Strong Intramolecular Hydrogen Bond Research Articles

Efficient Red-Emissive Organic Crystals with Amplified Spontaneous Emissions Based on a Single Benzene Framework.

Red-emissive fluorophores generally consist of large π-extended systems and thus encounter the problem of serious fluorescence quenching in the solid state. A series of structurally simple compounds 2,5-bis(alkylamino)terephthalates 1 a-c are reported that consist of a very small π-system (only a single benzene) but display efficient red emission in crystals. Crystal 1 a having a molecular weight of only 252 g mol-1 shows red emission with the maximum of 620 nm and a fluorescence quantum yield of 0.40. The unique emission property of crystal 1 a is mainly because of the planarization of skeleton dominated by the strong intramolecular hydrogen bonds and the packing structure with negligible π-π interactions contributed by the mini π-system. Moreover, besides efficient red emission, high crystallinity with co-planar facets endows crystal 1 a with significant amplified spontaneous emission.

Efficient Red‐Emissive Organic Crystals with Amplified Spontaneous Emissions Based on a Single Benzene Framework

AbstractRed‐emissive fluorophores generally consist of large π‐extended systems and thus encounter the problem of serious fluorescence quenching in the solid state. A series of structurally simple compounds 2,5‐bis(alkylamino)terephthalates 1 a–c are reported that consist of a very small π‐system (only a single benzene) but display efficient red emission in crystals. Crystal 1 a having a molecular weight of only 252 g mol−1 shows red emission with the maximum of 620 nm and a fluorescence quantum yield of 0.40. The unique emission property of crystal 1 a is mainly because of the planarization of skeleton dominated by the strong intramolecular hydrogen bonds and the packing structure with negligible π–π interactions contributed by the mini π‐system. Moreover, besides efficient red emission, high crystallinity with co‐planar facets endows crystal 1 a with significant amplified spontaneous emission.

Identification of Carboxylate, Phosphate, and Phenoxide Functionalities in Deprotonated Molecules Related to Drug Metabolites via Ion-Molecule Reactions with water and Diethylhydroxyborane.

Tandem mass spectrometry based on ion-molecule reactions has emerged as a powerful tool for structural elucidation of ionized analytes. However, most currently used reagents were designed to react with protonated analytes, making them suboptimal for acidic analytes that are preferentially detected in negative ion mode. In this work we demonstrate that the phenoxide, carboxylate, and phosphate functionalities can be identified in deprotonated molecules by use of a combination of two reagents, diethylmethoxyborane (DEMB) and water. A novel reagent introduction setup that allowed DEMB and water to be separately introduced into the ion trap region of the mass spectrometer was developed to facilitate fundamental studies of this reaction. A new reagent, diethylhydroxyborane (DEHB), was generated inside the ion trap by hydrolysis of DEMB on introduction of water. Most carboxylates and phenoxides formed a DEHB adduct, followed by addition of one water molecule and subsequent ethane elimination (DEHB adduct +H2O - CH3CH3) as the major product ion. Phenoxides with a hydroxy group adjacent to the deprotonation site and phosphates formed a DEHB adduct, followed by ethane elimination (DEHB adduct - CH3CH3). Deprotonated molecules with strong intramolecular hydrogen bonds or without the aforementioned functionalities, including sulfates, were unreactive toward DEHB/H2O. Reaction mechanisms were explored via isotope labeling experiments and quantum chemical calculations. The mass spectrometry method allowed the differentiation of phenoxide-, carboxylate-, phosphate-, and sulfate-containing analytes. Finally, it was successfully coupled with high-performance liquid chromatography for the analysis of a mixture containing hymecromone, a biliary spasm drug, and its three possible metabolites. Graphical Abstract ᅟ.

Modeling interactions between a β-O-4 type lignin model compound and 1-allyl-3-methylimidazolium chloride ionic liquid.

Aiming at understanding the molecular mechanism of the lignin dissolution in imidazolium-based ionic liquids (ILs), this work presents a combined quantum chemistry (QC) calculation and molecular dynamics (MD) simulation study on the interaction of the lignin model compound, veratrylglycerol-β-guaiacyl ether (VG) with 1-allyl-3-methylimidazolium chloride ([Amim]Cl). The monomer of VG is shown to feature a strong intramolecular hydrogen bond, and its dimer is indicated to present important π-π stacking and intermolecular hydrogen bonding interactions. The interactions of both the cation and anion of [Amim]Cl with VG are shown to be stronger than that between the two monomers, indicating that [Amim]Cl is capable of dissolving lignin. While Cl- anion forms a hydrogen-bonded complex with VG, the imidazolium cation interacts with VG via both the π-π stacking and intermolecular hydrogen bonding. The calculated interaction energies between VG and the IL or its components (the cation, anion, and ion pair) indicate the anion plays a more important role than the cation for the dissolution of lignin in the IL. Theoretical results provide help for understanding the molecular mechanism of lignin dissolution in imidazolium-based IL. The theoretical calculations on the interaction between the lignin model compound and [Amim]Cl ionic liquid indicate that the anion of [Amim]Cl plays a more important role for lignin dissolution although the cation also makes a substantial contribution.

Chalcogen substitution: Effect of oxygen-by-sulfur exchange on structural and spectroscopic properties of flavonols

Quantum chemical calculations using density functional theory have revealed that the substitution of the ketone group by a thioketone one doesn't significantly change the structure of 3-hydroxyflavone. Notably, the strong intramolecular hydrogen bond involving the hydroxyl function is preserved if the O atom is substituted by an S atom, but also by Se or Te atoms. However, fundamental modifications are observed in both absorption and emission electronic spectra upon substitution, particularly the non-fluorescent behavior of the flavothione. All these changes were explained by the optimizations of the different excited states carried out by time-dependent DFT method.

Predicting the Position of the Hydrogen Atom in the Short Intramolecular Hydrogen Bond of the Hydrogen Maleate Anion from Geometric Correlations

The position of the hydrogen atom inside the strong and short intramolecular hydrogen bond of the hydrogen maleate anion strongly varies depending on the crystalline environment. Therefore, it has not been possible in the past to accurately determine it using X-ray diffraction data although there are 292 hydrogen maleate crystal structures with different cations in the literature. In this study, a geometric correlation for the accurate prediction of the hydrogen position in the short intramolecular hydrogen bond is presented. The results used to derive the correlation are obtained from low-temperature neutron-diffraction studies on nine different hydrogen maleate salts that span the whole range from perfectly symmetric to highly asymmetric intramolecular hydrogen bonds. Since the only variable in the correlation as derived from the neutron data is the O···O distance, the hydrogen atom position in question can subsequently be predicted using information that is accurately available from routine X-ray data....

Simple Design of an Enzyme-Inspired Supported Catalyst Based on a Catalytic Triad

Summary Enzyme active sites afford an intricate interplay of functional groups to mediate complex organic and inorganic reactions. Many hydrolytic enzymes use a catalytic triad comprising three different functional residues—(Ser(-OH), Hist(-imidazole), Asp(-CO 2 H))—that catalyze the hydrolysis of numerous unique substrates. Inspired by this design, we have developed a simple one-step synthesis for preparing a new supported catalytic system in which the three reactive groups of the catalytic triad (alcohol, imidazole, and carboxylate) are incorporated into a single functional unit. These artificial active sites can be coupled to a solid-phase support (Merrifield resin) by copper(I)-catalyzed azide-alkyne cycloaddition "click chemistry," and their effectiveness as esterolysis catalysts was demonstrated. Furthermore, tuning the local hydrophobicity of the resin particles with an approach analogous to the native enzyme hydrophobic pocket increased the catalytic efficiency. Quantum mechanics and molecular dynamics computational modeling were used to probe the catalytic effect and suggested a concerted two-step mechanism and hydrophobic nanoenvironment similar to that of hydrolytic enzymes.

Substituent and intramolecular hydrogen-bond effect on the fluorescent emission of two easy-synthesizable fused rigid bicyclic octadiene derivatives

Tetra-tert-butyl-cis,cis-3,7-dihydroxybicyclo[3.3.0]octa-2,6-diene-2,4-exo-6,8-exo-tetracarboxylate (I) and tetra-tert-butyl-cis,cis-3,7-dimethoxybicyclo[3.3.0]octa-2,6-diene-2,4-exo-6,8-exo-tetracarboxylate (II) were prepared in a three step synthetic procedure. These compounds show two central five carbon atom rings, fused in such a way to define a central bicyclo[3.3.0]octa-2,6-diene core. The carbon atoms which fuse both rings have sp3 hybridization, then they are not coplanar. A dihedral angle of about 63° corresponds to butterfly conformation. UV–Vis spectra of I and II measured in solution show symmetrical bands centred around 245 nm (ε ∼ 104 M−1cm−1). These bands are consistent with π→π* transitions. TD-DFT simulated spectra over the DFT optimized in gas phase confirms this hypothesis, and additionally suggest a non-negligible contribution of n→π* transition for II. The slight dependence of λmax on solvent polarity experimentally observed for II is consistent with some nπ* character. After excitation at 250 nm, a weak emission around 400 nm was detected for both compounds, with quantum yield values below detection limit for I. The value of λem of II was observed to be sensible to the solvent polarity, confirming some relevant n→π* character. The almost fully quenching of the emission of I in solution would be attributed to a rather strong intramolecular hydrogen bond established between the hydroxyl group and the oxo-oxygen atom from tert-butoxy group, which is observed in the crystal structure of the compound (O⋯O ranges from 2.635(2) Å to 2.672(2) Å). We hypothesize that it is probably preserved in solution due to the molecular rigidity, and would be the responsible for the quenching of the emission in solvent solution.

NMR and IR Investigations of Strong Intramolecular Hydrogen Bonds.

For the purpose of this review, strong hydrogen bonds have been defined on the basis of experimental data, such as OH stretching wavenumbers, νOH, and OH chemical shifts, δOH (in the latter case, after correction for ring current effects). Limits for O–H···Y systems are taken as 2800 > νOH > 1800 cm−1, and 19 ppm > δOH > 15 ppm. Recent results as well as an account of theoretical advances are presented for a series of important classes of compounds such as β-diketone enols, β-thioxoketone enols, Mannich bases, proton sponges, quinoline N-oxides and diacid anions. The O···O distance has long been used as a parameter for hydrogen bond strength in O–H···O systems. On a broad scale, a correlation between OH stretching wavenumbers and O···O distances is observed, as demonstrated experimentally as well as theoretically, but for substituted β-diketone enols this correlation is relatively weak.

Theoretical insights into the ultrafast excited-state intramolecular proton transfer (ESIPT) mechanism in a series of amide-based N[sbnd]H⋯N hydrogen-bonding compounds

Excited-state intramolecular proton transfer (ESIPT) has many important potential applications. In this paper, the geometries of a series of amide-based NH⋯N hydrogen-bonding compounds in their ground S0 states and first excited singlet S1 states were optimized with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approaches. Both topological analysis and noncovalent interactions analysis show strong intramolecular hydrogen-bonds in the studied five systems and the electron density function ρ(r) exhibits good linear relationship with the distance of H⋯N2. The potential energy curves of the S0 and S1 states were scanned by using of DFT and TD-DFT to elucidate the ESIPT process. It reveals that all the systems considered here can undergo an ultrafast ESIPT reaction, with energy barrier of less than 0.05 eV, giving rise to the single fluorescence emission from the proton-transfer tautomer. It is also found that the shorter the hydrogen bond in the normal-form S1 state, the easier the ESIPT takes place.

Spectroscopic, DFT, and XRD Studies of Hydrogen Bonds in N-Unsubstituted 2-Aminobenzamides.

The structures of the mono- and the dihalogenated N-unsubstituted 2-aminobenzamides were characterized by means of the spectroscopic (1H-NMR, UV-Vis, FT-IR, and FT-Raman) and X-ray crystallographic techniques complemented with a density functional theory (DFT) method. The hindered rotation of the C(O)–NH2 single bond resulted in non-equivalence of the amide protons and therefore two distinct resonances of different chemical shift values in the 1H-NMR spectra of these compounds were observed. 2-Amino-5-bromobenzamide (ABB) as a model confirmed the presence of strong intramolecular hydrogen bonds between oxygen and the amine hydrogen. However, intramolecular hydrogen bonding between the carbonyl oxygen and the amine protons was not observed in the solution phase due to a rapid exchange of these two protons with the solvent and fast rotation of the Ar–NH2 single bond. XRD also revealed the ability of the amide unit of these compounds to function as a hydrogen bond donor and acceptor simultaneously to form strong intermolecular hydrogen bonding between oxygen of one molecule and the NH moiety of the amine or amide group of the other molecule and between the amine nitrogen and the amide hydrogen of different molecules. DFT calculations using the B3LYP/6-311++G(d,p) basis set revealed that the conformer (A) with oxygen and 2-amine on the same side predominates possibly due to the formation of a six-membered intramolecular ring, which is assisted by hydrogen bonding as observed in the single crystal XRD structure.

High-capacity hollow porous dummy molecular imprinted polymers using ionic liquid as functional monomer for selective recognition of salicylic acid

The existence of strong intramolecular hydrogen bond in salicylic acid (SA) weakens its intermolecular hydrogen bonding with functional monomer, then it is a challenge work to fabricate molecularly imprinted polymers (MIPs) for SA recognition with high capacity and good selectivity. Here, hollow porous dummy MIPs (HPDMIPs) were prepared using benzoic acid (BA) as dummy template, ionic liquid (i.e. 1-vinyl-3-methylimidazolium chloride) as functional monomer, and MCM-48 as sacrificial support. Factors that affected adsorption, such as type of template and porogen, mole ratio of template–functional monomer–cross-linker and type of binding solvent, were optimized in detail. Multiple strong interactions between SA and ionic liquid in HPDMIPs deduced higher binding capacity (29.75mg/g), imprinting factor (5.61) and selectivity than any previously reported MIPs by traditional or surface imprinting technology. The large surface area (543.9m2/g) with hollow porous structure resulted in faster kinetic binding (25min). The equilibrium data fitted well to Freundlich equation and the adsorption process could be described by pseudo-second order model. Finally, HPDMIPs were successfully applied to selectively extract and enrich SA from Actinidia chinensis with a relatively high recovery (84.6–94.5%).

Polymeric and monomeric copper(II) thiophene- and furancarboxylato complexes. Bridging and terminal coordination of 3-pyridylmethanol

The synthesis and characterization of eight new coordination compounds [Cu(2-tpc)2(μ-3-pyme)2]n (1), [Cu(3-Me-2-tpc)2(μ-3-pyme)2]n (2), [Cu(5-Me-2-tpc)2(μ-3-pyme)2]n (3), [Cu(5-Cl-2-tpc)2(3-pyme)2] (4), [Cu(2-fuc)2(μ-3-pyme)2]n (5), [Cu(3-fuc)2(μ-3-pyme)2]n (6), [Cu(2,5-Me2-3-fuc)2(μ-3-pyme)2]n (7), and [Cu(5-NO2-2-fuc)2(μ-3-pyme)2]n (8) (where 2-tpc is 2-thiophenecarboxylato, 3-Me-2-tpc is 3-methyl-2-thiophenecarboxylato, 5-Me-2-tpc is 5-methyl-2-thiophenecarboxylato, 5-Cl-2-tpc is 5-chloro-2-thiophenecarboxylato, 2-fuc is 2-furancarboxylato, 3-fuc is 3-furancarboxylato, 2,5-Me2-3-fuc is 2,5-dimethyl-3-furancarboxylato, 5-NO2-2-fuc is 5-nitro-2-furancarboxylato and 3-pyme is 3-pyridylmethanol) is reported and their X-ray structures were determined. X-ray analysis revealed that samples 1–3 and 5–8 are coordination polymers, whereas the complex 4 is monomeric. The polymeric extension is achieved through bridging N,O-3-pyridylmethanol molecules, resulting in the observation of 2-D (1–3, 5–7) or 1-D polymeric chains (8). In addition, the coordination polymers are also stabilized by strong intramolecular hydrogen bonds. On the other hand, the monomeric compound 4 with monodentate N-coordinated 3-pyridylmethanol ligands forms 1-D supramolecular chain due to strong intermolecular hydrogen bonds. Electronic, IR, EPR spectra and magnetic susceptibility measurements over the temperature range 1.8–300K are discussed in terms of known crystal and molecular structure. Based on the IR spectra it can be concluded that the type of coordination of 3-pyridylmethanol as well as hydrogen bonds strongly influence the position and the shape of the stretch vibrations bands assigned to hydroxyl group.

Fluorine atom influence on intramolecular hydrogen bonding, isomerization and methyl group rotation in fluorinated acetylacetones

1-Fluoro-pentane-2,4-dione (monofluoroacetylacetone, MFAA) is an asymmetric β-diketone with a strong intramolecular hydrogen bond similar to acetylacetone (AA) and its fluorinated analogs 1,1,1-trifluoro-(TFAA), and 1,1,1,5,5,5-hexafluoroacetylacetone (HFAA). The presence of a fluorine atom in MFAA has the potential to open an HF elimination channel in its gas-phase photochemistry motivating this study of MFAA hydrogen bonding by computer modeling using Density Functional Theory (DFT). As a context, we also report DFT modeling of AA and selected fluorinated acetylacetones: 1,1-difluoro-pentane-2,4-dione (difluoroacetylacetone, DFAA), TFAA, and HFAA. The most stable molecular structure for all three asymmetric β-diketones (MFAA, DFAA and TFAA) is the isomer with the fluoromethyl group proximal to the carbonyl carbon; in comparison to the proton transfer isomer, which has the fluoromethyl group proximal to the hydroxyl carbon, the carbonyl isomer is lower in energy by 5.1–5.5kJmol−1 (B3LYP/cc-pVTZ) and 2.1–3.7kJmol−1 (MP2/Aug-cc-pVTZ). We also report hydrogen bond strengths, barriers to proton transfer interconversion and barriers to rotation of the methyl/fluoromethyl groups. Our study, the first to report molecular structure information on MFAA and DFAA, indicates that the most stable chelated isomer of MFAA is not the structure with the strongest hydrogen bond as conventionally determined. Our modeling also reveals a coupling between proton transfer isomerization and methyl group rotation, and an unexpected double-minimum potential for the rotation of the fluoromethyl group of MFAA.

Role of Hydrogen Bonding on the Reactivity of Thiyl Radicals: A Mass Spectrometric and Computational Study Using the Distonic Radical Ion Approach.

Experimental and computational quantum chemistry investigations of the gas-phase ion-molecule reactions between the distonic ions +H3N(CH2)nS• (n = 2-4) and the reagents dimethyl disulfide, allyl bromide, and allyl iodide demonstrate that intramolecular hydrogen bonding can modulate the reactivity of thiyl radicals. Thus, the 3-ammonium-1-propanethiyl radical (n = 3) exhibits the lowest reactivity of these distonic ions toward all substrates. Theoretical calculations on this distonic ion highlight that its most stable conformation involves a six-membered ring configuration, and that it has the strongest intramolecular hydrogen bond. In addition, the calculations indicate that the barrier heights for radical abstraction by this hydrogen-bond-stabilized 3-ammonium-1-propanethiyl radical are the highest among the systems examined, consistent with the experimental observations.

Aminomethylation of naphthalen-2-ol and naphthalene-2,7-diol

Reactions of secondary aliphatic and heterocyclic diamines with formaldehyde and naphthalen-2-ol or naphthalene-2,7-diol afforded the corresponding Mannich bases. The formation of strong intramolecular hydrogen bonds O−H · · · N in the molecules of some products was detected by IR and 1H NMR spectroscopy.

Electronic effect on the photophysical properties of 2-(2-hydroxyphenyl)benzothiazole-based excited state intramolecular proton transfer fluorophores synthesized by sonogashira-coupling reaction

For excited-state intramolecular proton transfer (ESIPT) fluorophores, the intramolecular hydrogen bond strength has decisive influence on their photophysical properties. Herein, a series of 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives as new ESIPT fluorophores are synthesized through Sonogashira-coupling reaction, and their electronic structures are systematically tuned through various substitutions with different Hammett substituent constants (σ) in the 5-position of the phenol ring. Theoretical calculations and crystal analysis show that the HBT derivative with larger σ contains the stronger intramolecular hydrogen bond, which is less likely to be damaged by external factors, and that it has larger energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. The experimentally observed trends are well in line with the theoretical calculations. More importantly, unlike other HBT derivatives and parent HBT displaying anion emission in PBS medium, the two HBT derivatives bearing methyl ester and cyano substitutions, respectively, emit as keto tautomer, which are expected to be applied in biological aspects.

New insights into the conformal stability, influence of hydrogen bonding and vibrational analysis of 2,6- and 3,5-Dihydroxyacetophenone – A comparative study

The conformal stability of 2,6-Dihydroxyacetophenone (DHAP I) and 3,5-Dihydroxyacetophenone (DHAP II) were investigated using Potential Energy Surface scan studies and B3LYP/6-31G** level Density Functional Theory(DFT) calculations. The experimental (FT-IR and FT-Raman) spectra were recorded in the condensed phase in order to perform detailed vibrational analysis. The optimized geometry and vibrational wavenumbers were determined by using B3LYP/6-31G** level Density Functional Theory calculations. The effect of scaling on the calculated wavenumbers was analyzed by employing unscaled, uniform scaling and selective scaling procedures. A detailed comparative analysis on the effect of hydrogen bonding on the geometry, structural parameters and vibrational wavenumbers has been carried out using the results obtained from PES scan and DFT level calculations. The results from the above studies reveals that, the presence of strong intramolecular hydrogen bond in DHAP I has significantly altered the geometrical parameters and hence the vibrational wavenumbers. The hydrogen bonding in DHAP I is strong enough to make it more stable compared to that of DHAP II.

Synthesis and structure of 5,7-Di(tert-butyl)-2-(8-methanesulfonyloxyquinolin-2-yl)-1,3-tropolone

5,7-Di(tert-butyl)-2-(8-methanesulfonyloxyquinolin-2-yl)-1,3-tropolone whose structure was established by X-ray diffraction analysis and confirmed by 1H and 13C NMR, IR spectroscopy, and mass spectrometry, was obtained by the reaction of 2-methyl-8-methanesulfonyloxyquinoline with 3,5-di(tert-butyl)-1,2-benzoquinone. According to X-ray diffraction study, this sulfonyloxyquinolinotropolone exists in the NH tautomeric form with methanesulfonyloxy group in the exo position to the tropolone ring and the sixmembered chelate ring produced by the quinoline and tropolone moieties stabilized by strong intramolecular hydrogen bond. The strong intramolecular hydrogen bond between the phenolic oxygen atom and the six-membered chelate ring provides supplementary contribution to the stabilization of the NH tautomeric form.

DFT study of glycosyl group reactivity in quercetin derivatives

Density functional theory (DFT) is used to compute relevant electronic properties with the purpose of generating precise information which facilitates the best activity given by the positions of glycosyl group attached at all 3 different rings of quercetin such as Q3G (C- ring), Q7G (A-ring) and Q3′G (B-ring). Computed values of the OH BDE, frontier molecular orbitals (FMOs), molecular electrostatic potential (MEP), Density of states (DOS,PDOS,OPDOS) and electronic properties such as electron affinity (EA), ionization potential (IP), softness (S), hardness (η), electronegativity (χ) and electrophilic index (ω) indicate that the title compounds possess good radical scavenging activity. Charge delocalization and intramolecular hydrogen bonds are characterized using natural bond orbital (NBO) analysis. NBO accurately differentiate the weak and strong intramolecular hydrogen bond of quercetin-O-glycoside compounds. Results available from the computational investigation have proved that A-ring glycoside of quercetin is capable of donating electrons and acts as a good anti-oxidant than B-ring glycoside and C-ring glycoside of quercetin.