Reaction of Paramagnetic Synthon,Lithiated 4,4,5,5‐Tetramethyl‐4,5‐dihydro‐1H‐imidazol‐1‐oxyl 3‐oxide,with Cyclic Aldonitrones of the Imidazole Series

It was shown that dipole‐stabilized paramagnetic carbanion lithiated 4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazol‐1‐oxyl 3‐oxide can be attached in a nucleophilic manner to either isolated or conjugated aldonitrones of the 2,5‐dihydroimidazole 3‐oxide and 2H‐imidazole 1‐oxide series to afford adducts the subsequent oxidation of which leads to polyfunctional mono‐ and diradicals. According to XRD, at least two polymorphic modifications can be formed during crystallization of the resulting paramagnetic compounds, and for each of them, geometric parameters of the molecules are similar. An EPR spectrum of the diradical in frozen toluene has a complicated lineshape, which can be fairly well reproduced by using X‐ray diffraction structural analysis and the following set of parameters: D=14.9 mT, E=1.7 mT; tensor a(¹⁴N)=[0.260 0.260 1.625] mT, two equivalent tensors for the nitronyl nitroxide moiety a(¹⁴N)=[0.198 0.198 0.700] mT, and g≈2.007. According to our DFT and ab initio calculations, the intramolecular exchange in the diradical is very weak and most likely ferromagnetic.     

σ‐Hole Opposite to a Lone Pair: Unconventional Pnicogen Bonding Interactions between ZF3 (Z=N,P, As,and Sb) Compounds and Several Donors

The ability of several pnicogen sp³ derivatives ZF₃ (Z=N, P, As, Sb) to interact with electron‐rich entities by means of the opposite face to the lone pair (lp) is investigated at the RI‐MP2/aug‐cc‐pVQZ level of theory. The strength of the interaction ranges from ?1 to ?87 kJ mol^?1, proving its favorable nature, especially when the lp is coordinated to a metal center, whereby the strength of the interaction is significantly enhanced. NBO analysis showed that orbital effects are modest contributors to the global stabilization of the pnicogen σ‐hole bonded complexes studied. Finally, a selection of Cambridge Structural Database examples are shown that demonstrate the impact of this counterintuitive binding mode in the solid state.     

Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas‐Phase Radical–Radical Reaction O(3P)+(CH3)2CH→C3H6+OH

This paper reports on the gas‐phase radical–radical dynamics of the reaction of ground‐state atomic oxygen [O(³P), from the photodissociation of NO₂] with secondary isopropyl radicals [(CH₃)₂CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O(³P)+(CH₃)₂CH→C₃H₆ (propene)+OH, is examined by high‐resolution laser‐induced fluorescence spectroscopy in crossed‐beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X²Π) products with low‐ and high‐N′′ components in a ratio of 1.25:1. No significant spin–orbit or Λ‐doublet propensities are exhibited in the ground vibrational state. Ab initio computations at the CBS‐QB3 theory level and comparison with prior theory show that the statistical method is not suitable for describing the main reaction channel at the molecular level. Two competing mechanisms are predicted to exist on the lowest doublet potential‐energy surface: direct abstraction, giving the dominant low‐N′′ components, and formation of short‐lived addition complexes that result in hot rotational distributions, giving the high‐N′′ components. The observed competing mechanisms contrast with previous bulk kinetic experiments conducted in a fast‐flow system with photoionization mass spectrometry, which suggested a single abstraction pathway. In addition, comparison of the reactions of O(³P) with primary and tertiary hydrocarbon radicals allows molecular‐level discussion of the reactivity and mechanism of the title reaction.     

An Ab initio theoretical investigation on the geometrical and electronic structures of BAun−/0 (n = 1–4) clusters

An ab initio theoretical investigation on the geometrical and electronic structures and photoelectron spectroscopies (PES) of BAu_n^?/0 (n = 1–4) auroboranes has been performed in this work. Density functional theory and coupled cluster method (CCSD(T)) calculations indicate that BAu (n = 1–4) clusters with n‐Au terminals possess similar geometrical structures and bonding patterns with the corresponding boron hydrides BH. The PES spectra of BAu (n = 1–4) anions have been simulated computationally to facilitate their future experimental characterizations. In this series, the T_d BAu anion appears to be unique and particularly interesting: it possesses a perfect tetrahedral geometry and has the highest vertical electron detachment energy (VDE = 3.69 eV), largest HOMO‐LUMO gap (ΔE_gap = 3.0 eV), and the highest first excitation energy (E_ex = 2.18 eV). The possibility to use the tetrahedral BAu unit as the building block of Li⁺[BAu₄]^? ion‐pair and other [BAu₄]^?‐containing inorganic solids is discussed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Retro‐Diels–Alder Approach to the Synthesis of π‐Expanded Azuliporphyrins and Their Porphyrinoid Aromaticity

Bicyclo[2.2.2]octadiene (BCOD) fused azuliporphyrins were synthesized by 3+1 porphyrin synthesis of azulitripyrranes with diformylpyrroles. Subsequent retro‐Diels–Alder reaction of the BCOD‐fused azuliporphyrins afforded azulibenzo‐, azulidibenzo‐, and azulitribenzoporphyrins 1 – 5 . NMR and UV/Vis spectra, as well as nucleus‐independent chemical shift (NICS) calculations revealed that 1 – 5 and their diprotonated dications exhibit relatively low porphyrinoid aromaticity, which was dependent on the position and number of fused benzene rings present.     

Triplet‐State Aromaticity of 4nπ‐Electron Monocycles: Analysis of Bifurcation in the π Contribution to the Electron Localization Function

The π contribution to the electron localization function (ELF) is used to compare 4nπ‐ and (4n+2)π‐electron annulenes, with particular focus on the aromaticity of 4nπ‐electron annulenes in their lowest triplet state. The analysis is performed on the electron density obtained at the level of OLYP density functional theory, as well as at the CCSD and CASSCF ab initio levels. Two criteria for aromaticity of all‐carbon annulenes are set up: the span in the bifurcation values ΔBV(ELF_π) should be small, ideally zero, and the bifurcation value for ring closure of the π basin RCBV(ELF_π) should be high (≥ 0.7). On the basis of these criteria, nearly all 4nπ‐electron annulenes are aromatic in their lowest triplet states, similar to (4n+2)π‐electron annulenes in their singlet ground states. For singlet biradical cyclobutadiene and cyclooctatetraene constrained to D_4h and D_8h symmetry, respectively, the RCBV(ELF_π) at the CASSCF level is lower (0.531 and 0.745) than for benzene (0.853), even though they have equal proportions of α‐ and β‐electrons.     

Superlight and Superflexible Three‐Dimensional Semiconductor Frameworks A(X≡Y)4 (A=Si,Ge; X/Y=C,B, N) with Tunable Optoelectronic and Mechanical Properties from First‐Principles

Silicon carbide materials, as leading wide band gap semiconductors, hold significant importance in semiconductor technologies. Herein, diamond‐like 3D materials with low density, but high elasticity properties, have been designed from first‐principles calculations. They are porous single‐crystalline materials composed of sp³‐hybridized silicon (or germanium) and sp‐type C≡C (or B≡N) linear moieties; their stabilities are comparable to those of recently prepared SiC₄ materials. Moreover, such wide band gap semiconductors have strong absorption over a wide UV range and exhibit superlight, superflexible, and incompressible mechanical properties, and their optoelectronic and mechanical properties can be well tuned through structural modifications. Such features provide high potential for practicable application under extreme conditions, and suggest promising applications for the design of UV optoelectronic devices.     

Studying the Origin of the Antiferromagnetic to Spin‐Canting Transition in the β‐p‐NCC6F4CNSSN. Molecular Magnet

The crystal structure of the spin‐canted antiferromagnet β‐p‐NCC₆F₄CNSSN^. at 12 K (reported in this work) was found to adopt the same orthorhombic space group as that previously determined at 160 K. The change in the magnetic properties of these two crystal structures has been rigorously studied by applying a first‐principles bottom‐up procedure above and below the magnetic transition temperature (36 K). Calculations of the magnetic exchange pathways on the 160 K structure reveal only one significant exchange coupling (J(d1)=?33.8 cm^?1), which generates a three‐dimensional diamond‐like magnetic topology within the crystal. The computed magnetic susceptibility, χ(T), which was determined by using this magnetic topology, quantitatively reproduces the experimental features observed above 36 K. Owing to the anisotropic contraction of the crystal lattice, both the geometry of the intermolecular contacts at 12 K and the microscopic J_AB radical–radical magnetic interactions change: the J(d1) radical–radical interaction becomes even more antiferromagnetic (?43.2 cm^?1) and two additional ferromagnetic interactions appear (+7.6 and +7.3 cm^?1). Consequently, the magnetic topologies of the 12 and 160 K structures differ: the 12 K magnetic topology exhibits two ferromagnetic sublattices that are antiferromagnetically coupled. The χ(T) curve, computed below 36 K at the limit of zero magnetic field by using the 12 K magnetic topology, reproduces the shape of the residual magnetic susceptibility (having subtracted the contribution to the magnetization arising from spin canting). The evolution of these two ferromagnetic J_AB contributions explains the change in the slope of the residual magnetic susceptibility in the low‐temperature region.     

Coexistence of Three Ferroic Orders in the Multiferroic Compound [(CH3)4N][Mn(N3)3] with Perovskite‐Like Structure

The perovskite azido compound [(CH₃)₄N][Mn(N₃)₃], which undergoes a first‐order phase change at T_t=310 K with an associated magnetic bistability, was revisited in the search for additional ferroic orders. The driving force for such structural transition is multifold and involves a peculiar cooperative rotation of the [MnN₆] octahedral as well as order/disorder and off‐center shifts of the [(CH₃)₄N]⁺ cations and bridging azide ligands, which also bend and change their coordination mode. According to DFT calculations the latter two give rise to the appearance of electric dipoles in the low‐temperature (LT) polymorph, the polarization of which nevertheless cancels out due to their antiparallel alignment in the crystal. The conversion of this antiferroelectric phase to the paraelectric phase could be responsible for the experimental dielectric anomaly detected at 310 K. Additionally, the structural change involves a ferroelastic phase transition, whereby the LT polymorph exhibits an unusual and anisotropic thermal behavior. Hence, [(CH₃)₄N][Mn(N₃)₃] is a singular material in which three ferroic orders coexist even above room temperature.     

Cover Picture: Ab Initio Calculation of Parity‐Violating Chemical Shifts in NMR Spectra of Chiral Molecules (ChemPhysChem 4/2003)

The cover picture shows an intriguing effect in molecular systems, which is caused by the parity‐violating weak interactions: The chemical shifts of magnetic nuclei are predicted to differ for the two enantiomers of a chiral compound! While in the R enantiomer the nucleus (red) of the yellow center gives rise to the red NMR signal, the corresponding nucleus of the S enantiomer (green) is expected to absorb at a slightly different frequency. The ab initio approach presented by Laubender and Berger on pp. 395–399 allows for a prediction of the resulting parity‐violating line splitting (shown in the black curve) and for the identification of molecular candidates that are well‐suited to the first successful measurement of parity‐violating effects in molecules.     

Spin‐Resolved Rotational Energy Transfer for the CH B 2Σ−(v=0, N,F) State by Collisions with Ar

An experimental and theoretical investigation of rotational energy transfers (RET) of CH involving the B ²Σ^? (v=0, 0≤N≤5, F) state by collisions with Ar is undertaken, using the photolysis‐probe technique. Time‐resolved laser‐induced fluorescence resulting from an initially prepared fine‐structure label is dispersed using a step‐scan Fourier transform spectrometer. The spin‐resolved RET rate constants are evaluated with the simulation of a kinetic model. The quantum‐scattering method is used for the calculation of the fine‐structure‐resolved cross sections and rate constants in the rotationally inelastic collisions. The theoretical values are generally consistent with our experimental findings, both in the order of magnitude and trend of N and ΔN dependence. The propensity rules obtained from the experiments are essentially obeyed by theoretical calculations, and are also in accordance with those reported by Kind and Stuhl. The RET rate constants obtained for the v=0 level are smaller than those obtained previously for v=1. The discrepancy in the RET behavior may be caused by an anisotropy difference of the interaction potential resulting from vibrational excitation.