SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Bond order

36

We show that the different bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes can be distinguished by noncontact atomic force microscopy (AFM) with a carbon monoxide (CO)-functionalized tip. We found two different contrast mechanisms, which were corroborated by density functional theory calculations: The greater electron density in bonds of higher bond order led to a stronger Pauli repulsion, which enhanced the brightness of these bonds in high-resolution AFM images. The apparent bond length in the AFM images decreased with increasing bond order because of tilting of the CO molecule at the tip apex.

Concepts: Electron, Atom, Chemical bond, Carbon, Polycyclic aromatic hydrocarbon, Benzene, Quantum chemistry, Bond order

28

The bonding characteristics in cysteine-gold cluster complexes represented by thiolate (Au(n)·Cys(S) (n = 1, 3, 5, 7)) and thiol (Au(n)·Cys(SH) (n = 2, 4, 6, 8)) is investigated by density functional theory with 6-31G(d,p) and Lanl2DZ hybrid basis sets. The complexes exhibit very different bonding characteristic between these two forms. In the Au(n)·Cys(S) complexes, the charge transfers from gold clusters to sulfur atoms. The number of S-Au bonds in the Au(n)·Cys(S) complexes evolves from one to two when n is greater than three. For n equals three, i.e. Au(3)·Cys(S), its ground state only has one S-Au bond. While the only S-Au bond in Au(1)·Cys(S) is mainly covalent, the nature of the S-Au bond in other thiolates is featured with the combination of covalent and donor-acceptor interactions. In particular, one stable isomer of Au(3)·Cys(S) with two S-Au bonds, which is 2 kcal mol(-1) higher in energy than the corresponding ground state, consists of one covalent and one donor-acceptor S-Au bond explicitly. Moreover, the localized three center two electron bonds are formed within the Au clusters, which facilitates the formation of the two S-Au bonds in Au(5)·Cys(S) and Au(7)·Cys(S) complexes. In the Au(n)·Cys(SH) complexes, the donor-acceptor interaction prevails in the Au-SH bond by transferring lone pair electrons from the sulfur atom to the adjacent gold atom. Interestingly, the orbital with much more 6s-component in Au(4)·Cys(SH) enhances the donor-acceptor bonding character, thus yields the strongest bonding among all the Au(n)·Cys(SH) complexes studied in this paper. In general, the bonding strength between gold clusters and cysteine is positively correlated with the S-Au overlap-weighted bond order, but negatively correlated with the S-Au bond length. Lastly, the covalent and donor-acceptor S-Au bond strength is computed to be 48 and 18 kcal mol(-1), respectively.

Concepts: Electron, Atom, Chemical bond, Disulfide bond, Quantum chemistry, Bond order, Thiol, Bond length

28

Purpose: To evaluate a new method of simulated pulpal pressure in vitro in comparison with the conventional one. Materials and Methods: Four adhesives were analyzed: a three-step etch-and-rinse (Scotchbond Multi-Purpose [SBMP]), a two-step etch-and-rinse (Single Bond 2 [SB]), a two-step self-etching (Clearfil SE Bond [SE]), and a one-step self-etching (Clearfil S3 [S3]) system. Restorations were built up in flat, deep dentin from extracted molars. After two methods of simulated pulpal pressure or no pulpal pressure (control groups), the samples were cut into sticks and submitted to microtensile bond strength (µTBS) testing and nanoleakage evaluation. Results were analyzed with two-way ANOVA and Tukey’s test (p < 0.05). Results: In general, statistical analysis of µTBS showed SBMP>SB=SE>S3. For both methods of simulated pulpal pressure, the µTBS of SB and S3 was lower than in control groups. For SBMP and SE, the µTBS remained stable with simulated pulpal pressure. Conventional and experimental methods of simulating pulpal pressure resulted in similar µTBS (p = 1.00) and nanoleakage patterns. Silver impregnation was higher with SB and S3, especially after simulated pulpal pressure with both methods. Conclusion: The experimental simulated pulpal-pressure method tested here was similar to the conventional method and can be an alternative to it. The simplified adhesives show reduction in bond strength after simulated pulpal pressure. The multistep adhesives have stable bond strengths under simulated pulpal pressure. Therefore, the separate application of hydrophobic resin can achieve resistance to bonding deterioration after hydrostatic pressure.

Concepts: Scientific method, Simulation, James Bond, Analysis of variance, Simulated reality, Bond energy, Bond order, Bond strength

25

Calcium silicate-based sealers are known to have excellent sealing ability and bioactivities. They are typically recommended to be used in a single-cone (SC) technique. No studies have evaluated the effects of the thermoplastic obturation technique on the dentin interface of these sealers. The purpose of this study was to evaluate the push-out bond strengths of MTA Plus Sealer (Avalon Biomed Inc, Bradenton, FL) and EndoSequence BC Sealer (BC; Brasseler USA, Savannah, GA) when they were used in a thermoplastic technique.

Concepts: Effect, Effectiveness, Effects unit, Bond energy, Florida, Bond order, Bond strength, Sealant

3

We introduce a bond order potential (BOP) for stanene based on an ab-initio derived training data set. The potential is optimized to accurately describe the energetics, as well as thermal and mechanical properties of a free-standing sheet, and used to study diverse nanostructures of stanene, including tubes and ribbons. As a representative case study, using the potential, we perform molecular dynamics simulations to study stanene’s structure and temperature-dependent thermal conductivity. We find that the structure of stanene is highly rippled, far in excess of other 2-D materials (e.g., graphene), owing to its low in-plane stiffness (stanene: ~ 25 N/m; graphene: ~480 N/m). The extent of stanene’s rippling also shows stronger temperature dependence compared to that in graphene. Furthermore, we find that stanene based nanostructures have significantly lower thermal conductivity compared to graphene based structures owing to their softness (i.e. low phonon group velocities) and high anharmonic response. Our newly developed BOP will facilitate the exploration of stanene based low dimensional heterostructures for thermoelectric and thermal management applications.

Concepts: Energy, Molecular dynamics, Temperature, Thermodynamics, Heat, Thermal conductivity, Bond order, Bond order potential

2

While one might assume that the force to break a chemical bond gives a measure of the bond strength, this intuition is misleading. If the force is loaded slowly, thermal fluctuations may break the bond before it is maximally stretched, and the breaking force will be less than the bond can sustain. Conversely, if the force is loaded rapidly it is more likely that the maximum breaking force is measured. Paradoxically, no clear differences in breaking force were observed in experiments on gold nanowires, despite being conducted under very different conditions. Here we explore the breaking behaviour of a single Au-Au bond and show that the breaking force is dependent on the loading rate. We probe the temperature and structural dependencies of breaking and suggest that the paradox can be explained by fast breaking of atomic wires and slow breaking of point contacts giving very similar breaking forces.

Concepts: Energy, Measurement, Atom, Torque, Temperature, Difference, Bond energy, Bond order

0

The two-fold reduction of (cAAC)BHX2 (cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene; X = Cl, Br) provides a facile, high-yielding route to the dihydrodiborene (cAAC)2B2H2. The (chloro)hydroboryl anion reduction intermediate was successfully isolated using a crown ether. Overreduction of the diborene to its dianion [(cAAC)2B2H2]2- causes a decrease in the B-B bond order whereas the B-C bond orders increase.

Concepts: Hydrogen, Atom, Chemical bond, Ether, Ion, Valence electron, Bond order, Crown ether

0

To evaluate the effects on the surface properties (morphology, roughness, microhardness, composition) and bond strength to composite of four types of base cements (Equia-Fil/EQF, Angelus white MTA/MTA, Biodentin/ BDN and IRM/IRM) when treated with phosphoric acid etching (PAE) or two self-etch adhesives (Select One Prime & Bond and Clearfil S3 Bond).

Concepts: Oxygen, Acid, Water, Phosphoric acid, Materials science, Bond energy, Epoxy, Bond order

0

The purpose of this study was to investigate the effect of remaining dentin thickness (RDT) and long term water storage on dentin bond strength in-vitro. Twenty-seven third molars were randomly divided into 3 groups: Clearfil Bond SE ONE (SE1, Kuraray Noritake Dental, Okayama, Japan), G-Bond plus (GB, GC, Tokyo, Japan) and Clearfil Mega Bond (MB, Kuraray Noritake Dental). Bonded specimens were stored in water at 37ºC for 24 h. The teeth were then sectioned perpendicular to the adhesive interface to produce beams. RDT of each beam was measured digital calliper. Microtensile bond strength testing was carried out at a crosshead speed of 1 mm/min after 24 h and 1 year water storage. Thicker RDT produced higher bond strengths with one/two-step self-etch materials tested except for the group of 24 h MB. Nevertheless water storage time and RDT affected µTBS in all materials used.

Concepts: Time, Trigraph, Gh, Term, Teeth, Bond energy, Bond order, Bond strength

0

Reaction of the uranium silyl-phosphino-carbene complex [U{C(SiMe3)(PPh2)}(BIPM)(μ-Cl)Li(TMEDA)(μ-TMEDA)0.5]2 (1, BIPM = C(PPh2NSiMe3)2; TMEDA = Me2NCH2CH2NMe2) with the rhodium chloride compound [Rh(μ-Cl)(COD)]2 (COD = cyclo-octadiene) affords the unprecedented heterotrimetallic UIV-RhI2 complex [U(Cl)2{C(PPh2NSiMe3)(PPh[C6H4]NSiMe3)}{Rh(COD)}{Rh(CH(SiMe3)(PPh2)}] (2). Complex 2 exhibits one, very short, uranium-rhodium distance that is by some margin the shortest uranium-rhodium bond on record, one of the shortest absolute uranium-metal distances generally, and the shortest actinide-transition metal bond in terms of formal shortness ratio. Surprisingly, quantum chemical calculations on 2 reveal a remarkable RhI UIV net double dative bond interaction, involving RhI 4dz2- and 4dxy/xz-type donation into vacant UIV 5f-orbitals, resulting in a computed Wiberg/Nalewajski-Mrozek U-Rh bond order of 1.30/1.44, respectively. Despite being, formally, purely dative the uranium-rhodium bonding interaction in 2 is the most substantial actinide-metal multiple bond yet prepared under conventional experimental conditions, as consistently evidenced by structural, magnetic, and computational analyses.

Concepts: Electron, Molecule, Chemistry, Chemical bond, Quantum chemistry, Chemical compound, Linus Pauling, Bond order