Search Results (8)

We studied the boron-based composite cluster B₈Al₃⁺ doped with Al atoms. The global minimum structure of the B₈Al₃⁺ cluster is a three-layer structure, consisting of three parts: an Al₂ unit, a B₈ ring and an isolated Al atom. Charge calculations analysis shows that the cluster can be expressed as [Al]⁺[B₈]²⁻[Al₂]²⁺, has 6π/6σ double aromaticity and follows the (4n+2) Hückel rule. Born–Oppenheimer molecular dynamics (BOMD) simulation shows that the B₈Al₃⁺ cluster has dynamic fluxionality properties. Remarkably, at the single-point coupled cluster singles, doubles and triples (CCSD(T)) level, the energy barrier for intramolecular rotation is merely 0.19 kcal mol⁻¹. [B₈]²⁻ molecular wheels have magical 6π/6σ double aromaticity properties, providing a continuous cloud of delocalized electrons, which is a key factor in the dynamic fluxionality of the cluster. The B₈Al₃⁺ cluster provides a new example of dynamic structural fluxionality in molecular systems. Full article

Studies of the rotational barrier energy of the amide bond using quantum computing and nuclear magnetic resonance (NMR) are focused mainly on its use as a model of the peptide bond. The results of these studies are valuable not only in terms of the fundamental conformational properties of amide bonds, but also in the design of molecular machines, which have recently attracted interest. We investigate the fluxionality of the amide and enamide bonds of compound 3-[(E)-(dimethylamino)methylidene]-1,1-dimethylurea using advanced dynamic NMR experiments and a theoretical evaluation of the density functional theory (DFT) calculation. The dynamic NMR study shows restricted rotation around the amide group (16.4 kcal/mol) and a very high barrier around the enamine group (18.6 kcal/mol). In a structurally similar compound, (E)-3-(dimethylamino)-N,N-dimethylacrylamide (N atom is replaced by CH), the amide barrier is 12.4 kcal/mol and the enamine barrier is 11.7 kcal/mol. The DFT studies of both compounds reveal the electronic origin of this phenomenon. Theoretical calculations reveal the origin of the higher enamine barrier. The better delocalization of the lone pair of electrons on the end nitrogen atom into the antibonding orbital of the neighboring C–N double bond leads to the better stabilization of the ground state, and this leads to a greater increase in the enamine barrier. Full article

Doping alkali metals into boron clusters can effectively compensate for the intrinsic electron deficiency of boron and lead to interesting boron-based binary clusters, owing to the small electronegativity of the former elements. We report on the computational design of a three-layered sandwich cluster, Na₅B₇, on the basis of global-minimum (GM) searches and electronic structure calculations. It is shown that the Na₅B₇ cluster can be described as a charge-transfer complex: [Na₄]²⁺[B₇]³⁻[Na]⁺. In this sandwich cluster, the [B₇]³⁻ core assumes a molecular wheel in shape and features in-plane hexagonal coordination. The magic 6π/6σ double aromaticity underlies the stability of the [B₇]³⁻ molecular wheel, following the (4n + 2) Hückel rule. The tetrahedral Na₄ ligand in the sandwich has a [Na₄]²⁺ charge-state, which is the simplest example of three-dimensional aromaticity, spherical aromaticity, or superatom. Its 2σ electron counting renders σ aromaticity for the ligand. Overall, the sandwich cluster has three-fold 6π/6σ/2σ aromaticity. Molecular dynamics simulation shows that the sandwich cluster is dynamically fluxional even at room temperature, with a negligible energy barrier for intramolecular twisting between the B₇ wheel and the Na₄ ligand. The Na₅B₇ cluster offers a new example for dynamic structural fluxionality in molecular systems. Full article

A density functional theory study is performed to determine the stability and bonding in the neon dimer inside the B₃₀N₃₀ fullerene cage, the fluxional B₄₀ cage, and within non-fluxional cages such as B₁₂N₁₂ and C₆₀. The nature of bonding in the Ne₂ encapsulated B₄₀ is compared with the that in other cages in an attempt to determine whether any possible alterations are brought about by the dynamical nature of the host cage apart from the associated confinement effects. The bonding analysis includes the natural bond order (NBO), Bader’s Atoms-in-Molecules electron density analysis (AIM), and energy decomposition analysis (EDA), revealing the non-covalent nature of the interactions between the Ne atoms and that between the Ne and the cage atoms. The formation of all the Ne₂@cage systems is thermochemically unfavourable, the least being that for the B₃₀N₃₀ cage, which can easily be made favourable at lower temperatures. The Ne-Ne distance is lowest in the smallest cage and increases as the cage size increase due to steric relaxation experienced by the dimer. The dynamical picture of the systems is investigated by performing ab initio molecular dynamics simulations using the atom-centred density matrix propagation (ADMP) technique, which shows the nature of the movement of the dimer inside the cages, and by the fact that since it moves as a single entity, a weak bonding force holds them together, apart from their proven kinetic stability. Full article

We systematically explore the potential energy surface of the B₃Al₄⁺ combination of atoms. The putative global minimum corresponds to a structure formed by an Al₄ square facing a B₃ triangle. Interestingly, the dynamical behavior can be described as a Reuleaux molecular triangle since it involves the rotation of the B₃ triangle at the top of the Al₄ square. The molecular dynamics simulations, corroborating with the very small rotational barriers of the B₃ triangle, show its nearly free rotation on the Al₄ ring, confirming the fluxional character of the cluster. Moreover, while the chemical bonding analysis suggests that the multicenter interaction between the two fragments determines its fluxionality, the magnetic response analysis reveals this cluster as a true and fully three-dimensional aromatic system. Full article

Insufficient light supply for canopies is a constant issue during greenhouse production in most areas of Northern China. Applying supplemental lighting to plant canopies is an efficient method of solving this problem. Several studies were conducted to identify the optimal, economically efficient abaxial leafy supplemental lighting mode to produce high-quality greenhouse tomatoes. In this experiment, no supplemental treatment was used as a blank control (CK), while three supplemental lighting modes were used as treatments: T1, continuous supplemental lighting from 8:00–9:00 (at GMT+8, which is 6:00–7:00 local time, before the thermal insulation covers, abbreviated as TIC below, opening), and 20:00–22:00 (after TIC closing) with photosynthetic photon fluxion density (PPFD) of 200 μmol·m⁻²·s⁻¹; T2, dynamic altered supplemental lighting with PPFD rising from 100 μmol·m⁻²·s⁻¹ to 200 μmol·m⁻²·s⁻¹ before TIC opening and falling from 200 μmol·m⁻²·s⁻¹ to 100 μmol·m⁻²·s⁻¹ after TIC closing; and T3, intermittent supplemental lighting which was automatically conducted with PPFD of 100 μmol·m⁻²·s⁻¹ when indoor PPFD below 150 μmol·m⁻²·s⁻¹ from 8:00–22:00. The results demonstrated that abaxial leafy supplemental lighting treatment could improve both fruit yield and quality. The total yield in the T1 and T2 treatments was higher than in other treatments, though there was no significant difference. Differences in leaf carbon exportation showed the possibility of determining fruit yield from the 3rd leaf under the fruit. The overall appearance, flavor quality, nutrient indicators, and aroma of cherry tomato fruits under T1 and T2 treatments were generally higher than in other treatments. Correlation analysis of fruit yield and quality parameters suggested that they produce relatively high yield and fruit quality. Combined with a cost-performance analysis, dynamic altered supplemental lighting (T2) is more suitable for high-valued greenhouse cherry tomato production. Full article

The correlation consistent Composite Approach for transition metals (ccCA-TM) and density functional theory (DFT) computations have been applied to investigate the fluxional mechanisms of cyclooctatetraene tricarbonyl chromium ((COT)Cr(CO)₃) and 1,3,5,7-tetramethylcyclooctatetraene tricarbonyl chromium, molybdenum, and tungsten ((TMCOT)M(CO)₃ (M = Cr, Mo, and W)) complexes. The geometries of (COT)Cr(CO)₃ were fully characterized with the PBEPBE, PBE0, B3LYP, and B97-1 functionals with various basis set/ECP combinations, while all investigated (TMCOT)M(CO)₃ complexes were fully characterized with the PBEPBE, PBE0, and B3LYP methods. The energetics of the fluxional dynamics of (COT)Cr(CO)₃ were examined using the correlation consistent Composite Approach for transition metals (ccCA-TM) to provide reliable energy benchmarks for corresponding DFT results. The PBE0/BS1 results are in semiquantitative agreement with the ccCA-TM results. Various transition states were identified for the fluxional processes of (COT)Cr(CO)₃. The PBEPBE/BS1 energetics indicate that the 1,2-shift is the lowest energy fluxional process, while the B3LYP/BS1 energetics (where BS1 = H, C, O: 6-31G(d′); M: mod-LANL2DZ(f)-ECP) indicate the 1,3-shift having a lower electronic energy of activation than the 1,2-shift by 2.9 kcal mol⁻¹. Notably, PBE0/BS1 describes the (CO)₃ rotation to be the lowest energy process, followed by the 1,3-shift. Six transition states have been identified in the fluxional processes of each of the (TMCOT)M(CO)₃ complexes (except for (TMCOT)W(CO)₃), two of which are 1,2-shift transition states. The lowest-energy fluxional process of each (TMCOT)M(CO)₃ complex (computed with the PBE0 functional) has a ΔG^‡ of 12.6, 12.8, and 13.2 kcal mol⁻¹ for Cr, Mo, and W complexes, respectively. Good agreement was observed between the experimental and computed ¹H-NMR and ¹³C-NMR chemical shifts for (TMCOT)Cr(CO)₃ and (TMCOT)Mo(CO)₃ at three different temperature regimes, with coalescence of chemically equivalent groups at higher temperatures. Full article

Intermolecular chemical networks defined by the hydrogen bonds formed at the α-quartz|water interface have been data-mined using graph theoretical methods so as to identify and quantify structural patterns and dynamic behavior. Using molecular-dynamics simulations data, the hydrogen bond (H-bond) distributions for the water-water and water-silanol H-bond networks have been determined followed by the calculation of the persistence of the H-bond, the dipole-angle oscillations that water makes with the surface silanol groups over time, and the contiguous H-bonded chains formed at the interface. Changes in these properties have been monitored as a function of surface coverage. Using the H-bond distribution between water and the surface silanol groups, the actual number of waters adsorbed to the surface is found to be 0.6 H₂O/10 Ų, irrespective of the total concentration of waters within the system. The unbroken H-bond network of interfacial waters extends farther than in the bulk liquid; however, it is more fluxional at low surface coverages (i.e., the H-bond persistence in a monolayer of water is shorter than in the bulk) Concentrations of H₂O at previously determined water adsorption sites have also been quantified. This work demonstrates the complementary information that can be obtained through graph theoretical analysis of the intermolecular H-bond networks relative to standard analyses of molecular simulation data. Full article