Excited state properties of 7-hydroxy-4-methylcoumarin in the gas phase and in solution. A theoretical study

TDDFT/B3LYP and RI-CC2 calculations with different basis sets have been performed for vertical and adiabatic excitations and emission properties of the lowest singlet states for the neutral (enol and keto), protonated and deprotonated forms of 7-hydroxy-4-methylcoumarin (7H4MC) in the gas phase and in solution. The effect of 7H4MC-solvent (water) interactions on the lowest excited and fluorescence states were computed using the Polarizable Continuum Method (PCM), 7H4MC-water clusters and a combination of both approaches. The calculations revealed that in aqueous solution the pi pi* energy is the lowest one for excitation and fluorescence transitions of all forms of 7H4MC studied. The calculated excitation and fluorescence energies in aqueous solution are in good agreement with experiment. It was found that, depending on the polarity of the medium, the solvent shifts vary, leading to a change in the character of the lowest excitation and fluorescence transition. The dipole-moment and electron-density changes of the excited states relative to the ground state correlate with the solvation effect on the singlet excited states and on transition energies, respectively. The calculations show that, in contrast to the ground state, the keto form has a lower energy in the pi pi* state as compared to enol, demonstrating from this point of view the energetic possibility of proton transfer from the enol to the keto form in the excited state.     

The Richest Collection of Tautomeric Polymorphs: The Case of 2‐Thiobarbituric Acid

Five new polymorphs and one hydrated form of 2‐thiobarbituric acid have been isolated and characterised by solid‐state methods. In both the crystalline form II and in the hydrate form, the 2‐thiobarbituric molecules are present in the enol form, whereas only the keto isomer is present in crystalline forms I (reported in 1967 by Calas and Martinex), III , V and VI . In form IV , on the other hand, a 50:50 ordered mixture of enol/keto molecules is present. All new forms have been characterised by single‐crystal X‐ray diffraction, 1D and 2D (¹H, ¹³C, and ¹⁵N) solid‐state NMR spectroscopy, Raman spectroscopy and X‐ray powder diffraction at variable temperature. It has been possible to induce keto–enol conversion between the forms by mechanical methods. The role of hydrogen‐bond interactions in determining the relative stability of the polymorphs and as a driving force in the conversions has been ascertained. To the best of the authors’ knowledge, the 2‐thiobarbituric family of crystal forms represents the richest collection of examples of tautomeric polymorphism so far reported in the literature.     

DFT study on the isomerization in vitamin B6

Isomerization and tautomerism reactions of the active form of vitamin B6, pyridoxal phosphate, are studied at B3LYP level of theory using 6-311++G(2df,p) basis set in gas and aqueous phases. Twenty-three transition state (TS) structures for vitamin B6 isomerization are optimized, including 13 TS structures for O–H and C–C rotations, 8 TS structures for imine–enamine tautomerism, and 2 TS structures for keto–enol tautomerism. Activation energy (E _a), imaginary frequency (υ), and Gibbs free energy of activation (ΔG ^#) for the isomerization reactions are calculated. The activation energies of the imine–enamine tautomerism are in the range of 190–280 kJ/mol and of O–H and C–C rotations are mainly less than 60 kJ/mol. Also, our calculation shows that the imine forms of B6 are mainly more stable than the enamine forms. Effect of microhydration on the TS structures and activation energies is also investigated. It is found that the presence of water molecules catalyzes only the imine–enamine tautomerism.     

Keto-enol and enol-enol tautomerism in trifluoromethyl-β-diketones

The keto-enol (K?E) and enol-enol (E?E) equilibria of a variety of trifluoromethyl-β-diketones were investigated using ¹H, ¹³C, ¹⁹F NMR spectroscopy, infrared spectroscopy and ultraviolet-visible spectrophotometry in nonpolar solvents. In general, NMR, IR and UV spectral evidence indicates that trifluoromethyl-β-diketones exist as mixtures of two chelated cis-enol forms in nonpolar media. Infrared spectroscopy and ultraviolet spectrophotometry show the E?E equilibrium lies in the direction of the enol form which maximizes conjugation in most cases. Exceptions are noted and discussed.     

An ab initio study of excited states of guanine in the gas phase and aqueous media: Electronic transitions and mechanism of spectral oscillations

Ground state geometries of the four tautomeric forms keto‐N9H, keto‐N7H, enol‐N9H, and enol‐N7H of guanine were optimized in the gas phase at the RHF level using a mixed basis set consisting of the 4‐31G basis set for all the atoms except the nitrogen atom of the amino group for which the 6‐311+G* basis set was used. These calculations were also extended to hydrogen‐bonded complexes of three water molecules with each of the keto‐N9H (G9‐3W) and keto‐N7H (G7‐3W) forms of guanine. Relative stabilities of the four above‐mentioned tautomers of guanine as well as those of G9‐3W and G7‐3W complexes in the ground state in the gas phase were studied employing the MP2 correlation correction. In aqueous solution, relative stabilities of these systems were studied using the MP2 correlation correction and polarized continuum model (PCM) or the isodensity surface polarized continuum model (IPCM) of the self‐consistent reaction field (SCRF) theory. Geometry optimization in the gas phase at the RHF level using the 6‐31+G* basis set for all atoms and the solvation calculations in water at the MP2 level using the same basis set were also carried out for the nonplanar keto‐N9H and keto‐N7H forms of guanine. Thus, it is shown that among the different tautomers of guanine, the keto‐N7H form is most stable in the gas phase, while the keto‐N9H form is most stable in aqueous solution. It appears that both the keto‐N9H and keto‐N7H forms of guanine would be present in the ground state, particularly near the aqueous solution–air interface. Vertical excitation and excited state geometry optimization calculations were performed using configuration interaction involving single electron excitation (CIS). It is found that the absorption spectrum of guanine would arise mainly due to its keto‐N9H form but the keto‐N7H form of the same would also make some contribution to it. The enol‐N9H and enol‐N7H forms of the molecule are not expected to occur in appreciable abundance in the gas phase or aqueous media. The normal fluorescence spectrum of guanine in aqueous solution with a peak near 332 nm seems to originate from the lowest singlet excited state of the keto‐N7H form of the molecule while the fluorescence of oxygen‐rich aqueous solutions of guanine with a peak near 450 nm appears to originate from the lowest singlet excited state of the keto‐N9H form of the molecule. The origin of the slow damped spectral oscillation observed in the absorption spectrum of guanine has been explained. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 826–846, 2000     

An indo investigation of the structure and bonding in malondialdehyde and lithium malondialdehyde

The geometries of the enol forms of malondialdehyde and its lithium analog have been investigated using the INDO MO method in its original parametrization. The former molecule is predicted to have a planar, symmetric structure nearly identical with that of acetylacetone. Optimization of the geometry of lithium malondialdehyde yielded questionable results. The covalent bonding in both molecules is discussed with the aid of localized molecular orbitais and calculated interference energies.     

Theoretical study on keto–enol tautomerism and isomerization in pyruvic acid

Isomerization and tautomerism of 12 isomers of pyruvic acid including 4 keto and 8 enol forms were studied at the MP2 and B3LYP levels of theory using 6‐311++G(2df,p) basis set, separately. Activation energy (E_a), imaginary frequency (υ), and Gibbs free energy (ΔG^#) of the considered isomerization and tautomerism reactions were calculated. Interconversion of the enol forms proceeds through two paths: (i) proton transfer and (ii) internal rotation. Activation energies for the proton transfer paths were in the range of 125–145 kJ/mol and for the internal rotation paths were in the range of 5–45 kJ/mol. Keto–enol tautomerism of pyruvic acid proceeds only through proton transfer route and their activation energies were in the range of 200–300 kJ/mol. Effect of microhydration on the transition state structures and activation energies was also investigated. It was found that the presence of a water molecule catalyzes the isomerization and tautomerism reactions of pyruvic acid so that the activation energies decrease. © 2013 Wiley Periodicals, Inc.     

Features of synthesis and structure of ethyl (2<Emphasis Type="Italic">Z</Emphasis>)-3-hydroxy-(2,3,4,5-tetrafluorophenyl)-propyl-2-enoate

The structure of key intermediates in the synthesis of fluoroquinolone antibiotics: diethyl (2,3,4,5-tetrafluorobenzoyl)malonate and ethyl (2Z)-3-hydroxy-(2,3,4,5-tetrafluorophenyl)prop-2-enoate was for first time studied using X-ray diffraction (XRD) and ¹H, ¹⁹F NMR spectroscopy. In solution both the esters were shown to exist as a mixture of enol and ketone tautomeric forms with predominance of the latter. According to the XRD analysis, ethyl (2Z)-3-hydroxy-(2,3,4,5-tetrafluorophenyl)prop-2-enoate in the solid state exists entirely in the enol form.     

Solid‐State NMR Study of the Tautomerism of Acetylacetone Included in a Host Matrix

The tautomerism of the enol form of acetylacetone (=pentane‐2,4‐dione; 1 ) inside a host cavity has been studied by means of solid‐state ¹³C‐NMR spectroscopy (SSNMR) using the variable‐temperature CPMAS technique. It appears that the enol form, 4‐hydroxypent‐3‐en‐2‐one ( 1a ), exists in an equilibrium with an identical tautomer ( 1c ) trough O H ⋅⋅⋅O proton transfer. The experimental results (energy barrier and chemical shifts) were rationalized by means of MP2 and GIAO calculations.     

β-Diketones bearing electron-donating chromophores and a novel β-triketone: synthesis and reversible fluorescence behavior

Saturated and unsaturated β-diketones bearing 4-N,N-dimethylaminophenyl substituent and a novel β-triketone were synthesized. These β-diketones exist in both cis-enol and keto forms in solution, and their relative contents were determined by ¹H NMR measurements. In contrast, for the β-triketone, only enol form was observed in a solution. A strong fluorescence quenching was observed for unsaturated diketones as compared with the saturated ones. These β-diketones displayed reversible photoinduced ketonization as revealed by UV-vis spectroscopy. The fluorescence emission of the saturated β-diketones faded gradually during UV irradiation. The fluorescence emission of these saturated β-diketones can thus be reversibly switched between the enol form ‘ON’ and the keto form ‘OFF’ based on keto-enol tautomerization.     

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 Matanovi? I.; Dosli?, 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.     

Desmotropy,Polymorphism, and Solid‐State Proton Transfer: Four Solid Forms of an Aromatic o‐Hydroxy Schiff Base

The Schiff base derived from salicylaldehyde and 2‐amino‐3‐hydroxypyridine affords a diversity of solid forms, two polymorphic pairs of the enol‐imino ( D1 a and D1 b ) and keto‐amino ( D2 a and D2 b ) desmotropes. The isolated phases, identified by IR spectroscopy, X‐ray crystallography, and ¹³C cross‐polarization/magnetic angle spinning (CP/MAS) NMR spectroscopy, display essentially planar molecular conformations characterized by strong intramolecular hydrogen bonds of the O? H???N ( D1 ) or N? H???O ( D2 ) type. A change in the position of the proton within this O???H???N system is accompanied by substantially different molecular conformations and, subsequently, by divergent supramolecular architectures. The appearance and interconversion conditions for each of the four phases have been established on the basis of a number of solution and solvent‐free experiments, and evaluated against the results of computational studies. Solid phases readily convert into the most stable form ( D1 a ) upon exposure to methanol vapor, heating, or by mechanical treatment, and these transformations are accompanied by a change in the color of the sample. The course of thermally induced transformations has been monitored in detail by means of temperature‐resolved powder X‐ray diffraction and infrared spectroscopy. Upon dissolution, all forms equilibrate immediately, as confirmed by NMR and UV/Vis spectroscopy in several solvents, with the equilibrium shifted far towards the enol tautomer. This study reveals the significance of peripheral groups in the stabilization of metastable tautomers in the solid state.     

Tautomerism of bis(2,4-benzyloxy)-6-(5H)-one-1,3,5-triazine: A combined crystallographic and quantum-chemical investigation

In order to study the keto and enol forms of cyanuric acid derivatives in the solid state, we have synthesized bis(2,4-benzyloxy)-6-(5H)-one-1,3,5-triazine, 1. Computational investigations indicate that keto form is more stable than enol form in both gas phase and solution phase by 9.69–11.18 kcal mol⁻¹ IR and crystallographic analysis shows that 1 exists in keto form in the solid state also. To obtain the enol form in the solid state, we adopted co-crystallization with an organic base. The crystal structures of both keto/amine and the enol/imine forms (in a co-crystal) are reported. IICT Communication No. 2505/CMM-0022.     

Nickel(II) meso‐Hydroxyporphyrin Complexes Revisited: Palladium‐Catalysed Synthesis,Electronic Structures of Derived Oxy Radicals,and Oxidative Coupling to a Dioxoporphodimethene Dyad

We report the synthesis and characterisation of new examples of meso‐hydroxynickel(II) porphyrins with 5,15‐diphenyl and 10‐phenyl‐5,15‐diphenyl/diaryl substitution. The OH group was introduced by using carbonate or hydroxide as nucleophile by using palladium/phosphine catalysis. The NiPor?OHs exist in solution in equilibrium with the corresponding oxy radicals NiPor?O^.. The 15‐phenyl group stabilises the radicals, so that the ¹H NMR spectra of {NiPor?OH} are extremely broad due to chemical exchange with the paramagnetic species. The radical concentration for the diphenylporphyrin analogue is only 1 %, and its NMR line‐broadening was able to be studied by variable‐temperature NMR spectroscopy. The EPR signals of NiPor?O^. are consistent with somewhat delocalised porphyrinyloxy radicals, and the spin distributions calculated by using density functional theory match the EPR and NMR spectroscopic observations. Nickel(II) meso‐hydroxy‐10,20‐diphenylporphyrin was oxidatively coupled to a dioxo‐terminated porphodimethene dyad, the strongly red‐shifted electronic spectrum of which was successfully modelled by using time‐dependent DFT calculations.     

A theoretical study on tautomeric structures of 4′-nitroazobenzene-2,4-diol and 4-methyl-4′-nitroazobenzene-2,6-diol compounds

The goal of this study is to determine the most stable tautomeric forms, and their ground state conformers of 4′-nitroazobenzene-2,4-diol and 4-methyl-4′-nitroazobenzene-2,6-diol compounds. The calculations have shown that the most stable tautomeric forms of the compounds are hydrazo form for 4′-nitroazobenzene-2,4-diol and azo form for 4-methyl-4′-nitroazobenzene-2,6-diol. Besides, the vibrational frequencies, ¹H and ¹³C NMR shifts, frontier molecular orbital’s energies for the tautomeric forms of the compounds calculated by using density functional theory-B3LYP method with 6-311G(d) basis set were interpreted. All the assignments of the theoretical frequencies were identified by potential energy distribution (PED) analysis. Generally, theoretical spectral results were seen to be in a good agreement with the corresponding experimental data.     

Building blocks for the molecular expression of quantum cellular automata. Isolation and characterization of a covalently bonded square array of two ferrocenium and two ferrocene complexes

The suitability of [{(eta5-C5H5)Fe(eta5-C5H4)}4(eta4-C4)Co(eta5-C5H5)][PF6]2, [1][PF6]2, for use as a molecular quantum cellular automata (QCA) cell is demonstrated. To this end the structure of 1 in the solid state and the conversion of 1 to mono- and dicationic mixed-valence complexes have been accomplished. The latter compounds have been isolated as pure materials and characterized by IR, EPR, and M?ssbauer spectroscopies and single-crystal XRD (monocation only) and magnetic susceptibility measurements. Near-IR spectra demonstrate the mixed valence character of the cations (valence trapped on the IR, EPR and M?ssbauer time scales), and the energies of the intervalence charge-transfer bands provide a measure of the hole hopping frequency.     

一种苯并噻唑类稳定烯醇的合成及晶体结构

以2-甲基苯并噻唑和1,3-丙磺酸内酯为起始原料,合成了一种菁染料中间体2-(2-乙氧基-1-丁烯基)-1-(3-磺酸丙基)苯并噻唑(3).该染料中间体极易水解,对其水解产物(4e)采用红外光谱、核磁共振、质谱、元素分析和X射线单晶衍射分析等方法进行了表征.MS结果显示水解产物4e的分子量为327;在13C NMR谱图中,烯醇结构中与羟基相连的碳的化学位移为194.05,与之相连的另一个双键碳的化学位移为δ92.52;在IR谱图中,1618 cm-1处存在烯醇的特征吸收峰;通过X射线单晶衍射分析测定了水解产物的结构,进一步证实该水解产物是一种存在共轭芳基结构的稳定烯醇.     

13C-labeled N-acetyl-neuraminic acid in aqueous solution: detection and quantification of acyclic keto, keto hydrate, and enol forms by 13C NMR spectroscopy

Aqueous solutions of N-acetyl-neuraminic acid (Neu5Ac, 1) labeled with (13)C at C1, C2, and/or C3 were analyzed by (13)C NMR spectroscopy to detect and quantify the acyclic forms (keto, keto hydrate, enol) present at varying pHs. In addition to pyranoses, solutions contained the keto form, based on the detection of C2 signals at approximately 198 ppm (approximately 0.7% at pH 2). Spectra of [2-(13)C] and [3-(13)C] isotopomers contained signals arising from labeled carbons at approximately 143 and approximately 120 ppm, respectively, which were attributed to enol forms. Solution studies of [1,2,3-(13)C3] 1 substantiated the presence of enol (approximately 0.5% at pH 2). Enol was not detected at pH > 6.0. A C2 signal observed at approximately 94 ppm was identified as C2 of the keto hydrate (approximately 1.9% at pH 2), based partly on its abundance as a function of solution pH. Density functional theory (DFT) calculations were used to study the effect of enol and hydrate structure on J(CH) and J(CC) values involving C2 and C3 of these forms. Solvated DFT calculations showed that (2)J(C2,H3) in cis and trans enols have similar magnitudes but opposite signs, making this J-coupling potentially useful to distinguish enol configurations. Solvent deuterium exchange studies of 1 showed rapid incorporation of (2)H from (2)H2O at H3 axial in the pyranoses at p(2)H 8.0, followed by slower exchange at H3 equatorial. The acyclic keto form, which presumably participates in this reaction, must assume a pseudo-cyclic conformation in solution in order to account for the exchange selectivity. Weak (13)C signals arising from labeled species were also observed consistently and reproducibly in aqueous solutions of (13)C-labeled 1, possibly arising from products of lactonization or intermolecular esterification.     

Resonance Assisted Hydrogen Bonding Phenomenon Unveiled through Both Experiments and Theory: A New Family of Ethyl N-Salicylideneglycinate Dyes

Extensive experimental and theoretical investigations are reported on the nature of resonance-assisted hydrogen bonding phenomenon (RAHB) and its influence on photophysical properties of the newly designed dyes differing in donor–acceptor properties, namely ethyl N-salicylideneglycinate ( 1 ), ethyl N-(5-methoxysalicylidene)glycinate ( 2 ), ethyl N-(5-bromosalicylidene)glycinate ( 3 ) and ethyl N-(5-nitrosalicylidene)glycinate ( 4 ). All compounds are thermochromic in the solid state and they contain a typical intramolecular O−H⋅⋅⋅N hydrogen bond formed between the hydroxyl hydrogen atom and the imine nitrogen atom, yielding the enol form in the solid state. It is unveiled, that the magnitude of RAHB effect fine tunes the strength of the O−H⋅⋅⋅N bonding and accordingly the relative populations of the enol, cis-keto and trans-keto forms leading to variation of the photophysical properties of 1 – 4 . It is determined, that the electron-withdrawing NO₂ in 4 amplifies the most RAHB effect causing the breaking of the O−H⋅⋅⋅N hydrogen bond and accordingly formation of the dominant cis-keto isomer in both the solid state and EtOH. To this end, the UV/Vis spectra of 1 – 3 in EtOH revealed the exclusive presence of the enol form, while the prevalent contribution of the cis-keto form was found for 4 . Furthermore, only compound 4 is emissive in the solid state in ambient condition due to dual emission arising from the cis-keto* and trans-keto* forms, while 2 was found to be highly emissive in EtOH. It is revealed qualitatively and quantitatively, based on the ETS-NOCV charge and energy decomposition scheme and the EDDB population-based method, that RAHB is strongly a non-local phenomenon based on electrons pumping or sucking through both the π- and σ-channels, which accordingly exerts chemical bonding changes at both the phenyl ring and predominantly a distant O−H⋅⋅⋅N area.     

Synthesis, spectral characterization and crystal structure of N-2-hydroxy-4-methoxybenzaldehyde-N'-4-nitrobenzoyl hydrazone and its square planar Cu(II) complex

A new aroyl hydrazone, N-2-hydroxy-4-methoxybenzaldehyde-N'-4-nitrobenzoyl hydrazone (H2L) and its mixed ligand Cu(II) complex [CuLpy] [py, pyridine] have been prepared. The ligand is characterized by elemental analysis, electronic, infrared and NMR spectral studies and the complex by electronic, infrared, EPR spectral studies and the magnetic susceptibility data. The structures of the compounds were determined by single crystal X-ray diffraction studies. Both the ligand and the Cu complex crystallize into a triclinic lattice with a space group of PI. From the crystal studies, it is concluded that the ligand molecule exits in the keto form in the solid state, while at the time of complexation, it tautomerises into the enol form. The complex is formed by the double deprotonation of the ligand molecule--both the phenolic and the enolic protons.