Chemical Shifts Research Articles

Exploring the structure and dynamics of soft and hard cuticle of Bombyx mori using solid-state NMR techniques

This study conducts a comprehensive analysis and comparison of Bombyx mori cuticles across different developmental stages, ranging from larval to adult, utilizing advanced solid-state NMR techniques. The primary objective is to elucidate the underlying reasons for the contrasting hardness of adult cuticles and softness of larval cuticles. Notably, PXRD analysis reveals a prominent broad peak at 19.34°, indicating the predominantly amorphous nature of both larval and adult cuticles. Analysis of 13C CP-MAS SSNMR spectra highlights an elevated proportion of phenoxy carbon in adult cuticles (6.77%) compared to larval cuticles (1.24%). Furthermore, a distinctive resonance line at 144 ppm is exclusively observed in adult cuticles, due to catechols, suggesting potential biochemical pathway variations during development. Significant variations in the primary components of 13C chemical shift anisotropy (CSA) tensors for aliphatic carbons of amino acids, catechols, and lipids between adult and larval cuticles indicate alterations in electronic environments. Additionally, the shorter spin–lattice relaxation time of carbon nuclei in larval cuticles compared to adult cuticles implies slower motional dynamics with enhanced degree of sclerotization in adults. By investigating the internal structure and dynamics of cuticles, this research not only contributes to biomimetic material development but also enhances our understanding of structural changes across different developmental stages of B. mori.

The essential synergy of MD simulation and NMR in understanding amorphous drug forms.

Molecular dynamics (MD) simulations and chemical shifts from machine learning are used to predict 15N, 13C and 1H chemical shifts for the amorphous form of the drug irbesartan. The local environments are observed to be highly dynamic well below the glass transition, and averaging over the dynamics is essential to understanding the observed NMR shifts. Predicted linewidths are about 2 ppm narrower than observed experimentally, which is hypothesised to largely result from susceptibility effects. Previously observed differences in the 13C shifts associated with the two tetrazole tautomers can be rationalised in terms of differing conformational dynamics associated with the presence of an intramolecular interaction in one tautomer. 1H shifts associated with hydrogen bonding can also be rationalised in terms of differing average frequencies of transient hydrogen bonding interactions.

Expounding the application of nano and micro silica as a complementary additive in metakaolin phosphate geopolymer for ceramic applications—micro and nanoscale structural investigation

Metakaolin phosphate geopolymers comprising poly-phospho-siloxo units are known for their structural performance, additionally advancing their microstructure with the transformation of crystalline berlinite phases at elevated temperatures. The intrinsic reaction of Al of metakaolin in the acid exploited, but the reaction of secondary silica phases is limitedly known. Metakaolin as a primary precursor (M) with the addition of 2% and 5% of nano silica (MS2 and MS5) and micro silica (MM2 and MM5) cast using 8-M phosphoric acid was cured at 80 °C. To enhance the utilization of geopolymer in any high-temperature applications, the structural transformations were studied after heating to various temperatures (200, 400, 600 and 800 °C) by XRD, Raman, TGA-DTA, SEM, XPS, FTIR and MAS-NMR. Sample M attained a strength of 46.2 MPa enhanced to 63.6 MPa in MS5 and 54.2 MPa in MM5. This can be ascribed from the transformation of Si–O–Al–O–Si into Si–O–Al–O–P from Raman bands. Comparing the chemical shift of Al (IV) to control, micro silica addition shifts the signal to a lower field (53 to 50 ppm) related to the increase of the number of Al-connected Si to give a tougher network. Nanoindentation is visualized from hardness and elasticity, and the corresponding values are 1.4 to 2.1 GPa and 0.8 to 1.4 GPa for loads ranging from 20 to 100 mN in silica-reinforced samples that are much higher than M. The micro and macro hardness is due to the reinforcement of quartz in micro silica around the gel. TGA-DTA showed that the reduction of mass loss is as high as 25.4% in control whereas 17.2% in MS5 and 15.8% in the MM5. Further, shrinkage rate in MS5 and MM5 was as low as − 1.1% and − 0.8% throughout the temperature range from 25 to 1000 °C and thus provides the way of use of nano and micro form of silica for better thermal resistance.Graphical

Resolving unresolvable multiplets: 1H NMR spectral simulation of selected sesquiterpenoids from the acorenone-rich rhizome essential oil of sweet flag, Acorus calamus L.

The majority of published 1H NMR data for sesquiterpenes and other natural products are significantly affected by overlapping signals, especially those of higher order, making them largely unusable or only marginally useful. In this study, an acorenone-rich rhizome essential oil of Acorus calamus L. (Acoraceae, Acorales) yielded pure, potentially biologically significant sesquiterpenoids: preisocalamenediol, isocalamenediol, shyobunone, isoshyobunone, and acorenone. Due to the lack of complete NMR data, we performed a detailed assignment of the 1H and 13C NMR spectra of these compounds. A spin simulation of the 1H NMR spectra was conducted, allowing for the first time the complete determination of all relevant NMR data (1H NMR chemical shifts, 1H-1H coupling constants). This led to the confirmation of their relative configurations and the determination of their most stable conformations. The NMR-derived conformational data offer valuable insights into the structural properties of the oil's sesquiterpenic constituents, enabling molecular docking studies to predict their interactions with biological receptors and explain their observed biological activities.

Rapid Quantification of Protein Secondary Structure Composition from a Single Unassigned 1D 13C Nuclear Magnetic Resonance Spectrum.

The function of a protein is predicated upon its three-dimensional fold. Representing its complex structure as a series of repeating secondary structural elements is one of the most useful ways by which we study, characterize, and visualize a protein. Consequently, experimental methods that quantify the secondary structure content allow us to connect a protein's structure to its function. Here, we introduce an automated gradient descent-based method we refer to as secondary-structure distribution by NMR that allows for rapid quantification of the protein secondary structure composition of a protein from a single, 1D 13C NMR spectrum without chemical shift assignments. The analysis of nearly 900 proteins with known structure and chemical shifts demonstrates the capabilities of our approach. We show that these results rival alternative techniques such as FT-IR and circular dichroism that are commonly used to estimate secondary structure compositions. The resulting method requires only the primary sequence of the protein and its referenced 13C NMR spectrum. Each residue is modeled in an ensemble of secondary structures with percentage contributions from random coil, α-helix, and β-sheet secondary structures obtained by minimizing the difference between a simulated and experimental 1D 13C NMR spectrum. The capabilities of the method are demonstrated as applied to samples at natural abundance or enriched in 13C, acquired by either solution or solid-state NMR, and even on low magnetic field benchtop NMR spectrometers. This approach allows for rapid characterization of protein secondary structure across traditionally challenging to characterize states including liquid-liquid phase-separated, membrane-bound, or aggregated states.

Topological Heterogeneity of Protein Kinase C Modulators in Human T-Cells Resolved with In-Cell Dynamic Nuclear Polarization NMR Spectroscopy.

Phorbol ester analogs are a promising class of anticancer therapeutics and HIV latency reversing agents that interact with cellular membranes to recruit and activate protein kinase C (PKC) isoforms. However, it is unclear how these esters interact with membranes and how this might correlate with the biological activity of different phorbol ester analogs. Here, we have employed dynamic nuclear polarization (DNP) NMR to characterize phorbol esters in a native cellular context. The enhanced NMR sensitivity afforded by DNP and cryogenic operation reveals topological heterogeneity of 13C-21,22-phorbol-myristate-acetate (PMA) within T cells utilizing 13C-13C correlation and double quantum filtered NMR spectroscopy. We demonstrate the detection of therapeutically relevant amounts of PMA in T cells down to an upper limit of ∼60.0 pmol per million cells and identify PMA to be primarily localized in cellular membranes. Furthermore, we observe distinct 13C-21,22-PMA chemical shifts under DNP conditions in cells compared to model membrane samples and homogenized cell membranes, that cannot be accounted for by differences in conformation. We provide evidence for distinct membrane topologies of 13C-21,22-PMA in cell membranes that are consistent with shallow binding modes. This is the first of its kind in-cell DNP characterization of small molecules dissolved in the membranes of living cells, establishing in-cell DNP-NMR as an important method for the characterization of drug-membrane interactions within the context of the complex heterogeneous environment of intact cellular membranes. This work sets the stage for the identification of the in-cell structural interactions that govern the biological activity of phorbol esters.

Semisynthesis, Structure Elucidation and Anti-Mycobacterium marinum Activity of a Series of Marine-Derived 14-Membered Resorcylic Acid Lactones with Interesting Ketal Groups.

The incidence of Mycobacterium marinum infection is on the rise; however, the existing drug treatment cycle is lengthy and often requires multi-drug combination. Therefore, there is a need to develop new and effective anti-M. marinum drugs. Cochliomycin A, a 14-membered resorcylic acid lactone with an acetonide group at C-5' and C-6', exhibits a wide range of antimicrobial, antimalarial, and antifouling activities. To further explore the effect of this structural change at C-5' and C-6' on this compound's activity, we synthesized a series of compounds with a structure similar to that of cochliomycin A, bearing ketal groups at C-5' and C-6'. The R/S configuration of the diastereoisomer at C-13' was further determined through an NOE correlation analysis of CH3 or CH2 at the derivative C-13' position and the H-5' and H-6' by means of a 1D NOE experiment. Further comparative 1H NMR analysis of diastereoisomers showed the difference in the chemical shift (δ) value of the diastereoisomers. The synthetic compounds were screened for their anti-microbial activities in vitro. Compounds 15-24 and 28-35 demonstrated promising activity against M. marinum, with MIC90 values ranging from 70 to 90 μM, closely approaching the MIC90 of isoniazid. The preliminary structure-activity relationships showed that the ketal groups with aromatic rings at C-5' and C-6' could enhance the inhibition of M. marinum. Further study demonstrated that compounds 23, 24, 29, and 30 had significant inhibitory effects on M. marinum and addictive effects with isoniazid and rifampicin. Its effective properties make it an important clue for future drug development toward combatting M. marinum resistance.

Atomic-Level Structure of the Organic-Inorganic Interface of Colloidal ZnO Nanoplatelets from Dynamic Nuclear Polarization-Enhanced NMR.

Colloidal semiconductor nanoplatelets (NPLs) have emerged as a new class of nanomaterials that can exhibit substantially distinct optical properties compared to those of isotropic quantum dots, which makes them prime candidates for new-generation optoelectronic devices. Insights into the structure and anisotropic growth of NPLs can offer a blueprint for their controlled fabrication. Here, we present an atomic-level investigation of the organic-inorganic interface structure in ultrathin and stable benzamidine (bza)-supported ZnO NPLs prepared by the modified one-pot self-supporting organometallic approach. High-resolution transmission electron microscopy analysis showed a well-faceted hexagonal shape of ZnO NPLs with lateral surfaces terminated by nonpolar (101̅0) facets. The basal surfaces are flat and well-formed on one side and corrugated on the other side, which indicates that the layer-by-layer growth in the thickness of the NPLs likely occurs only in one direction via the expansion of 2D islands on the surface. The ligand coordination modes were elucidated using state-of-the-art dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy supported by density functional theory chemical shift calculations. Specifically, it was found that (101̅0) nonpolar facets are stabilized by neutral L-type bza-H ligands with hydrogen bond-supported η1-coordination mode, while polar (0001) and (0001̅) facets are covered by μ2-coordinated X-type anionic bza ligands with different conformations of aromatic rings. Moreover, the ligand packing on (101̅0) lateral facets was determined using 13C natural abundance (∼1.1%) homonuclear dipolar correlation experiments. Overall, an in-depth understanding of the growth mechanism and the unique bimodal X-type/L-type ligand coordination shell of ZnO NPLs is provided, which will facilitate further design of anisotropic nano-objects.

The impact of hetero-aromatic rings on optoelectronic and charge transport properties of fused polycyclic compounds

The structural, optoelectronic, and charge-transport properties of linearly fused polycyclic hydrocarbon molecules with heteroatoms (NH, O, S, and Se) in five-membered rings have been investigated using the DFT approach. The hybrid functional B3LYP in combination with the 6–311 + G (d,p) basis set is utilized in the Gaussian 16 W software package for all the calculations. The absorption and emission energies are investigated by using time-dependent density functional theory (TD-DFT) at the same level of theory. To investigate the stability of each molecule concerning its aromaticity, nucleus-independent chemical shift (NICS) values are computed. Electron affinity (EA), hole extraction potential (HEP), ionization potential (IP), and electron extraction potential (EEP) are also reported. The simulated hole (λ h) and electron (λ e) reorganization energies for the majority of the molecules are lower than the characteristics hole and electron-transporting materials. The reported fused polycyclic compounds may have potential applications in optoelectronic devices.

Uncovering Adrenal Incidentalomas – the role of imaging methods in diagnostic process

Introduction and purpose An adrenal incidentaloma is any lesion found unexpectedly during an imaging study performed for an unrelated reason. The detection rate of incidental adrenal tumors is steadily increasing. The clinical presentation of these tumors is influenced by the specific hormones production, which are determined by their origin. Most lesions are hormonally inactive and may go undetected by physicians, although some can be malignant. The aim of this work is to summarize the utility of different imaging modalities in clinical practice. State of knowledge All adrenal incidentalomas have to undergo an imaging procedure to determine if the mass is homogeneous and lipid-rich and therefore benign or if there are some malignant features. Noncontrast computed tomography is the primary imaging technique used to evaluate adrenal lesions based on their size, structure, and density. It is worth remembering that density <10 HU and homogenous appearance is consistent with a benign adrenal mass and no further imaging is required. When additional diagnostic clarification is needed, especially in malignancy suspicion, magnetic resonance imaging with a chemical shift technique should be conducted and it may be followed by nuclear medicine methods. Conclusions Imaging methods play crucial role in diagnostic process. Imaging studies provide easy follow-ups and allow to avoid unnecessary adrenalectomies and its complications. Therefore, they contribute to optimizing patients’ health management.

Structural changes of Natronomonas pharaonis halorhodopsin in its late photocycle revealed by solid-state NMR spectroscopy

Natronomonas pharaonis halorhodopsin (NpHR) is a light-driven Cl− inward pump that is widely used as an optogenetic tool. Although NpHR is previously extensively studied, its Cl− uptake process is not well understood from the protein structure perspective, mainly because in crystalline lattice, it has been difficult to analyze the structural changes associated with the Cl− uptake process. In this study, we used solid-state NMR to analyze NpHR both in the Cl−-bound and -free states under near-physiological transmembrane condition. Chemical shift perturbation analysis suggested that while the structural change caused by the Cl− depletion is widespread over the NpHR molecule, residues in the extracellular (EC) part of helix D exhibited significant conformational changes that may be related to the Cl− uptake process. By combining photochemical analysis and dynamic nuclear polarization (DNP)-enhanced solid-state NMR measurement on NpHR point mutants for the suggested residues, we confirmed their importance in the Cl− uptake process. In particular, we found the mutation at Ala165 position, located at the trimer interface, to an amino acid with bulky sidechain (A165V) significantly perturbs the late photocycle and disrupts its trimeric assembly in the Cl−-free state as well as during the ion-pumping cycle under the photo-irradiated condition. This strongly suggested an outward movement of helix D at EC part, disrupting the trimer integrity. Together with the spectroscopic data and known NpHR crystal structures, we proposed a model that this helix movement is required for creating the Cl− entrance path on the extracellular surface of the protein and is crucial to the Cl− uptake process.

Integrative Modeling of Protein-Polypeptide Complexes by Bayesian Model Selection using AlphaFold and NMR Chemical Shift Perturbation Data.

Protein-polypeptide interactions, including those involving intrinsically-disordered peptides and intrinsically-disordered regions of protein binding partners, are crucial for many biological functions. However, experimental structure determination of protein-peptide complexes can be challenging. Computational methods, while promising, generally require experimental data for validation and refinement. Here we present CSP_Rank, an integrated modeling approach to determine the structures of protein-peptide complexes. This method combines AlphaFold2 (AF2) enhanced sampling methods with a Bayesian conformational selection process based on experimental Nuclear Magnetic Resonance (NMR) Chemical Shift Perturbation (CSP) data and AF2 confidence metrics. Using a curated dataset of 108 protein-peptide complexes from the Biological Magnetic Resonance Data Bank (BMRB), we observe that while AF2 typically yields models with excellent consistency with experimental CSP data, applying enhanced sampling followed by data-guided conformational selection routinely results in ensembles of structures with improved agreement with NMR observables. For two systems, we cross-validate the CSP-selected models using independently acquired nuclear Overhauser effect (NOE) NMR data and demonstrate how CSP and NMR can be combined using our Bayesian framework for model selection. CSP_Rank is a novel method for integrative modeling of protein-peptide complexes and has broad implications for studies of protein-peptide interactions and aiding in understanding their biological functions.

Sensitivity Enhancement of Ultra-Wideline NMR by Progressive Saturation of the Proton Reservoir Under Magic-Angle Spinning.

Solid-state NMR of low-γ nuclides is often characterized by low sensitivity and by significant spectral broadenings induced by the quadrupolar and the chemical-shift anisotropy interactions. Herein, we introduce an indirect acquisition method, termed PROgressive Saturation of the Proton Reservoir Under Spinning (PROSPRUS), which could facilitate the acquisition of ultra-wideline NMR spectra under magic-angle spinning (MAS), in systems with a sufficiently long dipolar relaxation time, T1D. PROSPRUS NMR relies on the generation of so-called second-order dipolar order among abundant protons undergoing MAS, and on the subsequent depletion of this dipolar order by a series of looped cross-polarization events, transferring the proton order into polarization of the low-γ I-nuclei as a function of the latter's offsets. While the spin dynamics of the ensuing experiment is complex, particularly when dealing with narrow I spectral lines, it is shown that PROSPRUS can lead to faithful lineshapes for ultra-wideline spin-1/2 and spin-1 species, providing high sensitivity with extremely low RF power requirements. It is also shown that the ensuing 1H-detected PROSPRUS experiments can efficiently characterize I-spin lineshapes in excess of 1 MHz without having to retune electronics, while providing improvements in sensitivity per unit time over current broadband direct-detection methods by up to a factor of four.

Gas-Phase Studies of NMR Shielding and Indirect Spin-Spin Coupling in 13C-Enriched Ethane and Ethylene.

13C and 1H NMR spectra were observed as the function of density in 1,2-13C-enriched ethane and ethylene for the pure gaseous compounds and their binary mixtures with xenon and carbon dioxide gases as the solvents. All the chemical shifts and indirect spin-spin couplings were linearly dependent on the solvent density. The appropriate NMR parameters (σ and nJ) in isolated 13C2H6 and 13C2H4 molecules and the coefficients responsible for the binary molecular interactions were determined and compared with previous similar measurements and selected calculated shielding data. The newly obtained 13C shielding values in the isolated ethane and ethylene molecules suggest visible secondary isotope effects due to the additional carbon-13 atom. All the investigated shielding parameters depend on intermolecular interactions, and the dependence of 13C shielding is much more marked. In contrast, the indirect spin-spin couplings in 13C2H6 and 13C2H4 molecules are almost independent of solvent molecules. Their nJ values determined in liquids over sixty years ago are generally consistent with the same nJ parameters in isolated 13C2H6 and 13C2H4 molecules.

Metabolic imaging in living plants: A promising field for chemical exchange saturation transfer (CEST) MRI.

Magnetic resonance imaging (MRI) is a versatile technique in the biomedical field, but its application to the study of plant metabolism in vivo remains challenging because of magnetic susceptibility problems. In this study, we report the establishment of chemical exchange saturation transfer (CEST) for plant MRI. This method enables noninvasive access to the metabolism of sugars and amino acids in complex sink organs (seeds, fruits, taproots, and tubers) of major crops (maize, barley, pea, potato, sugar beet, and sugarcane). Because of its high signal detection sensitivity and low susceptibility to magnetic field inhomogeneities, CEST analyzes heterogeneous botanical samples inaccessible to conventional magnetic resonance spectroscopy. The approach provides unprecedented insight into the dynamics and distribution of sugars and amino acids in intact, living plant tissue. The method is validated by chemical shift imaging, infrared microscopy, chromatography, and mass spectrometry. CEST is a versatile and promising tool for studying plant metabolism in vivo, with many applications in plant science and crop improvement.

Oxygen vacancy modulation for enhanced hydrogen production via chemical looping water-gas shift

Chemical looping water gas shift (CL-WGS)is prospective to generate high-purity hydrogen with integrated CO2 capture. However, this technology is impeded by the lack of active oxygen carriers at mid-temperatures. Here, we synthesized several Ni-doped CoFe2O4-δ to modulate oxygen vacancies and investigate its effect on promoting hydrogen production reaction via chemical looping water gas shift at 650 °C. The findings delineate that doping Ni considerably lowers the energy barriers associated with the oxygen vacancies formation, thereby augmenting their concentration. The underlying mechanism elucidates that within the CL-WGS process, the transfer of lattice oxygen acts as the rate-limiting step. NixCo1-xFe2O4 lowers the formation energy of oxygen vacancies and facilitates the bulk lattice oxygen diffusion through the bulk. Hence, Ni0.5Co0.5Fe2O4 demonstrates the most reduction depth and reversibility via redox reactions, resulting in an elevated hydrogen yield (∼15.5 mmol g−1) at 650 °C, which surpasses the yield from undoped CoFe2O4 by 1.4 times. This performance remains consistently high with only a minimal decline over 100 cycles. The findings introduce a promising approach to promote the reactivity of oxygen carriers, particularly for mid-temperature applications.

Experimental, quantum chemical spectroscopic investigation, topological, molecular docking/dynamics and biological assessment studies of 2,6-Dihydroxy-4-methyl quinoline

The present work is a comprehensive investigation of the spectral, electronic structure, bonding, and reactivity of the 2,6-dihydroxy-4-methylquinoline (26DH4MQ) molecule through a wide range of experimental and quantum chemical spectroscopic calculations techniques, along with its applications as nonlinear optical materials and biological assessments. The study utilized density functional theory (DFT) and time-dependent density functional theory (TD-DFT) with the B3LYP method and the 6-311G++(d,p) basis set to analyze the structural and molecular properties of 26DH4MQ. The findings from UV–Vis, FT-IR, and FT-NMR spectroscopy show a strong agreement between the experimentally obtained vibrational frequencies and chemical shifts and those predicted by computational methods. Local reactivity descriptors, such as the dual descriptor, Fukui functions, and the molecular electrostatic potential (MEP) map, were used to identify the reactive regions of the molecule. Additionally, natural bond orbital (NBO) analysis provided insights into the charge transfer characteristics of 26DH4MQ, which helps to stabilize the molecular system. The study also revealed notable nonlinear optical (NLO) properties, with polarizability (18.52 × 10–24e.s.u.) and first-order hyperpolarizability (2.26 × 10–30e.s.u.) values that surpass those of standard organic compounds, suggesting significant potential for optoelectronic applications. The biological evaluation of 26DH4MQ assessed its drug-likeness, toxicity, enzyme inhibition, and ADME (Absorption, Distribution, Metabolism, and Excretion) parameters, highlighting its pharmaceutical potential. Furthermore, molecular docking and dynamics studies illustrated the compound's interaction with proteins, indicating its potential role as an insulin inhibitor.

Synthesis, DFT investigation, antioxidant, molecular docking and ADME-T properties of N′-(1-benzylpiperidin-4-ylidene) furan-2-carbohydrazide derivatives against SARS-CoV-2 spike proteins

ABSTRACT A novel synthetic reaction was used to produce a piperidine derivative, specifically N′-(1-benzylpiperidin-4-ylidene)furan-2-carbohydrazide (BPFC). The structure of the synthesised compound was confirmed through FT-IR, 1H-NMR and 13C-NMR spectral analysis. DFT calculations were performed to optimise structural parameters and investigate Natural Bond Orbital (NBO) analysis, Frontier Molecular Orbitals (FMOs), Molecular Electrostatic Potentials, Mulliken Charge Analysis, and Non-linear Optical properties. The predicted chemical shift values align well with the expected results. The antioxidant properties of the newly synthesised compound were assessed using DPPH, ABTS, and H2O2 radical scavenging assays. Molecular docking studies were also carried out to explore the binding mechanism of the compound with COVID-19-related proteins, particularly 6WCF and 6LU7. Additionally, the synthesised compound was assessed for drug-likeness and ADMET (Absorption, Distribution, Metabolism, Excretion, and Toxicity) properties, revealing favourable pharmacokinetic profiles without any signs of toxicity.

Schleyer hyperconjugative aromaticity in indene scaffolds

In the present research, the Schleyer hyperconjugative aromaticity was used to enhance the aromaticity of indene scaffolds. The first section of the research focused on examining the thermodynamic stability and electronic characteristics of the four structural isomers of indene. Results indicate that 1H-indene scaffolds are the most stable thermodynamic isomers of indene derivatives exceeding 100 kJ/mol. The hierarchy of stability is as follows: 1H-indenes > 2H-indenes > 5H-indenes > 4H-indenes. The aromaticity of 5- and 6-membered rings in the isomeric structures was investigated using the B3LYP/6–311 + G(d,p) method in the second section of the study. The hyperconjugative aromaticity of 5MR and 6MR was assessed using the harmonic oscillator model of aromaticity index, Bird index, Shannon index, aromatic fluctuation index, and the nucleus independent chemical shift. The results reveal that introducing electron-donating groups on 6MR enhances the aromaticity of 5MR. Moreover, adding two fluorine atoms on the Csp3 site negatively affects the aromaticity and induces antiaromaticity in the indene scaffold. Based on aromaticity data, the order of increasing aromaticity in the presence of different groups is F < CH3 < H < SiH3 < GeH3 < Si(CH3)3 < Ge(CH3)3. The extent of the Schleyer hyperconjugation interaction was also quantified using the E(2) parameter obtained from the NBO calculations.

1H, 13C, 15N NMR, and DFT Studies on Complex Formation of Zinc(II) Ion with Ethylenediamine in Ionic Liquid [C2mIm][TFSA

In bis(trifluoromethylsulfonyl)amide (TFSA-)-based ionic liquid (IL), 1-ethyl-3-methylimidazolium TFSA- ([C2mIm][TFSA]), the complex formation equilibria of zinc(II) ion (Zn2+) with ethylenediamine (EN) have been investigated. An EN molecule may coordinate with Zn2+ as a bidentate ligand. First, the formation of Zn2+-EN complexes in [C2mIm][TFSA] was confirmed from the difference of 1H and 13C NMR chemical shift values of EN molecules between [C2mIm][TFSA]-EN binary solvents and the 0.1 mol dm-3 Zn(TFSA)2/[C2mIm][TFSA]-EN solutions as a function of EN mole fraction xEN. Second, the stability constants of Zn2+-EN complexes formed in the IL were determined from the concentration ratio [EN]/[Zn2+] dependence of 15N NMR chemical shift values of the TFSA- N atom in the Zn2+/IL-EN solutions. In the IL, mono-, bis-, and tris-EN complexes are successively formed by 1:1 replacement of TFSA- anions coordinated with Zn2+ by EN molecules with increasing EN content. Third, 1H and 13C NMR measurements with the help of density functional theory (DFT) calculations were made on [C2mIm][TFSA]-EN binary solvents as a function of xEN to clarify key interactions to the mechanism of the complex formation. Fourth, the stability constants of Zn2+-EN complexes in the IL were compared with those in aqueous solutions. It was suggested that the hydrogen bonding of the EN molecule with the imidazolium ring H atoms and the TFSA- O atoms reduces the stability of the mono-EN complex in the IL. In contrast, the intracomplex hydrogen bonds between EN and TFSA- in the first coordination shell contribute to the higher stability of the bis-EN complex in the IL than that in aqueous solutions. The difference in the stability constants between the tris-EN complexes and hexaacetonitrile complexes, where acetonitrile (AN) molecules act as monodentate ligands, was interpreted in terms of the higher electron donicity of EN. Finally, to verify the present evaluation, the experimental 13C NMR chemical shift values of EN molecules in the solutions were compared with the theoretical values calculated by DFT using the stability constants determined.