Method and basis set dependence of the NICS indexes of aromaticity for benzene

A multi-descriptor analysis of substituent effects on the structure and aromaticity of benzene derivatives: π-Conjugation versus charge effects

This work provides a detailed multi-component analysis of aromaticity in monosubstituted (X = CH3, CH 2 - , CH 2 + , NH2, NH-, NH+, OH, O-, and O+) and para-homodisubstituted (X = CH3, CH2, NH2, NH, OH, and O) benzene derivatives. We investigate the effects of substituents using single-reference (B3LYP/DFT) and multireference (CASSCF/MRCI) methods, focusing on structural (HOMA), vibrational (AI(vib)), topological (ELFπ), electronic (MCI), magnetic (NICS), and stability (S0-T1 splitting) properties. The findings reveal that appropriate π-electron-donating and π-electron-accepting substituents with suitable size and symmetry can interact with the π-system of the ring, significantly influencing π-electron delocalization. While the charge factor has a minimal impact on π-electron delocalization, the presence of a pz orbital capable of interacting with the π-electron delocalization is the primary factor leading to a deviation from the typical aromaticity characteristics observed in benzene.

An overview of Helium-3 NMR: Recent developments and applications

The present review is focused on experimental and theoretical methods together with applications of helium NMR in chemistry and biochemistry. It comprises two main sections, the first dealing with standardization and instrumentation for 3He NMR spectroscopy and the second dealing with its practical applications, mainly those in general and organic chemistry with a special emphasis on the rapidly developing and exciting area of fullerenes encapsulating helium atoms. Several general applications of 3He NMR spectroscopy in physical chemistry and biomedicine are also briefly discussed.

Computational 1 H and 13 C NMR in structural and stereochemical studies

Present review outlines the advances and perspectives of computational ¹ H and ¹³ C NMR applied to the stereochemical studies of inorganic, organic, and bioorganic compounds, involving in particular natural products, carbohydrates, and carbonium ions. The first part of the review briefly outlines theoretical background of the modern computational methods applied to the calculation of chemical shifts and spin-spin coupling constants at the DFT and the non-empirical levels. The second part of the review deals with the achievements of the computational ¹ H and ¹³ C NMR in the stereochemical investigation of a variety of inorganic, organic, and bioorganic compounds, providing in an abridged form the material partly discussed by the author in a series of parent reviews. Major attention is focused herewith on the publications of the recent years, which were not reviewed elsewhere.

Aromaticity Concepts Derived from Experiments

Aromaticity, a very important term in organic chemistry, has never been defined unambiguously. Various ways to describe it come from different phenomena that have been experimentally observed. The most important examples related to some theoretical concepts are presented here.

On Aromaticity of the Aromatic α-Amino Acids and Tuning of the NICS Indices to Find the Aromaticity Order

The NICS aromaticity indices of the rings in flexible phenylalanine (Phe), tryptophan (Trp), tyrosine (Tyr), and histidine (His) chiral molecules were analyzed. These molecules have several dozens of conformers, and their rings are slightly non-planar. Therefore, the population-averaged NICS_pav index was defined, and the NICS scans had to be performed with respect to planes found by the least-squares routine. A rule differentiating an obverse and a reverse ring face in aromatic amino acids was formulated. The NICS scan minima corresponding to the obverse and reverse face were unequal, which prompted us to use the term ring face aromaticity/ring face tropicity. It appeared that for Phe, Trp, Tyr, and His, the reverse face has always had higher ring face aromaticity/ring face tropicity than the obverse one. Despite the NICS modifications, uncertainty about the amino acid aromaticity order remained. This motivated us to use the integral INICS index newly proposed by Stanger as well. Then, the following sequence was obtained: Trp(phenyl) > Phe > Trp(pyrrole) > His > Tyr. The juxtaposition of the INICS indices of amino acids with that of some model rings revealed a fair transferability of the values. Finally, analysis of the substituent effect on INICS demonstrated that the aromaticity of Tyr is the lowest due to the strength of the OH group π-electron-donating effect able to perturb enough the ring charge distribution and its magnetic aromaticity. The NICS calculations were executed using the ARONICS program written within the project.

A nitrogen-doped asymmetric phenalenyl with a zwitterionic ground state

Herein, an asymmetric N-doped phenalenyl (BTAP) compound has been synthesized and carefully studied for the first time. The synthesis of BTAP reveals a planar configuration and an unexpected zwitterionic ground state with the negative charge delocalized around the circumjacent part and the positive charge mainly localized on the center.

Benchmarking Magnetizabilities with Recent Density Functionals

We have assessed the accuracy of the magnetic properties of a set of 51 density functional approximations, including both recently published and already established functionals. The accuracy assessment considers a series of 27 small molecules and is based on comparing the predicted magnetizabilities to literature reference values calculated using coupled-cluster theory with full singles and doubles and perturbative triples [CCSD(T)] employing large basis sets. The most accurate magnetizabilities, defined as the smallest mean absolute error, are obtained with the BHandHLYP functional. Three of the six studied Berkeley functionals and the three range-separated Florida functionals also yield accurate magnetizabilities. Also, some older functionals like CAM-B3LYP, KT1, BHLYP (BHandH), B3LYP, and PBE0 perform rather well. In contrast, unsatisfactory performance is generally obtained with Minnesota functionals, which are therefore not recommended for calculations of magnetically induced current density susceptibilities and related magnetic properties such as magnetizabilities and nuclear magnetic shieldings. We also demonstrate that magnetizabilities can be calculated by numerical integration of magnetizability density; we have implemented this approach as a new feature in the gauge-including magnetically induced current (GIMIC) method. Magnetizabilities can be calculated from magnetically induced current density susceptibilities within this approach even when analytical approaches for magnetizabilities as the second derivative of the energy have not been implemented. The magnetizability density can also be visualized, providing additional information that is not otherwise easily accessible on the spatial origin of magnetizabilities.

On the aromaticity of uracil and its 5-halogeno derivatives as revealed by theoretically derived geometric and magnetic indexes

The problem of aromaticity in heterocyclic rings of uracil and its 5-halogenoderivatives (5XU) was analyzed theoretically by calculating modified harmonic oscillator model of aromaticity (HOMA) for Heterocycle Electron Delocalization (HOMHED), nucleus-independent chemical shift parameters (NICS) and the so-called scan experiments, using helium-3 atom as a magnetic probe. The impact of halogen electronegativity on C5 atom’s NBO charges was also investigated. Water, as a polar environment, has a negligible impact on 5XU aromaticity. The most stable diketo tautomer shows a very low aromaticity while the “rare” dihydroxy form (tautomer No 6) is aromatic and resembles benzene. This is in agreement with traditional drawing of chemical formula of uracil’s six-membered ring, directly showing three alternating single and double bonds in its tautomer No 6. No good correlation between magnetic and geometric indexes of aromaticity for the studied 5XU tautomers was found. Linear correlation between the magnitude of NICS minimum, as well as the distance of the minimum above uracil ring plane center from 3He NMR chemical shift scan plot with respect to halogen electronegativity were observed. A strong linear dependence of magnetic index of aromaticity and the electronegativity of 5X substituent was observed.

What is the form of muscimol from fly agaric mushroom (Amanita muscaria) in water? An insight from NMR experiment supported by molecular modeling

The biologically active alkaloid muscimol is present in fly agaric mushroom (Amanita muscaria), and its structure and action is related to human neurotransmitter _γ -aminobutyric acid (GABA). The current study reports on determination of muscimol form present in water solution using multinuclear ¹ H and ¹³ C nuclear magnetic resonance (NMR) experiments supported by density functional theory molecular modeling. The structures of three forms of free muscimol molecule both in the gas phase and in the presence of water solvent, modeled by polarized continuous model, and nuclear magnetic isotropic shieldings, the corresponding chemical shifts, and indirect spin-spin coupling constants were calculated. Several J-couplings observed in proton and carbon NMR spectra, not available before, are reported. The obtained experimental spectra, supported by theoretical calculations, favor the zwitterion form of muscimol in water. This structure differs from NH isomer, previously determined in dimethyl sulfoxide (DMSO) solution. In addition, positions of signals C3 and C5 are reversed in both solvents.

Local aromaticity in polyacenes manifested by individual proton and carbon shieldings: DFT mapping of aromaticity

Exponential dependencies between locally calculated geometric and magnetic indexes of aromaticity, harmonic oscillator model of aromaticity (HOMA) and nucleus independent chemical shifts (NICS)(0), NICS(1) and NICS(1)zz, and the number of conjugated benzene rings in linear acenes, from benzene to decacene were observed at B3LYP/6-311+G** level of theory. Correlations between HOMA and NICS indexes showed exponential dependencies and were fitted with simple three-parameter function. Similar correlations between both indexes of aromaticity and proton and carbon nuclear isotropic shieldings of individual acene rings were observed. Contrary to proton data, the predicted ¹³ C nuclear isotropic shieldings of carbon atoms belonging to inner rings in polyacenes were less shielded, indicating lower aromaticity and therefore, higher reactivity.

Three Queries about the HOMA Index

HOMA (Harmonic Oscillator Model of Aromaticity) is a simple, successful, and widely used geometrical aromaticity index. However, HOMA can also be used as a general molecular descriptor appropriate for any type of molecule. It reaches the global maximum for benzene, whereas the potent magnetic aromaticity NICS index has no lower or upper limits. Hence, questions arise and go beyond mere differences between the geometric and magnetic aspects of aromaticity: (1) Does a molecule of aromaticity greater than that of benzene, but undisclosed by the HOMA definition, exist? (2) Can the Kekuléne cyclohexatriene moiety with HOMA = 0 exist as a part of a larger system? (3) Can the geometrical aromaticity index be defined better? Our answer to the first query is "It is not likely enough", to the second, "Why not define HOMA using a less mysterious molecule than cyclohexatriene?", and to the third, "It is possible to construct another fair geometrical index, but is it better for evaluating aromaticity?" To find these answers, we have studied: (1) the HOMA and NICS indices of over 50 hexahomosubstituted benzenes, (2) the HOMA, as well as EN and GEO, indices of over 100 triply fused hexasubstituted benzenes, and (3) the HOMA and new Geometrical Auxiliary Index (GAI) , of different unsaturated and saturated, aromatic and aliphatic hydrocarbons including all alkane constitutional isomers composed of up to nine carbon atoms.

Local aromaticity mapping in the vicinity of planar and nonplanar molecules

We report on nucleus-independent magnetic shielding (NICS) scans over the centers of six- and five-membered rings in selected metal phthalocyanines (MPc) and fullerene C60 for more accurate characterization of local aromaticity in these compounds. Detailed tests were conducted on model aromatic molecules including benzene, pyrrole, indole, isoindole, and carbazole and subsequently applied to H₂ Pc, ZnPc, Al(OH)Pc, and CuPc. Similar behavior of three selected magnetic probes, Bq, ³ He, and ⁷ Li⁺ , approaching perpendicularly the ring centers, was observed. For better visualization of shielding zone over the centers of aromatic rings, we introduced a simple mathematical procedure: the first and second derivatives of scan curves with respect to magnetic probe position enabled their additional examination. It allowed an easier localization of curve minimum and discrimination between areas in space varying by the magnetic field magnitude and to illustrate local aromaticity of two different kinds of rings in MPc with better resolution. Our results supported earlier reports on very low aromaticity indexes of pyrrole ring incorporated into MPc and significant aromaticity of the central macrocycle. No direct dependence between harmonic oscillator model of aromaticity and NICS was observed. Instead, a correlation between position of scan curve minimum and its magnitude were observed. In addition, the NICS values and ³ He chemical shifts in the middle of neutral C60 and C60^6- anion agreed well with the reported experimental NMR values for He@C60 and He@C60^6- .

Computational protocols for calculating 13C NMR chemical shifts

The most recent results dealing with the computation of ¹³C NMR chemical shifts in chemistry (small molecules, saturated, unsaturated and aromatic compounds, heterocycles, functional derivatives, coordination complexes, carbocations, and natural products) are reviewed, paying special attention to theoretical background and accuracy, the latter involving solvent effects, vibrational corrections, and relativistic effects.

Quantifying the conceptual problems associated with the isotropic NICS through analyses of its underlying density

Despite correlations with other aromaticity descriptors, the NICS is based on fields that have no features chemists associate with aromaticity.