Mg–Mg bond polarization induced by a superbulky β-diketiminate ligand

Increasing the steric pressure in a low-valent β-diketiminate Mg(i) did not lead to radical formation but gave Mg–N bond cleavage and formation of an asymmetrically substituted Mg–Mg complex.

Conformational Investigations in Flexible Molecules Using Orientational NMR Constraints in Combination with 3J-Couplings and NOE Distances

The downscaling of NMR tensorial interactions, such as dipolar couplings, from tens of kilohertz to a few hertz in low-order media is the result of dynamics spanning several orders of magnitudes, including vibrational modes (~ns-fs), whole-molecule reorientation (~ns) and higher barrier internal conformational exchange (<ms). In this work, we propose to employ these dynamically averaged interactions to drive an “alignment-tensor-free” molecular dynamic simulation with orientation constraints (MDOC) in order to efficiently access the conformational space sampled by flexible small molecules such as natural products. Key to this approach is the application of tensorial pseudo-force restraints which simultaneously guide the overall reorientation and conformational fluctuations based on defined memory function over the running trajectory. With the molecular mechanics force-field, which includes bond polarization theory (BPT), and complemented with other available NMR parameters such as NOEs and scalar J-couplings, MDOC efficiently arrives at dynamic ensembles that reproduce the entire NMR dataset with exquisite accuracy and theoretically reveal the systems conformational space and equilibrium. The method as well as its potential towards configurational elucidation is presented on diastereomeric pairs of flexible molecules: a small 1,4-diketone 1 with a single rotatable bond as well as a 24-ring macrolide related to the natural product mandelalide A 2.

Computational investigations of 18-electron triatomic sulfur–nitrogen anions

MRCI-SD/def2-QZVP and PBE0/def2-QZVP calculations have been employed for the analysis of geometries, stabilities, and bonding of isomers of the 18-electron anions N2S2−, NS2−, and NSO−. Isomers of the isoelectronic neutral molecules SO2, S2O, S3, and O3 are included for comparison. The sulfur-centered acyclic NSN2−, NSS−, and NSO− anions are the most stable isomers of their respective molecular compositions. However, the nitrogen-centered isomers SNS− and SNO− lie close enough in energy to their more stable counterparts to allow their occurrence. The experimental structural information, where available, is in good agreement with the optimized bond parameters. The bonding in all investigated species is qualitatively similar, though electron density analyses reveal important quantitative differences that arise from bond polarization. Most of the investigated systems can be described with a single configuration wave function, the two notable exceptions being isomers SSS and OOO that show some diradical character. The computed MRCI-SD/def2-QZVP absorption maxima for SNS− and NSS− are 342 and 327 nm, respectively. The corresponding PBE0/def2-QZVP values in acetonitrile are 353 and 333 nm. These data support the proposed initial formation of SNS− from electrochemical or chemical reduction of SSNS− based on experimental UV–vis spectra. The interconversion of SNS− and NSS− is calculated to be facile and reversible, leading to an equilibrium mixture that also includes the remarkably stable dianion SNSNSS2−. Thus, salts of either SNS− or NSS− with bulky organic cations represent feasible synthetic targets.

Raman active modes in defective peapods

The vibrational properties of defective single-walled carbon nanotube filled with C60 fullerene is the subject of the current study. For this aim we use the spectral moments method in the framework of the bond-polarization theory to calculate the non-resonant Raman spectra of hexa-vacancy defective C60 peapods. Essentially, the vibrational properties are closely coupled with the atomic structure of the system. The evolution of the Raman spectrum as a function of the spatial arrangement of defects in carbon nanotubes is discussed. This work provides benchmark theoretical results to understand the experimental data of defective C60 peapods. #single_wall_carbon_nanotube #C60_peapods #hexa-vacancy_defect #Raman_spectroscopy #pectral_moment_method

Insight into the chromophore of rhodopsin and its Meta-II photointermediate by 19F solid-state NMR and chemical shift tensor calculations

19F solid-state NMR studies together with bond polarization theory chemical shift tensor calculations provide insight into the chromophore of rhodopsin and its active state Meta II.