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8-oxo-7,8-dihydro-2′-dexyoguanine (8-oxo-dG) can be tautomerized to a 6-enolate,8-keto tautomer through nearby-NH deprotonation at elevated pH. In this work, the N3-protonated 8-oxo-dG tautomers in deuterated pH-buffer solutions were studied using steady-state UV/Vis, FTIR, and ultrafast two-dimensional IR spectroscopies. The presence of 6,8-diketo and C6-anionic tautomers at neutral to basic conditions (pD = 7.4–12.0) was revealed by UV/Vis and FTIR results and was further confirmed by 2D IR signals in both diagonal and off-diagonal regions. However, the C6-enol tautomer, which may be an intermediate during the transition from 6,8-diketo to C6-enolate,C8-keto, was not observed appreciably due to its extreme low population. Furthermore, the neutral-to-anionic tautomeric transition of N3H-8-oxo-dG studied in this work occurs under more basic conditions than the N1H-8-oxo-dG reported previously, showing a higher pKa value for N3H than N1H. Finally, vibrational relaxation of the carbonyl stretching mode was found to be both molecular site dependent and pD dependent for 8oxo-dG. Taken together, this work shows that the ultrafast infrared spectroscopic method is effective for examining tautomers and their dynamics in nucleic acids. [ABSTRACT FROM AUTHOR]

Sirtinol is a known inhibitor of sirtuin proteins – a family of deacetylases involved in the physiology of aging. Its crystalline structure has never been determined except when bound to Fe(III) where it participates in the seven-fold coordination of the metal or to Cu(II) where it acts as a bidentate or tridentate ligand. Herein, we describe the structure of this important molecule, as follows: (a) the prevalent form of the keto-enol tautomerism in the solid state, and (b) in solution. Do they match? If not, how? The crystals of (R)-sirtinol are characterized by a large number of π–π bonded interactions linking molecules in infinite ribbons, which, in turn, are linked by additional π–π interactions of a variety of types, and by hydrogen bonds. In the latter case, we confirm by NMR that the X-ray determined position of an important H atom is on a N atom rather than on an O, which is how the molecule is usually depicted. [ABSTRACT FROM AUTHOR]

A pair of racemic oxacalix[4]arenes featuring planar chirality and a mesomer with a symmetric plane were conveniently synthesized under mild conditions via a one‐pot dynamic SNAr reaction and separated in 20 % and 49 % yields, respectively. X‐ray single crystal analyses confirmed their structures unambiguously. The racemic mixture could further be resolved using HPLC on a chiral column. Time‐dependent products distribution and interconversion between products with CsF as a catalyst were revealed by 1H NMR studies, convincing the dynamic nature of SNAr reaction. [ABSTRACT FROM AUTHOR]

This study investigates the chemical structure of profoxydim, focusing on its E–isomer, the main commercial form. The research aimed to determine the predominant tautomeric forms under various environmental conditions. Using proton and carbon–13 NMR spectroscopy alongside theoretical modeling, we examined tautomers and their conformers in different solvents (MeOD, DMSO, CDCl3, benzene) to mimic gas and aqueous phases. The findings reveal that the enolic form dominates in the gas phase, while the ketonic form prevails in aqueous environments, providing key insights into the herbicide's environmental behavior. We also observed an isomeric transition from E to Z under acidic conditions, which could affect profoxydim's reactivity in natural environments. The theoretical calculations indicated that in acidic conditions, the E and Z forms are nearly degenerate, with the E form remaining dominant in neutral environments. Additionally, QSAR models assessed the toxicity of various tautomers, revealing significant differences that could impact bioactivity and environmental fate. This research offers crucial insights into the structural dynamics of profoxydim, contributing to cyclohexanedione chemistry and the development of more effective herbicides. [ABSTRACT FROM AUTHOR]

The capability of bis(dioxolene) ligands to access multiple redox states makes them ideal candidates to tune the electronic properties of metal complexes, for example to achieve valence tautomerism (VT). In this study, a family of dinuclear cobalt complexes have been isolated with the bridging bis(dioxolene) thean− ligand in the cat2−‐cat2−, cat2−‐SQ⋅− and SQ⋅−‐SQ⋅− states (cat2−=catecholate, SQ⋅−=semiquinonate): [{CoIII(tpa)}2(theacat−cat)](PF6)2 (1), [{CoIII(tpa)}2(theacat−SQ)](PF6)3 (2), and [{CoIII(tpa)}2(theaSQ−SQ)](PF6)4 (3) (theaH4=2,3,6,7‐tetrahydroxy‐9,10‐dimethyl‐9,10‐dihydro‐9,10‐ethanoanthracene, tpa=tris(2‐pyridylmethyl)amine). Multi‐technique analysis confirms that 1, 2 and 3 adopt low spin‐CoIII containing {CoIII‐cat‐cat‐CoIII}, {CoIII‐cat‐SQ‐CoIII} and {CoIII‐SQ‐SQ‐CoIII} states, respectively. Compound 1 undergoes thermally‐induced {CoIII‐cat‐cat‐CoIII} ⇌ {CoIII‐cat‐SQ‐CoII} VT in the solid‐ and solution‐states above 300 K, involving high spin‐CoII. This interconversion is contrary to expectations, as tpa typically stabilizes low spin‐CoIII‐catecholate. Compound 2 is mixed‐valence class II/II–III, indicating a localized electronic structure, with electron transfer faster than the EPR and solvent rearrangement timescales, but slower than the infrared timescale. Compound 3 exhibits strong antiferromagnetic exchange. The overlap between the dioxolene π‐orbitals in thean− increases the accessibility of the cat2−‐SQ⋅− state, resulting in VT for 1. This study demonstrates that thean− can be isolated in multiple oxidation states in metal complexes, which is promising for applications in magnetic materials. [ABSTRACT FROM AUTHOR]

Azide-tetrazole tautomerism in a series of substituted 2-azidopyrimidines were studied by NMR spectroscopy methods. The structures of the tautomers existing in a solution were determined. By varying the temperature, the activation parameters of the studied tautomerism were determined from the integrated signal intensity in the NOESY/EXSY spectra. It was found that the phenyl substituents accelerate the tautomeric transformations. [ABSTRACT FROM AUTHOR]

Refrachor with a symbol F a new physical constant proposed for the first time in 1951 by Joshi and Tuli defined by the equation F = –P log( n D 20 –1) where P is the parachor and n D 20 is the refractive index, is now used in our present work to calculate the percentage composition of 1H- and 2H-tautomers of tetrazole, 1,2,3-triazole, and imine-enamine tautomerism of acetaldimine in their equilibrium mixtures. [ABSTRACT FROM AUTHOR]

With the aim of producing new heterocycle molecules, the previously reported 2-(aminomethyl)-2-(4-chlorophenyl)-2,3-dihydroquinazolin-4(1H)-one was converted efficiently by reacting with N,N′-dithiocarbonyldiimidazole (DTCI) to produce the substituted imidazolidine-2-thione moiety inserted in a three-fused-ring scaffold of the title compound. The molecular composition was confirmed by a high-resolution MS experiment, and its structure was elucidated by 1H, 13CNMR, and IR analyses. The thioacetamide form of the product was supported by density functional theory (DFT)–NMR analysis where 13C chemical shifts of the thioacetamide form and of its iminothiol tautomer were calculated in chloroform at the BP86/Jgauss-TZP2 level of theory. The very strong linear correlation between 13C chemical shifts from experimental findings and by calculation for the NHC=S form confirmed the structure. [ABSTRACT FROM AUTHOR]

The influence of the architecture of NH2-peripheral substitution of porphine derivatives on the intersystem T1 → S0-transition energy was studied theoretically. The molecular conformations of 15 porphine derivatives and 8 Zn-porphine derivatives in the ground singlet (S0) and lowest triplet (T1) states were optimized, the molecular orbital energies were determined, and the energies of the T1 → S0 transition were calculated using quantum chemical methods. The T1 → S0-transition energy was found to decrease from 11,700 to 6200 cm–1 upon increasing the number of NH2 groups in the macrocycle Cm-positions. The T1 → S0-transition energy was a linear function of the weighted sum of inductive and resonant Hammett constants 0.2σI + 0.8σR of the substituents. The ratio of inductive and resonant contributions of the NH2 groups depended on the method of attachment to the macrocycle, with the contribution of resonant interactions decreasing with increasing spacer length. The main reason for the bathochromic shift of the T1 → S0 transition was a significant increase in the energy of the b1-orbital, which had antinodes on the macrocycle Cm atoms. The dependence also held for Zn-porphyrins with the same peripheral substitution architecture. The energy of the T1 → S0 transition was noted to differ for both NH tautomers and conformers differing in the position of NH2 groups relative to the macrocycle mean plane. The calculations showed that experimental studies of aminoporphyrins were promising for obtaining new phosphors in the IR spectral region. A method for predicting the T1 → S0-transition energy for the synthesis of compounds with the required spectral and luminescent characteristics was proposed based on the results. [ABSTRACT FROM AUTHOR]

Multiconfigurational methods (CASSCF and CASPT2) were employed to gain a new understanding of the mechanism of the gas-phase phototautomerization of 2-pyridone/2-hydroxypyridine. Potential energy curves and crossing points of the low-lying excited states were analyzed. The results show that the tautomerization only occurs from 2-pyridone to 2-hydroxypyridine after electronic excitation to the S1 (ππ*) state. From this state, the system would be able to reach a conical intersection between S1 and the dissociative S2 (πσ*) due to vibrational effects. Then, it can evolve to the hydroxy form in its ground state by reaching an intersection seam between the S0 and the πσ* states. For this to happen, a roaming process responsible for the hydrogen atom migration would be required; otherwise, the system would revert to the 2-pyridone tautomer. The unfeasibility of the reverse process after optical excitation from the lactim to the lactam form is explained by the great amount of energy needed to reach the conical intersection between the ππ* and πσ* states. These findings would provide new insights into the understanding of the photophysics and photochemistry of a primordial heterocycle, considered a prebiotic model known to be found in interstellar clouds. [ABSTRACT FROM AUTHOR]

Corrole is a tetrapyrrolic dye with a structure that resembles porphyrin, apart from a single missing carbon. The absence of this carbon results in the re‐arrangement of the double bonds within the macrocycle, and the presence of three pyrrolic protons in the central cavity in its free‐base form. These protons lead to the existence of two distinct tautomeric structures that exist in a dynamic equilibrium. Although the ground‐state energies of the tautomers are similar, the excited states show a significant difference in energy which unbalances the equilibrium between the tautomers and results in rapid excited‐state tautomerization, favouring one tautomeric species over the other. Although the excited‐state tautomerization process has been known for a long time, very few studies have been performed on it, leaving many key aspects of the process poorly understood. Herein we show how ultrafast photoluminescence can be used to experimentally determine the rates of excited‐state tautomerization and activation energies of three free‐base corrole derivatives thus allowing us to completely describe the excited‐state dynamics of the unusual excited state of free‐base corrole and opening the door to the development of new materials that can exploit its unique characteristics. [ABSTRACT FROM AUTHOR]

A regiospecific synthesis of N9‐alkylated purines as novel acyclic nucleosides was developed. This approach is based on reconstructive methodology involving the construction of a target heterocyclic scaffold on a readily available 5‐aminotetrazole moiety, which is subsequently cleaved under reductive conditions due to azido‐tetrazole tautomerism. It appeared that the rate of reduction for the azide fragment in 6‐nitro‐7‐alkylaminotetrazolo[1,5‐a]pyrimidines is much greater than the rate of nitro group reduction. Treatment of these heterocycles with hydrogen over a palladium catalyst resulted in the formation of triaminopyrimidines in excellent yields through the reduction of both the azide and nitro group. Triaminopyrimidines were transformed into the desired N9‐alkylated purines, and an analog of the marketed drug penciclovir was synthesized by the developed method. [ABSTRACT FROM AUTHOR]

A Cu‐catalyzed multi‐component reaction of trifluoromethylated β‐amino ketones and nitriles using tert‐butyl nitrite as a diazotization reagent has been developed. Under the optimized conditions, N‐trifluoroalkyl amides were obtained with yields up to 87%. Control experiments and computational studies reveal that the reaction proceeds through the generation of diazo, in situ formation of nitrile ylide, water addition and final enol tautomerism. This approach features mild reaction conditions, wide substrate tolerance, and scale‐up applicability, which provides an efficient and practical strategy for amide synthesis. [ABSTRACT FROM AUTHOR]

A four-step synthesis of the natural product pseudane IX, starting from 3-oxododecanoic acid phenylamide and including only one chromatographic purification, was accomplished with an overall yield of 52%. The same synthetic sequence, but with a controlled partial reduction of a nitro group in the penultimate intermediate, led to the N-oxide of pseudane IX (NQNO). A shortened three-step variation of the synthesis allowed for the preparation of novel carboxamide analogs of the natural product. An agar diffusion assay against six different bacterial strains revealed significant antibacterial activity of the novel analogs against S. aureus at a concentration of 100 µg/mL. One of the novel compounds showed a remarkably broad spectrum of antibacterial activity, comparable to that of the positive control NQNO. [ABSTRACT FROM AUTHOR]

The effect of the external electric field on the ground-state tautomerism in 8-(benzo[d]thiazol-2-yl)quinolin-7-ol has been studied by using density functional theory. The compound exists as an enol tautomer (off state) and under the influence of the external electric field a long-range intramolecular proton transfer can occur, placing the tautomeric proton at the quinolyl nitrogen atom (on state). This is a result of the much higher dipole moment of the end keto tautomer and indicates that the external electric field can be used to mimic the implicit solvent effect in tautomeric systems. In the excited state, the further stabilization of the most polar on state leads to a situation when the excited-state intramolecular proton transfer becomes impossible, limiting the intramolecular rotation to the conical intersection region. [ABSTRACT FROM AUTHOR]

The structure of the title compound (4d), unexpectedly obtained in the reaction between o-phenylenediamine and 2-benzoylcyclohexanone instead of the target 3H-benzo[b][1,4]diazepine derivative 3d, was determined spectroscopically in solution and by a single-crystal X-ray diffraction (XRD) study. It involves two enantiomeric rotamers, called forms D and U, of which the structure was elucidated based on NMR spectra measured and predicted in DFT-GIAO calculations. An averaging of δCs for all tautomeric positions in the benzimidazole part of the 4d hydrate studied in wet (probably slightly acidic) CDCl3 unambiguously indicates tautomeric exchange in its imidazole unit. An XRD analysis of this material confirms the existence of only one tautomer in the solid phase. The non-covalent interactions forming between molecules of water and benzimidazole derivative are shorter than the sum of van der Waals radii and create an infinite-chain hydrogen bond motif along the b-axis. A possible mechanism for the observed cyclocondensation is also proposed. [ABSTRACT FROM AUTHOR]

Ammeline is a simple, readily available, molecular compound, which has been known for nearly 200 years. Despite that, no proper structural characterization of ammeline has been conducted so far. For this reason, the prevalent tautomeric form of ammeline in the solid remained unknown to this date. In the course of this study, its crystal structure was finally established by single‐crystal X‐ray diffraction. In this structure, ammeline is exclusively found as its 4,6‐diamino‐1,3,5‐triazin‐2(1H)‐one tautomer and adopts layered structure with an exceptionally high hydrogen bond density. Ammeline shows an interesting amphoteric behavior. Therefore, the synthesis and structural characterization of some of its salts were carried out to investigate the influence of the protonation degree on its molecular structure. In particular, the crystal structure of silver ammelinate monohydrate was solved as the first reported structure containing deprotonated ammeline. Moreover, the crystal structures of three different modifications of ammelinium perchlorate were elucidated and the transformation conditions between them were studied. Lastly, the crystal structure of ammelinediium diperchlorate monohydrate, containing unprecedented doubly protonated ammeline, was determined. The products' thermal behavior was studied by differential thermal analysis and thermogravimetric analysis. The perchlorate salts were additionally examined for their potential as insensitive high‐energy‐density materials. [ABSTRACT FROM AUTHOR]

We report an efficient synthetic route for the synthesis of tetrahydro indole-2,4-diones from readily available β-enaminones and glyoxal and subsequent I2/DMSO-mediated oxidation/aromatization of tetrahydro indole-2,4-dione to produce 4-hydroxyisatin derivatives. The key aspect of this protocol includes the dual catalytic activity of molecular iodine in the presence of DMSO. Mechanistic study suggested that this reaction is substrate selective as corresponding aromatization and sp3 C–H oxidation stops in the presence of 6,6-disubstituted tetrahydro indole-2,4-dione. Additionally, a thorough investigation into the synthesis of tetrahydro indole-2,4-dione follows acid-catalyzed cyclization/dehydration/enol–keto tautomerism from β-enaminones and glyoxal. [ABSTRACT FROM AUTHOR]

The investigation into approaches for achieving reversible control of topological phase transitions has attracted substantial research interest. So far, several system‐dependent physical control methods are developed, while a chemical strategy for efficient regulation of topological properties is still lack. Here, this study presents a chemical means to achieve reversible topological control in 2D organometallic lattices by introducing the lactam‐lactim tautomerization modules in the organic linkers. The strategy is validated in the theoretically designed 2D hexagonal framework with planar tricoordinated Co as nodes and 2,2′‐bithiazole derivatives as organic linkers, which transforms from a ferrimagnetic half metal with a local gap featuring Chern number of +1 to a ferromagnetic Chern insulator possessing Chern number of −1 after lactam‐lactim tautomerization. Simultaneously, a flip in the spin polarization of charge carriers between the two lattice tautomers is also observed. [ABSTRACT FROM AUTHOR]

The intermolecular arrangements leading to isomerization due to ring opening or cyclization is known as ring−chain tautomerism. The formation of some important heterocyclic compounds having one heteroatom via ring-chain tautomeric phenomenon is discussed. Most oxygenated or nitrogenous heterocyclic systems appeared isolated, condensed or in Spiro form. According to the size of the heterocyclic ring being formed via intramolecular addition of a heteroatom to a heteropolar multiple bond, indicated by a numerical prefix from three to six is reported in this review. [ABSTRACT FROM AUTHOR]

Novel 1-phenyl(1-thiophen-2-yl)-4-(diphenylphosphoryl)butane-1,3-diones were synthesized by the Claisen-type condensation of 1-(diphenylphosphoryl)propan-2-one with ethyl phenyl(thiophene-2-yl)carboxylates. The tautomeric equilibrium of diketone moiety of phosphine oxides was investigated by 1H and 13C{1H} NMR spectroscopy. Two species out of five possible tautomers (diketo and mono-enol) were observed in the solution of 1-phenyl(1-thiophen-2-yl)-4-(diphenylphosphoryl)butane-1,3-diones. The major tautomer was the mono-enol form with content 80–90%. Furthermore, the double bond position at C3 atom was determined by 13C{1H} NMR spectroscopy. [ABSTRACT FROM AUTHOR]

Herein, the classic organic chemistry experiment titled "Determination of Ethyl Acetoacetate Tautomers by 1H Nuclear Magnetic Resonance" was redesigned to enhance its scope and depth. This new experiment goes beyond simply recognizing the keto-enol tautomeric phenomenon. It delves into the kinetics, the impact of substituents, and the influence of solvents on the keto-enol tautomerization. It's worth noting that the hydrogen-deuterium exchange phenomenon can be observed, providing valuable insights into the ketone-enol tautomeric mechanism. This wellcrafted experiment is designed for senior undergraduate students pursuing chemistry-related majors. It not only deepens their comprehension of ketone properties but also fosters expertise in utilizing nuclear magnetic resonance spectroscopy in the field of organic chemistry. This, in turn, solidifies their theoretical knowledge in organic chemistry and instrumental analysis and hones their problem-solving and analytical skills. [ABSTRACT FROM AUTHOR]

The properties of natural polyphenol curcumin in water solutions of cationic Gemini surfactants decamethoxin and ethonium, which have similar positively charged nitrogen hydrophilic head groups but different spacer sizes and lengths of hydrophobic tails have been investigated in a wide range of concentrations by using UV–Vis spectroscopy and quantum‐chemical calculations. It was found that curcumin dissolves in the organized micellar media of these surfactants in the enol tautomeric form and its solubility increases 400 times compared to water. The binding constants of curcumin with decamethoxin and ethonium were determined by the solubility method. Significant differences in the influence of premicellar concentrations of cationic Gemini surfactants on the solubility and tautomeric transformations of curcumin were revealed. In contrast to decamethoxin, ethonium at a concentration below the CMC promotes a significant increase in the solubility of curcumin and a shift in the tautomeric equilibrium towards the formation of its keto form. [ABSTRACT FROM AUTHOR]

Breakdown of chlorophyll (Chl), as studied in angiosperms, follows the pheophorbide a oxygenase/phyllobilin (PaO/PB) pathway, furnishing linear tetrapyrroles, named phyllobilins (PBs). In an investigation with fern leaves we have discovered iso‐phyllobilanones (iPBs) with an intriguingly rearranged and oxidized carbon skeleton. We report here a key second group of iPBs from the fern and on their structure analysis. Previously, these additional Chl‐catabolites escaped their characterization, since they exist in aqueous media as mixtures of equilibrating isomers. However, their chemical dehydration furnished stable iPB‐derivatives that allowed the delineation of the enigmatic structures and chemistry of the original natural catabolites. The structures of all fern‐iPBs reflect the early core steps of a PaO/PB‐type pathway and the PB‐to‐iPB carbon skeleton rearrangement. A striking further degradative chemical ring‐cleavage was observed, proposed to consume singlet molecular oxygen (1O2). Hence, Chl‐catabolites may play a novel active role in detoxifying cellular 1O2. The critical deviations from the PaO/PB pathway, found in the fern, reflect evolutionary developments of Chl‐breakdown in the green plants in the Paleozoic era. [ABSTRACT FROM AUTHOR]

Methyl orange (p-Me2NC6H4N=NC6H4SO3Na; MO) is one of commonly used azo dyes known as an acid-base indicator, along with phenolphthalein. The chemical structural changes of MO with respect to its acid-base equilibrium in aqueous solutions have been learnt in chemical education. The reaction equations vary among analytical chemistry textbooks. Thus, details on the structures of MO are overviewed from historical perspectives for teaching materials in chemistry. Since protonation of MO potentially occurs not only at the azo group (-N=N-) but also at the dimethylamino group (Me2N-), two different chemical species can be considered; i.e., azonium (-N=NH+-) and ammonium (Me2NH+-). In fact, these tautomers of the two protonated MO species exist, and their equilibrium constants have been reported previously. However, ammonium species has not been presented as protonated MO in most textbooks. Misunderstandingly, another confusion concerning the 'resonance' between ammonium and quinonediiminium (p-Me2N+=C6H4=N-NHC6H4SO3-) of MO is also presented; exactly, there should be among azonium and quiononediiminium. In order to avoid misconceptions about the chemical structures of MO, these cases should be introduced to chemical educators and learners. Methyl orange (MO), as shown in Fig. 1, is an azo compound commonly used as an indicator of the neutralization, along with phenolphthalein. Since the proposal of the name of methyl orange and its use as an indicator by Lunge (1881), MO has been used not only as an acid-base indicator in analytical chemistry (Baker, 1964), but also as a learning material in chemical education (Shimada et al., 1992). MO undergoes a structural change from red in strongly acidic aqueous solutions to yellow at a pH 4 or higher (Barbosa, 2005). [ABSTRACT FROM AUTHOR]

Reactions of 12H-quinoxaline[2,3-b]phenoxazine (QOPO) derivatives with the CoII and ZnII hexafluoroacetylacetonates accompanied by protonation of the starting bases result in stable complexes [QOPOH]+[M(hfac)3]− whose structure was determined by X-ray diffraction and by 1H and 15N NMR spectroscopy. The complexes are characterized by long-wavelength absorption (560–664 nm) and fluorescence in the red region (λfl = 674–805 nm). The relative energies of the protonated tautomers N(14)—H and N(7)—H were determined using the results of B3LYP–D3BJ/6-311G++(d,p) density functional calculations with inclusion of D3BJ dispersion correction. A SQUID magnetometry study of the static and dynamic magnetic properties of the CoII complex revealed that it behaves as a field-induced single-ion magnet. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) mechanisms of nontraditional luminogens (NTLs) have attracted great interest, and they are generally explained with intra/intermolecular through‐space conjugation (TSC) of nonconventional chromophores. Here a new concept of nonaromatic through‐bond conjugation (TBC) is proposed and it is proved that it plays an important role in the PL of NTLs. The PL behaviors of the three respective isomers of cyclohexanedione and gemdimethyl‐1,3‐cyclohexanedione were studied and correlated with their chemical and aggregate structures. These compounds show different fluorescence emissions as well as different concentration, excitation and solvent‐dependent emissions. The compounds which undergo keto‐enol tautomerism and hence with a conjugated ketone‐enol structure (i.e., nonaromatic TBC) show more red‐shifted emissions. TBC effect reduces the energy gaps and facilitates the formation of stronger TSC in the aggregate state. The compounds in the ketone‐enol form are also prone to occur excited state intra/intermolecular proton transfer (ESIPT). The cooperative effect of nonaromatic TBC and TSC determines the PL behaviors of NTLs. This work provides a novel understanding of the PL mechanisms of NTLs and is of great importance for directing the design and synthesis of novel NTLs. [ABSTRACT FROM AUTHOR]

The current landscape of chiral covalent organic frameworks (COFs) predominantly centered on constructing asymmetric molecular-scale chirality, often introducing an inherent contradiction to the COF symmetry and limiting diversity. Herein, we overcome these challenges by achieving chiral transfer between one-dimensional (1D) imine linear polymers (LPs) and two-dimensional (2D) network β-ketoenamine COFs composed of achiral monomers. We successfully synthesize several 1D imine LPs with mesoscopic helical chirality, comprising achiral C2-symmetric terephthalaldehyde and diamine linkers in a chiral supramolecular transcription system. Leveraging the irreversible tautomerism mechanism within the linker replacement approach, terephthalaldehyde (TPA) units in these helical 1D LPs are substituted with C3-symmetric 1,3,5-triformylphloroglucinol (TP), yielding the corresponding 2D network β-ketoenamine COFs. Crystallinity and helicity of the resultant β-ketoenamine COFs intimately hinge on reaction conditions, including the aldehyde stoichiometry of Tp and TPA, as well as the quantity and concentration of the catalyst employed. Under optimized conditions, the nucleation and growth were precisely governed, achieving a harmonious equilibrium of crystallinity and helicity within the generated 2D network β-ketoenamine COFs, even with covalent bond rupture, recombination, and topological transition (from [C2 + C2] to [C3 + C2]). Impressively, the ground state chirality inherent to helical 1D LPs seamlessly transfers to helical 2D network β-ketoenamine COFs. This study not only offers new perspectives on the development of chiral functional COFs, but also provides fresh insights into the precise control of COFs' microscopic morphology. [ABSTRACT FROM AUTHOR]

The tautomerism in the title compound as a potential long-range proton transfer (PT) switch has been studied by using the DFT and TD-DFT approaches. The data show that in aprotic solvents, the enol tautomer dominates, while the increase in the content of the keto tautomer (short-range PT) rises as a function of polarity of the solvent. In ethanol, due to specific solute–solvent stabilization through intermolecular hydrogen bonding, a substantial amount of the keto forms exists in solution. The irradiation leads to two competitive processes in the excited state, namely ESIPT and trans/cis isomerization around the azomethine bond as in other structurally similar Schiff bases. The studied compound is not suitable for bistable tautomeric switching, where long-range PT occurs, due to the difficult enolization of the coumarin carbonyl group.

Oxidation of [(ArBIG-bian)2−Yb2+(dme)] (1) (ArBIG-bian = 1,2-bis[(2,6-dibenzhydryl-4-methylphenyl)imino]acenaphthene; dme = 1,2-dimethoxyethane) by 0.5 equivalent of Me2NC(S)S–S(S)CNMe2 in dme at ambient temperature affords a mixture of two products, [(ArBIG-bian)2−Yb3+{SC(S)NMe2}1−(dme)] and [(ArBIG-bian)1−Yb2+{SC(S)NMe2}1−(dme)], which represent two redox-isomers (2a and 2b, respectively). Their ratio in solution depends on the solvent as well as on the temperature. In the solid state, a decrease of temperature (350 → 100 K) caused an electron transfer from the Yb2+ ion to the ArBIG-bian radical-anion in isomer 2b to afford isomer 2a. Accordingly, the ratio of isomers 2a and 2b changes from 1 : 1 (350 K) to 3 : 1 (100 K). In contrast, in the dimer [(dme)(dpp-bian)1−Yb2+(μ-Cl)2Yb3+(dpp-bian)2−(dme)] (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene), which is the sole example of a lanthanide complex that reveals solid-state redox-isomerism (valence tautomerism) reported so far, the electron transfer from the Yb2+ ion to the dpp-bian radical-anion takes place at around 150 K and is completed within a temperature interval of ca. 7 K. [ABSTRACT FROM AUTHOR]

Although structural information on sugars is wide, experimental studies on the oxidation products of sugars in the gas phase, free from solvent interactions, have been rarely reported. We present an experimental work on the changes in the structure and interactions of two products of glucose oxidation (D -glucono-1,5-lactone (GlcL) and D -glucurono-6,3-lactone (GlcurL)) with respect to their precursor. Features such as intramolecular interactions, ring puckering and tautomerism were observed. [ABSTRACT FROM AUTHOR]

Herein, the molecular structures of amide ↔ imide tautomers of articaine (ART) were computationally investigated at the DFT/B3LYP/6–311++G (d,p) methodology. Experimental FT-IR and UV-Vis characterisation results of ART were compared with the theoretical results of amide (ART-A) and imide (ART-I) forms. Thermodynamic parameters computed for each phase revealed that the amide tautomer of ART was preferred with energy values in the range of 11.9–13.8 kcal/mol. The modification in surface properties as a function of prototropic tautomerism in the ART molecule was studied by electrostatic surface properties (ESP) analysis. Frontier molecular orbital (FMO), natural bond orbital (NBO), and nonlinear optical (NLO) analyses were performed. It is revealed that energy gap values of the ART-I (4.670–4.738 eV) for each phase are slightly higher than those of the corresponding phases in the ART-A (4.515–4.586 eV). Therefore, it can be concluded that the imide form exhibits lower chemical reactivity compared to the amide form. Regarding NLO characteristics, dipole moment (μtot), mean polarisability (αtot), and mean first-order hyperpolarisability (βtot) values of tautomers have been reported. It was determined that the βtot values computed for ART-A (0.653 × 10−30 esu) and ART-I (0.710 × 10−30 esu) were approximately twice the value of the standard urea. [ABSTRACT FROM AUTHOR]

Pyrazinacenes are linearly fused heteroaromatic rings, with N atoms replacing all apical CH moieties. Component rings may exist in a reduced state, having NH groups instead of N, causing cross-conjugation. These compounds have interesting optical and electronic properties, including strong fluorescence in the near-infrared region and photocatalytic properties, leading to diverse possible applications in bio-imaging and organic synthesis, as well as obvious molecular electronic uses. In this study, we investigated the behavior of seven-ring pyrazinacene 2,3,11,12-tetraphenyl-7,16-dihydro-1,4,5,6,7,8,9,12,13,14,15,16,17,18-tetradecaazaheptacene (Ph4H2N14HEPT), with an emphasis on protic processes, including oxidation, tautomerism, deprotonation, and protonation, and the species resulting from those processes. We used computational methods to optimize the structures of the different species and generate/compare molecular orbital structures. The aromaticity of the species generated by the different processes was assessed using the nucleus-independent chemical shifts, and trends in the values were associated with the different transformations of the pyrazinacene core. The computational data were compared with experimental data obtained from synthetic samples of the molecule tBu8Ph4H2N14HEPT. [ABSTRACT FROM AUTHOR]

The newly discovered growth self-inhibition phenomenon of tautomeric crystals is now generalized to the demostrope (form B) of irbesartan that displays intra-annular tautomerism in neutral aqueous solutions. The dynamic intra-annular tautomer inter-conversion on the surface is the key factor. Our findings provide implications for producing and engineering tautomeric materials. [ABSTRACT FROM AUTHOR]

We report the preparation and characterization of the dinuclear AuII hydroxide complex AuII2(L)2(OH)2 (L=N,N′‐bis (2,6‐dimethyl) phenylformamidinate) and study its reactivity towards weak X−H bonds. Through the interplay of kinetic analysis and computational studies, we demonstrate that the oxidation of cyclohexadiene follows a concerted proton‐coupled electron transfer (cPCET) mechanism, a rare type of reactivity for Au complexes. We find that the Au−Au σ‐bond undergoes polarization in the PCET event leading to an adjustment of oxidation levels for both Au centers prior to C(sp3)−H bond cleavage. We thus describe the oxidation event as a valence tautomerism‐induced PCET where the basicity of one reduced Au−OH unit provides a proton acceptor and the second more oxidized Au center serves as an electron acceptor. The coordination of these events allows for unprecedented radical‐type reactivity by a closed shell AuII complex. [ABSTRACT FROM AUTHOR]

Although identical in molecular formula and weight, curcumin and cyclocurcumin show remarkable differences in their reactivity. Both are natural compounds isolated from the rhizome of turmeric, the former is involved in the diketo/keto-enol tautomerism through the bis-α,β-unsaturated diketone unit according to the polarity of the solvent, while the latter could react by trans-cis isomerization due to the presence of the α,β-unsaturated dihydropyranone moiety. Even if curcumin is generally considered responsible of the therapeutical properties of Curcuma longa L. due to its high content, cyclocurcumin has attracted great interest over the last several decades for its individual behavior and specific features as a bioactive compound. Cyclocurcumin has a hydrophobic nature characterized by fluorescence emission, solvatochromism, and the tendency to form spherical fluorescent aggregates in aqueous solution. Molecular docking analysis reveals the potentiality of cyclocurcumin as antioxidant, enzyme inhibitor, and antiviral agent. Promising biological activities are observed especially in the treatment of degenerative and cardiovascular diseases. Despite the versatility emerging from the data reported herein, the use of cyclocurcumin seems to remain limited in clinical applications mainly because of its low solubility and bioavailability. [ABSTRACT FROM AUTHOR]

Tautomers coexisting in an equilibrium system have significant potential for regulating luminescent properties because of their structural differences. However, separating and stabilizing tautomers at room temperature is a considerable challenge. In this study, it is found that hydrogen‐bonded organic frameworks (HOFs) composed of Br‐ anions can effectively separate and stabilize two proton‐transfer tautomers of triarylformamidinium bromide: namely, the nitrogen cation (BA‐N) and carbon cation (BA‐C). The BA‐N crystal consisting of a dense anionic HOF and parallelly aligned organic cations exhibits green thermally activated delayed fluorescence and red room‐temperature phosphorescence (RTP). The BA‐C crystal contains acetone molecules that induce an antiparallel arrangement of the organic cations to form a loose HOF, producing blue prompt fluorescence and green RTP. Interestingly, switching of the HOFs between BA‐N and BA‐C can be achieved through the uptake and release of acetone, thereby dynamically adjusting multiple luminescent properties. Consequently, the HOF crystals can be used for the highly sensitive and specific sensing of acetone with a detection limit of 66.74 ppm. This study not only stabilizes tautomeric luminescent materials at room temperature, but also provides a new method for constructing smart HOFs with a sensitive response to a stimulus. [ABSTRACT FROM AUTHOR]

The Schiff base condensation of 5-methyl-4-imidazole carboxaldehyde, 5Me4ImCHO, and the anion of an amino acid, H2N-CH(R)CO2− (R = -CH3, -CH(CH3)2 and -CH2CH(CH3)2), gives the aldimine tautomer, Im-CH=N-CH(R)CO2−, while that of 5-methylimidazole-4-methanamine, 5MeIm-4-CH2NH2, with a 2-oxocarboxylate anion, R-C(O)-CO2−, gives the isomeric ketimine tautomer, Im-CH2-N=C(R)CO2−. All are isolated as the neutral nickel(II) complexes, NiL2, and are characterized by single crystal structure determination, IR, and positive ion ESI MS. In the cases of the 4 substituted imidazoles, either 5MeIm-4-CHO or 5MeIm-4-CH2NH2, both the aldimine and ketimine complexes are isolated cleanly with no evidence of an equilibrium between the two tautomers under the experimental conditions. The aldimines are blue while the tautomeric ketimines are green. In contrast, for the 2-substituted imidazoles, with either Im-2-CHO or Im-2-CH2NH2, the isolated product from the Schiff base condensation is the ketimine, which in the solid is green, as observed for the 4-isomer. These results suggest that for the 2-substituted imidazoles, there is a facile equilibrium between the aldimine and ketimine tautomers, and that the ketimine form is the thermodynamically favored tautomer. The aldimine tautomers of the 4-substituted imidazoles have three stereogenic centers, the nickel (Δ or Ʌ) and the two alpha carbon atoms (R or S). The observed pair of enantiomers is the ɅRR/ΔSS enantiomeric pair, suggesting that this pair is lower in energy than the others and that this is in general the preferred chiral correlation in these complexes. [ABSTRACT FROM AUTHOR]

The 2H‐tetrahydro‐4,6‐dioxo‐1,2‐oxazine (TDO) ring, a rare structural motif embedded in the molecular architecture of alchivemycins, is synthesized from 2,2,6‐trimethyl‐4H‐1,3‐dioxin‐4‐one via two routes. Both routes involve the intramolecular trapping of an acylketene intermediate by an internal hydroxy group to form the TDO ring as a key step. The first synthesis was achieved in 29 % overall yield from the starting material by a four‐step sequence featuring a DMAP−CaCl2‐mediated O‐ to C‐acyl migration reaction to install an acyl group at the C5 position of the TDO ring. The second synthesis was completed in 8 % overall yield from the same starting material as the first one through five steps by exploiting a photo‐induced double bromination reaction. [ABSTRACT FROM AUTHOR]

This study presents a straightforward solution to the challenge of elucidating the structures of nitrogen containing compounds undergoing isomerization. When spectral line broadening occurs related to isomerization, be it prototropic tautomerism or bond rotations, this poses a significant obstacle to structural elucidation. By adding acids, we demonstrate a simple approach to overcome this issue and effectively sharpen NMR signals for acid stable prototropic tautomers as well as the conformational isomers containing a morpholine or piperazine ring. [ABSTRACT FROM AUTHOR]

Purpose: This paper aims to synthesize 6-ethyl-4-hydroxyquinolin-2(1H)-one as a new enol-type coupling component in the preparation of some 3-arylazo-4-hydroxyquinolin-2(1H)-one dyes and evaluate the solvent effects on their absorption in ultraviolet-visible spectra. Design/methodology/approach: 6-Ethyl-4-hydroxyquinolin-2(1H)-one was synthesized by thermal cyclocondensation reaction of N, N′-bis(4-ethylphenyl) malonamide at 130–140°C in polyphosphoric acid. This compound was then applied in the azo-coupling reaction with some aniline-based diazonium salts, so as to prepare seven new mono-heterocyclic azo dyes. The structures of the compounds were confirmed using mass spectroscopic technique. Fourier transform infra red (FT-IR) and 1H proton nuclear magnetic resonance (1H NMR) and carbon-13 nuclear magnetic resonance (13 C NMR) studies on the structure of the azo compounds revealed that they exist as two E- and Z-isomers of hydrazone tautomer both in solid and solution state. The effects of acid and base on the visible absorption spectra of the dyes were also evaluated and discussed. Findings: Ultra violet-visible UV-vis absorption spectra of the dyes didn't show significant variation by changing of solvents because of intramolecular H-bonding between proposed hydrazone forms and 2- and 4-keto functions in their structures. The spectra of the dyes were not sensitive to the addition of acid but were very sensitive to base. Originality/value: The synthesized 3-arylazo-4-hydroxyquinolin-2(1H)-one dyes are new members in the 4-hydroxyquinolin-2(1H)-one azo dyes family, where very few details regarding the synthesis of such dyes are reported before in the literature. They are unique in terms of synthesis and spectral properties. [ABSTRACT FROM AUTHOR]

Variable temperature NMR spectroscopic measurements on (S)-warfarin [open-form: 3-(1′-phenyl-3′-oxobut-1′-yl)-4-hydroxycoumarin] in CDCl3, CD3OD and d6-DMSO generally showed tautomeric compositions in the order trans (2S,4S) coumarin hemiketal > cis (2R,4S) coumarin hemiketal > open (S) coumarin enol in slow dynamic equilibrium over temperature ranges rising modestly from ambient. A computational (DFT-M06-2X) examination of the lower energy tautomers including coumarin and chromone open and cyclic forms (gas phase, chloroform or DMSO fields) was consistent with the general solution compositions. The crystal and molecular structures for model compounds of the major solution tautomers are reported: (2S,4S)-warfarin methyl ketal [orthorhombic, P212121], (2R,4S)-warfarin methyl ketal [orthorhombic, P212121], (rac)-warfarin-4-methyl ether [monoclinic, P21/n], and the open chromone (S)-warfarin-2-methyl ether [monoclinic, P21, Z = 8]. A combination of direct integration and line-fitting methods were used to determine solution (S)-warfarin tautomer compositions. As temperatures were increased, the concentrations of the open coumarin form increased at the expense of the cyclic hemiketals. Equilibrium constants were used to determine the standard free-energy differences for the two open-cyclic equilibria (trans hemiketal ⇌ open, open ⇌ cis hemiketal, respectively) in three solvents: CDCl3 [+ 3.7(4), − 2.8(6) kJ/mol], CD3OD [+ 7.6(16), − 4.7(9) k/mol], d6-DMSO [+ 3.5(7), − 1.1(2) kJ/mol]. Standard enthalpy and entropy differences were also determined from van't Hoff analysis. Rates of the respective reactions were estimated from line-widths for the cyclic hemiketals and solution equilibrium compositions for each species. Eyring analysis gave ΔG‡, ΔH‡, and ΔS‡, respectively, for the forward and reverse reactions of coumarin trans hemiketal ⇌ open-form and for the open-form ⇌ cis hemiketal. Negative entropic contributions to the observed transition state energies were consistent with solvent or solute ordering in the prototropic reactions. Open-form NMR signals were broader than could be accounted for by the open-cyclic equilibria alone, increasingly so in polar and protic solvents and with rising temperatures. While a conformational equilibrium may operate, an increasingly faster intermediate dynamic equilibrium between open coumarin-chromone tautomers may be a more likely explanation. Structures of methylated warfarin tautomers and computational models enabled assignment of overlapping warfarin tautomeric NMR spectra and through variable temperature analysis, provided the thermodynamics of the tautomeric equilibria in three solvents. [ABSTRACT FROM AUTHOR]

The tautomerism in the title compound as a potential long-range proton transfer (PT) switch has been studied by using the DFT and TD-DFT approaches. The data show that in aprotic solvents, the enol tautomer dominates, while the increase in the content of the keto tautomer (short-range PT) rises as a function of polarity of the solvent. In ethanol, due to specific solute–solvent stabilization through intermolecular hydrogen bonding, a substantial amount of the keto forms exists in solution. The irradiation leads to two competitive processes in the excited state, namely ESIPT and trans/cis isomerization around the azomethine bond as in other structurally similar Schiff bases. The studied compound is not suitable for bistable tautomeric switching, where long-range PT occurs, due to the difficult enolization of the coumarin carbonyl group. [ABSTRACT FROM AUTHOR]

With the aim of producing new heterocycle molecules, the previously reported 2-(aminomethyl)-2-(4-chlorophenyl)-2,3-dihydroquinazolin-4(1H)-one was converted efficiently by reacting with N,N′-dithiocarbonyldiimidazole (DTCI) to produce the substituted imidazolidine-2-thione moiety inserted in a three-fused-ring scaffold of the title compound. The molecular composition was confirmed by a high-resolution MS experiment, and its structure was elucidated by 1H, 13CNMR, and IR analyses. The thioacetamide form of the product was supported by density functional theory (DFT)–NMR analysis where 13C chemical shifts of the thioacetamide form and of its iminothiol tautomer were calculated in chloroform at the BP86/Jgauss-TZP2 level of theory. The very strong linear correlation between 13C chemical shifts from experimental findings and by calculation for the NHC=S form confirmed the structure.

The 1H- and 2H-1,2,3-triazoles are isomeric five-membered ring, aromatic heterocycles that may undergo chemical equilibration by virtue of intramolecular hydrogen migration (tautomerization). Using millimeter-wave spectroscopy in the 130–375 GHz frequency range, we measured the spectroscopic constants for thirteen 1H-1,2,3-triazole and sixteen 2H-1,2,3-triazole isotopologues. Herein, we provide highly accurate and highly precise semi-experimental equilibrium (reSE) structures for the two tautomers based on the spectroscopic constants of each set of isotopologues, together with vibration–rotation interaction and electron-mass distribution corrections calculated using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The resultant structures are compared with a "best theoretical estimate" (BTE), which has recently been shown to be in exceptional agreement with the semi-experimental equilibrium structures of other aromatic molecules. Bond distances of the 1H tautomer are determined to <0.0008 Å and bond angles to <0.2°. For the 2H tautomer, bond angles are also determined to <0.2°, but bond distances are less precise (2σ ≤ 0.0015). Agreement between BTE and reSE values is discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] Reaction of 3-hydroxy-2,5-di- tert -butyl-1,4-benzoquinone with phenylhydrazine hydrochloride mainly leads to the corresponding hydrazone at the position 1 existing in tautomeric equilibrium with 3,6-di- tert -butyl-4-(phenyl- diazenyl)catechol. Thermodynamic parameters of the tautomerization were evaluated by NMR spectroscopy. The hydrazone structure was confirmed by XRD study. [ABSTRACT FROM AUTHOR]

Valence tautomerism (VT) involves the stimulated reversible intramolecular electron transfer between a redox-active metal and ligand. Dinuclear cobalt complexes bridged by bis(dioxolene) ligands can undergo thermally-induced VT with access to {CoIII-cat-cat-CoIII}, {CoIII-cat-SQ-CoII} and {CoII-SQ-SQ-CoII} states (cat2− = catecholate, SQ˙− = semiquinonate, CoIII refers to low spin CoIII, CoII refers to high spin CoII). The resulting potential for two-step VT interconversions offers increased functionality over mononuclear examples. In this study, the bis(dioxolene) ligand 3,3′,4,4′-tetrahydroxy-5,5′-dimethoxy-benzaldazine (thMH4) was paired with Mentpa (tpa = tris(2-pyridylmethyl)amine, n = 0–3 corresponds to methylation at 6-position of the pyridine rings) to afford [{Co(Mentpa)}2(thM)](PF6)2 (1a, n = 0; 2a, n = 2; 3a, n = 3). Structural, magnetic susceptibility and spectroscopic data show that 1a and 3a remain in the temperature invariant {CoIII-cat-cat-CoIII} and {CoII-SQ-SQ-CoII} forms in the solid state, respectively. In contrast, 2a exhibits incomplete thermally-induced VT between these two tautomeric forms via the mixed {CoIII-cat-SQ-CoII} tautomer. In solution, room temperature electronic absorption spectra are consistent with the assignments from the solid-state, with VT observed only for 2a. From electrochemistry, the proximity of the two 1e−-processes for the thMn− ligand indicates weak electronic communication between the two dioxolene units, supporting the potential for a two-step VT interconversion in thMn− containing complexes. Comparison of the redox potentials of the Co and thMn− processes suggests that only 2a has these processes in sufficient proximity to afford the thermally-induced VT observed experimentally. Density functional theory calculations are consistent with the prerequisite energy ordering for a two-step transition for 2a, and temperature invariant {CoIII-cat-cat-CoIII} and {CoII-SQ-SQ-CoII} states for 1a and 3a, respectively. This work presents the third example, and the first formally conjugated example, of a bridging bis(dioxolene) ligand that can afford two-step VT in a Co complex, suggesting new possibilities towards applications based on multistep switching. [ABSTRACT FROM AUTHOR]

A new family of phenanthroline ligands was prepared. Hydrolysis of 4,7-dihalogenated 1,10-phenanthroline-2,9-diamides in acidic media leads to the formation of the corresponding 4,7-oxygenated derivatives. These ligands can exist in different tautomeric forms. The tautomerism of 4,7-oxygenated phenanthroline diamides has been investigated using quantum chemical calculations. The unsymmetrical oxo-hydroxy tautomeric form was proved to be the most energetically advantageous according to the spectral data and X-ray analysis. It was shown that 4,7-difluoro derivatives undergo acid hydrolysis more easily when compared to 4,7-dichloro derivatives. The coordination chemistry of 4,7-oxygenated 1,10-phenanthroline-2,9-diamides toward some lanthanide nitrates was investigated. The structures of Ln-complexes thus formed were studied both in the solid state and in solution (XRD analysis and IR, NMR and UV spectroscopy). It was revealed that 4,7-oxygenated ligands adopt the dihydroxy tautomeric form upon coordination with lanthanide nitrates. [ABSTRACT FROM AUTHOR]

Light-induced polarization switchable molecular materials have attracted attention for decades owing to their potential remote manipulation and ultrafast responsiveness. Here we report a valence tautomeric (VT) complex with an enantiopure chiral ligand. By a suitable choice of counter anions, a significant improvement in photoconversion has been demonstrated, leading to novel photo-responsive polarization switching materials. [ABSTRACT FROM AUTHOR]

The irradiation of water solution of cytidine with UVC light showed that the compound participates in a tautomerism following the kinetics of a first‐order reaction. The PIDA mechanism showed that the proton detachments in the tautomers of cytidine occur through the repulsive 1πσ* excited states and lead to conical intersections S0/S1, whose structures were optimized. More probable is the IRC mechanism, which proceeds along 1nπ* excited state reaction paths. This mechanism includes a water molecule as a catalyst, which assists the phototransformation of amino‐oxo tautomer of cytidine into imino‐hydroxy one. [ABSTRACT FROM AUTHOR]