Nuclear Independent Chemical Shift Research Articles

A Theoretical Investigation of Novel Sila- and Germa-Spirocyclic Imines and Their Relevance for Electron-Transporting Materials and Drug Discovery.

A new class of spirocyclic imines (SCIs) has been theoretically investigated by applying a variety of quantum chemical methods and basis sets. The uniqueness of these compounds is depicted by various peculiarities, e.g., the incidence of planar six-membered rings each with two imine groups (two π bonds) and the incorporation of the isosteres carbon, silicon, or germanium spiro centers. Additional peculiarities of these novel SCIs are mirrored by their three-dimensionality, the simultaneous occurrence of nucleophilic and electrophilic centers, and the cross-hyperconjugative (spiro-conjugation) interactions, which provoke charge mobility along the spirocyclic scaffold. Substitution of SCIs with strong electron-withdrawing substituents, like the cyano group or fluorine, enhances their docking capability and impacts their reactivity and charge mobility. To gain thorough knowledge about the molecular properties of these SCIs, their structures have been optimized and various quantum chemical concepts and models were applied, e.g., full NBO analysis and the frontier molecular orbitals (FMOs) theory (HOMO-LUMO energy gap) and the chemical reactivity descriptors derived from them. For the assessment of the charge density distribution along the SCI framework, additional complementary quantum chemical methods were used, e.g., molecular electrostatic potential (MESP) and Bader's QTAIM. Additionally, using the aromaticity index NICS (nuclear independent chemical shift) and other criteria, it could be shown that the investigated cross-hyperconjugated sila and germa SCIs are spiro-aromatics of the Heilbronner Craig-type Möbius aromaticity.

Acoplanarity, Aromaticity, Chirality, and Helical Twisting Power of Chlorin e6 13(N)-Methylamide-15,17-dimethyl Ester Complexes: Effect of a Metal

The experimental and theoretical study of the influence of metal complexing on geometry, aromaticity, chirality, and the ability to twist the nematic phase by complexes based on modified natural chlorin e6 was carried out. The geometry optimization of the chlorin e6 13(N)-methylamide-15,17-dimethyl ester (MADMECl) and its Zn, Cu, and Ni complexes by DFT (CAM-B3LYP/6–31 G(d,p) functional) method was performed. Based on these calculations, the acoplanarity degree of the macrocyclic ligand and the distortion energy of its dianion were estimated, which allowed the arrangement of the MADMECl complexes in the series Ni > Cu > Zn. Aromaticity was evaluated using the NICS criterion (nuclear independent chemical shift). An increase in the degree of aromaticity of the macrocycle upon complex formation was established. At the same time, the aromaticity of the inner conjugation contour corresponds to the same series as the acoplanarity, while the outer π-delocalization is characterized by the reverse sequence. An experimental evaluation of the electron circular dichroism of the Soret and the Q-bands, as well as the g-factor of dissymmetry, was carried out. The growth of these parameters with an increase in the degree of acoplanarity and aromaticity of the internal conjugation contour was determined. The induction of helical phases in mixtures of nematic liquid crystals (LCs) based on cyanobiphenyls and MADMECl macrocyclic metal complexes was studied by polarization microscopy, and the clearance temperatures and helix pitch of the mesophases were measured. A strong effect of the metal on the phase transition temperature and helical twisting power was established.

Computational investigation of tuning the electronic ability and featured for heterofullerene based dye sensitized solar cells

The structural characteristics of the heteroatom substituted fullerene to improve its physical and chemical properties are discussed in this work, highlighting possible applications in aromaticity, photocatalysis, solar cells, and superconducting materials. The energy gap of doped fullerene lowers significantly, making C31Nb a more reactive material and transforming it into an efficient superconductor. The molecular structure, energy and relative stabilities of the heterofullerene were examined and evaluated to determine the material's identification. According to the results analysis, the extra niobium atom and substituted carbon atom improve the electronic stability of heterofullerene. Using 13C NMR nuclear independent chemical shift, the stability of each fullerene and aromatic found in nature is discovered. Furthermore, the simulated infrared spectra of fullerene are reviewed, and the major distinctive peaks are given to different functional groups. NBO study which shows the intermolecular charge transfer from the donor to the acceptor in doped fullerene, demonstrates that the strong intermolecular contact between carbon and niobium atoms makes this material a notable material for NLO property.

Electronic Materials: An Antiaromatic Propeller Made from the Four‐Fold Fusion of Tetraoxa[8]circulene and Perylene Diimides

The synthesis of an antiaromatic tetraoxa[8]circulene annulated with four perylene diimides (PDI), giving a dynamic non-planar π-conjugated system, is described. The molecule contains 32 aromatic rings surrounding one formally antiaromatic planarized cyclooctatetraene (COT). The intense absorption (ϵ=3.35×105 M−1 cm−1 in CH2Cl2) and emission bands are assigned to internal charge-transfer transitions in the combined PDI-circulene π-system. The spectroscopic data is supported by density functional theory calculations, and nuclear independent chemical shift calculation indicate that the antiaromatic COT has increased aromaticity in the reduced state. Electrochemical studies show that the compound can reversibly reach the tetra- and octa-anionic states by reduction of the four PDI units, and the deca-anionic state by reduction of the central COT ring. The material functions effectively in bulk hetero junction solar cells as a non-fullerene acceptor, reaching a power conversion efficiency of 6.4 %.

Electronic Materials: An Antiaromatic Propeller Made from the Four-Fold Fusion of Tetraoxa[8]circulene and Perylene Diimides.

The synthesis of an antiaromatic tetraoxa[8]circulene annulated with four perylene diimides (PDI), giving a dynamic non-planar π-conjugated system, is described. The molecule contains 32 aromatic rings surrounding one formally antiaromatic planarized cyclooctatetraene (COT). The intense absorption (ϵ=3.35×105  M-1  cm-1 in CH2 Cl2 ) and emission bands are assigned to internal charge-transfer transitions in the combined PDI-circulene π-system. The spectroscopic data is supported by density functional theory calculations, and nuclear independent chemical shift calculation indicate that the antiaromatic COT has increased aromaticity in the reduced state. Electrochemical studies show that the compound can reversibly reach the tetra- and octa-anionic states by reduction of the four PDI units, and the deca-anionic state by reduction of the central COT ring. The material functions effectively in bulk hetero junction solar cells as a non-fullerene acceptor, reaching a power conversion efficiency of 6.4 %.

A New Family of Tropylium Ions: Tropone–Triarylborane Complexes

AbstractAir‐ and thermally stable tropone–triarylborane 1 : 1 complexes were synthesized in excellent yields by treating tropones, including benzo‐fused derivatives, with an equimolar amount of B(C6F5)3. NMR and X‐ray analyses and DFT calculations including nuclear independent chemical shift (NICS) and anisotropy of the induced current density (ACID) studies revealed that the tropone–triarylborane complexes have betaine structures consisting of tropylium and borate moieties and that their aromaticity depends on the number of fused benzene rings.

Vacatadithiasapphyrins: Synthesis, Structure, and Spectral and Redox Properties.

Four examples of novel aromatic [22]vacatadithiasapphyrins were synthesized by refluxing appropriate [22]telluradithiasapphyrins in 1,2-dichlorobenzene in the presence of excess HCl followed by simple column chromatographic purification. The [22]vacatadithiasapphyrins can exist in three conformers "in", "out", and "zigzag" w.r.t the butadiene moiety, and under our experimental conditions, the "out" conformer was the major compound. The X-ray structure obtained for one of the "out" conformers of vacatadithiasapphyrins revealed that the macrocycle was planar similar to its parent telluradithiasapphyrin and showed effective π-delocalization over the entire macrocyclic core. NMR studies supported the formation of the "out" conformer and suggested that the vacatadithiasapphyrins were less aromatic than the parent telluradithiasapphyrins. Density functional theory, time-dependent DFT, and nuclear independent chemical shift studies indicated that the protonated form of vacatadithiasapphyrin was more aromatic than the parent protonated telluradithiasapphyrin. The absorption spectra of vacatadithiasapphyrins showed a typical strong Soret band at ∼488 nm and four relatively broad Q-bands in the region of 590-860 nm, and electrochemical studies suggest that vacatadithiasapphyrins were easier to oxidize and easier to reduce compared to the parent telluradithiasapphyrins.

Nanodiamond islands confined between two graphene sheets as perspective 2D quantum materials

Based on direct space DFT PBE0/6-31G* electronic structure calculations, the structure and properties of nanodiamond islands confined between two finite graphene fragments (NDI-c2G) was proposed and theoretically explored. DFT simulations revealed that fusion of planar aromatic molecules with two parent graphene fragments may form either cubic or hexagonal allotropes of NDI-c2Gs lattices accompanied with formation of local corrugated sp3 sites with substituted dangling bonds embedded into graphene sublattices. It was shown that at the DFT level of theory, low distortion energies of NDI-c2Gs lattices are comparable or smaller than the energy of van-der-Waals interactions, which allows the NDI-c2G lattices to be stabilized by a support and finally synthesized. The Nuclear Independent Chemical Shift calculations, shows that in the vicinity of NDI regions, graphene lattices are estimated to be either low-, or anti-aromatic. Furthermore, the formation of NDI-c2Gs leads to localization of HOMO and LUMO states at different sites of the NDI-c2Gs lattices. The NDI-c2G regions with confined frontier orbitals can be considered as arrays of quantum dots isolated from each other by NDI scattering centres of 3.82 Å dimension. The results demonstrates that NDI-c2G should be considered as strongly-correlated entangled hybrid quantum dots which may form extended quantum ensembles with great potential for advanced quantum applications.

Synthesis and Studies of Core-Modified Tellura Dithiasapphyrins.

Four different aromatic meso-substituted tellurophene-containing dithiasapphyrins were synthesized by acid-catalyzed [3+2] condensation of 16-telluratripyrrane with bithiophene diol in 22-23% yields. The structural, spectral, and electrochemical properties of tellura dithiasapphyrins were studied and compared with those of previously reported aza (pyrrole)- and other heterocycle (furan, thiophene, and selenophene)-containing dithiasapphyrins. Nuclear magnetic resonance studies indicated that the tellurophene ring in tellura dithiasapphyrins is in the normal conformation, facing toward the inner core, but flips in diprotonated derivatives to an inverted conformation, facing away from the macrocyclic core, unlike aza- and other heterocycle-containing dithiasapphyrins in which pyrrole and the corresponding heterocycle ring always prefer to be in inverted conformation in their neutral and protonated forms. The crystal structure obtained for one of the tellura dithiasapphyrins showed that the macrocycle is highly planar and the tellurophene ring is in the normal conformation. The absorption spectra of tellura dithiasapphyrins exhibited slight hypsochromic shifts compared to those of pyrrole- and other heterocycle-containing dithiasapphyrins. The redox studies indicated that the tellura dithiasapphyrins are electron deficient and readily undergoes reductions. Density functional theory and nuclear independent chemical shift studies indicated that the macrocycles are aromatic, and the computational results closely matched the experimental observations.

Calculated Magnetic Properties of a Recently Synthesized Corannulene-Dibenzobistetracene Hybrid.

Comparison is made between two calculated magnetic properties of a recently synthesized 24-ring corannulene-dibenzobistetracene hybrid (Corannulene-DBT): nuclear independent chemical shifts published by Xu et al. and Hückel-London-Pople-McWeeny (HLPM) ring currents and bond currents that are presented here. There is agreement in 18 of the 24 rings in Corannulene-DBT. This is seen as further evidence that the HLPM approach has a surprising ability to reproduce complex patterns of current in large polycyclic hydrocarbons.

The theoretical investigation on properties of paeonol and its isomers

Paeonol (2-hydroxy-4-methoxyacetophenone) is the main active ingredient of Traditional Chinese Medicine Moutan Cortex and has a wide range of pharmacological activities. Its isomer 2-hydroxy-5-methoxyacetophenone has also been shown to own the potential for drug. The exploration of the structure and properties of these compounds is helpful for drug design, especially the influence of intramolecular hydrogen bonds on the properties of compounds that medicinal chemists are interested in. In this study, the geometric structure parameters, vibration frequency, topological parameters of electron density, nuclear independent chemical shift, electrostatic potential and reactivity index of paeonol and its isomers in the gas and water phase were calculated by means of quantum chemical calculation (DFT/B3LYP). The equilibrium structure, infrared spectroscopy, intramolecular hydrogen bonding, and reactivity of the two compounds were analysed, and the effect of water solvent was considered. This study reveals the structure and properties of selected compounds at the atomic level and will provide theoretical guidance and inspiration for paeonol-based drug design.

Electride Characteristics of Some Binuclear Sandwich Complexes of Alkaline Earth Metals, M2(η5-L)2 (M = Be, Mg; L = C5H5-, N5-, P5-, As5-).

Ab initio calculations have been performed for a series of binuclear sandwich complexes, M2(η5-L)2. It has been observed that the eclipsed and staggered conformations have almost equal amount of energies. The M-M bond lengths are comparable with those in the free M2 molecules (M = Be, Mg). The nuclear-independent chemical shift (NICS) values indicate the aromaticity of these complexes. The stability of Be2(η5-L)2 complexes is higher than that of the Mg2(η5-L)2 complexes. The natural bond orbital (NBO) analysis and electron density descriptors proved the existence of a single covalent M-M bond in an M22+ fragment. It has been observed that each M-M bond contains a non-nuclear attractor (NNA) at the center of the respective bond. The Laplacian of electron density [∇2ρ(r)] is negative at the NNAs. The energy decomposition analysis (EDA) showed that M22+ and 2L- represent the bonding interaction in the complexes. All of the designed binuclear sandwich complexes behave as electrides.

A DFT quest for effects of fused rings on the stability of remote N-heterocyclic carbenes

Assuming aromaticity (cyclic continuous conjugation, planarity, and obeying the Huckel 4n + 2 rule), effects of one and two fused six-membered heterocyclic rings are investigated on the energy lowering (stabilization) of 22 novel singlet (s) and triplet (t) carbenes, at B3LYP/AUG-cc-pVTZ and M06-2X/AUG-cc-pVTZ. Results display that (1) exclusive of triplet pyridine-4-ylidene, and s and t states, other species appear as ground state, so every s Hammick carbene exhibits more stability than its corresponding t state; (2) the highest stability is demonstrated by unsubstituted pyridine-4-ylidene as reference carbene, and the lowest stability is shown by carbene situated between two nitrogen heteroatoms of two fused rings, in a “W” arrangement; (3) regarding the relationship between carbenic center (CC) and substituted heteroatom, the order of stabilization for fused rings is meta > para > ortho; (4) regardless of how organized, fusion of one six-membered ring, in a given arrangement, has more stabilizing effect than two six-membered rings; (5) contrary to our expectation, t Hammick carbenes show higher band gap (ΔEHOMO-LUMO) than their corresponding s species; (6) based on the NICS (nuclear independent chemical shift) results, the least stable carbene has the most aromaticity in its pyridine ring; and (7) according to proposed homomolecular isodesmotic reactions, all s states are stabilized via π-donor/σ-acceptor substitution more than the t states.

Factors Governing the Chemical Stability and NMR Parameters of Uracil Tautomers and Its 5-Halogen Derivatives.

We report on the density functional theory (DFT) modelling of structural, energetic and NMR parameters of uracil and its derivatives (5-halogenouracil (5XU), X = F, Cl, Br and I) in vacuum and in water using the polarizable continuum model (PCM) and the solvent model density (SMD) approach. On the basis of the obtained results, we conclude that the intramolecular electrostatic interactions are the main factors governing the stability of the six tautomeric forms of uracil and 5XU. Two indices of aromaticity, the harmonic oscillator model of aromaticity (HOMA), satisfying the geometric criterion, and the nuclear independent chemical shift (NICS), were applied to evaluate the aromaticity of uracil and its derivatives in the gas phase and water. The values of these parameters showed that the most stable tautomer is the least aromatic. A good performance of newly designed xOPBE density functional in combination with both large aug-cc-pVQZ and small STO(1M)−3G basis sets for predicting chemical shifts of uracil and 5-fluorouracil in vacuum and water was observed. As a practical alternative for calculating the chemical shifts of challenging heterocyclic compounds, we also propose B3LYP calculations with small STO(1M)−3G basis set. The indirect spin–spin coupling constants predicted by B3LYP/aug-cc-pVQZ(mixed) method reproduce the experimental data for uracil and 5-fluorouracil well.

MnB6-: An Open-Shell Metallaboron Analog of 3d Metallabenzenes.

We report a study of the structure and bonding of a transition-metal-doped boron cluster, MnB6-, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra indicate a significant geometry change between the anionic and neutral ground states of MnB6. The electron affinity of MnB6 is measured to be 2.4591(5) eV, and vibrational frequencies for five of its vibrational modes were determined. The experimental data are combined with theoretical calculations to determine the structure and bonding of MnB6-, which is found to be planar with a B-centered hexagonal structure (C2v symmetry) and a quintet spin state (5A2). Nuclear-independent chemical shift calculations indicate that MnB6- is aromatic. Molecular orbital analyses reveal that MnB6- contains three π orbitals, one of which is singly occupied. Hence, MnB6- can be considered as an open-shell metallaboron analog of 3d metallabenzenes.

Theoretical investigation on radical anion promoted electrocyclization in photochromes

Rapid electrocyclization is proposed under radical anionic conditions in organic photochromes. DFT calculations have been performed to investigate the radical anion mediated electrocyclization in different organic photochromes. Furthermore, the activation barriers under radical anionic conditions are compared with those in neutral and radical cationic conditions. The nuclear independent chemical shift (NICS(0)) and synchronicity calculations have been performed for the confirmation of concerted nature and aromatic character of transition states, respectively. The activation barrier for thermal return of cyclophanediene (CPD) to dihydropyrene (DHP) under radical anionic conditions is very lower (ΔH = 5.92 kcal/mol, ΔG = 6.97 kcal/mol) than under neutral conditions, but higher than that in radical cationic conditions (ΔH = 3.13 kcal/mol, ΔG = 4.0 kcal/mol). Similarly, the other prominent classes of photochromes; dithienylethene (ΔH = 20.12 kcal/mol, ΔG = 21.55 kcal/mol) and vinylheptafulvene (ΔH = 23.72 kcal/mol, ΔG = 24.82 kcal/mol) have shown decreased activation barrier under radical anionic condition. However, activation barrier of fulgide under radical anionic conditions is not different than those under neutral and radical cationic conditions. Synchronicity and NICS(0) values for organic photochromes also show significant changes under radical anionic conditions.

Novel tetrazol‐5‐germavinylidenes with triplet monomeric, normal, and abnormal forms studied using DFT method

AbstractTriplet ground state‐monomeric 1,4‐disubstituted‐tetrazole‐5‐germavinylidenes (normal, 1R) and their corresponding 1,3‐disubstituted‐tetrazole‐5‐germavinylidenes (abnormal, 2R) are computed and compared at B3LYP/6‐311++G** level, with R = H, CN, CF3, SH, NH2, OMe, and OH. Every triplet germavinylidene is more stable than its corresponding singlet form (except for 1CF3). Likewise, every triplet abnormal isomer (2R) emerges more stable than its corresponding normal isomer (1R). The most stable triplet monomeric species are 1H and 2H. All abnormal 2R isomers show higher nucleophilicity (N) and smaller band gaps (ΔEHOMO‐LUMO) than their corresponding normal 1R isomers. Electron‐donating species increase N in both 1R and 2R (R = H > NH2 > SH > OCH3 > OH > CF3 > CN). The nuclear independent chemical shift (NICS) results indicate that every 1R is more aromatic than its corresponding 2R.

What is the driving force behind molecular triangles and their guests? A quantum chemical perspective about host-guest interactions.

The physical nature of host-guest (HG) interactions occurring between molecular triangles and linear anions was explored using density functional theory (DFT) calculations combined with energy decomposition analyses (EDA), nuclear independent chemical shift (NICS), and non-covalent interaction index (NCI). We demonstrate that: (i) in addition to the host being significantly rigid, the strain energies are not negligible, especially for host 2; (ii) halogen anions interact mainly by electrostatic forces (ΔEelst > ΔE > ΔEdisp), meanwhile; (iii) trihalogen anions interact mostly by dispersion forces (ΔEdisp > ΔEelst≈ΔE). The NICS and NCI calculations corroborate the idea that HG interactions are considerably mediated through dispersion terms, and also indicate an antiaromatic character inside the host walls.

Significance of five membered heterocycles in fine tuning of HOMO-LUMO gap of simple donor-acceptor system as organic solar cell material: A DFT approach

The significance of five membered heterocycles in simple donor-acceptor system was elaboratively studied by density functional theory (DFT) and time dependent density functional theory (TDDFT). By effectively substituting eight different rings the HOMO-LUMO gap can be reduced considerably. The nuclear independent chemical shift (NICS) calculations help to predict the aromaticity of central heterocyclic systems. The natural bond orbital (NBO) analysis reveals the charge contribution. The close observation of frontier molecular orbitals (FMO) gives a detailed explanation of delocalization.

Novel triplet germylenes in focus: normal vs. abnormal triplet exocyclic tetrazol-5-vinylidene germylenes at DFT.

Substituent effects on stability (assumed as the singlet and triplet energy gaps, ΔΕS-T) of novel 1,4-disubstituted-tetrazole-5-vinylidene germylenes (normal, 1R) and their corresponding 1,3-disubstituted-tetrazole-5-vinylidene germylenes (abnormal, 2R) are computed and compared, at B3LYP/6-311++G** and M06/6-311++G**, where R = H, CN, CF3, F, SH, C6H6, OMe, and OH. Interestingly, every triplet vinylidene germylene shows more stability than its corresponding singlet. Also, every triplet abnormal isomer (2R) emerges to be more stable than its corresponding normal (1R). All abnormal 2R isomers show broader band gaps (ΔEHOMO-LUMO) and higher nucleophilicity (N), but less electrophilicity (ω) than their corresponding normal 1R isomers. The NICS (nuclear-independent chemical shift) results indicate that every 1R (except singlet 2Ph) emerges more aromatic than its corresponding 2R. Our Hammet studies indicate that 1R is more sensitive to the electronic effects of substituents, R, than 2R. Electron-donating species increase N in both 1R and 2R, while electron-withdrawing groups increase stability.