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• It is shown that the tautomeric state plays a crucial role for the optical properties of azo dyes. • Currently there is no suitable DFT approach to predict the optical spectra of tautomeric azo dyes. • Do not neglect the tautomeric state. In a paper of Zhu and co-workers ("Study of the electronic effect and quantitative spectra predictions of o-methoxyanilineterminated monoazonaphthols: a combined experimental and DFT study" by Guoxun Zhu, Yan Lin, Wenxian Zhou, Huacan Song and Zhengquan Li, RSC Advances, 2023, 13 , 33736), the authors derived a protocol for the absorption spectra prediction is sulphonated azo naphthols assuming that they exist as single azo tautomers. However, as it is well known before and as shown by the available experimental data and the calculations, performed in this note, the studied dyes exist almost solely in hydrazone tautomeric form, which unavoidably affects the conclusions and linear relations made in the original paper. A detailed benchmarking on the spectra prediction, involving 39 density functionals (33 of them hybrid and 6 – pure) indicates that there is no a functional correctly predicting the absorption spectra of the tautomers in azo naphthols. [ABSTRACT FROM AUTHOR]

Two-photon absorption (TPA) properties of noncentrosymmetric π-conjugated stilbenoid molecules with D-π-A structures, TPA spectra of which have been reported [L. Antonov et al., Phys. Chem. Chem. Phys. 5, 1193 (2003)], have been investigated theoretically by ab initio molecular orbital methods. The difference in the observed one-photon absorption and TPA spectra among compounds with the same donor (D) and acceptor (A) units is well reproduced by the present calculations, although the calculated excitation energies are overestimated by the configuration interaction with single excitation method used. It was found that the spectral differences among the compounds were mainly due to the deviation from the planar structure by intramolecular rotation around the N[Single_Bond]C (phenyl) bond of the N-benzilideneanilines having the C==N linkage as the central π bridge. Substitution of the end donor or acceptor groups with weaker ones leads to a decrease in the TPA intensity of the lowest π-π* TPA states, resulting mainly from the decrease in the dipole moment of the excited states. The total TPA cross section spectra have been separated into contributions of the dipolar term, which appear only in noncentrosymmetric systems, and the three-state term, which appear in any systems irrespective of symmetry. The dipolar term predominates only for the lowest π-π* state, while for the higher excited states the three-state terms become predominant. An analysis employing the index Rf defined with the transition polarizability shows that the TPA properties of the higher excited states are well described by the three-state approximation mediated by the lowest π-π* state. The differences found between the centrosymmetric and dipolar molecules in the enhancement mechanism of the TPA intensity by substituting the end groups with strong donors are discussed by comparison with the TPA properties of azobenzenes symmetrically substituted with the same donors. [ABSTRACT FROM AUTHOR]

The tautomeric optical sensors based on 4-(phenyldiazenyl)naphthalen-1-ol exist in their pure enol tautomeric form as free ligands, while the addition of metal ion fully shifts the equilibrium towards the keto tautomer allowing a red shift in the measured absorbance. This effect is achieved when a side ionophore group is connected to a tautomeric backbone by a spacer in a way that stabilizes the enol form via hydrogen boding. When the ionophore captures the metal ion the keto form is stabilized due to C─O tautomeric group participation in the complex. In the current study, we model theoretically the effect of symmetric tweezer like ionophores (RCOXCOR, where X, being CH or N, is the linker to the tautomeric backbone) on the tautomeric state and complexation ability of 4-(phenyldiazenyl)naphthalen-1-ol containing ligands. It was found that enol form stabilisation is achieved when R = NMe2, independing on the linker. Both ligands are unsuitable for capturing alkali metal ions. The calculations predict that the complexation with alkali earth metal ions could lead to a full shift of the tautomeric equilibrium towards keto tautomer. [ABSTRACT FROM AUTHOR]

Abstract: The effect of the complex formation with calix[4]arene on the tautomerism of curcumin was studied in water by means of theoretical calculations at M062X/6‐31G(d,p) level of theory. The results show that both tautomeric forms can enter and leave the host cavity without sterical problems. The binding of the diketo tautomeric form was found to be more favorable (by 2.1 kcal mol−1 in the gas phase and 1.7 kcal mol−1 in water) than the binding to the same host system of the preferred keto‐enol tautomeric form. This results in attenuation of the energy difference between the tautomers in the complex, but does not shift dramatically the position of the tautomeric equilibrium in the guest molecule. The predicted spectral characteristics of the individual tautomers and their complexes reproduce well the experimental observations. [ABSTRACT FROM AUTHOR]

Pharmacological and physicochemical studies ofN-unsubstituted indazole-5-carboxamides (subclass I) and their structurally optimised N1-methylated analogues (subclass II), initially developed as drug and radioligand candidates for the treatment and diagnosis of Parkinson’s disease (PD), are presented. The compounds are highly brain permeable, selective, reversible, and competitive monoamine oxidase B (MAO-B) inhibitors with improved water-solubility and subnanomolar potency (pIC50 >8.8). Using a well-validated, combined X-ray/modelling technology platform, we performed a semi-quantitative analysis of the binding modes of all compounds and investigated the role of the indazole N1 position for their MAO-B inhibitory activity. Moreover, compounds NTZ-1006, 1032, and 1441 were investigated for their ability to bind Fe2+and Fe3+ions using UV-visible spectroscopy. [ABSTRACT FROM PUBLISHER]

The acid dissociation constant of three benzimidazoles, namely 2,2′-bibenzo[d]imidazole, 2,5′-bibenzo[d]imidazole, and 5,5′-bibenzo[d]imidazole, have been investigated by means of density functional theory calculations in gas phase and in aqueous solution. The theoretical approach was validated by the comparing of predicted and experimentally determined pKa values in imidazole, benzimidazole, and 2-phenylbenzimidazole. From the studied compounds, 2,2′-bibenzo[d]imidazole was found to be the most acidic, which made it a valuable candidate as a material for polymer electrolyte membrane fuel cells. [ABSTRACT FROM AUTHOR]

Tautomerization energies of a seriesof isomeric [(4-R-phenyl)azo]naphtholsand the analogous Schiff bases (R = N(CH3)2,OCH3, H, CN, NO2) are calculated by LPNO-CEPA/1-CBSusing the def2-TZVPP and def2-QZVPP basis sets for extrapolation.The performance of various density functionals (B3LYP, M06-2X, PW6B95,B2PLYP, mPW2PLYP, PWPB95) as well as MP2 and SCS-MP2 is evaluatedagainst these results. M06-2X and SCS-MP2 yield results close to theLPNO-CEPA/1-CBS values. Solvent effects (CCl4, CHCl3, CH3CN, and CH3OH) are treated by avariety of bulk solvation models (SM8, IEFPCM, COSMO, PBF, and SMD)as well as explicit solvation (Monte Carlo free energy perturbationusing the OPLSAA force field). [ABSTRACT FROM AUTHOR]

The performance of several functionals (M06-2X, B97D, ωB97 family and B2PLYPD), accounting long range correlation, was evaluated in respect of the tautomerism of 1-phenylazo-4-naphthol. Three sets of experimental data were used to define the evaluation criteria: reproduction of the bond lengths in the structures of the individual tautomers, description of the non-planarity of the enol tautomer and prediction of the position of the tautomeric equilibrium (∆G value) at 298 K. The results show that M06-2X at most of the basis sets reasonably covers all requirements. In most of the cases, the empirical dispersion correction tends to stabilize the keto tautomer independently on the basis set. The obtained results have been used to select M06-2X, ωB97X and B2PLYP functionals to test their performance for a set of azonaphthols (1-phenylazo-4-napthol, 1-phenylazo-2-naphthol and 2-phenylazo-1-naphthol) where experimental data for the Gibbs free energies at room temperature are available. As a result, it should be pointed out the M06-2X functional slightly overestimates the enol tautomer, but at 6-31G**, TZVP and def2TZVP basis sets, the deviation is below 0.5 kcal.mol−1. The same result, but in favor of the keto tautomer, is obtained at B2PLYP/D95++**. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In this review, we have attempted to briefly summarize the influence of an external electric field on an assembly of tautomeric molecules and to what experimentally observable effects this interaction can lead to. We have focused more extensively on the influence of an oriented external electric field (OEEF) on excited-state intramolecular proton transfer (ESIPT) from the studies available to date. The possibilities provided by OEEF for regulating several processes and studying physicochemical processes in tautomers have turned this direction into an attractive area of research due to its numerous applications. [ABSTRACT FROM AUTHOR]

In the tautomeric Schiff bases, derived from 7-hydroxyquinoline, two competitive channels are possible upon excitation of the enol tautomer, namely proton transfer (PT) through intramolecular hydrogen bonding to the corresponding keto form and trans–cis isomerization around the azomethine double bond. The former leads to switching, based on twist-assisted excited state intramolecular PT, where the long-range proton transfer can occur as a targeted process. The latter, determined by the flexibility of the crane part, reduces the efficiency of the main targeted process. In previously studied molecular switches based on the 7-hydroxyquinoline skeleton, only the intramolecular PT photo-process undergoing from the excited enol form towards the keto tautomer, which is in most cases barrierless, has been discussed. Therefore, in the current study, the ground state PT properties and isomerization of (E)-8-((phenylimino)methyl)quinolin-7-ol and (E)-8-(((pentafluorophenyl)imino)methyl)quinolin-7-ol are investigated in depth using the MP2 methodology, while the excited state energy profiles are calculated with the ADC(2) method. The obtained results are discussed in light of the existing experimental data. [ABSTRACT FROM AUTHOR]

Abstract: A series of azo dyes, possessing amide fragments with restricted flexibility tethered to 4-(phenyldiazenyl)naphthalen-1-ol, was obtained from 1-hydroxy-2-naphthoic acid by subsequent conversion to amides and diazo coupling. It was shown that the position of the tautomeric equilibrium in solution strongly depends on the solvent in both UV and NMR concentration scale. The compounds exist as pure enol forms in chloroform and hydrocarbons, while in polar solvents (acetone, acetonitrile, alcohols) a tautomeric mixture is observed. According to the quantum-chemical calculations the aggregation of the keto tautomer is the possible reason for this shift in the position of the tautomeric equilibrium. To support the theoretical predictions, it was found that from acetone the keto form crystallizes as a dimer with hydrogen bonding between N1–H in the one molecule and amide C O in the other forming a three-dimensional structure. The importance of the side-chain nitrogen atom on the dimer formation was confirmed by solution and solid state study of 4-(phenyldiazenyl)-2-acetylnaphthalen-1-ol. The results indicate that the new azo-dyes obtained could be suitable candidates for switching and sensing applications in non-polar solvents. [Copyright &y& Elsevier]

Abstract: The reliability in the description of the tautomerism of 1-phenylazo-4-naphthol by using of HF and MP2 ab initio levels of theory and DFT methods with variety of pure GGA (OLYP), hybrid (B3LYP and B3PW91), long range corrected (LC-BLYP) and double-hybrid (B2PLYP and mPW2PLYP) functionals with large number of basis sets was estimated. In this evaluation three criteria were used: reproduction of the bond lengths in the structures of the individual tautomers, description of the non-planarity of the enol tautomer and prediction of the position of the tautomeric equilibrium (∆G value) at 298K. The results show that in substantial number of cases HF reasonably covers all requirements. The tested pure (OLYP) and hybrid functionals (B3LYP and B3PW91) fail in the prediction of the position of the equilibrium independent on the basis set. The situation is slightly better at the long range corrected functional (LC-BLYP), which give predominance of the enol tautomer at 6-31+G** and D95++**. The double hybrid functionals give very good description with D95++** basis set, but at substantial computational costs. [Copyright &y& Elsevier]

Abstract: A new sensing molecule containing aza-15-crown-5 as a receptor and 4-(phenyldiazenyl)naphthalen-1-ol as a signal converter has been synthesized. In the free ligand, the hydrogen bonding between the tautomeric OH group and the nitrogen atom from the macrocycle fully shifts the tautomeric equilibrium towards the enol form. The complexation reverses the equilibrium as a result of the strong interaction between the metal ion captured in the macrocyclic moiety and the tautomeric carbonyl group. The complex formation is accompanied by strong bathochromic (between 60 and 100nm) and hyperchromic effects on the absorption spectra. Obviously, the tautomeric OH/Cplay the role of an active side-arm in the complexation, supporting it via formation of a 3D cavity and giving possibility for ejection of the guest compound under stimuli to shift the tautomeric process back. [Copyright &y& Elsevier]

The relative strength of the intramolecular hydrogen bonding (IHB) in the title compounds has been investigated by means of ab initio quantum chemical calculations, UV–Vis spectral study and linear solvation energy relationship analysis (LSER) of the obtained tautomeric constants. It has been found that in acetone (and in all used solvents with substantial proton acceptor abilities) the tautomeric constant in the azonaphthol compound is lower than could be expected. The fact is explained with the breakage of the IHB and the coexistence of closed and open enol tautomers. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The aim of the current report is to shed light on the tautomerism of 4-((Phenylimino)methyl) naphthalene-1-ol in solution, which was studied by UV–Vis spectroscopy and quantum chemical calculations. It was found that this compound does not have the typical tautomeric behavior of its analog 4-Phenylazo-naphthalen-1-ol. The complicated equilibrium between the enol- and keto-like forms and two kinds of dimers that can exist in solution, is strongly dependent on the proton acceptor/donor abilities of the solvent. Using advanced data treatment quantitative information about the tautomeric and dimeric equilibrium constants was obtained. Copyright © 2007 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Linear (one-photon) and nonlinear (two-photon) spectral properties of 4-N,N-dimethylamino-4′-aminoazobenzene have been investigated under protonation in DMSO/water solvent mixtures. Observed spectral changes have been discussed in terms of ammonium–azonium tautomerism. The corresponding tautomeric constants have been determined for the first time for such kind of tautomeric equilibria. The process of protonation leads to changes of the two-photon absorption properties, suggesting that such process could be used as a tool for change of third-order nonlinear optical activity in solution. [Copyright &y& Elsevier]

An LSER study on the tautomerism of some aromatic Schiff bases and related azo compounds has vindicated the picture presented by previous ab initio studies and added further detail of its own. As predicted, the aminoenone tautomer is always the more polar (positive π* term) and, in addition, is specifically favoured by proton donor solvents (α term positive) through binding to the second lone pair on carbonyl. As expected for chelates, the Schiff bases are unaffected by proton acceptors (β term zero) but owing to the poor proton acceptor properties of the azo linkage, those of the azo compounds are partially broken. Tautomer preference as earlier calculated by ab initio methods is linearly related to log KT in the gas phase as estimated by LSER. Comparison with other compounds clearly shows the effect of aromatization in reducing KT; for 1-[(E)- methyliminomethyl]naphthalen-2-ol this is quantitatively similar to that for 2-pyridone, of which it is a kind of vinylogue. However, the effect of benzo fusion is similar only in part, and a further apparent anomaly lies in the near-identical KT values, for water, of Schiff bases and azo compounds. This is discussed in terms of their differing ‘intrinsic’ tautomeric bias, leading to a fortuitous cancellation of effects. Copyright © 2005 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The enol imine ⇄ enaminone tautomerization constants, KT, and thermodynamic parameters, ΔHT and ΔST, of 1-hydroxy-2-naphthaldehyde Schiff bases are determined by UV/vis spectroscopy. Polar solvents shift the equilibrium toward the quinone form (for the unsubstituted derivative 1c, KT = 0.20 (cyclohexane) and KT = 1.49 (ethanol)). Both donor (MeO, NMe2) and acceptor (CN, NO2) substituents lead to a decreased KT independent of solvent polarity. In apolar solvents, for all derivatives 1a−1e, the enol imine ⇄ enaminone equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of ΔGT to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with these extrapolated ΔGT values. Solvent effects on 1c are also successfully reproduced by the calculations. Geometric (O···N distance) and energetic criteria (conformer energy differences, homodesmotic reactions) establish the importance of intramolecular hydrogen bonding for the tautomerism of these compounds. The results obtained for 1a−1e are compared with tautomeric properties of the isomeric naphthaldehyde anils 2−4, the monocyclic analogues 5 and 6, the corresponding azo derivatives 7−9, and the N-alkyl derivative 10. [ABSTRACT FROM AUTHOR]

In this short review, we attempt to unfold various aspects of excited-state intramolecular proton transfer (ESIPT) from the studies that are available up to date. Since Weller's discovery of ESIPT in salicylic acid (SA) and its derivative methyl salicylate (MS), numerous studies have emerged on the topic and it has become an attractive field of research because of its manifold applications. Here, we discuss some critical aspects of ESIPT and tautomerization from the mechanistic viewpoint. We address excitation wavelength dependence, anti-Kasha ESIPT, fast and slow ESIPT, reversibility and irreversibility of ESIPT, hydrogen bonding and geometrical factors, excited-state double proton transfer (ESDPT), concerted and stepwise ESDPT. [ABSTRACT FROM AUTHOR]

There is no experimental information about the tautomerism of Favipiravir (T-705). Therefore, its tautomeric state was predicted by using density functional theory in gas phase and in solution (toluene, acetonitrile and water). The results have shown that, in neutral state, the enol form is strongly dominating in both gas phase and solution. The carboxamide group is easily protonated in the presence of acid, which leads to shift of the tautomeric equilibrium toward the keto tautomer. In order to validate the theoretical predictions, 2-hydroxy pyridine and 2-hydroxy pyrazine were also included in the set of studied compounds. The available experimental data about their tautomerism are in very good agreement with the theoretical predictions, which validate the conclusions made for T-705. [ABSTRACT FROM AUTHOR]

The performance of 26 hybrid density functionals was tested against a tautomeric dataset (TautData), containing experimental information for the keto-enol tautomeric equilibrium in 16 tautomeric azodyes and Schiff bases in cyclohexane, carbon tetrachloride and acetonitrile. The results have shown that MN12-SX, BHandH and M06-2X can be used to describe the tautomeric state of the core structures in the frame of ~0.5 kcal/mol error and correctly predict the tautomeric state in respect of dominating tautomeric form. Among them MN12-SX is the best performer, although it fails to describe the nonplanarity of some of the enol tautomers. The same experimental dataset was used to develop and test a special DFT functional (TautLYP) aimed at describing the tautomeric state in azo- and azomethyne compounds in solution when nonspecific solvents are used. [ABSTRACT FROM AUTHOR]

The ability of oxime-containing acetylcholinesterase reactivators BT-07 (K005), BT-08 (K033), BT-07-4M (K074), and obidoxime (H2LX2, X = Cl-, Br-)to bind palladium(II) or platinum(II) cations was evaluated in aqueous solutions (pH 7.4) at metal-to-ligand molar ratio varying from 1:10 to 10:1. Spectral changes were monitored within a week after mixing the reagents in the range from 220 nm to 500 nm. Results showed that depending on chemical structure and reaction time, oximes form two complex species of composition, [MHL]3+ or [M2 L]4+ (M = Pd(II), Pt(II)). It was also found that complexation of platinum(II) ions proceeds slower as compared to palladium(II). Conditional stability constants of new complex species were determined spectrophotometrically using an advanced chemometric procedure, based on the resolution of overlapping bands technique. [ABSTRACT FROM AUTHOR]

The tautomeric properties of an N -(3,4-dichlorophenyl)-1 H -indazole-5-carboxamide (NTZ-1006, 2 ) derivative, developed as highly potent, reversible and selective MAO-B inhibitor useful for the treatment of Parkinson's disease (PD) and other neurological disorders, have been studied both experimentally and theoretically. The theoretical data (M06–2X, B3LYP and MP2-4 quantum chemical calculations) have shown that due to aromaticity reasons the 1 H tautomer strongly dominates over the 2 H form. There are no substantial spectral changes by changing the solvent and the concentration, which leads to a conclusion that compound 2 exists in solution as 1 H tautomer and its tautomerism is not influenced by the solvents and the concentration. The results are in line with the understanding for the tautomerism of 1 H -indazole and shows that substitution at the C5 position in the indazole unit does not influence the tautomeric state. The isolated crystal structure of 2 is in an excellent agreement with the computation in respect of the most stable tautomer. Combined single X-ray/molecular modeling studies including HYdrogen-DEsolvation (HYDE) analysis provided not only insights into the enzyme–inhibitor interaction within the binding site of the human MAO-B isoform, but also a valuable information regarding the most stable 1H-indazole tautomeric form of NTZ-1006 that contributes to its high potency against hMAO-B enzyme (IC 50 0.586 nm) and selectivity (>17000-fold) over the hMAO-A isoenzyme. [ABSTRACT FROM AUTHOR]

Rosa damascena Mill. is one of the most important plants from Rosaceae family, with a long historical use in the traditional medicine and as a valuable oil bearing plant. The rose scent derived as oil, concrete or absolute, is one of the world most sought-after products and the development of reliable analytical methods for the origin assessment and quality assurance is of substantial fundamental and practical interest. A comprehensive chemical profiling of Bulgarian rose absolute ( Rosa damascena Mill.) was performed by means of GC/MS and GC–FID. A protocol for simultaneous analysis of compounds with high structural diversity and volatility, involving a multistep temperature gradient and trimethylsilyl derivatives was applied for this purpose. As a result 132 compounds, mainly mono- and sesquiterpenoids were identified. The main constituents, representing 80.0–95.5% of the total content of the detected compounds, were quantified by means of GC-FID. The developed approach was applied to rose absolute samples from Egypt and Morocco, as well. The specific chromatographic fingerprint, characteristic for the country of production (climatic-geographical origin) could be used for a fast classification of the rose absolute and for authenticity control. [ABSTRACT FROM AUTHOR]

A series of OLED-relevant compounds, consisting of 1,3,5-triazine core linked to various aromatic arms by amino group, has been synthesized and characterized. The studied compounds exist in solution as a mixture of two conformers, a symmetric propeller and asymmetric conformer, in which one of the aromatic arms is rotated around the C-N bond. At temperatures below −40 °C, the VT NMR spectra in DMF-d7 are in a slow exchange regime, and the signals of two conformers can be elucidated. At temperatures above 100 °C, the VT NMR spectra in DMSO-d6 are in a fast exchange regime, and the averaged spectra can be measured. The ratio of symmetric and asymmetric conformers in DMF-d7 varies from 14:86 to 50:50 depending on the substituents. The rotational barriers of symmetric and asymmetric conformers in DMF-d7 were measured for all compounds and are in the interval from 11.7 to 14.7 kcal/mol. The ground-state energy landscapes of the studied compounds, obtained by DFT calculations, show good agreement with the experimental rotational barriers. The DFT calculations reveal that the observed chemical exchange occurs by the rotation around the C(1,3,5-triazine)-N bond. Although some of the compounds are potentially tautomeric, the measured absorption and emission spectra do not indicate proton transfer neither in the ground nor in the excited state. [ABSTRACT FROM AUTHOR]

The tautomeric structures of 4-((2-hydroxynaphthalen-1-yl)diazenyl)-3,5-dinitrobenzoic acid have been revised by using quantum chemical (M06-2X/TZVP) calculations showing that the previously suggested structures are incorrect. By using theoretically predicted relative energies and Uv–Vis spectral analysis of a structurally similar compound (1-((2-nitrophenyl)diazenyl)naphthalen-2-ol), it has been estimated that the tautomeric equilibrium is shifted to the keto tautomer, but not completely. [ABSTRACT FROM AUTHOR]

The tobacco absolute usually obtained from tobacco waste, generated at various stages of post-harvest processing of the tobacco leaves and the cigarettes production, can be an important source of natural additives for tobacco compositions and cosmetics. Therefore, the chemical profile of Bulgarian tobacco absolutes derived from the three main type tobacco, grown in Bulgaria (Burley, Virginia and Oriental) was studied by means of gas chromatography-mass spectrometry (GC/MS) and gas chromatography with flame ionization detection (GC-FID). A protocol for simultaneous analysis of compounds with high structural diversity and volatility was developed involving a multistep temperature gradient and trimethylsilyl derivatives. As a result 90 compounds, mainly terpenes, but also organic acids, waxes and steroids were identified. The main constituents, representing 94.5-97.5% of the total content of the detected compounds, were quantified by means of GC-FID. Although the most important terpenoid classes, forming the heart of tobacco flavour, were found in all three samples, significant differences were observed as a reflection of the tobacco type. In Virginia and Burley absolutes only cembranoids were detected, while in Oriental compounds representative for both cembranoids and labdanoids were found. This defines a specific chromatographic fingerprint pattern that could be used for a fast identification of the tobacco absolute type. [ABSTRACT FROM AUTHOR]

The tautomerism of curcumin has been investigated in ethanol/water binary mixtures by using UV-Vis spectroscopy and advanced quantum-chemical calculations. The spectral changes were processed by using advanced chemometric procedure, based on resolution of overlapping bands technique. As a result, molar fractions of the tautomers and their individual spectra have been estimated. It has been shown that in ethanol the enol-keto tautomer only is presented. The addition of water leads to appearance of a new spectral band, which was assigned to the diketo tautomeric form. The results show that in 90% water/10% ethanol the diketo form is dominating. The observed shift in the equilibrium is explained by the quantum chemical calculations, which show that water molecules stabilize diketo tautomer through formation of stable complexes. To our best knowledge we report for the first time quantitative data for the tautomerism of curcumin and the effect of the water. [ABSTRACT FROM AUTHOR]

The reimagined concept of long‐range tautomeric proton transfer using crane subunits is shown by designing and synthesising two new acylhydrazones containing a 7‐hydroxyquinoline (7‐OHQ) platform. The acylhydrazone subunits attached to the 7‐OHQ at the 8th position act as crane arms for delivering proton cargo to the quinoline nitrogen. Light‐induced tautomerization to their keto forms leads to Z/E isomerization of the C=C axle bond, followed by proton delivery to the quinoline nitrogen by the formation of covalent or hydrogen bonds. The axle's being either an imine or ketimine bond is the structural difference between the studied compounds. The −CH3 group in the latter provides steric strain, resulting in different proton transport pathways. Both compounds show long thermal stability in the switched state, which creates a tuneable action of bidirectional proton cargo transport by using different wavelengths of irradiation. Upon the addition of acid, the quinoline nitrogen is protonated; this results in E/Z configuration switching of the acylhydrazone subunits. This was proven by single‐crystal X‐ray structure analysis and NMR spectroscopy. [ABSTRACT FROM AUTHOR]

The photoinduced birefringence of two 4-substituted phthalimide 2-hydroxy Schiff bases, containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties has been investigated in PMMA matrix. Their optical behaviour as nanocomposite films was revealed by combined use of DFT quantum chemical calculations (in ground and excited state) and experimental optical spectroscopy (UV–Vis and fluorescence). The results have shown that solid-state reversible switching takes place by enol/keto tautomerization and Z/E isomerization. Birefringence study was performed in the PMMA nanocomposite films using pump lasers at λrec = 355 nm and λrec = 442 nm. Fast response time and high stability of anisotropy up to 58% for (4) and 95% for (5) after turning off the excitation laser, was observed, which makes these materials appropriate candidates for cutting-edge optical information technology materials. The possibility for multiple cycles of recording, reading and optical erasure of the photoinduced birefringence at λrec = 442 nm in 5 has been demonstrated. The obtained results have shown that the maximum value of the measured birefringence is close to the anisotropic characteristics of the most frequently used azo materials. [ABSTRACT FROM AUTHOR]

Comprehensive chemical profiling of threeRosa damascena Mill. essential oil samples and two extracts (supercritical and subcritical) was performed by means of GC/MS/FID. As a result 85 compounds, representing 87.9-99.2% of the detected compounds, were identified by GC/MS and simultaneously quantified by GC-FID.The main aroma constituents in the essential oil sampleswere monoterpene alcohols citronellol (20.8-30.8%), geraniol (19.3-23.3%) and nerol (8.9-10.8%), followed by the stearopten fraction with nonadecane (12.0-17.5%), nonadecene (1.0-4.0%), heneicosane (4.0-5.8%) and heptadecane (1.8-2.5%) found in highest concentration. The chemical composition of the extracts was dominated by b-phenylethyl alcohol - 56.6% found in the subcritical sample and 46.7% in the supercritical one. Fatty acids esters, waxes and triterpenoids were found in the extracts as well. In addition, sensory evaluation of the rose aroma products was performed, demonstrating intense citrus, floral and balsamic notes for the essential oils and slight herbaceous and fruity notes for the extracts. [ABSTRACT FROM AUTHOR]

The tautomerism of 8-(phenyldiazenyl)quinolin-5-ol has been studied by a combination of theoretical (DFT calculations) and experimental (UV–Vis and NMR) methods. The detailed study of neutral molecules has shown that the hydrazone tautomeric form, stabilized by an intramolecular N–H⋯N hydrogen bond, is solely present in most of the solvents. In strong proton acceptor solvents, besides the dominant hydrazone form, the deprotonated form also appears. Solvent effects on the absorption maxima of the hydrazone form are interpreted by the linear solvation energy relationship concept, using Kamlet-Taft and Catalán models. Upon deprotonation, a substantial structural transformation is observed in the studied compound leading to a slow rotation around the Cquin-N bond. The process, as monitored by 1H NMR, is strongly solvent assisted and facilitated by proton acceptor solvents. Consequently, the investigated dye could be considered as a base-activated rotary switch. Image 1 • Tautomerism of quinoline azo dye is studied by combination of theory and experiment. • Hydrazone form observed in solutions is stabilized by intramolecular hydrogen bond. • Upon deprotonation, slow rotation around the Cquin-N bond is observed. • Deprotonation is solvent assisted and facilitated by proton acceptor solvents. • The studied dye could be considered as rotary switch activated by addition of base. [ABSTRACT FROM AUTHOR]

• Discrimination among different M2+ ions in neutral ML 2 complexes was achieved by SRCD. • The presence of new positively charged complexes ML+ was confirmed for the first time. • Excess of M2+ increases the coordination ability of the ligand in ML+ species. • Individual spectra of ML+ and ML 2 and their stability constants were calculated. • Monensin A affinity to bind M(II) as ML+ species was evaluated. Complexation reactions of polyether ionophorous antibiotic Monensin A with divalent metal ions were studied in methanolic solutions by means of synchrotron radiation circular dichroism (SRCD) spectroscopy. Data showed that in the presence of organic base (Et 4 NOH) Monensin A forms two types of metal complexes depending on metal-to-ligand molar ratio. Complexes of composition [M(Mon) 2 (H 2 O) 2 ] (Mon– = Monensinate A anion; M = Co2+, Mn2+, Mg2+, Ca2+, Ni2+, Zn2+, Cd2+) exist mainly in the presence of ligand excess (M2+:Mon– ~ 1:10–1:2), while the increase of metal(II) ion concentration results in the formation of new [M(Mon)(H 2 O)]+ species (M2+:Mon– ~ 1:1–6:1), not detected up to now with these metal ions. Experimental data showed that coordination of divalent metal ions to Monensinate A anion significantly influences the SRCD spectral pattern in different manner. Based on sets of SRCD experiments, species distribution, individual spectra of [M(Mon) 2 (H 2 O) 2 ] and [M(Mon)(H 2 O)]+ as well as their conditional stability constants were calculated. The relative affinity of Monensin A to bind divalent metal ions demonstrates the metal ion selectivity of the ligand. [ABSTRACT FROM AUTHOR]

A comprehensive study was performed for the first time to compare two structurally related substance classes, namely indazole-5-carboxamides (11 – 16) and (indazole-5-yl)methanimines (17 – 22). Both chemical entities are potent, selective and reversible MAO-B inhibitors and, therefore, may serve as promising lead structures for the development of drug candidates against Parkinson's disease (PD) and other neurological disorders. Compounds 15 (K i = 170 pM, SI = 25907) and 17 (K i = 270 pM, SI = 16340) were the most potent and selective MAO-B inhibitors in both series. To investigate the multi-target inhibitory activity, all compounds were further screened for their potency against human AChE and BuChE enzymes. Compound 15 was found to be the most potent and selective AChE inhibitor in all series (h AChE IC 50 = 78.3 ± 1.7 μM). Moreover, compounds 11 and 17 showed no risk of drug-induced hepatotoxicity and a wider safety window, as determined in preliminary cytotoxicity screening. Molecular modeling studies into the human MAO-B enzyme-binding site supported by a HYDE analysis suggested that the imine linker similarly contributes to the total binding energy in methanimines 17 – 22 as the amide spacer in their carboxamide analogs 11 – 16. Amplified photophysical evaluation of compounds 17 and 20 , including single X-ray analysis, photochemical experiments, and quantum-chemical calculations, provided insights into their more favourable isomeric forms and structural features, which contribute to their biologically active form and promising drug-like properties. Image 1 • A comparative study of carboxamides vs. methaimines as selective and reversible MAO-B inhibitors has been performed. • Methanimines 17 – 22 exhibit similar or even higher potency against h MAO-B than their carboxamide analogs 11 – 16. • Compounds 11 and 17 showed no risk of drug-induced hepatotoxicity. [ABSTRACT FROM AUTHOR]

Hydrazones are popular building blocks in the development of functional materials because of their simple structure, straightforward synthesis, hydrolytic stability and tunable properties. Although a significant body of experimental information has been accumulated on the hydrazone-based rotary switches, mechanistic studies are still scarce. The effect of different stator units (phenyl-, naphthyl- and quinolinyl) on the on/off state distribution is studied in details at various computational levels in order to select a reliable computational level for a further computational study of the switching mechanism and rotary action of these systems. [ABSTRACT FROM AUTHOR]

The title compound potentially can exist as four isomers in solution. Recently Lycka has proposed a protocol for distinguishing two of them based on 15N NMR. This approach has been confirmed theoretically, in the current study, and further developed into a logical scheme that allows the existence of each of the isomers to be proven in solution. The experimental data, obtained by NMR and UV–Vis spectroscopy, have shown that the studied compound exists as a mixture of two solvent stabilized isomers in diluted solutions of dimethyl sulfoxide. However, the concentration effect also plays a substantial role allowing formation of linear aggregates as the X-ray analysis and theoretical calculations show. • The title compound exists as two isomers in dimethyl sulfoxide. • 15N NMR approach is able distinguish them. • The aggregation of the E′ isomer is proven by X-ray analysis. [ABSTRACT FROM AUTHOR]

Abstract An extensive study was performed to develop a series of (pyrrolo-pyridin-5-yl)benzamides as reversible MAO-B inhibitors. Compounds 14 (NTZ-2020, h MAO-B IC 50 = 1.11 nM, K i = 0.56 nM, >9000-fold selective versus MAO-A) and 15 (NTZ-2027, h MAO-B IC 50 = 3.27 nM, K i = 1.45 nM, SI > 3058) are identified as the most promising BBB permeable derivatives within the series of (1 H -pyrrolo[3,2- b ]pyridine-5-yl)benzamides, combining both high potency and selectivity with optimal physicochemical and drug-like properties required for CNS active drugs. Extended photophysical analysis, including single X-ray analysis, quantum-chemical calculations and spectroscopic experiments provided insights into their tautomerism and structural behavioral, which relates to their biologically active form. The reversible MAO-B inhibitor 14 (NTZ-2020) exhibits a neuroprotective effect on cortical neuron survival and induces neurite network outgrowth. These effects are associated with a good BBB penetration of 14 that was confirmed in a triple cell neurovascular unit (NVU) model. Graphical abstract Image 1 Highlights • Selective and reversible MAO-B inhibitors have been developed. • (Pyrrolo-pyridin-5-yl)benzamides 10 , 14 – 16 show high BBB in vitro permeability. • Compound 15 exhibits also excellent water solubility. • Compound 14 showed a large neuroprotective effect on cortical neurons. • The neuroprotective effect of 14 observed also in a cell-based neurovascular unit model (+BBB). [ABSTRACT FROM AUTHOR]

The modulatory interactions between neurotensin (NT) and the dopaminergic neurotransmitter system in the brain suggest that NT may be associated with the progression of Parkinson's disease (PD). NT exerts its neurophysiological effects by interactions with the human NT receptors type 1 (hNTS1) and 2 (hNTS2). Therefore, both receptor subtypes are promising targets for the development of novel NT-based analogs for the treatment of PD. In this study, we used a virtually guided molecular modeling approach to predict the activity of NT(8–13) analogs by investigating the docking models of ligands designed for binding to the human NTS1 and NTS2 receptors. The importance of the residues at positions 8 and/or 9 for hNTS1 and hNTS2 receptor binding affinity was experimentally confirmed by radioligand binding assays. Further in vitro ADME profiling and in vivo studies revealed that, compared to the parent peptide NT(8–13), compound 10 exhibited improved stability and BBB permeability combined with a significant enhancement of the motor function and memory in a mouse model of PD. The herein reported NTS1/NTS2 dual-specific NT(8–13) analogs represent an attractive tool for the development of therapeutic strategies against PD and potentially other CNS disorders. [Display omitted] • NT(8–13) analogs as dual hNTS1 and hNTS2 receptor ligands have been developed. • Replacement of Arg8 and/or Arg9 by Lys and/or Cav residues led to compounds 10 – 14. • All peptide mimetics showed sub-nanomolar activity at both hNTS1/2 receptors. • The hNTS1R full agonist 10 exhibits improved stability and BBB permeability. • Compound 10 demonstrated in vivo efficacy in a mouse model of Parkinson's disease. [ABSTRACT FROM AUTHOR]

The title compound is potentially tautomeric and its tautomerism was studied by means of molecular spectroscopy ( 1 H and 13 C NMR and UV–Vis) in DMSO as well as by quantum chemical calculations (M06-2X/TZVP). The detailed assignment of the NMR signals supported by the theoretical calculations clearly shows that the previous interpretation, available in the literature, about the coexistence of two tautomeric forms is not correct. The compound exists as major and minor isomer of a single tautomeric form. In addition, a 2-methoxy derivative (the OH group replaced by a methoxy group) is also investigated and show similar trends. [ABSTRACT FROM AUTHOR]

In two consecutive studies the tautomerism in 4-((2-hydroxynaphthalen-1-yl)diazenyl)-3,5-dinitrobenzoic acid and the structurally similar 1-((2-nitrophenyl)diazenyl)naphthalen-2-ol has been considered from viewpoint of theoretical chemistry, UV–Vis spectroscopy and NMR. Although the theoretical data (at M062X level) show that both compounds exist only as a keto tautomer, the experiment proves existence of the enol form. The difference in the results obtained by UV–Vis spectroscopy and NMR requires a deeper consideration and verification of the NMR approach, which is based on using model compounds to provide the NMR signal pattern of individual tautomers. [ABSTRACT FROM AUTHOR]

The pharmacological and physicochemical analysis of structurally optimized N -alkyl-substituted indazole-5-carboxamides, developed as potential drug and radioligand candidates for the treatment and diagnosis of Parkinson's disease (PD) and other neurological disorders, is reported. Recent efforts have been focused on the development of subnanomolar potent, selective MAO-B (N1-alkyl-substituted compounds 12a – 14a and 15 ) and dual active MAO-A/B (N2-methylated compounds 12b – 14b ) inhibitors with nanomolar potency towards MAO-B and moderately active against MAO-A enzyme, respectively. The most promising drug-like derivatives in both series were N -(3-chloro-4-fluorophenyl)-1-methyl-1 H -indazole-5-carboxamide ( 13a , NTZ-1441, IC 50 hMAO-B 0.662 nM, >15000-fold selective versus MAO-A) and N -(3-chloro-4-fluorophenyl)-2-methyl-2 H -indazole-5-carboxamide ( 13b , NTZ-1442, IC 50 hMAO-B 8.08 nM, IC 50 hMAO-A 0.56 μM, SI = 70). Moreover, compounds 13a and 13b were predicted to cross both the gastrointestinal tract (at pH 2.0, 5.5, and 7,4) and the blood-brain barrier (BBB) in vitro with appropriate drug-like properties required for CNS active drugs. Combined single X-ray/molecular modeling studies provided insights into the enzyme–inhibitor interactions within both MAO isoforms and the rationale for their inhibitory activity with controlled MAO-A/B selectivity – despite their small structural differences. The binding modes of 12a , b and 13a , b confirmed that the major interactions with hMAO-B were established via the flexible carbonyl group of the carboxamide linkage and the electron-donating nitrogens N1 or N2 of the indazole moiety, allowing further exploration of the alkyl side chain for next step lead optimization efforts. [ABSTRACT FROM AUTHOR]

A simple approach for the synthesis of multifunctionalized N1-alkyl 7-amino-6-azaoxindole derivatives was developed and investigated. Formation of 5-amino- and 7-amino-6-aza-2-oxindoles 12a and 13a , respectively, was achieved using an intramolecular reductive cyclization as a key step. Subsequent alkylation of the pyrrole N1 atom in 12a led to the desired N1-alkylated compounds 22a–24 comprising different functionalities. Alkylation of 5-amino-substituted regioisomer 13a under the same conditions as used for 12a did not resulted in N1-alkylated products. To find a plausible explanation for the observed differences in reactivity, we investigated the possible tautomers of 12a and 13a and the distribution of their neutral and ionized forms in a gas phase. The relevant physicochemical properties of compounds 12a and 23 were determined. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two new molecular switches, based on benzothiazole, are designed expecting different action upon irradiation. • Both compounds exist as carboxamide tautomers in ground state. • In the isonicotinamide compound two rotation pathways are possible. • ESIPT is observed in both compounds upon irradiation. • Both compounds rotate in the same way due to the presence of water. The ground state tautomerism and excited state intramolecular proton transfer in two new molecular switches, namely N-(benzo[d]thiazol-2-yl)picolinamide and N-(benzo[d]thiazol-2-yl)isonicotinamide, are studied in acetonitrile by the combined use of steady state and time dependent spectroscopy and DFT calculations. Although, according to the theoretical calculations, two potential switching pathways are possible in N-(benzo[d]thiazol-2-yl)picolinamide, either through the pyridine or through the benzothiazole moiety, the experimental results show the same photodynamics as in N-(benzo[d]thiazol-2-yl)isonicotinamide. This indicates that the excited state proton transfer from the amide NH to the betzothiazole nitrogen is preferred in both compounds. [ABSTRACT FROM AUTHOR]

The tremendous therapeutic potential of curcumin as a chemopreventive, antineoplastic and chemosensitizing agent has failed to progress towards clinical development and commercialization due to its unfavorable physicochemical properties, low aqueous solubility, chemical instability, and pharmacokinetics. The present contribution is focused on the feasibility of using PEGylated calixarene, in particular polyoxyethylene-derivatized tert-butylcalix[4]arene, to prepare various platforms for delivery of curcumin such as inclusion complex, supramolecular aggregates, and hybrid liposomal systems. The inclusion complex is characterized by UV-vis and FT-IR spectroscopy as well as thermal gravimetrical analysis and differential scanning calorimetry. At concentrations exceeding the critical micellization concentration of PEGylated calixarene, the tremendous solubility enhancement of curcumin is attributed to additional solubilization and hydrophobic non-covalent interactions of the drug with supramolecular aggregates. A hybrid liposomal system is created via encapsulation of the inclusion complex in dipalmitoylphosphatidylcholine:cholesterol liposomes. Bare and liposomal curcumin:BEC-X inclusion complexes, as well as free curcumin were additionally investigated for cytotoxicity and apoptogenic activity against human tumor cell lines. [ABSTRACT FROM AUTHOR]

Abstract: It has recently been suggested that 1,1′,1′′-(2,4,6-trihydroxybenzene-1,3,5-triyl)triethanone may be tautomeric. Using 13C NMR chemical shifts and deuterium isotope effects on 13C chemical shifts, it is demonstrated that this is not the case. This compound occurs as a strongly hydrogen bonded benzene structure with hydrogen bonds between OH groups and the acetyl groups in both non-polar and hydrogen donating solvents. Quantum-chemical calculations using MP2 and M06-2X methods show substantial preference for the phenol structure in both the gas phase, and in cyclohexane and methanol. In addition, conventional UV–vis spectroscopy data suggest not tautomeric, but aggregation behaviour of the molecule in methanol and acetonitrile. [Copyright &y& Elsevier]

The thermal reaction of cis-3,4-benzo-1,2-diisopropyl-1,2-dimethyl-1,2-disilacyclobut-3-ene(1a) with tert-butyl alcohol at 300°C for 24 h proceeded with high stereospecificity to give erythro-1-(tert-butoxyisopropylmethylsilyl)-2-(isopropylmethylsilyl)benzene(2a) in 71% yield, as a single stereoisomer. A similarreaction of trans-benzodisilacyclobutene (1b) with tert-butyl alcohol produced the threoisomer 2bin 81% yield, as the sole product. The photolysisof 1a,bin the presence of tert-butyl alcohol afforded a mixture of two adducts formed by the reactionof 1-[2-(isopropylmethylsilyl)phenyl]-1-isopropylsilene with tert-butyl alcohol, respectively. The reaction of 1awith ethylene in an autoclave at 300 °C for 24 h gave3,4,9,10-dibenzo(r-1)-trans-2,trans-5,cis-8-tetraisopropyl-1,2,5,8-tetrasilacyclododeca-3,9-diene(10a), as a single stereoisomer in 71% yield. The reactionof 1bwith ethylene under the same conditions, however,produced poly[1,2-bis(isopropylmethylsilylene)phenylene–ethylene],whose molecular weight was determined to be 61000 (Mw/Mn= 2.32), relative topolystyrene standards. The reaction of 1awith 1-hexyne, tert-butylacetylene, phenylacetylene, and (trimethylsilyl)acetylenein benzene in a sealed degassed tube at 150 or 200 °C proceededstereospecifically to give the respective cis-5,6-benzo-1,4-diisopropyl-1,4-dimethyl-1,4-disilacyclohexa-2,5-dienederivatives (11a–14a) in high yieldsas the sole products. The reaction of 1bwith the alkynesunder the same conditions afforded the respective trans-benzodisilacyclohexadienes (11b–14b), as the sole stereoisomers in high yields. Theoretical treatmentfor the reaction of 3,4-benzo-1,1,2,2-tetramethyl-1,2-disilacyclobut-3-ene(1c) with tert-butyl alcohol and acetyleneindicated that the substrates react directly with 1ctogive the respective adducts. For the reaction of 1cwithacetylene, it has been shown that the acetylene molecule inserts directlyinto a silicon–silicon bond in 1cto give thebenzodisilacyclohexadiene system. Such direct reactions of the substrateswith 1cwere shown to be energetically more advantageousthan the reaction of o-quinodisilane, 1,2-bis(dimethylsilylene)cyclohexa-3,5-diene,arising from a conrotatory ring-opening reaction of 1cwith substrates. [ABSTRACT FROM AUTHOR]

Two novel, 7-hydroxyquinoline based, Schiff bases have been synthesized and their spectral properties have been investigated by combined use of optical and NMR spectroscopy and theoretical DFT calculations. The results indicate that 8-((phenylimino)methyl)quinolin-7-ol exists as a mixture of enol and two keto tautomers in solution, while 8-(((pentarfluorophenyl)imino)methyl)quinolin-7-ol is presented as a single enol form. Upon irradiation, in both compounds, through excited state intramolecular proton transfer mechanism, a rotation around the C quin -CH bond occurs, indicating that they are suitable to be used as bistable switches. The back relaxation is faster comparing to the theoretical expectations, due to the additional flexibility around the azomethine bond. As a result of forming complexes with the acidic counter ion, no intramolecular rotation happens upon protonation with trifluoroacetic acid. [Display omitted] • Two new 7-OH quinoline Schiff bases, containing C 6 H 5 and C 6 F 5 groups, are designed. • The first one exists as a mixture of enol and two keto forms in solution. • The second one is presented solely as an enol tautomer. • Z/E switching, assisted by ESIPT, is observed upon irradiation. • The relaxation back is faster due to flexibility of the azomethyne part. [ABSTRACT FROM AUTHOR]

Background: The title compound belongs to the class of bis-azomethine pigments. On the basis of comparative studies on similar structures, insight into the complex excited state dynamics of such compounds has been gained. It has been shown, for example, that only compounds that possess hydroxyl groups are fluorescent, and that the possibility for cis-trans isomerisation and/or bending motions of the central bis-azomethine fragment allows for different non-radiative decay pathways. Results: The compound, 4,4'-dihydroxy-1,1'-naphthaldazine (1) was synthesized and characterized by means of spectroscopic and quantum chemical methods. The tautomerism of 1 was studied in details by steady state UV-Vis spectroscopy and time resolved flash photolysis. The composite shape of the absorption bands was computationally resolved into individual subbands. Thus, the molar fraction of each component and the corresponding tautomeric constants were estimated from the temperature dependent spectra in ethanol. Conclusions: According to the spectroscopic data the prevalent tautomer is the diol form, which is in agreement with the theoretical (HF and DFT) predictions. The experimental data show, however, that all three tautomers coexist in solution even at room temperature. Relevant theoretical results were obtained after taking into account the solvent effect by the so-called supermolecule-PCM approach. The TD-DFT B3LYP/6-31 G** calculated excitation energies confirm the assignment of the individual bands obtained from the derivative spectroscopy. [ABSTRACT FROM AUTHOR]