Chelated Enol Forms Research Articles

Exploring substituents and solvent effects on the reduction potential and molecular properties of five derivatives of hydroxybenzophenone (HBP) with their possible conformations and isomers

Hydroxybenzophenone (HBP) molecules have their applications in commercial sunscreens as an UV chromophore because of their photostability and lack of reactivity after several hours of irradiation. They are usually photoexcited from the ground state chelated enol form to excited state where it undergoes proton transfer to chelated keto form and undergoes dihedral twisting to the ground state twisted keto form before going back to enol form. In this research, the three proposed isomers of five substituted 2-hydroxybenzophenones were modelled with additional three conformational changes in the ground state chelated enol and the results are related to the available experimental observations in solvents acetonitrile, n-dimethylformamide and dimethylsulfoxide. A significant difference in reduction potential, electron affinity and molecular properties was observed within the four ground state conformations and also the two excited-state proton transfer geometries for each of the five derivatives. The nature of the solvents also had significant effects on the electron affinity, HOMO, LUMO and energy bandgap of the molecules. The best possible conformation from the theoretical models that better described the experimental observations is the ground state trans chelated enol (named M1d) because of the close range of its reduction potential values and their strong level of correlation to that of the experiment and also a close range of its λmax values to that of the experiment.

Application of Hammett equation to intramolecular hydrogen bond strength in para-substituted phenyl ring of trifluorobenzoylacetone and 1-aryl-1,3-diketone malonates

The stability of two stable cis-enol forms in two categories of β-diketones, including para-substituted of trifluorobenzoylacetone (X-TFBA) and 1-aryl-1,3-diketone malonates (X-ADM, X: H, NO2, OCH3, CH3, OH, CF3, F, Cl, and NH2) has been obtained by different theoretical methods. According to our results, the energy difference between the mentioned stable chelated enol forms for the titled compounds is negligible. The theoretical equilibrium constants between the two stable cis-enol of the mentioned molecules are in excellent agreement with the reported experimental equilibrium constant. In addition, the effect of different substitutions on the intramolecular hydrogen bond strength has been evaluated. The correlation between Hammett para-substituent constants, σp. with the theoretical and experimental parameters related to the strength of hydrogen bond in p-X-TFBA and p-X-ADM molecules also investigated by means of density functional theory calculations. The electronic effects of para-substitutions on the intramolecular hydrogen bond strength were determined by NMR and IR data related to intramolecular hydrogen bond strength, geometry, natural bond orbital results, and topological parameters. These parameters were correlated with the Hammett para-substituent constants, σp. Good linear correlations between σp and the several parameters related to the hydrogen bond strength, in this study were obtained.

Evaluation the origin of conformational preferences in trifluoroacetylacetaldehyde by detail analysis of the intramolecular hydrogen bond and π-electron delocalization in the ground and first excited states

In the current research, the origin of conformational preferences in trifluoro-acetylacetaldehyde (TFAAD) by detail analysis of the intramolecular hydrogen bond (IMHB) and π-electron delocalization (π-ED) was investigated. In this regard, all of the possible conformations of keto and enol tautomers of TFAAD at various theoretical levels were optimized. Our results reveal that the chelated enol forms (E11, E21) have extra stability with respect to the other conformations, and at all of the computational levels E21 is more stable than the E11 and identifies as the global minimum. We expected that the extra stability of E21 is probably related to the stronger intramolecular hydrogen bond (IMHB), but quantum chemical calculations confirm the stronger IMHB of E11. Since, the chelated forms are resonance assisted hydrogen bond (RAHB) systems, it seems that the π-ED concept can probably justify this duality. The significance of π-ED by the geometrical factor of Gilli (λ) and the harmonic oscillator model of aromaticity (HOMA) were assessed. According to these indicators, E21 has higher π-ED than E11, in excellent agreement with their stability order. Finally, the population analysis by the natural bond orbital method and atoms in molecules theory were carried out. The stabilization charge transfer energies of RAHB systems also indicate more π- ED in E21. Consequently the π-electron delocalization effect is the superior factor determining the global minimum. The IMHB of the chelated forms, in the first singlet excited state, were also studied. Surprisingly, the results show that the IMHB in E21 is stronger than in E11, in contrast to the ground state.

A new insight into the photochemistry of avobenzone in gas phase and acetonitrile from ab initio calculations.

Avobenzone (4-tert-butyl-4'-methoxydibenzoylmethane, AB) is one of the most widely used filters in sunscreens for skin photoprotection in the UVA band. The photochemistry of AB includes keto-enol tautomerization, cis-trans isomerization, rotation about the single bond and α bond cleavages of carbonyl groups. In this contribution we study chelated and non-chelated enol, rotamers Z and E, and keto tautomers of AB in the ground and excited states in gas phase and acetonitrile by means of a coupled cluster method. Our findings suggest that torsion around the double C2-C3 bond of photoexcited chelated enol leads to internal conversion to the ground state and formation of rotamer E. In addition, opening of the chelated hydrogen ring by torsion of the hydroxyl group creates non-chelated enol. The possible mechanisms of rotamer Z formation are discussed. The solvent dependent photolability is related to the relative order of the lowest triplet ππ* and nπ* states of the keto tautomer.

Ultrafast UV-induced photoisomerization of intramolecularly H-bonded symmetric β-diketones.

In photoinduced molecular reaction dynamics, the effects of electronic charge redistribution can lead to multiple pathways that are determined by the nature of the initial structures involved and the environment the molecule of interest is studied in. The β-diketones are a common example of this complexity. They show keto-enol tautomerism that is almost totally shifted toward the enolic form. However, compared to the gas phase, the photochemistry proceeds completely differently by virtue of the solvent environment for these compounds, which are used in commercial sunscreen agents due to a high absorption in the ultraviolet (UV) and fast deactivation processes. We disclose these dynamics by investigating three symmetrical β-diketones in various solvents. To observe these effects on an ultrafast time scale directly in the UV spectral region where the relevant electronic transitions take place, we have developed and employed femtosecond transient absorption with detection capability in the deep UV. Our studies confirm that electronic excitation of the chelated enol form does not lead to any ultrafast photochemistry other than proton transfer followed by rotamerization. The formation of the nonchelated conformers takes place on a picosecond time scale through a dark state, whereas the recovery to the stable chelated enol form is a comparably slow process.

Intramolecular hydrogen bonding and vibrational assignment of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione

Intramolecular hydrogen bonding (IHB) and vibrational frequencies of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione (TFDMHD) have been investigated by means of density functional theory (DFT) calculations, Atoms in Molecules (AIM) analysis, natural bond orbital (NBO) theory, and IR and Raman spectroscopies. The results are compared with those of 1,1,1-trifluoro-2,4-pentanedione (TFAA) and 5,5-dimethyl hexane-2,4-dione (DMHD). The hydrogen bonding energies (EHB) of the most stable chelated enol forms, obtained by AIM theory are in the range of 17.1–20.0kcal/mol. The IR and Raman spectra of TFDMHD and its deuterated analog were clearly assigned. Comparing the calculated and experimental band frequencies and intensities suggests coexisting of three stable cis-enol forms in comparable proportions in the sample. The theoretical calculations and spectroscopic results indicate that the IHB strength of TFDMHD is between those of TFAA and DMHD.

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.

Conformation, molecular structure, and intramolecular hydrogen bonding of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione

Conformations, molecular structure, keto-enol content, and intramolecular hydrogen bonding of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione (also named as (Z)-6,6,6-trifluoro-5-hydroxy-2,2-dimethylhex-4-en-3-one), known as pivaloyltrifluoroacetone, have been investigated by means of the density functional theory calculations and experimental NMR, IR and Raman spectroscopies. The geometries and the electronic energies of different cis-enol conformers of this molecule have been obtained at BLYP, B3LYP, BPW91, B3PW91, G96LYP, G96PW91 and also MP2 levels of theory, using various basis sets. The results of conformational stabilities, molecular structures, NBO atomic charges, estimated energies of intramolecular hydrogen bonding, barrier height for proton transfer, and some spectroscopic properties related to the intramolecular hydrogen bonding for stable chelated enol forms of the target molecule have been compared with each other and also with those of some similar compounds. This is suggested by the theoretical studies that this β-diketone is completely in the enol form, in the best agreement with the experimental analyses done. Also, the theoretical calculations besides the 1H NMR and vibrational spectroscopic results indicate that the intramolecular hydrogen bond strength of this molecule is considerably weaker than that of 5,5-dimethyl-2,4-hexanedione, but stronger than that of 1,1,1-trifluoro-2,4-pentanedione.

Photochemistry of acetylacetone isolated in parahydrogen matrices upon 266 nm irradiation

The photochemistry of the chelated enol form of acetylacetone (AcAc) was investigated by UV excitation of the S(2) state at 266 nm in parahydrogen matrices, complemented by experiments in neon and normal hydrogen matrices. Infrared (IR) spectroscopy, combined with theoretical calculations, was used to identify the photoproducts. Isomerization towards various non-chelated forms (no intramolecular H-bond) of AcAc is the dominant channel whereas fragmentation is very minor. The isomerization kinetics is monitored by IR spectroscopy. Among the seven non-chelated conformers of AcAc, only three are formed in parahydrogen matrices, whereas four are observed in normal hydrogen matrices. This difference suggests that an active tunnelling process between conformers occurs in parahydrogen but is quenched in normal hydrogen where guest-host interactions are stronger. Fragmentation and isomerization of excited AcAc are discussed in the light of these new data. The role of the intermediate triplet state in the S(2)→ S(0) relaxation is confirmed, as the importance of phonons in the condensed phase.

Conformational stability, molecular structure, intramolecular hydrogen bonding, and vibrational spectra of 5,5-dimethylhexane-2,4-dione

Complete conformational analyses of all possible keto and enol forms, molecular structure, intramolecular hydrogen bonding (IHB), and vibrational frequencies of 5,5-dimethyl hexane-2,4-dione (DMHD, also known as acetylpinacolin and pivalylacetone), were investigated by means of ab initio calculations and IR and Raman spectroscopies. The results are compared with those of acetylacetone (AA) and 2,2,6,6-tetramethyl-3,5-heptanedione (TMHD). The energy differences between three stable E1, E2, and E3 chelated enol forms are negligible. Comparing the calculated and experimental band frequencies and intensities suggests the coexisting of these three conformers in comparable proportions in the sample. The vibrational frequencies of DMHD and its deuterated analogue were also clearly assigned. According to the theoretical calculations, the stable cis-enol conformers of DMHD have an average hydrogen bond strength of 16.6 kcal/mol, calculated at the B3LYP/6-311++G ** level, which is about 0.7 kcal/mol stronger than that of AA. This enhancement in the IHB strength is also consistent with the experimental results of the band frequency shifts of OH/OD and O⋯O stretching and OH/OD out of plane bending frequencies. The theoretical calculations and spectroscopic results indicate that the IHB strength of DMHD is between those of AA and TMHD.

Structure, intramolecular hydrogen bonding, and vibrational spectra of 2,2,6,6-tetramethyl-3,5-heptanedione

Molecular structure, intramolecular hydrogen bonding (IHB) and vibrational frequencies of 2,2,6,6-tetramethyl-3,5-heptanedione (TMHD, also known as dipivaloylmethane), have been investigated by means of the density functional theory (DFT) calculations and IR and Raman spectroscopies. In addition, the geometry of the cis - and trans -enol conformers were also optimized at the MP2/6-31G ∗∗ level of theory. The results are compared with those of acetylacetone (AA), benzoylacetone (BA), and dibenzoylmethane (DBM). The energy differences between two stable E1 and E2 chelated enol forms, calculated at different levels of theory, are negligible. Comparing the calculated and experimental band frequencies and intensities suggests coexisting of two stable cis -enol conformers in comparable proportions in the sample. Also the IR and Raman spectra of TMHD and its deuterated analogue were clearly assigned. According to the theoretical calculations, TMHD has a hydrogen bond strength of about 17.8–18.0 kcal/mol, calculated at the B3LYP/6-311++G ∗∗ level of theory, which is about 3.0 kcal/mol stronger than that of AA. This enhancement in the IHB strength is also consistent with the experimental results of the band frequency shifts for the modes of OH/OD and O ⋯ O stretching and OH/OD out-of-plane bending frequencies. The theoretical calculations and spectroscopic results indicate that the IHB strength of TMHD is between those of AA and DBM.

Hydrogen bonding in acetylacetaldehyde: Theoretical insights from the theory of atoms in molecules

AbstractAll the possible conformations of tautomeric structures (keto and enol) of acetylacetaldehyde (AAD) were fully optimized at HF, B3LYP, and MP2 levels with 6‐31G(d,p) and 6‐311++G(d,p) basis sets to determine the conformational equilibrium. Theoretical results show that two chelated enol forms have extra stability with respect to the other conformers, but identification of global minimum is very difficult. The high level ab initio calculations G2(MP2) and CBS‐QB3) also support the HF conclusion. It seems that the chelated enol forms have equal stability, and the energy gap between them is probably lies in the computational error range. Finally, the analysis of hydrogen bond in these molecules by quantum theory of atoms in molecules (AIM) and natural bond orbital (NBO) methods fairly support the ab initio results. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Vibrational assignment and structure of trifluorobenzoylacetone: A density functional theoretical study

Molecular structure and vibrational frequencies of 4,4,4-trifluoro-1-phenyl-1,3-butanedione, known as trifluorobenzoylacetone (TFBA), have been investigated by means of density functional theory (DFT) calculations. The results were compared with those of benzoylacetone (BA), acetylacetone (AA), and trifluoroacetylacetone (TFAA). Comparing the calculated and experimental band frequencies and intensities suggests coexisting of both stable cis-enol conformers in comparable proportions in the sample. The energy difference between the two stable chelated enol forms is negligible, 0.96 kcal/mol, calculated at B3LYP/6-311++G ** level of theory. The molecular stability and the hydrogen bond strength were investigated by applying the natural bond orbital (NBO) theory and geometry calculations. The theoretical calculations and spectroscopic results indicate that the hydrogen bond strength of TFBA is between those of TFAA and AA, considerably weaker than that of BA.

Vibrational assignment and structure of benzoylacetone: A density functional theoretical study

Molecular structure and vibrational frequencies of benzoylacetone (BA) have been investigated by means of density functional theory (DFT) calculations. In addition, the geometry of the cis- and trans-enol conformers were also optimized at MP2/6-31G** level of theory. The results were compared with those of acetylacetone (AA), the parent molecule. IR and Raman spectra of BA and its deuterated analogue were clearly assigned. Comparing the calculated and experimental band frequencies and intensities suggests coexisting of both stable cis enol conformers in comparable proportions in the sample. The energy difference between the two stable chelated enol forms is negligible, about 0.3 and 0.1 kcal/mol, calculated at B3LYP/6-311++G** and MP2/6-31G** levels of theory, respectively. The calculated hydrogen bond energies for these tautomers are about 16 kcal/mol, calculated at B3LYP/6-311++G** level of theory. The molecular stability and the hydrogen bond strength were investigated by applying the NBO and geometry calculations. The theoretical calculations indicate that the hydrogen bond in BA is relatively stronger than that in AA. The averaged geometrical parameters, obtained by superposition of calculated corresponding geometrical parameters for 2BA and 4BA conformers, are in good agreement with the X-ray and neutron diffraction data.

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.

Recovery mechanism of the reaction intermediate produced by photoinduced cleavage of the intramolecular hydrogen bond of dibenzoylmethane

The recovery mechanism of the reaction intermediate (non-chelated enol form) produced by photoinduced cleavage of the intramolecular hydrogen bond of dibenzoylmethane was studied in various solvents by nanosecond laser flash photolysis. The recovery rate and mechanism depend strongly on the nature of the solvent. Unimolecular recovery of the intermediate to the chelated enol form takes place in acetonitrile, diethyl ether and dimethylsulphoxide with extremely small rate constants (1.1, 1.5 and 6.6 s −1 respectively) despite the small activation energy (3.6 kcal mol −1 in 3-methylpentane). The slow unimolecular recovery rate can be ascribed to the small frequency factor (7.0 × 10 5 s −1), i.e. the large negative entropy change for the formation of the chelated enol form. In non-polar aliphatic hydrocarbon solvents, a bimolecular recovery process via hydrogen-bond interactions between two intermediate molecules is included in addition to unimolecular recovery. In alcohols, a solvent-assisted recovery process by mutual hydrogen exchange between the intermediate and alcohol molecule(s) accelerates the recovery rate. Basic catalysts, e.g. KOH in ethanol and triethylamine in acetonitrile, increase the recovery rate considerably by an additional process through the enolate anion.

Z-E isomerization of .BETA.-methoxychalcones: Preferred existence of E-isomers in naturally occurring .BETA.-methoxychalcones.

Methylation of dibenzoylmethanes with diazomethane always gave Z-β-methoxychalcones, which are the products of kinetically controlled methylation of chelated enol forms of β-hydroxychalcones. Z-β-Methoxychalcones were thermodynamically unstable and readily isomerized into more stable E-isomers on contact with silica gel, on keeping in polar solvents, or on exposure to light. The structures of both isomers of the simplest β-methoxychalcone were determined by X-ray crystal structure analysis. All β-methoxychalcones prepared were fully characterized by spectroscopic methods, revealing that the E- and Z-isomers are distinguishable in terms of ultraviolet and 13 C-nuclear magnetic resonance spectra, and nuclear Overhauser effect between the β-methoxyl group and H-8 proton. The spectral data reported for natural β-methoxychalcones, methylpongamol and praecansone A, are attributable to the E-isomers

Studies on the lowest triplet state of mono- and dicarbonyl compounds. II

The modified CNDO method of Del Bene and Jaffé has been applied to the study of the phosphorescence spectra of some carbonyl compounds, namely, benzaldehyde, acetophenone, propiophenone, and various forms of the β-diketone studied in the first paper of this series: the keto form and two chelated enol forms of benzoylacetone. The results are quite satisfactory. The phosphorescence spectra of the carbonyl compounds and the keto form of benzoylacetone has been assigned to a n → π* transitions whereas the phosphorescence spectrum of the preponderant chelated enol form of benzoylacetone has been assigned to a π → π* transition.

Studies of the Triplet State of Carbonyl Compounds. I. Phosphorescence of β-Diketones

The phosphorescence intensity of the diketone form and of one of the two chelated enol forms of β-diketones in solution at 77°K depends largely on the solvent used. The phosphorescence of the diketone form (seen principally in polar solvent) is similar to that of aromatic monoketones and arises from a Tnπ* state. The phosphorescence of one of the two chelated enol forms in nonpolar solvent is emitted from a Tπ π* state as suggested by the external and internal heavy atom effects. However the shortness of the phosphorescence lifetime shows that the lowest triplet state is probably mixed with some higher energy triplet state of nπ* type. The diketone form seems much more luminescent than the chelated enol form. This result is consistent with the suggestion of Lamola [A. A. Lamola and L. J. Sharp, J. Phys. Chem. 70, 2634 (1966)] that a very fast radiationless decay takes place in molecules possessing an intramolecular hydrogen bond between the carbonyl oxygen and the hydroxyl hydrogen. The comparison of chelated enol spectra with those of crotonophenone and benzalacetone indicates that phosphorescent chelated enol has structure II, the other form being nonradiatively deactivated or nonexistent. Benzoyltrifluoroacetone gives a spectrum with a long lifetime which seems to originate from a nonchelated enol form.

Steric Influences on the Structure of Chelating Agents and on the Stability of Metal Chelates

2-PHENACYLPYRIDINE and its derivatives are of interest because of their possible bacteriostatic activity1 and their power to chelate metals. In addition, it has been shown2 that in the solid state they exist, with accountable exceptions, in the resonance-stabilized conjugate chelated enol form (I) rather than in the keto form (II). The purpose of this communication is to record some results of a study of their infra-red spectra, and to point out that they provide interesting examples of the influence of steric hindrance on molecular structure and on the stability of metal chelates.