Isotropic Magnetic Shielding Research Articles

Detailed Visualization of Aromaticity Using Isotropic Magnetic Shielding.

For many years, Clar's aromatic sextet theory has served as a qualitative method for assessing the aromatic character of polycyclic aromatic hydrocarbons. A new approach, based on the calculation of isotropic magnetic shielding (IMS) contour plots, is shown to provide a feature-rich picture of aromaticity that is both quantitative yet still easily interpreted. Chemists are visual creatures who are adept at discerning reactivity and chemical behavior from molecular structures. To quote Roald Hoffmann, "People like pictures. Chemists live off them." Thus, the detailed image analysis we present simultaneously provides quantitative assessment of electronic structure, which is still easy-to-understand through visual inspection, embedded in an aesthetically appealing and intuitive picture that draws the reader in. We provide novel computed IMS contour plots for a representative selection of aromatic molecules. Where Clar's static drawings capture only a partial sketch of the electronic properties of a molecule, IMS contour plots present a detailed, global landscape of a molecule that sums all possible resonance structures. This novel analysis allows us to correct certain drawbacks of Clar's analysis with respect to polycyclic aromatics and quantitatively assess the bonding and electronic structure of acene hydrocarbons.

Detailed Visualization of Aromaticity Using Isotropic Magnetic Shielding

AbstractFor many years, Clar's aromatic sextet theory has served as a qualitative method for assessing the aromatic character of polycyclic aromatic hydrocarbons. A new approach, based on the calculation of isotropic magnetic shielding (IMS) contour plots, is shown to provide a feature‐rich picture of aromaticity that is both quantitative yet still easily interpreted. Chemists are visual creatures who are adept at discerning reactivity and chemical behavior from molecular structures. To quote Roald Hoffmann, “People like pictures. Chemists live off them.” Thus, the detailed image analysis we present simultaneously provides quantitative assessment of electronic structure, which is still easy‐to‐understand through visual inspection, embedded in an aesthetically appealing and intuitive picture that draws the reader in. We provide novel computed IMS contour plots for a representative selection of aromatic molecules. Where Clar's static drawings capture only a partial sketch of the electronic properties of a molecule, IMS contour plots present a detailed, global landscape of a molecule that sums all possible resonance structures. This novel analysis allows us to correct certain drawbacks of Clar's analysis with respect to polycyclic aromatics and quantitatively assess the bonding and electronic structure of acene hydrocarbons.

Assessment of the Performance of Optimally Tuned Range‐Separated Hybrid Functionals for Nuclear Magnetic Shielding Calculations

AbstractThe performance of optimally tuned range‐separated hybrid (OT‐RSH) functional calculations in predicting accurate isotropic nuclear magnetic shielding (σ) and chemical shift values is examined. To that end, the results of OT‐RSH and other approximate density functional theory calculations are assessed against recently published benchmark CCSD(T) calculations for a test set consisting of several molecules and bond types. It is found that for atoms in single bonds with a large paramagnetic contribution to σ, OT‐RSH offers a significant improvement in prediction of shielding constants over popular semi‐local and hybrid density functionals, yielding non‐empirical results that are as accurate as those of semi‐empirical density functionals often used for prediction of shielding constants. This success is attributed to the improved fundamental gap prediction of the OT‐RSH approach. For atoms in multiple bonds, however, larger errors often persist. By comparing OT‐RSH and recently reported double‐hybrid functional results, the remaining difficulties are traced to significant non‐local correlation.

31P nuclear magnetic resonance, optical and thermal spectra in MP3 (M = Ir, Co, Rh, Ni) compounds by DFT

We present the relation between Nuclear Magnetic Resonance (NMR) of 31P, optoelectronic and thermoelectric properties of MP3 (M = Ir, Co, Rh, Ni) compounds. Full potential linearized augmented plane wave (FP-LAPW) method and PBE-GGA/ mBJ plus spin–orbit coupling (SOC) approach are used for prediction of all spectra. Obtained 31P isotropic magnetic shielding value decreases from 60.5 ppm for IrP3 to −13.35 ppm for NiP3 due to the decreasing band gap as IrP3 > CoP3 > RhP3 > NiP3. The results demonstrate linear relationships between the magnetic shielding and chemical shifts. It is found that Plasmon energy decreases from about 24.43 for IrP3 to 21.97 eV for NiP3. Obtained results show that isotropic magnetic shielding for 31P has a dominating effect on the optoelectronic and thermoelectric properties of these compounds.

Response to "Comment on 'Nuclear relaxation in an electric field enables the determination of isotropic magnetic shielding'" [J. Chem. Phys. 152, 037101 (2020)

Response to "Comment on 'Nuclear relaxation in an electric field enables the determination of isotropic magnetic shielding'" [J. Chem. Phys. 152, 037101 (2020)

Theoretical Analysis of Solvent Effect on the NMR Parameters in trans-(NHC)PtI2Py Complex: A Platinum-Based Anticancer Drug

In this study, quantum chemical calculations using MPW1PW91 method were applied to analyze the solvent effect on the NMR parameters for a platinum-based anticancer drug, namely complex trans-(NHC)PtI2Py. The solvent effects on isotropic magnetic shielding tensor data (σiso) for Pt and CNHC atoms and spin–spin coupling constants of Pt–C and Pt–N in the complex were studied by the self-consistent reaction field theory (SCRF) based on the Polarizable Continuum Model (PCM). The linear correlations between these parameters and dielectric constants of solvents were studied. The σiso (Pt) values in different solvents were correlated with the Kirkwood–Bauer–Magat equation (KBM).

Demonstration of Baird's rule complementarity in the singlet state with implications for excited-state intramolecular proton transfer.

The aromatic character of an arene is proposed to switch from aromatic in the ground state (S0) to antiaromatic in the S1 and T1 excited states. This behavior is known as Baird's rule and has been invoked to explain excited-state properties, primarily in the triplet state, whereas rationalization of antiaromaticity in the singlet state is less developed. This work demonstrates the first application of Baird's rule to rationalize previously unexplained experimental behavior of the singlet state process known as excited-state intramolecular proton transfer (ESIPT). Further, by analyzing the variations in isotropic magnetic shielding around the base arenes (benzene and naphthalene) of ESIPT fluorophores in the S0 and S1 electronic states, different shielding distributions indicate a complementarity to Baird's rule: greater aromaticity in S0 leads to greater antiaromaticity in S1 and vice versa. These findings have immediate application in the design of functional ESIPT fluorophores and, more generally, for photochemical reactions that are driven by the relief of antiaromaticity in the excited state. Notably, a tenet of traditional chromophore design states that expansion of conjugation generally leads to a red-shift in absorbance and emission wavelengths. The results of this study show that ESIPT fluorophores run contrary to those conventional design principles and this behavior can only be rationalized by considering Baird's rule.

Magnetic Shielding, Aromaticity, Antiaromaticity and Bonding in the Low-Lying Electronic States of S2 N2.

Aromaticity, antiaromaticity and chemical bonding in the ground (S0 ), first singlet excited (S1 ) and lowest triplet (T1 ) electronic states of disulfur dinitride, S2 N2 , were investigated by analysing the isotropic magnetic shielding, σiso (r), in the space surrounding the molecule for each electronic state. The σiso (r) values were calculated by state-optimized CASSCF/cc-pVTZ wave functions with 22 electrons in 16 orbitals constructed from gauge-including atomic orbitals (GIAOs). The S1 and T1 electronic states were confirmed as 11 Au and 13 B3u , respectively, through linear response CC3/aug-cc-pVTZ calculations of the vertical excitation energies for eight singlet (S1 -S8 ) and eight triplet (T1 -T8 ) electronic states. The aromaticities of S0 , S1 and T1 were also assessed using additional magnetic criteria including nucleus-independent chemical shifts (NICS) and magnetic susceptibilities calculated at several levels of theory, the highest of which were CCSDT-GIAO/cc-pVTZ for S0 and CASSCF(22,16)-GIAO/aug-cc-pVQZ for S1 and T1 . The results strongly suggest that: 1) the S0 electronic ground state of S2 N2 is aromatic but less so than the electronic ground state of benzene; 2) S1 is profoundly antiaromatic, to an extent that removes any bonding interactions that would keep the atoms together; and 3) T1 is also antiaromatic, but its antiaromaticity is more moderate and similar to that observed in the electronic ground state of square cyclobutadiene. S2 N2 is the first example of an inorganic ring for which theory predicts substantial changes in aromaticity upon vertical transition from the ground state to the first singlet excited or lowest triplet electronic states.

Influence of Solvent Representation on Nuclear Shielding Calculations of Protonation States of Small Biological Molecules.

In this study, we assess the influence of solvation on the accuracy and reliability of isotropic nuclear magnetic shielding calculations for amino acids in comparison to experimental data. We focus particularly on the performance of solvation methods for different protonation states, as biological molecules occur almost exclusively in aqueous solution and are subject to protonation with pH. We identify significant shortcomings of current implicit solvent models and present a hybrid solvation approach that improves agreement with experimental data by taking into account the presence of direct interactions between amino acid protonation state and water molecules.

Quantum chemical investigations on hydrogen bonding interactions established in the inclusion complex β-cyclodextrin/benzocaine through the DFT, AIM and NBO approaches

Structure and stability of an inclusion complex formed by Benzocaine (BZC) and β-cyclodextrin (β-CD) were investigated computationally using different levels of theory. The conformational research based on PM6 method allowed reach two minimum-energy structures: model A and model B. The lowest conformers have been exposed to fully geometry optimization employing four DFT functionals: B3LYP, CAM-B3LYP, M05-2X and M06-2X. The performed DFT calculations have identified the model B, in which the amino group is located at the primary face of β-CD, as the most stable complex by an amount up to −40 kcal/mol. Further, the greater stabilization of model B in respect to model A, has been ascertained through AIM and NBO analyses which clarified the main hydrogen bonds HBs interactions governing the reactivity of BZC inside the hydrophobic cavity of β-CD. Finally, the estimated isotropic 1H nuclear magnetic shielding constants generated from the gauge-including-atomic-orbital calculation have been analyzed and then compared with the available experimental data.

Magnetic Shielding Studies of C2 and C2 H2 Support Higher than Triple Bond Multiplicity in C2.

The carbon-carbon bonds in the ground states of C2 and C2 H2 , at their equilibrium geometries, are compared by analysing the changes in the off-nucleus magnetic shielding tensor within the space surrounding each of these molecules. A wide range of quantum-chemical approaches, including full-valence CASSCF-GIAO, CCSD(T)-GIAO and CCSDT-GIAO, all with the cc-pVQZ basis set, as well as HF-GIAO and MP2-GIAO, with the cc-pVQZ, cc-pV5Z and cc-pV6Z basis sets, show that the surroundings of the carbon-carbon bond in C2 are more shielded than those of the carbon-carbon bond in C2 H2 . The additional shielding of the carbon-carbon bond in C2 is found to be due to a larger paramagnetic contribution to the component of the shielding tensor which is perpendicular to the molecular axis. The analysis of the off-nucleus shielding data indicates that the carbon-carbon bond in C2 is "bulkier" and, therefore, of a higher multiplicity, but weaker than the corresponding bond in C2 H2 . According to the results of the shielding calculations, the carbon nuclei in C2 should be much more shielded than those in C2 H2 , with 13 C isotropic magnetic shieldings in the ranges of around 224-227 ppm and around 123-125 ppm for C2 and C2 H2 , respectively.

Spectroscopic and DFT studies of bis-3-hydroxypyridinium and bis-3-hydroxymethylpyridinium dibromides with tetramethylene linker

Experimental and theoretical IR, Raman, UV–Vis, 1H and 13C NMR spectra of 1,4-di(3-hydroxypyridinium)butane dibromide and 1,4-di(3-hydroxymethylpyridinium)butane dibromide were obtained and analyzed. Optimized geometrical structures of the studied compounds were calculated by B3LYP method using 6-311++G(d,p) basis set and employed to determine the theoretical wavenumbers and intensities of IR and Raman spectra. The frequency assignments were supported by the potential energy distribution (PED) analysis. The significant role of the intermolecular interactions and the hydrogen bond was revealed on the basis of IR spectra. The calculated GIAO/B3LYP/6-311++G(d,p) isotropic magnetic shielding constants were used to predict the 1H and 13C chemical shifts for the optimized structures. Accuracy of the prediction of 1H and 13C chemical shifts was significantly improved by a simulation of the solvent in calculations. On the basis of UV—Vis spectra the acid-base equilibrium in the water solution of 1,4-di(3-hydroxypyridinium)butane dibromide was found.

An NMR Crystallographic Investigation of the Relationships between the Crystal Structure and 29Si Isotropic Chemical Shift in Silica Zeolites.

NMR crystallography has recently been applied to great effect for silica zeolites. Here we investigate whether it is possible to extend the structural information available from routine NMR spectra via a simple structure–spectrum relationship. Unlike previous empirically derived relationships that have compared experimental crystal structures for (often disordered) silicates with experimental NMR spectra, where the structure may not be an accurate representation of the material studied experimentally, we use NMR parameters calculated by density functional theory (DFT) for both model Si(OSi(OH)3)4 clusters and also extended zeolitic SiO2 frameworks, for which the input structure corresponding to the NMR parameters is known exactly. We arrive at a structure–spectrum relationship dependent on the mean Si–O bond length, mean Si–O–Si bond angle, and the standard deviations of both parameters, which can predict to within 1.3 ppm the 29Si isotropic magnetic shielding that should be obtained from a DFT calculation. While this semiempirical relationship will never supersede DFT where this is possible, it does open up the possibility of a rapid estimation of the outcome of a DFT calculation where the actual calculation would be prohibitively costly or otherwise challenging. We also investigate the structural optimization of SiO2 zeolites using DFT, demonstrating that the mean Si–O bond lengths all tend to 1.62 Å and the distortion index tends to <2.0°, suggesting that these metrics may be suitable for rapid validation of whether a given crystal structure represents a realistic local geometry around Si, or merely a bulk average with contributions from several different local geometries.

Magnetic Shielding, Aromaticity, Antiaromaticity, and Bonding in the Low-Lying Electronic States of Benzene and Cyclobutadiene.

Aromaticity, antiaromaticity, and their effects on chemical bonding in the ground states (S0), lowest triplet states (T1), and the first and second singlet excited states (S1 and S2) of benzene (C6H6) and square cyclobutadiene (C4H4) are investigated by analyzing the variations in isotropic magnetic shielding around these molecules in each electronic state. All shieldings are calculated using state-optimized π-space complete-active-space self-consistent field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs), in the 6-311++G(2d,2p) basis. It is shown that the profoundly different shielding distributions in the S0 states of C6H6 and C4H4 represent aromaticity and antiaromaticity "fingerprints" which are reproduced in other electronic states of the two molecules and allow classification of these states as aromatic (S0 and S2 for C6H6, T1 and S1 for C4H4) or antiaromatic (S0 and S2 for C4H4, T1 and S1 for C6H6). S2 C6H6 is predicted to be even more aromatic than S0 C6H6. As isotropic shielding isosurfaces and contour plots show very clearly the effects of aromaticity and antiaromaticity on chemical bonding, these can be viewed, arguably, as the most succinct visual definitions of the two phenomena currently available.

Theoretical Screening of Novel 5-picrylamino- 1,2,3,4-tetrazole (PAT) and 5,5′-styphnylamino-1,2,3,4-tetrazole (SAT) Derivatives: A New Molecular Design Strategy of Multi-Nitrogen Energetic Materials by Introducing Intermolecular Hydrogen Bonds and π–π Stacking Interactions

ABSTRACTOn the basis of a design strategy that results in the introduction of intramolecular hydrogen-bonds and π–π stacking interactions leading to low-sensitive and high-energy materials (LSHMs), –NH2/–NO2 fused derivatives of 5-picrylamino- 1,2,3,4-tetrazole (PAT) and 5,5′-styphnylamino- 1,2,3,4-tetrazole (SAT) were designed and investigated theoretically. Density functional theory has been explored to investigate the geometric, electronic structures, band gaps, and heats of formation. The detonation performance was evaluated by using Kamlet-Jacobs equations based on the calculated densities and enthalpy of formation (EOF). The thermal stability of these compounds was studied by calculating bond dissociation energies and energy gaps. Proper numbers of 5-aminotetrazole (5-AT) moieties can enhance the EOFs of explosives, further improving the detonation performance as well as nitro group. Though introducing more nitro-groups into TATB frameworks, NH-moieties can retain the intramolecular hydrogen-bond, which can enhance the crystal-packing and physico-chemical stability. Meanwhile, noncovalent interaction (NCI) plots reflects that intermolecular interaction also has a crucial influence on the mechanical sensitivity for the designed derivatives, except for the energy gap (ΔE) and bond dissociation energy (BDE). UV/Vis spectra and 15N isotropic magnetic shielding were further computed for characterizing and predicting the molecular structure and reaction mechanism towards the future experiments. The predicted performance implies that the designed molecules are expected to be promising candidates for multi-nitrogen energetic materials.

Nuclear relaxation in an electric field enables the determination of isotropic magnetic shielding

It is shown that in contrast to the case of nuclear relaxation in a magnetic field B, simultaneous application of the magnetic field B and an additional electric field E causes transverse relaxation of a spin-1/2 nucleus with the rate proportional to the square of the isotropic part of the magnetic shielding tensor. This effect can contribute noticeably to the transverse relaxation rate of heavy nuclei in molecules that possess permanent electric dipole moments. Relativistic quantum mechanical computations indicate that for 205Tl nucleus in a Pt-Tl bonded complex, Pt(CN)5Tl, the transverse relaxation rate induced by the electric field is of the order of 1 s−1 at E = 5 kV/mm and B = 10 T.

Exploring Chemical Bonds through Variations in Magnetic Shielding.

Differences in nuclear isotropic magnetic shieldings give rise to the chemical shifts measured in NMR experiments. In contrast to existing NMR experimental techniques, quantum chemical methods are capable of calculating isotropic magnetic shieldings not just at nuclei, but also at any point in the space surrounding a molecule. Using s-trans-1,3-butadiene, ethane, ethene, and ethyne as examples, we show that the variations in isotropic magnetic shielding around a molecule, represented as isosurfaces and contour plots, provide an unexpectedly clear picture of chemical bonding, which is much more detailed than the traditional description in terms of the total electron density.

GIAO⿿DFT isotropic magnetic shielding constants and spin⿿spin coupling of tartaric acid in water solution

We investigate the nuclear isotropic shielding constants and spin⿿spin coupling for oxygen and carbons atoms of isomers of tartaric acid in gas phase and in water solutions by Monte Carlo simulation and quantum mechanics calculations using the GIAO-B3LYP approach. Solute polarization effects are included iteratively and play an important role in the quantitative determination of shielding constants. Our MP2/aug-cc-pVTZ results show substantial increases of the dipole moment in solution as compared with the gas phase results (61⿿221%). The solvent effects on the Ͽ(13O) values are in general small. More appreciable solvent effects can be seen on the Ͽ(17O) and J(CO).

Shielding in and around Oxazole, Imidazole, and Thiazole: How Does the Second Heteroatom Affect Aromaticity and Bonding?

Isotropic magnetic shielding distributions in the regions of space surrounding oxazole, imidazole, and thiazole are used to investigate aromaticity and bonding in these five-membered heterocycles with two heteroatoms. This is achieved by constructing HF-GIAO and MP2-GIAO (Hartree-Fock and second-order Møller-Plesset perturbation theory with gauge-including atomic orbitals) isotropic shielding plots, within the 6-311++G(d,p) basis, using regular two-dimensional 0.05 Å grids in the molecular plane and in planes 0.5 and 1 Å above it. The extent of isotropic shielding delocalization in the contour plots in planes 1 Å above the molecular plane, which is a new sensitive two-dimensional aromaticity criterion, indicates that aromaticity decreases in the order thiazole > imidazole > oxazole; in combination with previous results on furan, pyrrole, and thiophene ( J. Org. Chem. 2013 , 78 , 8037 - 9043 ), the aromaticity ordering in the six five-membered heterocycles becomes thiophene > thiazole > pyrrole > imidazole > furan > oxazole. The results suggest that the inclusion of a second heteroatom in a five-membered heterocycle has a detrimental effect on its aromaticity, which is very minor in oxazole, when compared to furan, and small but noticeable in imidazole and pyrrole and in thiazole and thiophene.

DFT study of molecular structures and 13C NMR parameters of two fluorinated biphenyls and their η6-tricarbonylchromium complexes

The molecular structures of 2,2′-difluoro-6,6′-dimethylbiphenyl, 4,5-difluoro-9,10-dihydrophenanthrene and of their η6-tricarbonylchromium complexes have been discussed in the light of the results of molecular energy calculations. Also the isotropic magnetic shielding constants and carbon–fluorine spin–spin coupling constants for these objects have been calculated and compared with the experimental values of 13C NMR chemical shifts and J constants. The calculational methods used were: DFT/BHandH/6-311++G(2d,p) and/or DFT/B3LYP/6-311++G(2d,p). It has been confirmed that experimental 13C NMR chemical shifts for η6-arene tricarbonylchromium complexes can be satisfactorily predicted using both methods, although the method exploiting BHandH functional is not able to reproduce the 13C NMR chemical shifts of Cr(CO)3 carbon atoms. On the other hand, this method provides the J(13C, 19F) values which are close to the experimental ones.