Stable Enol Form Research Articles

Hidden Isomer of Trifluoroacetylacetone Revealed by Matrix Isolation Infrared and Raman Spectroscopy.

Enol forms of trifluoroacetylacetone (TFacac) isolated in molecular and rare gas matrices were studied using infrared (IR) and Raman spectroscopy. Additionally, calculations using DFT B3LYP and M06-2X as well as MP2 methods were performed in order to investigate the possibility of coexistence of more than one stable enol form isomer of TFacac. Calculations predict that both stable enol isomers of TFacac, 1,1,1-trifluoro-4-hydroxy-3-penten-2-one (1) and 5,5,5-trifluoro-4-hydroxy-3-penten-2-one (2), could coexist, especially in matrices where the room temperature population is frozen, 1 being the most stable one. Raman and IR spectra of TFacac isolated in nitrogen (N2) and carbon monoxide (CO) matrices exhibit clear absorption bands, which cannot be attributed to this single isomer. Their relative band positions and intensity profiles match well with the theoretical calculations of 2. This allows us to confirm that in N2 and CO matrices both isomers exist in similar amounts. Careful examination of the spectra of TFacac in argon, xenon, neon, normal, and para-hydrogen (Ar, Xe, Ne, nH2, and pH2 respectively) matrices revealed that both isomers coexist in all the explored matrices, whereas 2 was not considered in the previous spectroscopic works. The amount of the second isomer (2) in the as-deposited samples depends on the host. The analysis of TFacac spectra in the different hosts and under various experimental conditions allows the vibrational characterization of both chelated isomers. The comparison with theoretical predictions is also investigated.

Selective photoisomerisation of 2-chloromalonaldehyde.

Isomerization of 2-chloromalonaldehyde (2-ClMA) is explored giving access to new experimental data on this derivative of malonaldehyde, not yet studied much. Experiments were performed isolating 2-ClMA in argon, neon, and para-hydrogen matrices. UV irradiation of the matrix samples induced isomerization to three open enolic forms including two previously observed along with the closed enolic form after deposition. IR spectra of these specific conformers were recorded, and a clear assignment of the observed bands was obtained with the assistance of theoretical calculations. UV spectra of the samples were measured, showing a blue shift of the π* ← π absorption with the opening of the internal hydrogen bond of the most stable enol form. Specific sequences of UV irradiation at different wavelengths allowed us to obtain samples containing only one enol conformer. The formation of conformers is discussed. The observed selectivity of the process among the enol forms is analyzed.

Elucidating the Catalytic Role of Mg(II) in the Intramolecular Proton Transfer Reaction in Thymine

In the framework of the reaction force analysis a study of the mechanism of the 1-3 intramolecular proton transfer in free and Mg (II) coordinated Thymine was done in terms of the reaction electronic flux and Wiberg bond orders. Profiles of these properties evidentiated differences between the proton transfer mechanisms induced by the presence of Mg(II) cation. A significative lowering in the activation energy put forward the catalytic effect of Mg. The reaction force analysis allows a precise identification of the catalytic effect thus uncovering the physical nature of activation energies. While in the free Thymine electronic polarization and transfer processes are present separately, in the Mg(II) coordinated Thymine both effects are observed simultaneously and are localized on the ring in the molecular topology. It is argued that the difference in the charge transfer mechanism leads to a more stable enol form in the Mg(II) coordinated Thymine.

Conformational analysis, intramolecular hydrogen bonding, and vibrational assignment of 4,4-dimethyl-1-phenylpentane-1,3-dione

Molecular structure, conformational stabilities, and intramolecular hydrogen bonding (IHB) of 4,4-dimethyl-1-phenylpentane-1,3-dione (DMPD), have been investigated by means of density functional theory (DFT) calculations and experimental results. The geometries and electronic energies of different cis-enol forms of DMPD have been obtained with the ab initio (MP2 level) and DFT (B3LYP and TPSSh levels) methods, using various basis sets. The energy differences between three stable E1, E2, and E3 chelated enol forms are negligible. According to the theoretical calculations, DMPD has a hydrogen bond strength of about 16.8kcal/mol, calculated at the B3LYP/6-311++G** level, which is about 0.7kcal/mol stronger than that of benzoylacetone (BA). The theoretical and experimental results obtained for stable enol forms of DMPD have been compared with each other and also with those of BA and 5,5-dimethylhexane-2,4-dione (DMHD). The molecular stability and the hydrogen bond strength were investigated by applying the NBO, topological analysis, geometry calculations, and spectroscopic results.

Conformational analysis, tautomerization, IR, Raman, and NMR studies of ethyl benzoylacetate

Abstract A complete conformational analysis of the keto and enol forms of ethyl benzoylacetate (EBA), a β-ketoester, was carried out by ab initio calculations, at the density functional theory (DFT) level. The relative stabilities of cis-enol and keto forms were calculated in the gas phase and in solution. The intramolecular hydrogen bond characters of the most stable enol forms of EBA are discussed and compared with those of benzoylacetone (BA). Harmonic vibrational frequencies and 1 H and 13 C NMR chemical shifts of the most stable enol and keto forms were also calculated at the B3LYP/6-311++G ** level and compared with the experimental data.

Nonlinear Optical Switching Behavior in the Solid State: A Theoretical Investigation on Anils

The linear (χ(1)) and second-order nonlinear (χ(2)) optical properties of two anil crystals, [N-(4-hydroxy)-salicylidene-amino-4-(methylbenzoate) and N-(3,5-di-tert-butylsalicylidene)-4-aminopyridine, denoted 4A and 4P, respectively], as well as the optical contrasts upon switching between their enol (E) and keto (K) forms, have been investigated by combining the molecular responses calculated using quantum chemistry methods and an electrostatic interaction scheme to account for the local field effects. It is found that intermolecular interactions impact differently the K/E optical contrasts in the two systems, which illustrates the importance of the supramolecular organization on the macroscopic responses. In 4A, the surrounding effects on the (hyper)polarizabilities are similar in the enol and keto forms, leading to optical contrasts very close to those of the isolated molecule. In contrast, an enhancement of the second-order susceptibility is observed in the keto form of 4P, leading to a large χ(2)(K)/χ(2)(E) contrast. Moreover, the χ(2)(4A)/χ(2)(4P) ratio for the most stable enol forms is obtained to be in good agreement with previous experimental investigations, which supports the reliability of the computational procedure.

Photoinduced disruption of the strong intramolecular H-bond in the enol form of acetylacetone: Mechanisms of radiationless decay

We propose mechanisms which describe the photoinduced disruption of the intramolecular H-bond in the stable enol form of acetylacetone. The study was performed with the second-order approximate coupled cluster singles and doubles model CC2. The basis set aug-cc-pVDZ was applied. The excited-state reaction paths of the rotations around C C, C C and C O bonds were investigated using both coordinate-driven (relaxed scan) and linear interpolation in internal coordinates (LIIC) approaches. We show that the 1ππ ∗ excited states of the rotations around the C C double bond relax in, a completely barrierless manner, to the ground state via S 0–S 1 conical intersections. This is the main relaxation channel of the excited states associated with the disruption of the strong H-bond in the enol form of acetylacetone. The C C and C O rotations did not show such mechanisms.

Tautomerism of the natural thymidine nucleoside and the antiviral analogue D4T. Structure and influence of an aqueous environment using MP2 and DFT methods

A comparative theoretical conformational analysis of the tautomerism of the four most stable conformers of the antiviral analogue D4T (stavudine) and natural thymidine (Thy) nucleosides was carried out, by using the B3LYP and MP2 quantum chemical methods. The calculated structure data and energy values of the tautomers were compared with the T1 keto form and with the results determined for the thymine molecule. For each conformer, only two stable enol forms, T3 and T5, were obtained when considering different positions of hydrogen around the base. Tautomer T3 always appears more stable than T5, and more stable in the natural nucleoside Thy than in thymine or in D4T.The effect of water on the tautomers was estimated by using an explicit number of up to five water molecules to surround the nucleoside, and also by Tomasi's polarized continuum model (PCM). A total of about 200 clusters were optimised and the geometrical parameters and energies discussed. The water net differs in the three tautomers of D4T and Thy. Depending on the nature of the tautomers, cyclic, distributed water molecules, or clustered structures are formed. The deformation and interaction counterpoise (CP)-corrected energies between the nucleoside and water molecules were determined. The relative stabilities of all tautomers were established. The microhydrated environment stabilizes remarkably the enol forms more than the canonical keto one, although this one continues being the most stable. The changes in the intramolecular H-bondings and in the total atomic charges were discussed. Intramolecular H-bonds being stronger in D4T than in Thy could indicate a higher molecular flexibility for Thy.

Luminescence and Raman Spectra of Acetylacetone at Low Temperatures

Raman spectra of acetylacetone were recorded for molecules isolated in an argon matrix at 10 K and for a polycrystalline sample. In the solid sample, broad bands appear superimposed on a much weaker Raman spectrum corresponding mainly to the stable enol form. The position of these bands depends on the excitation wavelength (514.5 and 488.8 nm argon ion laser lines were used), sample temperature, and cooling history. They are attributed to transitions from an excited electronic state to various isomer states in the ground electronic state. Laser photons have energies comparable to energies of a number of excited triplet states predicted for a free acetylacetone molecule (Chen, X.-B.; Fang, W.-H.; Phillips, D. L. J. Phys. Chem. A 2006, 110, 4434). Since singlet-to-triplet photon absorption transitions are forbidden, states existing in the solid have mixed singlet/triplet character. Their decay results in population of different isomer states, which except for the lowest isomers SYN enol, TS2 enol (described in Matanović I.; Doslić, N. J. Phys. Chem. A 2005, 109, 4185), and the keto form, which can be detected in the Raman spectra of the solid, are not vibrationally resolved. Differential scanning calorimetry detected two signals upon cooling of acetylacetone, one at 229 K and one at 217 K, while upon heating, they appear at 254 and 225 K. The phase change at higher temperature is attributed to a freezing/melting transition, while the one at lower temperature seems to correspond to freezing/melting of keto domains, as suggested by Johnson et al. (Johnson, M. R.; Jones, N. H.; Geis, A; Horsewill. A. J.; Trommsdorff, H. P. J. Chem. Phys. 2002, 116, 5694). Using matrix isolation in argon, the vibrational spectrum of acetylacetone at 10 K was recorded. Strong bands at 1602 and 1629 cm(-1) are assigned as the SYN enol bands, while a weaker underlying band at 1687 cm(-1) and a medium shoulder at 1617 cm(-1) are assigned as TS2 enol bands.

WITHDRAWN: Spectroscopic studies of stable enol form of barbituric acid

WITHDRAWN: Spectroscopic studies of stable enol form of barbituric acid

Generation, Isolation, and Characterization of a Stable Enol from Grignard Addition to a Bis-Ester. A Microscale Experiment for the Undergraduate Organic Chemistry Laboratory

A microscale experiment for the undergraduate organic chemistry laboratory that consists of preparing and characterizing an α-ketoester and its corresponding, remarkably stable enol form, has been developed. The reaction is that of a Grignard reagent with a bis-ester. A difference in reaction temperature is responsible for the selective generation of the α-ketoester and the enol ester. Analysis of spectral data (1H NMR and IR) and a knowledge of organic reactions allows the students to determine the detailed structure of the two reaction products and also to suggest a mechanism for their formation. This experiment introduces students to the concept of stability of the tetrahedral intermediate in acyl-transfer reactions. It also gives them a taste of the unexpected.

The Decarbonylation of the Acetamide Radical Cation and the Enolization of its Dimer by Self-Catalysis

The acetamide radical cation, CH3C(=O)NH2•+, and its enol, CH2=C(OH)NH2•+, undergo several unimolecular reactions in the μs time-frame of which decarbonylation is predominant. This reaction produces the ylid ion CH2NH3•+, rather than CH3NH3•+ [ J. Am. Chem. Soc. 109, 4819 (1987)]. A previously proposed mechanism via ion–dipole complexes is confirmed by the present CBS-QB3 calculations. These calculations reveal the existence of a second mechanism which proceeds via the enol ion and the distonic ion •CH2C(=O)NH3+. Both mechanisms can account for previously reported isotopic labeling experiments. Tandem mass spectrometry based experiments do not provide evidence that the non-decomposing acetamide ions rearrange to any significant extent to the more stable enol form. However, this transformation occurs smoothly by interaction with a neutral acetamide molecule (“self-catalysis”). By integration of experimental data (MS/MS/MS and labeling experiments) and ab initio calculations [CBS-Q (RHF/DZP)] three mechanisms for this assisted tautomerization have been traced. In the first mechanism the neutral acetamide component of the dimer ion accepts a C–H proton from its ionic partner and then donates the proton back to the oxygen atom. This is an example of “proton-transport catalysis”. In the second mechanism, isomerization takes place within the ionic partner via a conventional 1,3-H shift. The neutral partner serves only to lower the energy of the transition state by ion–dipole attractions. This is an example of the “Spectator” mechanism. In the third mechanism, proton transfer from the ionic partner to its neutral counterpart is followed by back-donation of a hydrogen atom. This is an example of the “Quid-pro-Quo” mechanism. The behavior of the acetamide dimer ion is compared to that of the acetone dimer ion which undergoes only proton-transport catalysis.

Molecular structure and vibrational assignment of dimethyl oxaloacetate

A complete conformational analysis of the keto and chelated enol forms of dimethyl oxaloacetate (DMOA), a β-dicarbonyl compound, was carried out by ab initio calculations, at the density functional theory (DFT) level. In addition to nine stable enol conformers, which are stabilized by intramolecular hydrogen bonds, twelve stable keto conformers were also obtained. The considerably higher energy of the keto compared to that of the most stable enol conformer makes the presence of keto form, at least in the gas phase, unlikely. Theoretical calculations in the solution, using the Onsager Method, suggest two coexisting enol conformers in the solution. This finding is in agreement with the experimental data. The hydrogen bond strength of the most stable conformer of DMOA is compared with that of acetylacetone (AA). Harmonic vibrational frequencies of this stable enol form and its deuterated analog were also calculated and compared with the experimental data. According to the theoretical calculations, the enolated proton in dimethyl oxaloacetate moves in an asymmetric single minimum potential with a hydrogen bond strength of 31.1 kJ/mol, 35.3 kJ/mol less than that of AA. This weakening of hydrogen bond is consistent with the frequency shifts for OH/OD stretching, OH/OD out-of-plane bending and O⋯O stretching modes. The calculated O ⋯O distance is about 0.07–0.08 Å longer than that of its parent AA.

Flash-photolysis of 2-(2′-hydroxyphenyl)-3-H-indole. Ground-state keto-enol tautomerization by mutual hydrogen exchange and by proton catalysis

Efficient excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxy-5′-methylphenyl)-3,3-dimethyl-3H-indole (HBC) leads to the formation of an unstable trans-keto tautomer 1Ktr whose transient absorption can be observed in flash experiments. The assignment of this transient absorption to the trans-keto tautomer is corroborated by the observation that a similar transient is not formed from BHBC, which has been synthesized as a bridged, rotationally blocked counterpart of HBC. In hydrocarbon solvents, two 1Ktr molecules revert by a mutual hydrogen-exchange reaction to the stable enol form of HBC. This second-order reaction is diffusion-controlled and its rate constant is proportional to T/η over a wide range of the solvent viscosity η. In the presence of alcohols (ROH) a first-order, pseudo-monomolecular proton-catalysed re-enolization reaction competes with the second-order reaction. With methanol and ethanol as proton donors it is shown that two donor molecules are involved in this process. The rate of the proton-catalysed reaction increases with decreasing temperatures. A pre-equilibrium between a nonreactive 1Ktr and a reactive 1Ktr::: (ROH)2 complex is the reason for this unusual temperature dependence of the first-order 1Ktr decay rate.

ChemInform Abstract: 5‐HYDROXY‐7B‐METHYL‐7BH‐CYCLOPENT(CD)INDENE, A STABLE ANNULENOL

AbstractAnnulenole (IVa), (IX) und (XII) werden auf den im Formelschema skizzierten Wegen dargestellt.

5-Hydroxy-7b- methyl-7bH-cyclopent[cd]indene, a stable annulenol

5-Hydroxy-7b-methyl-7bH-cyclopent[cd]indene (1), the diester (6), and dialdehyde (11a) exist in the stable enol form and are thus the first isolable higher annulenols.

Reaction of dimethylphosphorous acid with some ?, ?-unsaturated ketones

1. Depending on the amount of sodium methylate present, dimethylphosphorous acid adds to furfuralacetone and to the benzylidene derivatives of cyclopentanone and cyclohexanone either at the C=O group to give α-hydroxyphosphonates or to the C=C bond to giveΒ-ketophosphonates. Diethylamine, and in the case of benzalcyclopentanone and benzalcyclohexanone also triethylamine, facilitate the formation of the α-hydroxyphosphonate. 2. The reaction of dimethylphosphorous acid with benzalacetophenone, benzal-α-tetralone, and 3,3-diphenyl-2-benzalhydrindone was studied. The presence of an aromatic ring α to the C=O group in the benzylidene derivatives of ketones hinders the formation of the α-hydroxyphosphonates and the reaction goes by the 1,4-addition scheme to give either theΒ-ketophosphonates or the stable enol forms.