Ring Centroid Distance Research Articles

Effects of structural parameters of a novel burner scheme with annularly combined multiple airflows on performance of a 660 MW tangentially fired boiler

Based on a previously validated 660 MW tangentially-fired boiler model, effects of equivalent ring diameter (ERD) and ring centroid offset distance (RCOD) of the proposed burner with annularly combined multiple airflows (ACMA) on boiler performance are investigated. Results show that both ERD and RCOD have great influences on ACMA boiler performance. Small ERD enhances the mutual entrainments of combined airflows, but the interweave of combined airflows hinders flue gas to get into their interior. Although large ERD weakens airflows’ mutual entrainments, flue gas can enter the interior of combined airflows to avoid the large pressure difference from the fireside to the backfire sides, so airflow deflection is alleviated. For coal combustion process, small ERD only promotes the intense combustion inside each ACMA burner, while increased ERD makes the intense coal combustion process more continuous at the burner gaps. The increased RCOD adversely affects both the aerodynamics and coal combustion performance, resulting in large airflow deflections and deteriorated coal combustion process. For the investigated boiler, ERD of 3.6 m and RCOD of 5.0 m are recommended. These findings deepen the understanding of ACMA burners and, together with future experimental research results, can provide guidance for its application in practical boilers.

Lanthanide-mediated tuning of electronic and magnetic properties in heterotrimetallic cyclooctatetraenyl multidecker self-assemblies.

The synthesis of a novel family of homoleptic COT-based heterotrimetallic self-assemblies bearing the formula [LnKCa(COT)3(THF)3] (Ln(iii) = Gd, Tb, Dy, Ho, Er, Tm, and Yb) is reported followed by their X-ray crystallographic and magnetic characterization. All crystals conform to the monoclinic P21/c space group with a slight compression of the unit cell from 3396.4(2) Å3 to 3373.2(4) Å3 along the series. All complexes exhibit a triple-decker structure having the Ln(iii) and K(i) ions sandwiched by three COT2− ligands with an end-bound {Ca2+(THF)3} moiety to form a non-linear (153.5°) arrangement of three different metals. The COT2− ligands act in a η8-mode with respect to all metal centers. A detailed structural comparison of this unique set of heterotrimetallic complexes has revealed consistent trends along the series. From Gd to Yb, the Ln to ring-centroid distance decreases from 1.961(3) Å to 1.827(2) Å. In contrast, the separation of K(i) and Ca(ii) ions from the COT-centroid (2.443(3) and 1.914(3) Å, respectively) is not affected by the change of Ln(iii) ions. The magnetic property investigation of the [LnKCa(COT)3(THF)3] series (Ln(iii) = Gd, Tb, Dy, Ho, Er, and Tm) reveals that the Dy, Er, and Tm complexes display slow relaxation of their magnetization, in other words, single-molecule magnet (SMM) properties. This behaviour is dominated by thermally activated (Orbach-like) and quantum tunneling processes for [DyKCa(COT)3(THF)3] in contrast to [ErKCa(COT)3(THF)3], in which the thermally activated and Raman processes appear to be relevant. Details of the electronic structures and magnetic properties of these complexes are further clarified with the help of DFT and ab initio theoretical calculations.

(1R*,3S*,4S*)-3-(2-Hy-droxy-benzo-yl)-1,2,3,4-tetra-hydro-1,4-ep-oxy-naphthalen-1-yl]methyl 4-nitro-benzoate.

The relative stereo- and regiochemistry of the racemic title compound, C25H19NO7, were established from the crystal structure. The fused benzene ring forms dihedral angles of 77.3 (1) and 60.3 (1)° with the hy-droxy-substituted benzene ring and the nitro-substituted benzene ring, respectively. The dihedral angle between the hy-droxy-substituted benzene ring and the nitro-substituted benzene ring is 76.4 (1)°. An intra-molecular O-H⋯O hydrogen bond closes an S(6) ring. In the crystal, weak C-H⋯O hydrogen bonds connect the mol-ecules, forming layers parallel to (100). Within these layers, there are weak π-π stacking inter-actions with a ring centroid-ring centroid distance of 3.555 (1) Å.

1,1′-Bis(4-bromophenyl)-3,3′-diphenylferrocene

In the title compound, [Fe(C17H12Br)2], the FeII atom is sandwiched between the eclipsed five-membered rings, with Fe...ring centroid distances of 1.649 (3) and 1.652 (3) Å. In one of the ligands, the Br atom is disordered over two locations [site occupancies = 0.839 (2) and 0.161 (2)].

Can Stacking Interactions Exist Beyond the Commonly Accepted Limits?

Systematic study of π···π interactions of structurally characterized compounds containing parallel benzene and/or pyridine rings was carried out. The gathered geometrical parameters were analyzed in statistical terms. The quantum mechanical calculations were made for the above-mentioned ring systems in different arrangements and the calculated interaction energy values were referred to the statistical data. The maximum bonding energy of the studied systems is about 3 kcal/mol. The ring rotation about the vertical axis has almost no influence on the system binding energy. In the specific ring arrangements, the stacking interactions can be bonding even for ring centroid distances larger than 6 A. The results prove that the application of the generally accepted geometrical criteria of stacking interactions leads to the omission of the multiple bonding intermolecular interactions during the interpretation of the reactivity, self-assembly, as well as the properties of the supramolecular compounds.

The intricacies of the stacking interaction in a pyrrole–pyrrole system

Extensive calculations of potential energy surfaces for parallel-displaced configurations of pyrrole–pyrrole systems have been carried out by the use of a dispersion-corrected density functional. System geometries associated with the energy minima have been found. The minimum interaction energy has been calculated as −5.38 kcal/mol. However, bonding boundaries appeared to be relatively broad, and stacking interactions can be binding even for ring centroid distances larger than 6 A. Though the contribution of the correlation energy to intermolecular interaction in pyrrole dimers appeared to be relatively small (around 1.6 smaller than it is in a benzene–benzene system), this system’s minimum interaction energy is lower than those calculated for benzene–benzene, benzene–pyridine and even pyridine–pyridine configurations. The calculation of the charges and energy decomposition analysis revealed that the specific charge distribution in a pyrrole molecule and its relatively high polarization are the significant source of the intermolecular interaction in pyrrole dimer systems.

Structural, spectroscopic, and theoretical comparison of traditional vs recently discovered Ln(2+) ions in the [K(2.2.2-cryptand)][(C5H4SiMe3)3Ln] complexes: the variable nature of Dy(2+) and Nd(2+).

The Ln(3+) and Ln(2+) complexes, Cp'3Ln, 1, (Cp' = C5H4SiMe3) and [K(2.2.2-cryptand)][Cp'3Ln], 2, respectively, have been synthesized for the six lanthanides traditionally known in +2 oxidation states, i.e., Ln = Eu, Yb, Sm, Tm, Dy, and Nd, to allow direct structural and spectroscopic comparison with the recently discovered Ln(2+) ions of Ln = Pr, Gd, Tb, Ho, Y, Er, and Lu in 2. 2-La and 2-Ce were also prepared to allow the first comparison of all the lanthanides in the same coordination environment in both +2 and +3 oxidation states. 2-La and 2-Ce show the same unusual structural feature of the recently discovered +2 complexes, that the Ln-(Cp' ring centroid) distances are only about 0.03 Å longer than in the +3 analogs, 1. The Eu, Yb, Sm, Tm, Dy, and Nd complexes were expected to show much larger differences, but this was observed for only four of these traditional six lanthanides. 2-Dy and 2-Nd are like the new nine ions in this tris(cyclopentadienyl) coordination geometry. A DFT-based model explains the results and shows that a 4f (n)5d(1) electron configuration is appropriate not only for the nine recently discovered Ln(2+) ions in 2 but also for Dy(2+) and Nd(2+), which traditionally have 4f (n+1) electron configurations like Eu(2+), Yb(2+), Sm(2+), and Tm(2+). These results indicate that the ground state of a lanthanide ion in a molecule can be changed by the ligand set, a previously unknown option with these metals due to the limited radial extension of the 4f orbitals.

Conformation and crystal structures of 1-amino-cyclo-hexa-neacetic acid (β(3,3)Ac6c) in N-protected derivatives.

N-Protected derivatives of 1-amino-cyclo-hexa-neacetic acid (β(3,3)-Ac6c), namely Valeroyl-β(3,3)-Ac6c-OH [2-(1-pentanamidocyclohexyl)acetic acid, C13H23NO3], (I), Fmoc-β(3,3)-Ac6c-OH [2-(1-{[(9H-fluoren-9-yloxy)carbonyl]amino}cyclohexyl)acetic acid, C23H25NO4], (II), and Pyr-β(3,3)-Ac6c-OH {2-[1-(pyrazine-2-amido)cyclohexyl]acetic acid, C13H17N3O3}, (III), were synthesized and their conformational properties were determined by X-ray diffraction analysis. The backbone torsion angles (ϕ, θ) for β(3,3)-Ac6c-OH are restricted to gauche conformations in all the derivatives, with a chair conformation of the cyclo-hexane ring. In the crystal structure of (I), the packing of mol-ecules shows both carb-oxy-lic acid R 2 (2)(8) O-H⋯O and centrosymmetric R (2) 2(14) N-H⋯O hydrogen-bonding inter-actions, giving rise to chains along the c-axis direction. In (II), centrosymmetric carb-oxy-lic acid R 2 (2)(8) O-H⋯O dimers are extended through N-H⋯O hydrogen bonds and together with inter-ring π-π inter-actions between Fmoc groups [ring centroid distance = 3.786 (2) Å], generate a layered structure lying parallel to (010). In the case of compound (III), carb-oxy-lic acid O-H⋯Npyrazine hydrogen bonds give rise to zigzag ribbon structures extending along the c-axis direction.

(2,2′-Bipyridyl-κ2N,N′)bis(η5-pentamethylcyclopentadienyl)barium

In the title compound, [Ba(C10H15)2(C10H8N2)], the Ba—N distances are 2.798 (3) and 2.886 (3) Å, and the Cp ring centroid distances to Ba2+ are 2.7291 (7) and 2.7192 (9) Å. The angle between the N atoms in the bypyridine ligand and the metal ion is 56.80 (8)° and the N—C—C—N torsion angle in the bi­pyridine ligand is 1.7 (4)°. The bi­pyridine ligand is almost planar, the dihedral angle formed by the intersection of the planes defined by the pyridyl rings being 3.04 (19)°, and the angle between the plane defined by the Ba2+ ion and the two bipyridyl N atoms and the plane defined by the 12 atoms of the bi­pyridine ligand is 10.2 (3)°. The average Ba—N and Ba—centroid distances are 0.16 and 0.14 Å longer, respectively, than the equivalent distances in the isotypic strontium compound [Kazhdan et al. (2008 ▶). Acta Cryst. E64, m1134]. This difference is in accord with the difference between the ionic radii of 0.16 Å suggested by Shannon [Acta Cryst. (1976 ▶), A32, 751–767].

2,4-Dimeth-oxy-6-[(E)-2-(4-meth-oxy-phenyl)ethen-yl]benzaldehyde.

There are two conformationally similar mol­ecules in the asymmetric unit of he title compound, C18H18O4, in which the dihedral angles between the benzene rings are 23.54 (12) and 31.11 (12)°. In the crystal, C—H⋯π inter­actions (minimum H⋯ring centroid distance = 2.66 Å) link the mol­ecules into a layered structure extending down a.

A New Di-Copper(II) Complex of Schiff-Base and 1,3-Di(pyridin-4-yl)propane Ligands: Synthesis, Spectral, Thermal Properties, and Crystal Structure

A new binuclear copper(II) complex with the Schiff-base ligand N-(2-hydroxybenzyl)aminomethanesulfonic acid (H2Saes) has been synthesized, [Cu(Saes) (1,3-dpp)]2 (1,3-di(pyridin-4-yl)propane abbreviation as 1,3-dpp) (1), and characterized by single-crystal X-ray diffraction, elemental, IR spectra, and thermogravimetric analyses. Complex 1 shows phenoxo-bridged dinuclear Cu2O2 core with intermetallic distance of 3.19 (8) Å. In the crystal structure, 1D infinite chain structure is formed through weak π-π stacking interactions between the phenyl rings of adjacent dimmer units with ring-centroid distances of 3.787 (4) Å. In addition, 2-D layers structure are formed by intermolecular C-H···O hydrogen bonds with the separate C···O distances of 3.278 (8)-3.533 (9) Å. Molecules from neighboring stacks further interdigitate with each other in the c-direction, leading to an interwoven three-dimensional network held together by C-H···O interactions and π-stacking and stabilize the coordination complex.

Methyl 3′,4′,5′-trimethoxybiphenyl-4-carboxylate

In the title compound, C17H18O5, the dihedral angle between the benzene rings is 31.23 (16)°. In the crystal, the mol­ecules are packed in an anti­parallel fashion in layers along the a axis. In each layer, very weak C—H⋯O hydrogen bonds occur between the meth­oxy and methyl ester groups. Weak C—H⋯π inter­actions between the 4′- and 5′-meth­oxy groups and neighbouring benzene rings [meth­oxy-C–ring centroid distances = 4.075 and 3.486 Å, respectively] connect the layers.

(Nitrato-κ2O,O′)bis(1,10-phenanthroline-κ2N,N′)copper(II) tricyanomethanide

The title compound, [Cu(NO3)(C12H8N2)2][C(CN)3], is formed of discrete [Cu(NO3)(phen)2]+ complex cations (phen is 1,10-phenanthroline) and C(CN)3 − counter-anions. The CuII atom has an asymmetric tetragonal–bipyramidal (4 + 1+1) stereochemistry with a pseudo-C 2 symmetry axis bis­ecting the nitrate ligand and passing through the CuII atom between the two phen ligands. The four N atoms of the phen ligands coordinate to the CuII atom with Cu—N distances in the range 1.974 (2)–2.126 (2) Å, while the two O atoms coordinate at substanti­ally different distances [2.154 (2) and 2.586 (2) Å]. The structure is stabilized by C—H⋯O hydrogen bonds and weak π–π inter­actions between nearly parallel benzene and pyridine rings of two adjacent phen mol­ecules, with centroid–centroid distances of 3.684 (2) and 3.6111 (2) Å, and between π-electrons of the tricyano­methanide anion and the pyridine or benzene rings [N⋯(ring centroid) distances = 3.553 (3)–3.875 (3) Å].

4-({(Z)-5-[(Z)-3-Ethoxy-4-hydroxybenzylidene]-3-methyl-4-oxo-1,3-thiazolidin-2-ylidene}amino)benzoic acid dimethylformamide monosolvate

The mol­ecular structure of the title compound, C20H18N2O5S·C3H7NO, represents an essentially planar 5-benzyl­idene-thia­zolidine moiety (r.m.s. deviation from planarity without ring substituents = 0.095 Å) to which the 4-amino­benzoic acid fragment is inclined at 76.23 (1)°. In the crystal, the benzoic acid mol­ecules are arranged in layers parallel to [001] which are built up from inversion dimers held together by head-to-tail phenol–carb­oxy O—H⋯O hydrogen bonds and head-to-tail π–π stacking inter­actions between the 5-benzyl­idene-thia­zolidine moieties (ring centroid distance = 3.579 Å). These layers are separated by the dimethyl­formamide solvent mol­ecules which are firmly anchored via a short O—H⋯O hydrogen bond [O⋯O = 2.5529 (10) Å] donated by the –COOH group.

2-Oxo-2H-chromen-4-yl 4-methoxybenzoate

In the title mol­ecule, C17H12O5, the chromen-2-one ring and the 4-meth­oxy­benzoate side chain are inclined to one another at a dihedral angle of 69.82 (9)°. The crystal structure features parallel sheets of centrosymmetric R 2 2(6) dimers joined by a C(7) chain, resulting in centrosymetric tetra­mers of hydrogen-bonded mol­ecules with graph-set motif R 4 4(40). These centrosymetric tetra­mers are connected by a pair of hydrogen bonds described by an R 2 2(8) ring motif and a C(7) chain via C—H⋯O inter­actions. In the structure, there are also π–π stacking inter­actions between chromene benzene and the six-membered heterocyclic rings [centroid–centroid distance = 3.691 (2) Å] and weak C=O⋯π inter­actions [O⋯(ring centroid) distance = 3.357 (3) Å].

Expanding Rare-Earth Oxidation State Chemistry to Molecular Complexes of Holmium(II) and Erbium(II)

The first molecular complexes of holmium and erbium in the +2 oxidation state have been generated by reducing Cp'(3)Ln [Cp' = C(5)H(4)SiMe(3); Ln = Ho (1), Er (2)] with KC(8) in the presence of 18-crown-6 in Et(2)O at -35 °C under argon. Purification and crystallization below -35 °C gave isomorphous [(18-crown-6)K][Cp'(3)Ln] [Ln = Ho (3), Er (4)]. The three Cp' ring centroids define a trigonal-planar geometry around each metal ion that is not perturbed by the location of the potassium crown cation near one ring with K-C(Cp') distances of 3.053(8)-3.078(2) Å. The metrical parameters of the three rings are indistinguishable within the error limits. In contrast to Ln(2+) complexes of Eu, Yb, Sm, Tm, Dy, and Nd, 3 and 4 have average Ln-(Cp' ring centroid) distances only 0.029 and 0.021 Å longer than those of the Ln(3+) analogues 1 and 2, a result similar to that previously reported for the 4d(1) Y(2+) complex [(18-crown-6)K][Cp'(3)Y] (5) and the 5d(1) La(2+) complex [K(18-crown-6)(Et(2)O)][Cp″(3)La] [Cp″ = 1,3-(Me(3)Si)(2)C(5)H(3)]. Surprisingly, the UV-vis spectra of 3 and 4 are also very similar to that of 5 with two broad absorptions in the visible region, suggesting that 3-5 have similar electron configurations. Density functional theory calculations on the Ho(2+) and Er(2+) species yielded HOMOs that are largely 5d(z(2)) in character and supportive of 4f(10)5d(1) and 4f(11)5d(1) ground-state configurations, respectively.

2-(4-Sulfamoylphenyl)hydrazin-1-ium chloride

The hydrazinium residue in the cation of the title salt, C6H10N3O2S+·Cl−, is twisted out of the plane of the benzene ring to which it is attached [N—N—C—C torsion angle = 25.9 (2)°] and the amino group is almost perpendicular to the benzene ring [N—S—C—C torsion angle = 88.71 (16)°]. In the crystal, the cations are linked by N—H⋯O hydrogen bonds and π–π inter­actions [ring centroid distance = 3.7280 (11) Å], forming layers in the bc plane that are connected by N—H⋯Cl hydrogen bonds.

Carbon Nanotube Inhibits the Formation of β-Sheet-Rich Oligomers of the Alzheimer's Amyloid-β(16-22) Peptide

Alzheimer's disease is associated with the abnormal self-assembly of the amyloid-β (Aβ) peptide into toxic β-rich aggregates. Experimental studies have shown that hydrophobic nanoparticles retard Aβ fibrillation by slowing down the nucleation process; however, the effects of nanoparticles on Aβ oligomeric structures remain elusive. In this study, we investigate the conformations of Aβ(16-22) octamers in the absence and presence of a single-walled carbon nanotube (SWCNT) by performing extensive all-atom replica exchange molecular-dynamics simulations in explicit solvent. Our simulations starting from eight random chains demonstrate that the addition of SWCNT into Aβ(16-22) solution prevents β-sheet formation. Simulation starting from a prefibrillar β-sheet octamer shows that SWCNT destabilizes the β-sheet structure. A detailed analysis of the Aβ(16-22)/SWCNT/water interactions reveals that both the inhibition of β-sheet formation and the destabilization of prefibrillar β-sheets by SWCNT result from the same physical forces: hydrophobic and π-stacking interactions (with the latter playing a more important role). By analyzing the stacking patterns between the Phe aromatic rings and the SWCNT carbon rings, we find that short ring–centroid distances mostly favor parallel orientation, whereas large distances allow all other orientations to be populated. Overall, our computational study provides evidence that SWCNT is likely to inhibit Aβ(16-22) and full-length Aβ fibrillation.

Bis(μ-azido-κ2N1:N1)bis{(acetato-κ2O,O′)[2,4,6-tris(2-pyridyl)-1,3,5-triazine-κ3N2,N1,N6]lead(II)}

The complete dinuclear title complex, [Pb2(C2H3O2)2(N3)2(C18H12N6)2], is generated by the application of a crystallographic centre of inversion. The PbII atom is coordinated by three N atoms of the tridentate ligand, two O atoms derived from an asymmetrically coordinating acetate ligand, and two azido-N atoms derived from two asymmetrically bridging azido ligands. The metal coordination geometry can be described as a square anti-prism with one position occupied by an unseen lone pair of electrons. In the ligand, the two coordinating pyridine rings are almost co-planar with the central pyrazine ring [dihedral angles = 0.47 (17) and 0.83 (18)°], but the terminal ring is twisted [dihedral angle = 19.76 (18)°]. In the crystal, the presence of π–π inter­actions [ring centroid distance between pyridyl rings = 3.581 (2) Å] leads to supra­molecular chains along the a-axis direction.

2-Methyl-3-[(4-methylphenyl)sulfonyloxy]-2-{[(4-methylphenyl)sulfonyloxy]methyl}propyl 4-methylbenzenesulfonate

The title mol­ecule, C26H30O9S3, adopts an extended conformation whereby two approximately parallel benzene rings [dihedral angle = 8.32 (10)°] are orientated in opposite directions along the pseudo-threefold axis through the central quaternary C atom, while a third ring occupies a position mid-way and face-on to these rings [dihedral angles = 82.28 (10) and 78.81 (7)°]. The crystal packing is dominated by C—H⋯O contacts and π–π inter­actions [ring centroid distance = 3.6902 (12) Å].