Crystal structure of methyl 1,3-benzoxazole-2-carboxylate

The title compound, C9H7NO3, crystallizes in the monoclinic (P21) space group. In the crystal, the almost planar molecules display a flattened herringbone arrangement. Stacking molecules are slipped in the lengthwise and widthwise directions and are linked by π–π interactions [d(Cg...Cg = 3.6640 (11) Å]. The structure is characterized by strong C—H...N and weak C—H...O hydrogen bonds, and further stabilized by C–O...π interactions.

Metal atom–guided conformational analysis of single polynuclear coordination molecules

Microscopic observation of single molecules is a rapidly expanding field in chemistry and differs from conventional characterization techniques that require a large number of molecules. One of such form of single-molecule microscopy is high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), which is especially suitable for coordination compounds because of its atomic number–dependent contrast. However, to date, single-molecule observations using HAADF-STEM has limited to simple planar molecules. In the present study, we demonstrate a direct structural investigation of nonplanar dendronized polynuclear Ir complexes with subnanometer resolution using Ir as an atomic label. Decreasing the electron dose to the dendrimer complexes is critical for the single-molecule observation. A comparison with simulated STEM images of conformational isomers is performed to determine the most plausible conformation. Our results enlarge the potential of electron microscopic observation to realize structural analysis of coordination macromolecules, which has been impossible with conventional methods.

DFT Investigation of the Molecular Properties of the Dimethylglyoximato Complexes [M(Hdmg)2] (M = Ni, Pd, Pt)

Important applications of the NiII, PdII and PtII complexes [M(Hdmg)2] (H2dmg = dimethylglyoxime) stem from their metal...metal stacked virtually insoluble aggregates. Given the virtual insolubility of the materials, we postulated that the rare reports on dissolved species in solution do not represent monomolecular species but oligomers. We thus studied the structural and spectral properties of the monomolecular entities of these compounds using density functional theory (DFT) and time-dependent DFT computations in dimethyl sulfoxide (DMSO) as a solvent. The molecular geometries, IR and UV-vis spectra, and frontier orbitals properties were computed using LANL2DZ ecp and def2TZVP as basis sets and M06-2X as the functional. The results are compared with the available experimental and other calculated data. The optimised molecular geometries proved the asymmetric character of the two formed O–H…O bonds which connect the two Hdmg‒ ligands in the completely planar molecules. Calculated UV-vis spectra revealed the presence of three absorptions in the range 180 to 350 nm that are red-shifted along the series Ni–Pd–Pt. They were assigned to essentially ligand-centred π−π* transitions in part with metal(d) to ligand(π*) charge transfer (MLCT) contributions. The notorious d‒p transitions dominating the colour and electronics of the compounds in the solid-state and oligomeric stacks are negligible in our monomolecular models strongly supporting the idea that the previously reported spectroscopic observations or biological effects in solutions are not due to monomolecular complexes but rather to oligomeric dissolved species.

Unavoidable Failure of Point Charge Descriptions of Electronic Density Changes for Out-of-Plane Distortions

<div>Population analyses based on point charge approximations accurately estimating the equilibrium dipole moment will systematically fail when predicting infrared intensities of out-of-plane vibrations of planar molecules, whereas models based on both charges and dipoles will always succeed. It is not a matter of how the model is devised, but on its number of degrees of freedom. Population analyses based on point charges are very limited in terms of the amount of meaningful chemical information they provide, whereas models employing both atomic charges and atomic dipoles should be preferred for molecular distortions. A good model should be able to correctly describe not only static, equilibrium structures but also distorted geometries in order to correctly assess information from vibrating molecules. The limitations of point charge models also hold for distortions much larger than those encountered vibrationally.</div>

Unavoidable Failure of Point Charge Descriptions of Electronic Density Changes for Out-of-Plane Distortions

<div>Population analyses based on point charge approximations accurately estimating the equilibrium dipole moment will systematically fail when predicting infrared intensities of out-of-plane vibrations of planar molecules, whereas models based on both charges and dipoles will always succeed. It is not a matter of how the model is devised, but on its number of degrees of freedom. Population analyses based on point charges are very limited in terms of the amount of meaningful chemical information they provide, whereas models employing both atomic charges and atomic dipoles should be preferred for molecular distortions. A good model should be able to correctly describe not only static, equilibrium structures but also distorted geometries in order to correctly assess information from vibrating molecules. The limitations of point charge models also hold for distortions much larger than those encountered vibrationally.</div>

Magnetically induced currents and aromaticity in ligand-stabilized Au and AuPt superatoms

Understanding magnetically induced currents (MICs) in aromatic or metallic nanostructures is crucial for interpreting local magnetic shielding and NMR data. Direct measurements of the induced currents have been successful only in a few planar molecules but their indirect effects are seen in NMR shifts of probe nuclei. Here, we have implemented a numerically efficient method to calculate gauge-including MICs in the formalism of auxiliary density functional theory. We analyze the currents in two experimentally synthesized gold-based, hydrogen-containing ligand-stabilized nanoclusters [HAu9(PPh3)8]2+ and [PtHAu8(PPh3)8]+. Both clusters have a similar octet configuration of Au(6s)-derived delocalized “superatomic” electrons. Surprisingly, Pt-doping in gold increases the diatropic response of the superatomic electrons to an external magnetic field and enhances the aromaticity of [PtHAu8(PPh3)8]+. This is manifested by a stronger shielding of the hydrogen proton in the metal core of the cluster as compared to [HAu9(PPh3)8]2+, causing a significant upfield shift in agreement with experimental proton NMR data measured for these two clusters. Our method allows the determination of local magnetic shielding properties for any component in large 3D nanostructures, opening the door for detailed interpretation of complex NMR spectra.

Magnetically Induced Ring-Current Strengths of Planar and Nonplanar Molecules: New Insights from the Pseudo-π Model

<p>The π-contribution to the magnetically induced current densities, ring-current strengths, and induced magnetic fields of large planar molecules (as kekulene) and three-dimensional molecules (as [10]cyclophenacene and chiral toroidal nanotubes C₂₀₁₆and C₂₁₉₆) have been computed using the pseudo-π model with the gauge-including magnetically induced currents method. The magnetic response analysis shows that π-electrons are the main actors of the electron delocalization in carbon systems regardless of their size, suggesting that the π- component of the ring-current strengths can be used for assessing the aromatic character of this kind of molecules. Computations using the pseudo-π model yield current densities and induced magnetic fields that are not contaminated by contributions from core and σ-electrons allowing investigations of large molecular structures as polycyclic aromatic hydrocarbons and cylindrical or toroidal carbon nanotubes.</p>

Magnetically Induced Ring-Current Strengths of Planar and Nonplanar Molecules: New Insights from the Pseudo-π Model

<p>The π-contribution to the magnetically induced current densities, ring-current strengths, and induced magnetic fields of large planar molecules (as kekulene) and three-dimensional molecules (as [10]cyclophenacene and chiral toroidal nanotubes C₂₀₁₆and C₂₁₉₆) have been computed using the pseudo-π model with the gauge-including magnetically induced currents method. The magnetic response analysis shows that π-electrons are the main actors of the electron delocalization in carbon systems regardless of their size, suggesting that the π- component of the ring-current strengths can be used for assessing the aromatic character of this kind of molecules. Computations using the pseudo-π model yield current densities and induced magnetic fields that are not contaminated by contributions from core and σ-electrons allowing investigations of large molecular structures as polycyclic aromatic hydrocarbons and cylindrical or toroidal carbon nanotubes.</p>