Reduced Density Gradient Isosurfaces Research Articles

Effects of amino‐ and dimethylamine‐groups on the fluorescent properties of novel flavone‐based derivatives with ESIPT behavior: A TD‐DFT study

AbstractIn this work, the effects of different functional groups on the excited‐state intramolecular proton transfer behavior and fluorescent features of 3‐hydroxy‐2‐(naphthalen‐2‐yl)‐4H‐chromen‐4‐one (HFN) are explored in detail. Three new compounds (HFN‐1, HFN‐2, and HFN‐3) are designed based on the HFN by introducing the electron‐donating groups (–NH2, –N(CH3)2). The mainly geometrical parameters of optimized configuration showed that the intramolecular hydrogen bonds of the studied compounds are enhanced in the excited (S1) state, confirming by the ground (S0) and excited states infrared spectra, electron densities and reduced density gradient isosurfaces. The S0 → S1 transitions of HFN derivatives are dominated by ππ* charge transfer transition. The introduction of electron‐donating group (–NH2, –N(CH3)2) weakens the intramolecular hydrogen bond, increases the excited‐state intramolecular proton transfer barrier and red‐shifts the absorption/fluorescence peak. The coexistence of –NH2 and –N(CH3)2 make the absorption and fluorescence wavelengths of HFN‐1/HFN‐2 red‐shift/blue‐shift.

ZnCl2:ChCl:2urea as a new ternary deep eutectic for clean production of high content zwitterionic micro- or nano-cellulose by passing to the binary DES of ZnCl2:2urea

Deep eutectic solvents (DESs) are clean alternatives for chemical reagents and reaction media. Herein, a novel method for zwitterionzation of micro-/nano-cellulose (MC/NC) by a novel ternary DES (TDES) from ZnCl2, choline chloride (ChCl), and urea was established. In the characterized TDES of ZnCl2:ChCl:2urea, by phase diagram, viscosity, density, FT-IR, and TGA analyses, ZnCl3−Ch+ is hydrogen bond-acceptor (HBA) and two urea molecules are hydrogen bond-donors (HBDs). The theoretical investigations on donor–acceptor mechanism by Functional Density Theory, the reduced density gradient isosurface, and electrostatic potential surface (EPS demonstrate the co-existences of the H-bonding and electrostatic-interactions in formation of ZnCl2:ChCl:2urea. These interactions between ZnCl3−Ch+ complex as HBA and urea HBDs provide a higher equilibrium than the related binary DESs. By this new TDES, alkalized MC or NC was zwitterionized without any additional reagent and the products were characterized by FT-IR, FESEM, EDX, PZC, TGA, and XRD analyses. Dual positively/negatively sites on cellulose and decrease in crystallinity-index (7%) and thermal-stability detected for the given zwitterionized celluloses. In this new protocol, the simply prepared ZnCl2:ChCl:2urea served as both of reaction medium for dissolution of cellulose and cholinating agent for the zwitterionization of cellulose by nucleophilic attack of the alkalized MC/NC to activated-ChCl by TDES.

Noncovalent Interactions and Crystal Structure Prediction of Energetic Materials.

The crystal and molecular structures, intermolecular interactions, and energy of CL-20, HATO, and FOX-7 were comparatively predicted based on molecular dynamic (MD) simulations. By comparison, the 2D fingerprint plot, Hirshfeld surface, reduced density gradient isosurface, and electrostatic potential surface were studied to detect the intermolecular interactions. Meanwhile, the effects of vacuum and different solvents on the crystal habit of CL-20, HATO, and FOX-7 were studied by AE and MAE model, respectively. The energy calculation was also analysed based on the equilibrium structures of these crystal models by MD simulations. Our results would provide fundamental insights for the crystal engineering of energetic materials.

Noble-gas compounds: A general procedure of bonding analysis.

This paper accounts for a general procedure of bonding analysis that is, expectedly, adequate to describe any type of interaction involving the noble-gas (Ng) atoms. Building on our recently proposed classification of the Ng-X bonds (X = binding partner) [New J. Chem. 44, 15536 (2020)], these contacts are first distinguished into three types, namely, A, B, or C, based on the topology of the electron energy density H(r) and on the shape of its plotted form. Bonds of type B or C are, then, further assigned as B-loose (Bl) or B-tight (Bt) and C-loose (Cl) or C-tight (Ct) depending on the sign that H(r) takes along the Ng-X bond path located from the topological analysis of ρ(r), particularly at around the bond critical point (BCP). Any bond of type A, Bl/Bt, or Cl/Ct is, finally, assayed in terms of contribution of covalency. This is accomplished by studying the maximum, minimum, and average value of H(r) over the volume enclosed by the low-density reduced density gradient (RDG) isosurface associated with the bond (typically, the RDG isosurface including the BCP) and the average ρ(r) over the same volume. The bond assignment is also corroborated by calculating the values of quantitative indices specifically defined for the various types of interactions (A, B, or C). The generality of our taken approach should encourage its wide application to the study of Ng compounds.

New benzyltriethylammonium/urea deep eutectic solvent: Quantum calculation and application to hyrdoxylethylcellulose modification

In this paper, a new deep eutectic solvent (DES) has been successfully synthesized that is based on benzyltriethylammonium bromide as a hydrogen bond acceptor (HBA) and urea as a hydrogen bond donor (HBD). However, its usability in modifying cellulose derivatives, especially acylating hydroxyethylcellulose (HEC) was investigated. The chemical modification (acetylation) of HEC was carried out in BTEAB/urea DES system without any additional conventional solvent or catalyst. However, the proposed structure of acetylated HEC (HECA) was confirmed according to the structural spectra analyses FTIR-ATR, 1H, 13C, and APT-NMR. The crystalline behavior of acetylated and unmodified HEC in the DES system has been evaluated using XRD patterns, where the thermal stability was evaluated basing on the TD-TGA thermograms. Hence, SEM images and EDX spectra were recorded to prove the changes that are expected at the morphological level and elemental profile. Yet, the nanometric sheets aspect was observed. The Functional Density Theory (DFT) was investigated as a useful computational tool to understand mechanism and donor-acceptor interactions. The topological parameters (electron density Laplacian, kinetic energy density, potential energy density, and energy density) at the bond critical points (BCP), between TBEAB and urea, are deducted according to Quantum Bader's theory, and Atoms-in-molecules (AIM). The non-covalent interactions and steric effect in the DES system were studied using the reduced density gradient isosurface (RDG). Theoretical and computational calculations revealed that the H-bonds and the electrostatic coexist, as predominant interactions in the BTEAB-based DES resulting chemical structure, and mechanism formation. The physical interactions between the component entities of DES lead to a new equilibrium that is more stable than that of HBA and HBD in their separate states.

Systematic theoretical investigation of two novel molecules BtyC-1 and BtyC-2 based on ESIPT mechanism

Inexperiment, Song et al. have successfully synthesizedtwo novel molecules BtyC-1 and BtyC-2 and observedasingle and dual fluorescence peaks in these two molecules respectively. (Song et al. Tetrahedron Lett. 2019, 60, 1696–1701) However, they still lack a detailed and reasonable theoretical explanation. Then we wonder why these two similar structures behave so much differently? In this work, we focus on explaining the photochemical and photophysical properties of BtyC-1 and BtyC-2 by studying the excited state intramolecular proton transfer (ESIPT) mechanisms. Based on the optimized geometric configurations, the calculated infrared spectra indicate the intramolecular hydrogen bonding interactions are heightened in their excited states. The frontier molecular orbitals reflect the charge redistribution in photoinduced process, which explains that the driving force of ESIPT process is provided by enhanced hydrogen bonding interactions. In the meantime, the calculations of potential energy curves vividly explain the principle of the experimental dual fluorescence phenomenon. The analysis of Mulliken charges deepens the discussion of molecular structures on the potential energy barriers. Calculated absorption spectra via using density functional theory and emission spectra via using time-dependent density functional theory are consistent with the experimental data, which confirms the correctness of our calculation methods. The reduced density gradient isosurfaces help us distinguish the complex non-covalent bonds. Base on the above analyses, we conclude that there is no stable structure for BtyC-1 in excited state, which make it occur the ESIPT reaction spontaneously. BtyC-2 exists a stable normal structure in excited state. Its dual fluorescence signals are emitted by its normal and isomer structures, respectively.

Substituent Effects on Excited-State Intramolecular Proton Transfer Reaction of 2-Aryloxazoline Derivatives

Different substituents and benzene ring numbers had significant effects on the fluorescence phenomenon of 2-aryloxazoline derivatives as observed in an experiment. Here, we select five 2-aryloxazoline derivatives with different substituents and benzene ring numbers (2u, 2ad, 2af, 2ai, and 2ah) to analyze the effects on the fluorescence phenomena. For 2ad, 2ah, and 2ai, first, the geometric structures are optimized based on the density functional theory and time-dependent density functional theory methods. The analysis of the obtained bond parameters reveals the variation of hydrogen bond interactions from S0 to S1 states. Second, the calculated absorption and emission spectra are consistent with the experimental values, which proves that the theoretical method is feasible. Finally, through the analysis of the infrared vibrational spectrum, reduced density gradient isosurfaces, frontier molecular orbitals, and potential energy curves, the strengthening mechanism of the hydrogen bond interaction and the ability of the excited-state intramolecular proton transfer (ESIPT) reaction to occur are further explained. Since the proton transfer reactions of 2u and 2af occur spontaneously under photoexcitation, they have no stable structures in the S1 state. In conclusion, due to the different substituents, 2u is more prone to the proton transfer reaction than 2ad. For 2af, 2ai, and 2ah with different benzene ring numbers, the ESIPT reaction is more difficult to occur as the number of benzene rings increases. The ability of the ESIPT reaction to occur follows the order 2af → 2ah → 2ai. For 2-aryloxazoline derivatives with different substituents or different benzene ring numbers, the hydrogen bond strengthening mechanism has been authenticated, which promotes the occurrence of the ESIPT reactions.

Theoretical investigation of fluorescence changes caused by methanol bridge based on ESIPT reaction* *Project supported by the National Basic Research Program of China (Grant No. 2019YFA0307701), the National Natural Science Foundation of China (Grant No. 11874180), the Young and Middle-aged Scientific and Technological Innovation Leaders and Team Projects in Jilin Province (Grant No. 20200301020RQ).

The different fluorescence behavior caused by the excited state proton transfer in 3-hydroxy-4-pyridylisoquinoline (2a) compound has been theoretically investigated. Our calculation results illustrate that the 2a monomer in tetrahydrofuran solvent would not occur proton transfer spontaneously, while the 2a complex in methanol (MeOH) solvent can undergo an asynchronous excited state intramolecular proton transfer (ESIPT) process. The result was confirmed by analyzing the related structural parameters, infrared vibration spectrum and reduced density gradient isosurfaces. Moreover, the potential curves revealed that with the bridging of single MeOH molecular the energy barrier of ESIPT was modulated effectively. It was distinctly reduced to 4.80 kcal/mol in 2a-MeOH complex from 25.01 kcal/mol in 2a monomer. Accordingly, the ESIPT process induced a fluorochromic phenomenon with the assistant of proton-bridge. The elucidation of the mechanism of solvent discoloration will contribute to the design and synthesis of fluorogenic dyes as environment-sensitive probes.

A detecting Al3+ ion luminophor 2-(Anthracen-1-yliminomethyl)-phenol: Theoretical investigation on the fluorescence properties and ESIPT mechanism

2-(Anthracen-1-yliminomethyl)-phenol (AYP) had been synthesized recently and used as a chemosensor to detect Al3+ ion, while its fluorescent properties and excited-state intramolecular proton transfer (ESIPT) process were not investigated in detail. In this study, the molecular absorption and emission spectra were accurately reproduced by using TDDFT/CAM-B3LYP/6-31 + G(d,p) computational method. The ESIPT- chromophore photochemical behaviors and detecting Al3+ ion photophysical changes were explained for the first time at the molecular level. As driving force of ESIPT reaction, the bond parameters and vibrational frequencies of intramolecular hydrogen bond were analyzed by optimizing structures and calculating infrared spectra, analysis of frontier molecular orbitals and reduced density gradient isosurfaces. To further elucidate the proton transfer reactive paths, we scanned the potential energy curves of AYP chemosensor in different electronic states. By comparing potential barriers of the S0 and S1 states, the proton transfer is confirmed to occur in the S1 state. In addition, the experimentally unpresented AYP-Enol fluorescence signal was assigned via analyzing molecular fluorescent properties. Moreover, the calculated fluorescence spectra were employed to explain carefully the mechanism of detection of Al3+ ion.

Theoretical investigation into the cooperativity effect between the intermolecular π∙π and H-bonding interactions in the curcumin∙cytosine∙H2O system.

In order to reveal the mechanism of drug action and design of DNA/RNA-targeted drugs containing aromatic rings, the cooperativity effects between the intermolecular π∙∙∙π and H-bonding interactions in curcumin(drug)∙∙∙cytosine(DNA/RNA base)∙∙∙H2O were investigated by the B3LYP-D3 and MP2(full) methods with the 6-311++G(2d,p) basis set. The π∙∙∙π interaction plays an important role in stabilizing the linear ternary complexes with the cooperativity effects, and the cyclic structures suffer the anticooperativity effects. The cooperativity or anticooperativity effects are notable, which could lead to a possible significant change in drug activity. The hydration is essentially the cooperativity or anticooperativity effect. These results were confirmed by the atoms in molecules (AIM), reduced density gradient (RDG), and surface electrostatic potentials analyses. The cyclic complexes are more stable, from which it can be deduced that the drug always links with the DNA/RNA base and H2O by the π∙∙∙π or H-bonding interactions, and only in this way can the drug activity be shown. Therefore, the designed DNA/RNA-targeted drugs should possess a certain number of hydrophilic groups in contact with the DNA/RNA base and H2O to reconcile drug activity by the cooperativity effect between the π∙∙∙π and H-bonding interactions, as is in agreement with many of the drugs in use. Graphical abstract RDG isosurface of ternary complex.

Theoretical Study of the ESIPT Process for a New Natural Product Quercetin.

The investigation of excited-state intramolecular proton transfer (ESIPT) has been carried out via the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) method for natural product quercetin in dichloromethane (DCM) solvent. For distinguishing different types of intramolecular interaction, the reduced density gradient (RDG) function also has been used. In this study, we have clearly clarified the viewpoint that two kinds of tautomeric forms (K1, K2)originated from ESIPT processconsist inthe first electronic excited state (S1). The phenomenon of hydrogen bonding interaction strengtheninghas been proved by comparing the changes of infrared (IR) vibrational spectra and bond parameters of the hydrogen bonding groups in the ground state with that in the first excited state. The frontier molecular orbitals (MOs)provided visual electron density redistribution have further verified the hydrogen bond strengthening mechanism. It should be noted that the ESIPT process of the K2 form is easier to occur than that of the K1 form via observing the potential energy profiles. Furthermore, the RDG isosurfaces has indicated that hydrogen bonding interaction of the K2 form is stronger than that of the K1 formin the S1 state, which is also the reason why the ESIPT process of the K2 form is easier to occur.

Energetics of non-covalent interactions from electron and energy density distributions

Non-covalent interactions dictate how molecules interact with their surroundings. Enhancing their knowledge is crucial to explain phenomena of utmost importance like self-assembly, chemical reactivity and crystallization. In this work, the possibility of investigating Non-Covalent Interactions (NCIs) by using the Reduced Density Gradient (RDG) in tandem with energy densities descriptors is explored. A sample of 30 molecular adducts, spanning dispersive, hydrogen bonds and XH⋯π interactions was considered. Potential relationships among molecule⋯molecule stabilization energies and energy densities were sought for. Adducts characterized by NCIs having similar physical origins exhibit an excellent linear correlation between stabilization energies and kinetic energy densities integrated over the volume enclosed by low-value RDG isosurfaces. Estimating stabilization energies this way is computationally unexpensive and applicable also to electron densities derived from experiment, where a reliable approximation to the kinetic energy density in the intermolecular regions is afforded through Abramov’s functional. Potential energy densities, when averaged over the basins enclosed by low-value RDG isosurfaces, assume different values according to the kind of interaction class (dispersive, HBs, XH···π, and so on). This observation provides a new recipe to disentangle how the various NCIs contribute to the total stabilization energy. Implications on the possibility of retrieving quantitative thermodynamic information from the topology of suitable scalar fields are discussed.

NCImilano: an electron-density-based code for the study of noncovalent interactions

NCImilano, a Fortran90 code for applying the reduced density gradient (RDG) descriptor to a real-space study of noncovalent interactions, is presented. This code has been specifically designed for the X-ray charge density community, as it can deal with both gas-phase and solid-state electron densities as evaluated by popular multipolar (XD2006) and Gaussian-based quantum mechanical (Gaussian03/09,CRYSTAL) computational platforms. Moreover, it implements for the first time the possibility of plotting energy densities over RDG isosurfaces.

Non-covalent interaction via the reduced density gradient: Independent atom model vs experimental multipolar electron densities

The novel non-covalent interaction (NCI) descriptor proposed by Johnson et al. (J. Am. Chem. Soc. 2010, 132, 6498–6506) and based on the reduced density gradient (RDG), is applied to three molecular crystals, spanning a wide range of NCI classes. The key-aspect of this work is to contrast the NCI description obtained using the RDG of the electron density (ED) derived from single-crystal X-ray diffraction data with that calculated from the RDG of the corresponding Independent Atom Model (IAM) density. It is shown that the RDG isosurfaces obtained from the simpler, unperturbed model density are often able to highlight the same interactions revealed by the corresponding surfaces using the crystalline density, but differ systematically in size and are generally much less structured than these latter. A qualitative rationale for such differences is provided for the first time. Our results question the use of the simpler density model to study NCI in large biological systems.

Quantum chemical topology investigation on structure, electronic properties and interaction of CuNg n + (n = 1–3, Ng = He, Ne)

The structures and stabilities of title clusters have been investigated at CCSD(T) computational level. For the title systems, the geometry with high symmetry is preferred and the n = 2 systems are more stable than its neighbors. For the Cu–Ng interaction, topological analysis of the electron density field, electron localization function, and positive local energy density represent the intermediate interaction type $$ \left( {\nabla^{2}{}_{(3,\; - 1)} \rho > 0\; {\text{and}}\;E\left( r \right) < 0} \right) $$ . The interaction region is located by generating reduced density gradient isosurface in the real molecular space.