Keto Tautomeric Form Research Articles

(E)-1-[(2-Fluoro-phen-yl)imino-meth-yl]-2-naphthol-(Z)-1-[(2-fluoro-phen-yl)amino-methyl-idene]naphthalen-2(1H)-one (0.57/0.43).

The title Schiff base compound, 0.57C17H12FNO·0.43C17H12FNO, reveals both the enol (OH) and keto (NH) tautomeric forms with occupancies of 0.57 (6) and 0.43 (6), respectively. The tautomeric forms are stabilized by intra­molecular O—H⋯N (enol) and N—H⋯O (keto) hydrogen bonds. The dihedral angle between the naphthalene ring system and the benzene ring is 32.76 (1)°.

Switching the Conductance of a Single Molecule by Photoinduced Hydrogen Transfer

A new mechanism for optical switching of nanoscale single-molecule junctions between different conductance states is proposed. The mechanism is based on photoinduced excited state hydrogen transfer in the molecular bridge. Employing first principles electronic structure and transport calculations for prototype examples it is shown that the keto and enol tautomeric forms of the molecular bridge exhibit very different conductance properties, which realize the ‘on’ and ‘off’ states of the molecular switch. The feasibility and robustness of the mechanism is analyzed in some detail.

Two thiosemicarbazones derived from salicylaldehyde: very specific hydrogen-bonding interactions of the N—H...S=C type

The molecular structures of two salicylaldehyde thiosemicarbazone derivatives, namely salicylaldehyde 4-phenylthiosemicarbazone, C(14)H(13)N(3)OS, (I), and 4-methoxysalicylaldehyde 4-phenylthiosemicarbazone, C(15)H(15)N(3)O(2)S, (II), both of potential pharmacological interest, are found in the keto (thione) tautomeric form. The first compound represents a second triclinic polymorph of composition beta-C(14)H(13)N(3)OS. Although both polymorphs crystallize in the same space group (P1), the alpha-polymorph [Seena, Kurup & Suresh (2008). J. Chem. Crystallogr. 38, 93-96] differs from the beta form in its unit-cell volume at 293 K. The molecules in the crystal structures of (I) and (II) are linked into centrosymmetric R(2)(2)(8) dimers by hydrogen bonds of the N-H...S=C type. These dimers are connected through pi-pi stacking and T-shaped C-H...pi interactions into three-dimensional networks.

Synthesis and structure of cobalt(II) complexes with hydroxyl derivatives of pyridinecarboxylic acids: Conformation analysis of ligands in the solid state

Cobalt(II) complexes of 6-hydroxypicolinic acid (6-OHpicH), namely [Co(6-OHpic) 2(H 2O) 2] ( 1) and [Co(6-OHpic) 2(4-pic) 2]·4-pic ( 2), and of 2-hydroxynicotinic acid (2-OHnicH), [Co(2-OHnic) 2(H 2O) 2] ( 3) were prepared. The crystal structures of free 6-hydroxypicolinic acid monohydrate 6-OHpicH·H 2O ( 4), and the novel polymorph of 2-hydroxynicotinic acid 2-OHnicH ( 5) and complex 2 were determined by X-ray crystal structure analysis. All compounds were characterized by IR-spectroscopy and thermal methods (TGA/DSC) and data are in agreement with the structure analysis. It was established that 4 and 5 exist in solid state in keto tautomeric form. For 2, structure analysis revealed N,O-chelating mode of 6-hydroxypicolinic acid.

3-Methyl-1-phenyl-4-{phenyl[4-(trifluoromethyl)anilino]methylene}-1H-pyrazol-5(4H)-one

In the title compound, C24H18F3N3O, the mol­ecule exists in the enamine–keto tautomeric form. The pyrazolone ring makes dihedral angles of 31.8 (2), 45.4 (2) and 71.2 (2)° with the 1-phenyl, aniline and methylene-bound phenyl rings, respectively. An intra­molecular N—H⋯O hydrogen bond helps to stabilize the mol­ecular conformation.

3-Methyl-1-phenyl-4-[phenyl(8-quinolylamino)methylene]pyrazol-5(4H)-one

The title compound, C26H20N4O, was synthesized by the reaction of 1-phenyl-3-methyl-4-benzoyl­pyrazol-5-one and 8-amino­quinoline. The mol­ecule exists in the enamine–keto tautomeric form.

Tautomeric Equilibria and Pi Electron Delocalization for Some MonohydroxyarenesQuantum Chemical Studies

Keto-enol tautomeric interconversions and variations of the pi-electron distribution were studied for 11 isolated monohydroxyarenes at the DFT(B3LYP)/6-311++G(2df,2p) level. For two monohydroxyarenes (phenol and 9-anthrol), the PCM model of solvation (water) was also applied to the DFT geometries. The geometry-based HOMA index was applied to estimate pi-electron delocalization in the keto and enol tautomeric forms. Thermodynamic parameters of tautomeric interconversions (DeltaET, DeltaGT, TDeltaST, pKT) were calculated to estimate relative stabilities of individual tautomers and their percentage contents in the tautomeric mixtures. In almost all cases, the aromatic enol forms are strongly favored. An exception is 9-anthrol, which prefers its keto form. The resonance stabilization of this form comes from the central ring. Generally, aromaticity is the main factor that influences tautomeric equilibria in monohydroxyarenes. Hydration effect is considerably smaller and it does not change the tautomeric preference.

Chemo-selective hydrolysis of the iminic moiety in salicylaldehyde semicarbazone promoted by ruthenium

ortho-Hydroxybenzaldehyde semicarbazone (salicylaldehyde semicarbazone) undergoes chemo-selective hydrolysis of the iminic carbon nitrogen double bond through its reaction with [RuCl 2(dmso) 4] in ethanol in the presence of water, yielding free salicylaldehyde and semicarbazide that remains coordinated to the ruthenium ion as a bidentate N,O-donor to afford [RuCl 2(dmso) 2(semicarbazide)] · 2H 2O complex. The ruthenium–semicarbazide complex has been characterized by 1H NMR and FTIR spectroscopies and X-ray diffraction methods. Related semicarbazones, derived from p-hydroxybenzaldehyde and benzaldehyde, were not hydrolyzed under the same conditions, suggesting a significant role of the structural o-hydroxy motive in the reaction. Theoretical studies were performed in order to gain further insight on the mechanism of reaction. Results support the hypothesis that the ortho-hydroxy moiety, in the keto tautomeric form, participates in the chemo-selective hydrolysis promoted by [RuCl 2(dmso) 4].

Solid-state NMR study of ureidopyrimidinone model compounds

High-resolution (1)H and (15)N{(1)H} solid-state NMR experiments were conducted on two ureidopyrimidinone model compounds: dimeric 2-butylureido-6-methyl-4-pyrimidinone (1) and its bifunctional analogue N,N-1,6-hexanediyl(2-ureido-6-methyl-4-pyrimidinone) (4). High magic angle spinning rates and (1)H decoupling schemes were used to increase the proton spectral resolution. Upon heating 1 to 440 K, an increase in mobility was observed for non-hydrogen-bonded protons; the dimer remained in keto tautomeric form, which is capable of much stronger intermolecular hydrogen bonding than the enol tautomer. From these findings, it was concluded that this ureidopyrimidinone moiety should allow the design of strongly bonded molecular assemblies whose thermal stability compares favourably with that of conventional engineering polymers.

Theoretical and experimental investigations on the chemical reactions of positively charged phenyl radicals with cytosine and 1-methylcytosine.

The chemical behavior of positively charged phenyl radicals toward cytosine, 1-methylcytosine, and some pyrimidine analogues in the gas phase was investigated both theoretically by performing molecular orbital calculations and experimentally by using FT/ICR mass spectrometry. The phenyl radicals react with cytosine and 1-methylcytosine predominately by hydrogen abstraction and addition. For cytosine, the preferred site for hydrogen abstraction appears to be the amino group, and addition occurs preferentially at the N3 and N1 positions of the keto and enol tautomeric forms, respectively. For 1-methylcytosine, the methyl group is the thermodynamically favored site for hydrogen abstraction and N3 for addition. Possible structures and formation mechanisms are suggested for two unknown product ions formed upon the reaction of cytosine with the 3-dehydro-N-phenylpyridinium radical cation.

4-{[3,4-Dihydro-5-methyl-3-oxo-2-phenyl-2H-pyrazol-4-ylidene](phenyl)methylamino}-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one

In the title compound, C28H25N5O2, the carbonyl group of the 5-methyl-2-phenyl­pyrazol-3-one moiety, the adjacent double bond and the amine N atom of antipyrine are essentially coplanar, the largest deviation from the mean plane being 0.049 (2) Å. The compound is a neutral tridentate ligand in an en­amine–keto tautomeric form, due to a strong intramolecular N—H⋯O hydrogen bond. The dihedral angle between the two pyrazolone rings is 86.2 (3)°, reducing steric hindrance.

Pairing of the nucleobase adenine studied by IR-UV double-resonance spectroscopy and ab initio calculations

In this paper we present detailed R2PI spectra with IR–UV and UV–UV double resonance measurements of the guanine dimer (GG) and its methylated derivatives. We show that there are two isomers of GG in the investigated wavelength range from 32565 to 33600 cm−1. We were able to assign the two isomers to specific structures based on comparison of the intermolecular vibronic patterns of the dimers with and without methylation, on analysis of the IR spectra in the range of the OH and NH stretching vibrations and on comparison with ab initio calculated dimer stabilities and vibrational frequencies. In both structures both guanine moieties are in the keto tautomeric form, even though the enol tautomers are also present in the beam. One isomer exhibits nonsymmetric hydrogen bonding with HNH⋯N, NH⋯N and CO⋯HNH interactions (K9K7-2). The other isomer has a symmetrical hydrogen bond arrangement with CO⋯NH/NH⋯OC bonding (K9K7-1). The most stable guanine dimer forms CO⋯NH/NH⋯OC hydrogen bonds and has C2h symmetry (K9K9-1). Due to its strong exciton splitting the allowed S0–S2 transition is outside the investigated spectral range. We did not observe any keto–enol or enol–enol dimers in the investigated wavelength region. The calculations predict these dimers to be considerably less stable.

Pairing of the nucleobase guanine studied by IR–UV double-resonance spectroscopy and ab initio calculations

In this paper we present detailed R2PI spectra with IR–UV and UV–UV double resonance measurements of the guanine dimer (GG) and its methylated derivatives. We show that there are two isomers of GG in the investigated wavelength range from 32565 to 33600 cm−1. We were able to assign the two isomers to specific structures based on comparison of the intermolecular vibronic patterns of the dimers with and without methylation, on analysis of the IR spectra in the range of the OH and NH stretching vibrations and on comparison with ab initio calculated dimer stabilities and vibrational frequencies. In both structures both guanine moieties are in the keto tautomeric form, even though the enol tautomers are also present in the beam. One isomer exhibits nonsymmetric hydrogen bonding with HNH⋯N, NH⋯N and CO⋯HNH interactions (K9K7-2). The other isomer has a symmetrical hydrogen bond arrangement with CO⋯NH/NH⋯OC bonding (K9K7-1). The most stable guanine dimer forms CO⋯NH/NH⋯OC hydrogen bonds and has C2h symmetry (K9K9-1). Due to its strong exciton splitting the allowed S0–S2 transition is outside the investigated spectral range. We did not observe any keto–enol or enol–enol dimers in the investigated wavelength region. The calculations predict these dimers to be considerably less stable.

X-ray Diffraction Analysis of the Structure of 3-Nicotinehydroxamic Acid and Pyrazinecarbohydroxamic Acid Monohydrate

X-ray diffraction analysis of the crystal structure of 3-nicotine- and pyrazinecarbohydroxamic acids revealed that the molecules were present in keto tautomeric form and possessed Z-configuration. The system of the intermolecular hydrogen bonds is the most affected by the character of the aromatic substituent and the presence of crystallization water molecules in the structure.

P-methoxybenzaldehyde benzoylhydrazone monohydrate

The crystal structure of the title compound, C(15)H(14)N(2)O(2). H(2)O, is in the keto tautomeric form and the configuration at the azomethine C=N double bond is E. The molecule is non-planar, with a dihedral angle of 27.3 (1) degrees between the aromatic rings. The crystal structure is stabilized by extensive hydrogen bonding involving the water molecule and hydrazone moiety.

4,5-Diazafluoren-9-one benzoylhydrazone monohydrate.

The title compound, C(18)H(12)N(4)O.H(2)O, adopts the keto tautomeric form and the azomethine C=N double bond is in the E configuration. The dihedral angle between the planes of the diazafluorene moiety and the phenyl ring is 11.3 (1) degrees. In the solid state, the molecules form infinite chain-like structures via O-H.N hydrogen bonds involving the water molecules and diazafluorene moieties.

1,1'-Diacetylferrocene bis(thiosemicarbazone) monohydrate

X-ray analysis reveals that both thiosemicarbazone groups of the title compound, [Fe(C 8 H 10 N 3 S) 2 ].H 2 O, are in the keto tautomeric form and that the configuration of the azomethine C=N double bond is E. The two cyclopentadienyl rings are parallel and nearly eclipsed. The crystal structure is stabilized by extensive intra- and intermolecular hydrogen bonding involving the water molecule and the thiosemicarbazone moieties.

P-Methoxybenzaldehyde isonicotinoylhydrazone monohydrate

X-ray analysis reveals that the title compound, C 14 H 13 N 3 O 2 .H 2 O, is in keto tautomeric form and the configuration at the azomethine C=N double bond is E. The pyridine plane makes a dihedral angle of 39.9 (1)° with the plane through the central hydrazone bridge. The crystal structure is stabilized by extensive hydrogen bonding involving the water molecule and the hydrazone bridge.

O-Chlorobenzaldehyde nicotinoylhydrazone dihydrate

The title compound, C 13 H 10 ClN 3 O.2H 2 O, adopts the keto tautomeric form and the azomethine C=N double bond is in the E configuration. In the solid state, the molecules form a column-like supramolecular structure in which hydrogen bonds link the molecules to each other through water molecules.

O-Nitrobenzaldehyde Isonicotinoylhydrazone

The title compound, C13H10N4O3, is planar and exists in the keto tautomeric form. The crystal packing is stabilized by weak intermolecular hydrogen bonds involving the N3⋯N4 and C2⋯O1 atoms of neighbouring molecules.