupling multinuclear solid state NMR, powder XRD and ab initio calculations

The results from one-dimensional multinuclear (19F, 207Pb and 67Zn) magic-angle spinning nuclear magnetic resonance experiments combined with the use of the ISODISPLACE program allow for the space group determination of beta-Pb2ZnF6 (no. 138 P4(2)/ncm). The structure was refined from X-ray powder diffraction data (a = 5.633 (1) A and c = 16.247 (1) A, Z = 4). beta-Pb2ZnF6 has one six-fold coordinated Zn, one eleven-fold coordinated Pb and five F non-equivalent crystallographic sites and is built from alternated layers parallel to the (a, b) plane; tilted ZnF4(2-) layers of corner sharing ZnF6(4-) octahedra and FPb+ layers of edge sharing FPb4(7+) tetrahedra. The structure of beta-Pb2ZnF6 was then optimized using the ab initio code WIEN2k and the calculated 67Zn EFG is in agreement with the NMR results. 19F-19F proximities and 19F-207Pb connectivities were evidenced using through-space and through-bond NMR correlation experiments, respectively, and support the proposed structure. 19F-207Pb J-coupling was also used to select fluorine resonances depending on the number of neighbouring lead ions, leading to an unambiguous assignment of the different 19F resonances.     

Ab initio and molecular mechanics (MM3) calculations of protonated-neutral diamine hydrogen bonds

Ab initio calculations of cation-neutral diamine complexes have been carried out at the MP2/6-311+G** level. The geometry and energetics of the charge-reinforced hydrogen bond are analyzed with respect to the alkyl substitution of both the protonated and neutral nitrogen atoms, and these results have been used to improve the quality of the MM3(2000) force field. In addition, specialized hydrogen bond parameters optimized for MM3(2000) are presented. These parameters allow very accurate gas-phase modeling of the charge-neutral diamine environment. Molecular mechanics calculations can model effectively protonated amine-neutral amine hydrogen bonds in the gas phase and solution (continuum dielectric) through a combination of charge-dipole interactions and explicit hydrogen-bonding terms.