Direct Amination of Benzene with Molecular Nitrogen Enabled by Plasma-Liquid Interactions.

Nitrogen fixation is industrially realized by mass production of ammonia, the principal intermediate nitrogen source for N-containing organic molecules. Instead, direct C-N bond formation from dinitrogen (N2 ) is of great interest but remains a challenge. Here, by virtue of unique plasma-liquid interactions, we developed an environmentally benign one-pot approach to directly couple benzene and N2 , two naturally abundant yet chemically inert molecules, into value-added arylamines. Under the optimal conditions, an amination yield of 45 % was rapidly achieved, far better than the reported benzene amination efficiency using ammonia. A tentative reaction mechanism was proposed involving the long-lived N2 (A3 Σ u + ) and N2 + species, as evidenced by the key intermediates detected. With a deeper mechanistic understanding and by further optimizing the plasma reactor, the realization of cost-effective electrical amination of benzene with N2 could become reality.

[Effect of low frequency pulsed electromagnetic field on peak bone mass in young rats].

OBJECTIVE: To compare effects of low frequency pulsed electromagnetic fields on bone quality in growing rats between 1 h and 1.5 h. METHODS: Thirty male SPF SD rats aged 4 weeks selected, which with the average body weight (115.8± 1.5) g, were divided into three groups according to random number table, 10 rats in each group. Control groups put rats into electromagnetic field device with 1.5 h every day, the other two groups put rats in electromagnetic field for 1 h and 1.5 h with a 50 Hz 0.6 mT intensity pulsed. The body weight of rats was weighed every 2 weeks and detected bone mineral density. Rats were sacrificed after 6 weeks to measure bone mineral density and biomechanical value of the right femur and lumbar vertebrae. Serum osteocalcin (OC) and tartrate-resistant acid phosphatase 5b (TRACP 5b) concentrations were determined by ELISA methods. After the magenta-picric acid staining, the micro tissue structure of the right tibia was observed, and the parameters of trabecular bone were analyzed by IPP 6.0 software. RESULTS: There were no statistical difference in body weight and organ coefficient among each groups at different times. Bone mineral density results showed that the body thickness of the 1.5 h group was significantly increased compared with control group at 6 weeks, and bone mineral density of femoral and vertebra in 1.5 h group were higher than that of in 1 h group. The results of three point femoral bending and vertebral compression test showed that the maximum load value of femur and vertebrae in 1.5 h group increased significantly compared with control group, and the maximum femoral load value in 1.5 h group was significantly higher than that of 1 h group, while there was no difference in elastic modulus values among each groups. Results of serum biochemical indicators showed that level of OC in 1.5 h group was significantly increased compared with control group, and significantly higher than that of 1 h group, while no significant difference in TRACP 5b values among each groups. Bone histomorphometry analysis results showed that there was no statistical difference in trabecular thickness, number and resolution between 1 h group and control group, trabecular bone thickness and number in 1.5 h group were increased, and trabecular bone resolution was decreased; The thickness and number of trabecular bone in 1.5 h group were also significantly increased compared with 1 h group, and the degree of resolution was reduced, and had significant difference between two groups. CONCLUSION: Intervention of 50 Hz 0.6 mT low frequency pulsed electromagnetic field for 1.5 h could effectively increase peak bone mineral density and bone microstructure in young rats, enhance biomechanical properties of bone, promote concentration of bone formation markers in rat blood. The results indicating that pulsed electromagnetic field could be used as a good way to prevent and treat osteoporosis.

[Different Types of Low-frequency Electromagnetic Fields Resist Bone Loss Caused by Weightlessness].

Objective To compare the effects of 50-Hz 0.6-mT low-frequency pulsed electromagnetic fields(PEMFs) and 50-Hz 1.8-mT sinusoidal alternating electromagnetic fields(SEMFs) in preventing bone loss in tail-suspended rats,with an attempt to improve the prevention and treatment of bone loss caused by weightlessness.Methods Tail-suspension rat models were used to simulate microgravity on the ground. Forty rats were randomly divided into four groups[control group,hindlimb-suspended(HLS) group,HLS+PEMFs group,and HLS+SEMFs group],with 10 rats in each group. In the PEMFs treatment group and SEMFs treatment group,the intervention was 90 min per day. Rats were sacrificed after four weeks. Bone mineral density(BMD) of femur and vertebra was measured by dual-energy X-ray absorptiometry and biomechanical strength by AG-IS biomechanical instrument. Serum osteocalcin(OC),tartrate-resistant acid phosphatase 5b(Tracp 5b),parathyroid hormone(PTH),and cyclic adenosine monophosphate(cAMP) were detected by ELISA. The microstructure of bone tissue was observed by Micro-CT and HE staining.Results The BMD of the femur(P=0.000) and vertebrae(P=0.001) in the HLS group was significantly lower than in the control group;the BMD of the femurs(P=0.001) and vertebrae(P=0.039) in the HLS+PEMFs group was significantly higher than in the HLS group;the BMD of the femurs in the HLS+SEMFs group was significantly higher than in the HLS group(P=0.003),but the BMD of the vertebrae showed no significant difference(P=0.130). There was no significant difference in the BMD of the femur(P=0.818) and vertebrae(P=0.614) between the HLS+PEMFs group and the HLS+SEMFs group. The maximum load(P=0.000,P=0.009) and elastic modulus(P=0.015,P=0.009) of the femurs and vertebrae in the HLS group were significantly lower than those in the control group;the maximum load of the femur(P=0.038) and vertebrae(P=0.087) in the HLS+PEMFs group was significantly higher than that in the HLS group,but the elastic modulus was not significantly different from that in the HLS group(P=0.324,P=0.091). The maximum load(P=0.190,P=0.222) and elastic modulus(P=0.512,P=0.437) of femurs and vertebrae in the HLS+SEMFs group were not significantly different from those in the HLS group. There were no significant differences in the maximum load and elastic modulus of femurs(P=0.585,P=0.948) and vertebrae(P=0.668,P=0.349) between the HLS+PEMFs group and the HLS+SEMFs group. The serum OC level in the HLS group was significantly lower than that in the control group(P=0.000),and the OC level in HLS+PEMFs group(P=0.000) and HLS+SEMFs group(P=0.006) were significantly higher than that in the HLS group. The serum Tracp 5b concentration in the HLS group was significantly higher than that in the control group(P=0.011). There was no significant difference between the HLS+PEMFs group(P=0.459) and the HLS+SEMFs group(P=0.469) compared with the control group.Serum Tracp 5b concentrations in the HLS+PEMFs group(P=0.056) and the HLS+SEMFs group(P=0.054) were not significantly different from those in the HLS group. The PTH(P=0.000) and cAMP concentrations(P=0.000) in the HLS group were significantly lower than those in the control group. The PTH(P=0.000,P=0.000) and cAMP concentrations(P=0.000,P=0.000) in the HLS+PEMFs group and the HLS+SEMFs group were significantly higher than in the HLS group. The femoral cancellous bone of the HLS group was very sparse and small compared with the control group. The density and volume of the cancellous bone were similar among the control group,HLS+PEMFs group,and HLS+SEMFs group. Compared with the control group,the HLS group had lower BMD(P=0.000),bone volume (BV)/tissue volume(TV)(P=0.000),number of trabecular bone (Tb.N)(P=0.000),and trabecular thickness(Tb.Th)(P=0.000) and higher trabecular bone dispersion(Tb.Sp)(P=0.000) and bone surface area(BS)/BV(P=0.000). Compared with the HLS group,the HLS+PEMFs group and the HLS+SEMFs group had significantly lower Tb.Sp(P=0.000,P=0.000) and BS/BV(P=0.000,P=0.000) and significantly increased BMD(P=0.000,P=0.000),BV/TV(P=0.001,P=0.004),Tb.Th(P=0.000,P=0.001),and Tb.N(P=0.000,P=0.001). The trabecular thickness significantly differed between the HLS+PEMFs group and the HLS+SEMFs group(P=0.024). The HLS group(P=0.000),HLS+PEMFs group(P=0.000),and HLS+SEMFs group(P=0.000) had the significantly lower osteoblast density on the trabecular bone surface than the control group;however,it was significantly higher in the HLS+SEMFs group(P=0.000) and the HLS+PEMFs group(P=0.000) than in the HLS group. The HLS group had significantly lower density of osteoblasts in the endothelium than the control group(P=0.000);however,the density of osteoblasts was significantly higher in HLS+PEMFs group(P=0.000) and HLS+SEMFs group(P=0.000) than HLS group and was significantly higher in HLS+PEMFs group than in HLS+SEMFs group(P=0.041). Compared with the control group,a large number of fatty cavities were produced in the bone marrow cavity in the HLS group,but the fat globules remarkably decreased in the treatment groups,showing no significant difference from the control group. The number of adipose cells per mm 2 bone marrow in the HLS group was 4 times that of the control group(P=0.000);it was significantly smaller in the HLS+PEMFs group(P=0.000) and HLS+SEMFs group(P=0.000) than in the HLS group,whereas the difference between the HLS+PEMFs group and the HLS+SEMFs group was not statistically significant(P=0.086). Conclusions 50-Hz 0.6-mT PEMFs and 50-Hz 1.8-mT SEMFs can effectively increase bone mineral density and biomechanical values in tail-suspended rats,increase the concentration of bone formation markers in rat blood,activate the cAMP pathway by affecting PTH levels,and thus further increase the content of osteoblasts to prevent the deterioration of bone micro-structure. In particular,PEMFs can prevent the reduction of bone mineral density and maximum load value by about 50% and increase the bone mass of tail-suspended rats by promoting bone formation.

Pulsed electromagnetic fields prevented the decrease of bone formation in hindlimb-suspended rats by activating sAC/cAMP/PKA/CREB signaling pathway.

Microgravity is one of the main threats to the health of astronauts. Pulsed electromagnetic fields (PEMFs) have been considered as one of the potential countermeasures for bone loss induced by space flight. However, the optimal therapeutic parameters of PEMFs have not been obtained and the action mechanism is still largely unknown. In this study, a set of optimal therapeutic parameters for PEMFs (50 Hz, 0.6 mT 50% duty cycle and 90 min/day) selected based on high-throughput screening with cultured osteoblasts was used to prevent bone loss in rats induced by hindlimb suspension, a commonly accepted animal model to simulate the space environment. It was found that hindlimb suspension for 4 weeks led to significant decreases in femoral and vertebral bone mineral density (BMD) and their maximal loads, severe deterioration in bone micro-structure, and decreases in levels of bone formation markers and increases in bone resorption markers. PEMF treatment prevented about 50% of the decreased BMD and maximal loads, preserved the microstructure of cancellous bone and thickness of cortical bone, and inhibited decreases in bone formation markers. Histological analyses revealed that PEMFs significantly alleviated the reduction in osteoblast number and inhibited the increase in adipocyte number in the bone marrow. PEMFs also blocked decreases in serum levels of parathyroid hormone and its downstream signal molecule cAMP, and maintained the phosphorylation levels of protein kinase A (PKA) and cAMP response element-binding protein (CREB). The expression level of soluble adenylyl cyclases (sAC) was also maintained. It therefore can be concluded that PEMFs partially prevented the bone loss induced by weightless environment by maintaining bone formation through signaling of the sAC/cAMP/PKA/CREB pathway. Bioelectromagnetics. 39:569-584, 2018. © 2018 Wiley Periodicals, Inc.

Non-Transition-Metal Catalytic System for N2 Reduction to NH3: A Density Functional Theory Study of Al-Doped Graphene.

The prevalent catalysts for natural and artificial N2 fixation are known to hinge upon transition-metal (TM) elements. Herein, we demonstrate by density functional theory that Al-doped graphene is a potential non-TM catalyst to convert N2 to NH3 in the presence of relatively mild proton/electron sources. In the integrated structure of the catalyst, the Al atom serves as a binding site and catalytic center while the graphene framework serves as an electron buffer during the successive proton/electron additions to N2 and its various downstream NxHy intermediates. The initial hydrogenation of N2 can readily take place via an internal H-transfer process with the assistance of a Li+ ion as an additive. In view of the recurrence of H transfer in the first step of N2 reduction observed in biological nitrogenases and other synthetic catalysts, this finding highlights the significance of heteroatom-assisted H transfer in the design of synthetic catalysts for N2 fixation.

Validation of density functionals for pancake-bonded π-dimers; dispersion is not enough.

π-Stacking pancake bonding between radicals poses special challenges to density functional theories (DFTs) due to their shorter than van der Waals contact distances, their multireference singlet ground states and the concurrently important dispersion interactions. We examined over 50 DFTs including 22 with dispersion corrections on four different π-dimerized pancake-bonded systems exploring the performances of these DFTs in the very short intermolecular contact regime. We examined crucial energetic as well as geometric parameters against available high-level multireference average quadratic coupled cluster (MR-AQCC) results. Overall we did not find an omnipotent DFT applicable for all four pancake-bonded π-dimers. However, some DFTs were found to perform well for each individual system: M05-2X and PBE0-MBD are the only DFTs that work well for the phenalenyl π-dimer; BLYP is the only appropriate DFT for the 1,2,4,6-thiatriazine π-dimer; O3LYP works best on the double pancake-bonded 1,3,2,4,6-dithiatriazine π-dimer with a few acceptable ones and MN15L is the best method for K+TCNE- π-dimer in addition to a few acceptable ones. We believe our findings can deliver insights towards the design and characterization of the pancake-bond based materials and the development of new DFTs.

Pancake π-π Bonding Goes Double: Unexpected 4e/All-Sites Bonding in Boron- and Nitrogen-Doped Phenalenyls.

Chemical bonding interactions are the main driving force for the formation of molecules and materials from atoms. The two-electron/multicenter pancake π-π bonding found in phenalenyl (PLY, 1) radical π-dimers is intriguing due to its unconventional nature of covalent bonding for molecular aggregations and its propensity to induce unique optical, electronic, and magnetic properties. By using high-level quantum chemistry calculations, we show that the B- or N-doped PLYs (2 and 4), usually considered closed-shell and therefore trifling, can be rendered open-shell singlet by proper edge substitutions (3 and 5). The resulting two unpaired valence electrons on each molecular unit contribute to the formation of a genuine pancake-shaped 4e/all-sites double π-π bonding upon intermolecular π-dimerization, in contrast to the 2e/half-sites single π-π bonding in the parent PLY π-dimers. The unusual double π-π bonding motif discovered in these PLY analogues may broaden the landscape of, and find new applications for, intermolecular covalent bonding interactions.

Nitrogen Doping Enables Covalent-Like π-π Bonding between Graphenes.

The neighboring layers in bilayer (and few-layer) graphenes of both AA and AB stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW) interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in terms of a special chemical bonding between the giant graphene "molecules". Through rigorous theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping levels can form an unusual two-dimensional (2D) π-π bonding in bilayer NGPs bringing the neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies can be enhanced by up to 50% compared to the pristine graphene bilayers that are characterized by only vdW interactions. Such an unusual chemical bonding arises from the π-π overlap across the vdW gap while the individual layers maintain their in-plane π-conjugation and are accordingly planar. The existence of the resulting interlayer covalent-like bonding is corroborated by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the 3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.

Ambient ionization and direct identification of volatile organic compounds with microwave-induced plasma mass spectrometry.

An innovative method of volatile organic compounds analysis by using microwave-induced plasma ionization (MIPI) source in combination with an ambient ion trap mass spectrometer is presented here. Using MIPI for direct sample vapor, analysis was achieved without any sample preparation or subsequent heating. The relative abundance of the target compounds can be obtained almost instantly within a few seconds. The ionization processes of different volatile compounds was optimized, and the limits of detection were identified in the range of 0.15-4.5 pptv or 0.73-8.80 pg ml(-1). The relative standard deviation (RSD) is in the range of 4-14%, while correlation coefficients of the working curves (R(2)) are better than 0.98. The new method possesses advantages of ease operation, time-saving, high sensitivity and inexpensive setup. In addition, the ionization processes of short n-alkane chains were investigated with the MIPI technique, and a unique [M + 13](+) was detected, which has not been reported in detail by any other related ionization techniques. An ionization mechanism was proposed on the basis of the experimental results obtained in this work and available information in literatures, in which the n-alkanes in the plasma environment possibly generate protonated cyclopentadiene [M - 5](+) or alkyl-substituted analogues as well as hydrous ions [M + 13](+) and [M + 13 + 18](+), as shown in Scheme 1 in the main text.

How does Nishibayashi's molybdenum complex catalyze dinitrogen reduction to ammonia?

Recently, Nishibayashi et al. reported a dimolybdenum-dinitrogen complex that is catalytic for complete reduction of dinitrogen to ammonia. This catalyst is different from the Schrock molybdenum catalyst in two fundamental aspects: it contains two metal centers, and the oxidation state is Mo(0) instead of Mo(III). We show that a remarkable feature of the bimetallic complex is the bond-mediated delocalized electronic states, resulting from the two metal centers bridged by a dinitrogen ligand. Using first-principles calculations, we found that this property makes the bimetallic complex the effective catalyst, as opposed to the originally postulated monometallic fragment. A favorable reaction pathway is identified, and the nature of the intermediates is examined. Furthermore, studies of the intermediate states led us to propose possible deactivation processes of the catalyst. The finding that the central bimetallic unit (Mo-N2-Mo) is relevant for catalytic activity may provide a guideline for the development of more efficient dinitrogen-reducing catalysts.

On the anisotropy of van der Waals atomic radii of O, S, Se, F, Cl, Br, and I.

The Cambridge Structural Database (CSD) was used to obtain flattening factors to describe the overall anisotropy of nonbonding van der Waals (vdW) contacts between several main group elements. The method for obtaining the flattening factors is based on a novel minimization process. Results show that the vdW contact distances are significantly dependent on the environment and the orientations of the surrounding covalently bonded atoms: head-on vdW contacts are generally shorter than sideways contacts in overall agreement with earlier results by Nyburg and Faerman (Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 274-279). With the exception of Se, we find flattening factors that are somewhat smaller than those found earlier. High-level ab initio quantum chemical calculations using Ar and Ne as a probe also confirm the flattening effect and its dependency on the environment. A dozen popular long-range corrected and dispersion supplemented density functionals are compared with the CCSD(T) data. While several of them perform quite poorly, four DFT-D methods, especially B3LYP-GD3BJ, provided vdW flattening similar to those found by the CCSD(T) theory and experiment.

Synthesis and characterization of a tetrathiafulvalene-salphen actinide complex.

A new tetrathiafulvalene-salphen uranyl complex has been prepared. The system was designed to study the electronic coupling between actinides and a redox active ligand framework. Theoretical and experimental methods--including DFT calculations, single crystal X-ray analysis, cyclic voltammetry, NMR and IR spectroscopies--were used to characterize this new uranyl complex.

Exploring electrochemical windows of room-temperature ionic liquids: a computational study.

Room-temperature ionic liquids (RTILs) are regarded as green solvents due to their low volatility, low flammability, and thermal stability. RTILs exhibit wide electrochemical windows, making them prime candidates as media for electrochemically driven reactions such as electro-catalysis and electro-plating for separations applications. Therefore, understanding the factors determining edges of the electrochemical window, the electrochemical stability of the RTILs, and the degradation products is crucial to improve the efficiency and applicability of these systems. We present here computational investigations of the electrochemical properties of a variety of RTILs covering a wide range of electrochemical windows. We proposed four different approaches with different degrees of approximation and computational cost from gas-phase calculations to full explicit solvation models. It was found that, whereas the simplest model has significant flaws in accuracy, implicit and explicit solvent models can be used to reliably predict experimental data. The general trend of electrochemical windows of the RTILs studied is well reproduced, showing that it increases in the order of imidazolium < ammonium < pyrrolidinium < phosphonium giving confidence to the methodology presented to use it in screening studies of ionic liquids.

Bonds or not bonds? Pancake bonding in 1,2,3,5-dithiadiazolyl and 1,2,3,5-diselenadiazolyl radical dimers and their derivatives.

Unusually long bonds or short intermolecular contacts occur in the title compounds reminiscent of pancake bonding. Pancake bonding interactions seem analogous to π-stacking interactions, but they display much shorter contact distances than normally seen in van der Waals (vdW) dimers. The interpretation of these SN and SeN containing structures has been an outstanding challenge for some time. The antibonding (π*) singly occupied molecular orbital (SOMO) of the radical is the source of two-electron multicenter bonding (2e/mc). Preferred conformations thus can be traced back to SOMO-SOMO overlap. We used several computational methods to understand the nature of pancake bonding in the title compounds including four wave function methods (WFT) and a dozen density functional theories (DFT) including empirical dispersion corrections. We used experimental data and high level CCSD(T)/6-311++G(d,p) and MRPT2/6-311++G(d,p) calculations for comparison. The analysis provided the interpretation a wealth of experimental data including conformational preferences of these SN and SeN containing radical dimers leading to a better overall understanding of pancake bonding. Analysis of the various components of the inter-radical interactions showed that SOMO-SOMO bonding interaction and dispersion interaction contribute to the binding energy and neither of these interactions alone is sufficient to bind the dimer. The dimer is predicted to show weak diradical character.

Charge shift bonding concept in radical π-dimers.

We show that pancake bonding in radical π-dimers display features of charge shift (CS) bonding. While the CS bonding concept has been developed to interpret the unusual aspects of σ-bonds around centers with a large number of lone pairs, such as F(2) and HOOH, we find a similar role played by the nonbonding or slightly bonding π-electron pairs in π-stacking radical dimers. Arguments and computational evidence indicate that the CS bonding concept developed by Shaik and Hiberty et al. captures essential features of the intermolecular bonding in radical π-dimers in which the overlap of the two radical centered singly occupied molecular orbitals (SOMOs) play a crucial role. By using the tetracyanoethylene anion dimer, [TCNE](2)(2-), as a model, we show that compared to CAS(2,2) calculations, significant binding contributions are recovered in the calculations simply by including selected intrapair excitations of the SOMO-SOMO bonding orbitals and the nonbonding π-orbitals. This observation is the basis for the analogy of chemical bonding between pancake bonded radical π-dimers and other charge shift bonded molecules, such as F(2). By extending the CS bonding concept to a new class of molecules, we find a novel application of the lone pair bond weakening effect (LPBWE) in which the doubly occupied π-orbitals play the role of lone pairs.

Is there a lower limit to the CC bonding distances in neutral radical pi-dimers? The case of phenalenyl derivatives.

Two-electron multicenter (2e/mc) bonding of phenalenyl (PHYL) pi-dimers was found to be significantly affected by the electron density on the bonding active sites. The computational analysis shows that, upon appropriate beta-substitutions, the newly introduced dimers have the shortest and strongest covalent bonding interactions seen in any neutral pi-dimer. The unusual strengthening of the bonding was attributed to the reduced lone pair bond weakening effect, LPBWE, upon substitutions with electron-withdrawing groups.

Why is there no in-plane H-atom transfer from aryloxy radicals? A theoretical and experimental investigation.

A combined experimental and theoretical study of the mechanisms and energies associated with intramolecular H-atom transfers from methyl groups with varying numbers of phenyl substituents to oxygen atoms of aryloxy radicals is reported. It is shown that the transfers within the six aryloxy radicals investigated would have high activation energies and, in all but one case, are endothermic. A detailed analysis of the calculated reaction coordinates indicates proton-coupled electron transfers as the favored mechanisms.

Bimolecular hydrogen transfer in phenalene by a stepwise ene-like reaction mechanism.

For the hydrogen transfer of phenalene, a bimolecular ene-like mechanism is proposed, which is preferable over the hypothesized unimolecular rearrangement in the literature. Unique SOMO-SOMO pi-bonding of phenalenyl reduces the barriers of pericyclic reactions significantly. Pi-bonding between radicals is being recognized as a novel type of bonding interaction. This paper adds to the use of this interaction by pointing out its effect on reaction barriers.

Fluxional sigma-bonds of 2,5,8-tri-tert-butyl-1,3-diazaphenalenyl dimers: stepwise [3,3], [5,5] and [7,7] sigmatropic rearrangements via pi-dimer intermediates.

Inspired by experimental evidence of the thermally accessible pi-dimer of the title compound, DAzPh (7), we propose that the sigma-dimer (8) can undergo a variety of sigma-bond shifts representing very unusual multi-faceted fluxional bonding between two neutral pi-radicals. In this paper, we present a theoretical study of the sigmatropic rearrangement of the DAzPh sigma-dimers. Out of the six sigma-bonded tautomers three are competitive: a degenerate pair resulting from a [5,5] sigmatropic rearrangement and a non-degenerate product of a [3,3] sigmatropic rearrangement with barriers of 10.21 kcal mol(-1) and 10.00 kcal mol(-1), respectively. Both of these rearrangements occur stepwise through a pi-dimer intermediate (9), which is 1.33 kcal mol(-1) higher in energy than the sigma-dimer (8). These data are consistent with optical and paramagnetic susceptibility experiments and offer a natural interpretation for the unusual C-C contact distance of 2.153 A obtained by X-ray diffraction by Morita et al. Another new sigma-dimer (15) with a different dipole-dipole stacking pattern is predicted, the energy of which is very close to that of 8, and is likely to be isolable under suitable conditions. The new sigma-dimer (15) is expected to undergo stepwise [7,7] sigmatropic rearrangement. Thus we observed a complete spectrum of sigmatropic rearrangement reactions in these DAzPh dimers. The pi-dimers 4, 9 and 17 show decreasing order of SOMO-SOMO splittings consistent with the UV-vis absorbance. The calculated paramagnetism is in good agreement with experiments providing further evidence for the presented interpretation of fluxional bonding in the DAzPh sigma-dimers.