Carbon Bond Strength Research Articles

Study on the thermodynamic mechanism of external magnetic field on gas explosion characteristics

To investigate the thermodynamic mechanism of the magnetic field on the explosion characteristics of gases, experiments were carried out to study the influence of the applied magnetic field of 300 mT on the explosion characteristics of 5.7 % C2H6, 6.5 % C2H4 and 7.7 % C2H2 by volume. CHEMKIN-PRO software was used to simulate the three kinds of gas explosion chain reaction process. The chemical bond strength, stability and bond energy of the three gases were calculated and analyzed by Materials Studio software to derive the strength of chemical bond stability of the three gas molecules in the explosion process. The results show that the magnetic field weakened the breaking rate of CC, CC, and CC, the maximum explosion pressures of C2H6, C2H4, and C2H2 were reduced by 12.14 %, 16.47 %, and 18.71 %. The three gases carbon–carbon bond strength was weaker than the strength of the carbon-hydrogen bond, carbon-hydrogen bond was not easy to break, which in turn reduced the rate of generation of ·H in the chain initiation stage of the cleavage reaction, thus presenting a significant inhibitory effect of the magnetic field on C2H6, C2H4, and C2H2 explosions. The COOP of CC is 0.377 Ha, and the bond energy is 839 KJ/mol, which was greater than CC and CC, so the number of free radicals generated in the C2H2 cleavage reaction was lower, leading to a slower chain reaction rate which led to a stronger inhibitory effect of the magnetic field on C2H2 explosions than that on C2H6 and C2H4.

Does Additive Pressurized Carbon Dioxide Lavage Improve Cement Penetration and Bond Strength in Cemented Arthroplasty?

The modern cementing technique in cemented arthroplasty is a highly standardized and, therefore, safe procedure. Nevertheless, aseptic loosening is still the main reason for revision after cemented total knee or cemented total hip arthroplasty. To investigate whether an additional carbon dioxide lavage after a high-pressure pulsatile saline lavage has a positive effect on the bone–cement interface or cement penetration, we set up a standardized laboratory experiment with 28 human femoral heads. After a standardized cleaning procedure, the test implants were cemented onto the cancellous bone. Subsequently, the maximum failure load of the bone–cement interface was determined using a material testing machine to pull off the implant, and the cement penetration was determined using computed tomography. Neither the maximum failure load nor cement penetration into the cancellous bone revealed significant differences between the groups. In conclusion, according to our experiments, the additive use of the carbon dioxide lavage after the high-pressure pulsatile lavage has no additional benefit for the cleaning of the cancellous bone and, therefore, cannot be recommended without restrictions.

Synthesis of a Ga-Stabilized As-Centered Radical and a Gallastibene by Tailoring Group 15 Element-Carbon Bond Strengths.

A convenient synthetic route to Ga-stabilized pnictogen-centered radicals and gallapnictenes by manipulation of pnictogen-carbon bond strengths is presented. Two equivalents of LGa (L = HC[C(Me)N(Dip)]2, Dip = 2,6-i-Pr2C6H3) react with CpArAsCl2 (CpAr = C5(4-t-BuC6H4)5) with formation of the arsenic-centered radical [L(Cl)Ga]2As·1. In contrast, the analogous reaction with TerSbCl2 (Ter = 2,6-Mes2C6H3; Mes = 2,4,6-Me3C6H2) yields the gallastibene LGa═SbTer (2) containing a Ga-Sb double bond, whereas reactions of DipSbCl2 with one and two equivalents of LGa give the monoinsertion and bisinsertion products L(Cl)GaSb(ClDip) (3) and [L(Cl]Ga]2SbDip (4), respectively. 1-4 were structurally characterized by single crystal X-ray diffraction. The description of 1 as an arsenic-centered radical is consistent with results of electron paramagnetic resonance and density functional theory (DFT) studies. The π-bonding in LGa═SbTer (2) is estimated to 10.68 kcal mol-1 by variable-temperature (VT) NMR spectroscopy, and DFT studies reveal a significant π-bonding interaction between Sb and Ga.

Metal–carbon bond strengths under polymerization conditions: 2,1-insertion as a catalyst stress test

Quantitative agreement between experimentally determined M–C bond dissociation energies (BDE) and DFT predictions (M06-2X/TZ//TPSSTPSS/DZ) can be reached by choosing the correct anchor for experimentally derived BDE. For the example of the archetypical metallocene catalyst Cp2TiCl2, it is shown that titanium–carbon bonds are very weak under polymerization conditions and fluctuate; steric strain is introduced after 2,1 insertion and via olefin capture. Thus, homolysis can become competitive with chain propagation. Depending on the catalyst and temperature, 2,1 insertion can be only a temporary inconvenience (dormancy) or a definitive decay event. It is then shown for a set of nine common Ti and Zr polymerization catalysts how ligand variation affects the metal–carbon BDE. Predicted stabilities of the M(IV) oxidation state with respect to homolysis are in nice agreement with the experimentally observed temperature tolerance of the various catalysts: homolysis is easier for Ti than for Zr, and cyclopentadienyl groups in particular facilitate homolysis, especially in bis-cyclopentadienyl systems.

Establishing the most favorable metal–carbon bond strength for carbon nanotube catalysts

We have studied a wide range of transition metals to find potential carbon nanotube (CNT) catalysts for chemical vapor deposition (CVD) production.

Understanding the Mechanisms of Unusually Fast HH, CH, and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid CC reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for HH, CH, and CC bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52 °C, about 82 % of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.

Thermo-mechanical properties of a piezoelectric polyimide carbon nanotube composite: Assessment of composite theories

In this work, we have characterized the thermomechanical properties of carbon nanotube based piezoelectric polymer nanocomposite using a hybrid force field for all atomistic molecular dynamic simulations. In addition, applicability of some of the well-known micromechanics composite theory in estimating carbon nanotube based polymer nanocomposite properties were assessed. We found that the primary reason for the strengthening effect of a nanocomposite with incorporation of a nanotube is the carbon–carbon bond and angle strength. The orientation of the nanotube in the polymer matrix is key to its reinforcement effect on a nanocomposite. We also observed that a perfect axial orientation does result in improving the axial modulus, but in the radial direction any strengthening for such a unidirectional composite does not seem possible without any bonding at the interface between the filler and the matrix material. The self-consistent field theory was found to be the closest with the atomistic simulation results for predicting mechanical properties of a polymer nanocomposite. The Halpin–Tsai model also demonstrated reasonable capability in predicting the strengthening effect. Mori–Tanaka model, however, underestimated the strengthening effect of carbon nanotube on the polymer matrix. It was also found that the existing composite theories are better at estimating low weight percentage nanotube strengthening effect.

Hydrothermal degradation of model sulfonic acid compounds: Probing the relative sulfur–carbon bond strength in water

Development of heterogeneous catalysts for the biorenewable industry requires hydrothermal degradation resistance. However, the relationship between hydrothermal stability and the immediate electronic hybridization of the carbon atoms adjacent to the sulfonic acid active group is not fully known. We systematically tested model compounds containing sulfonic acid groups linked to aromatic, alkane, or cycloalkane carbon atoms. We subjected them to hydrothermal conditions. The compounds' structural integrity was monitored with solution NMR. The aromatic-sulfonic acid compounds degraded readily, while the hydrolysis of the alkyl sulfonic acid linkages was negligible. Therefore, hydrothermally stable sulfonic-acid catalysts need sulfonic acid attached via alkyl linkers.

Synthesis and energetics of Tp′Rh[P(OMe)3](R)H: a systematic investigation of ligand effects on C–H activation at rhodium

The thermal precursor Tp′Rh[P(OMe)3](Me)H was used to generate the active [Tp′Rh[P(OMe)3]] fragment, which activates C–H bonds of various hydrocarbons to form products of the type Tp′Rh[P(OMe)3](R)H (Tp′ = tris-(3,5-dimethylpyrazolyl)borate). Only one single activation product was observed in each case. The structures of Tp′Rh[P(OMe)3](R)X (X = H, Br, Cl) have been characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The kinetics of reductive elimination of RH from Tp′Rh[P(OMe)3](R)H as well as competition experiments between substrates allow measurement of the Rh–C bond strengths relative to the Rh–Ph bond strength. Two separate linear correlations of the Rh–C bond energies versus H–C bond energies were found based on whether the alkyl group is α-substituted or not. While the correlation for α-substituted substrates gives a slope of 1.45, smaller than the slope (1.55) for unsubstituted hydrocarbons, the Rh–C bond energies of the former group are higher by ∼7 kcal mol−1. In comparison with the analogous [Tp′Rh(PMe3)] and [Tp′Rh(CNneopentyl)] systems, replacing the spectator ligand with a more electronic donating group slightly increases metal–carbon bond strengths as the trend in slopes of the correlations follows an order of CNneopentyl < P(OMe)3 ≤ PMe3.

Kinetic and Thermodynamic Selectivity of Intermolecular C–H Activation at [Tp′Rh(PMe3)]. How Does the Ancillary Ligand Affect the Metal–Carbon Bond Strength?

Tp'Rh(PMe3)(CH3)H was synthesized as a precursor to produce the coordinatively unsaturated fragment [Tp'Rh(PMe3)], which reacts with benzene, mesitylene, 3,3-dimethyl-1-butene, 2-methoxy-2-methylpropane, 2-butyne, acetone, pentane, cyclopentane, trifluoroethane, fluoromethane, dimethyl ether, and difluoromethane at ambient temperature to give only one product in almost quantitative yield in each case. All of the complexes Tp'Rh(PMe3)(R)H were characterized by NMR spectroscopy, and their halogenated derivatives were fully characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The active species [Tp'Rh(PMe3)] was also able to activate the alkynyl C-H bond of terminal alkynes to give activation products of the type Tp'Rh(PMe3)(C≡CR)H (R = t-Bu, SiMe3, hexyl, CF3, Ph, p-MeOC6H4, and p-CF3C6H4). The measured relative rhodium-carbon bond strengths display two linear correlations with the corresponding carbon-hydrogen bond strengths, giving a slope of 1.54 for α-unsubstituted hydrocarbons and a slope of 1.71 for substrates with α-substitution. Similar trends of energy correlations were established by DFT calculated metal-carbon bond strengths for the same groups of substrates.

C–H Activation of Terminal Alkynes by Tris-(3,5-dimethylpyrazolyl)boraterhodiumneopentylisocyanide: New Metal–Carbon Bond Strengths

C-H bond activation of terminal alkynes by [Tp'Rh(CNneopentyl)] (Tp' = hydridotris-(3,5-dimethylpyrazolyl)borate) resulted in the formation of terminal C-H bond activation products Tp'Rh(CNneopentyl)(C≡CR)H (R = t-Bu, SiMe(3), hexyl, CF(3), p-MeOC(6)H(4), Ph, and p-CF(3)C(6)H(4)). A combination of kinetic selectivity determined in competition reactions and activation energy for reductive elimination has allowed for the calculation of relative Rh-C(alkynyl) bond strengths. The bond strengths of Rh-C(alkynyl) products are noticeably higher than those of Rh-C(aryl) and Rh-C(alkyl) analogues. The relationship between M-C and C-H bond strengths showed a linear correlation (slope R(M-C/H-C) = 1.32), and follows energy correlations previously established for unsubstituted sp(2) and sp(3) C-H bonds in aliphatic and aromatic hydrocarbons.

Characterization and FTIR spectral studies of human urinary stones from Southern India

Urinary stones resected from urinary bladders of patients hailing from Kollam district of Kerala State, India were analyzed by SEM, XRD and by thermal analysis techniques. The analytical results indicate that, stones have different composition, i.e., calcium phosphate, calcium phosphate hydroxide and sodium calcium carbonate. Infrared spectral studies also reveal the presence of phosphates or carbonates in these samples. Further, IR spectral investigations have revealed that amorphous carbonated species are occupied in PO 4 sites in calcium phosphate type stone and OH sites in calcium phosphate hydroxide sample. Thermal studies of these samples also reveal that, carbon dioxide is released from carbonated samples upon heating which is related to amount of carbon content and bond strength. Crystals with defects and irregular morphology are grown inside the urinary bladder due to variation in crystal growth conditions.

Deposition of metal clusters on single-layer graphene/Ru(0001): Factors that govern cluster growth

Fabrication of nanoclusters on a substrate is of great interest in studies of model catalysts. The key factors that govern the growth and distribution of metal on graphene have been studied by scanning tunneling microscopy (STM) based on different behaviors of five transition metals, namely Pt, Rh, Pd, Co, and Au supported on the template of a graphene moiré pattern formed on Ru(0001). Our experimental findings show that Pt and Rh form finely dispersed small clusters located at fcc sites on graphene while Pd and Co form large clusters at similar coverages. These results, coupled with previous findings that Ir forms the best finely dispersed clusters, suggest that both metal–carbon (M–C) bond strength and metal cohesive energies play significant roles in the cluster formation process and that the M–C bond strength is the most important factor that affects the morphology of clusters at the initial stages of growth. Furthermore, experimental results show Au behaves differently and forms a single-layer film on graphene, indicating other factors such as the effect of substrate metals and lattice match should also be considered. In addition, the effect of annealing Rh on graphene has been studied and its high thermal stability is rationalized in terms of a strong interaction between Rh and graphene as well as sintering via Ostwald ripening.

Diversity and design of metal-based carbon monoxide-releasing molecules (CO-RMs) in aqueous systems: revealing the essential trends

The CO-releasing ability of a diverse library of primary metal carbonyl complexes has been assessed using a deoxymyoglobin-carbonmonoxymyglobin assay. A wide spectrum of rates for the CO-release process was observed in aqueous systems. For octahedral d(6) complexes, the rate was found to decrease in the sequence FeI(2)(CO)(4) > [NEt(4)][V(CO)(6)] > MnBr(CO)(5) > Cr(CO)(6) implying that CO-release is not controlled by the metal-carbon bond strengths. Within the series, [NEt(4)][MX(CO)(5)] (M = Cr, Mo, W; X =Cl, Br, I), the rate of CO-release was found to decrease down the group (Cr > Mo > W), whilst within the chromium series a similar trend was observed for the halide (Cl > Br > I). The d(4) complexes [NEt(4)][MI(3)(CO)(4)] (M = Mo, W) exhibit faster release than their d(6) congeners. A mechanistic investigation into the [NEt(4)][MX(CO)(5)] series revealed the intermediacy of [[M(CO)(5)](2)(mu-X)](-) in the CO-release process and that the hydrolysis of the M-X bond, rather than the intrinsic strength of M-CO bonds, controls the rate of CO-release in aqueous systems.

Sluggish σ-Bond Formation between sp-Hybridized Carbons: Acetylide Coupling on Ag(111)

Chemisorbed methylacetylide intermediates were generated on clean Ag(111) to study the mechanism and kinetics of C−C σ-bond formation between sp-hybridized carbon centers. Following the adsorption of propynyl iodide (C3H3I) at 110 K, surface heating led to the cleavage of the C−I bond at 200 K, and the resulting surface Cα⋮Cβ−CH3 species were tilted and coordinated to the surface via a metal−carbon σ-bond with weak π-interactions, confirmed by reflection absorption infrared spectroscopy and theoretical calculations. Temperature programmed reaction (TPR) showed that a Csp−Csp coupling reaction occurred at ∼500 K to afford 2,4-hexadiyne through head-to-head dimerization involving Cα carbons. We attribute the relatively slow acetylide−acetylide coupling rate against those from vinyl−vinyl (Csp2−Csp2) and alkyl−alkyl (Csp3−Csp3) couplings to the stronger metal−carbon (M−Cα) bond strength as well as the anionic character bestowed upon the adsorbed acetylides, which all correlate with the increased s-character ...

Free radical polymerization with catalytic chain transfer: Using NMR to probe the strength of the cobalt–carbon bond in small molecule model reactions

AbstractThis work examines cobalt–carbon bond formation between the cobalt (II) macrocycle, (tetrakis(p‐methoxyphenyl)porphyrinato)cobalt (II), (TAP)Co, and a variety of radicals derived from vinyl compounds to facilitate a better understanding of the various factors affecting the cobalt–carbon bond strength in catalytic chain transfer polymerization. The reaction of (TAP)Co with the following vinylic molecules was studied: methacrylonitrile, cyclohexene, methyl methacrylate, styrene, methyl acrylate, vinyl acetate, vinyl benzoate, methyl crotonate, cis‐2‐pentenenitrile, and ethyl α‐hydroxymethacrylate. Different concentrations of each vinylic compound were added to (TAP)Co and 2,2′‐azobis(isobutyronitrile) in CDCl3 at 60 °C. The ratio of Co(III) to Co(II) and the nature of the radical bound to the cobalt macrocycle were determined via nuclear magnetic resonance measurements. Several factors are shown to affect the reaction of the radical and the cobalt (II) species (and hence the strength of the cobalt–carbon bond in the resulting compound). These factors are as follows: the number of pathways by which a radical may be derived from the vinyl compound; the variety of radicals that can be produced from the vinylic molecule; the stability of the radical(s) generated; and the relative propagation rate of the vinyl compound. A discussion on the relevance of this study to the behavior of different monomers in catalytic chain transfer reactions is included. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6171–6189, 2006

The metal–carbon stretching frequencies in methyl complexes of Rh, Ir, Ga and In with porphyrins and a tetradentate pyridine–amide ligand

Interactions of trans-[Rh(bpb)(CH 3)(H 2O)] ( 1) [bpb=1,2-bis(2-pyridinecarboxamido)benzene] with Lewis bases L afford the respective adducts trans-[Rh(bpb)(CH 3)(L)], where L=4-substituted pyridine 4-Xpy (X=H ( 2), t Bu ( 3), NMe 2 ( 4) CN ( 5)), PMe 2Ph ( 6) or benzimidazole ( 7). The structures of complexes 3 and 6 have been established by X-ray crystallography. The Rh–C distance in complex 6 (2.095(6) Å) is longer than that in complex 3 (2.02(1) Å), indicating that PMe 2Ph has a stronger trans influence than 4- t Bupy. The metal–carbon stretching frequencies for trans-[Rh(bpb)(CH 3)(L)] and [M(TTP)(CH 3)] [TTP=5,10,15,20-tetrakis(4-methylphenyl)porphyrin dianion; M=Rh, Ir, Ga, In] have been determined by near IR FT-Raman spectroscopy. Complex 1 exhibits ν(Rh–C) at 562 cm −1, which downshifts to 532 cm −1 upon deuteriation of the axial methyl group. Replacement of the aquo ligand in complex 1 with nitrogen ligands or phosphine resulted in downshift in ν(Rh–C). The ν(Rh–C) for trans-[Rh(bpb)(CH 3)(L)] was found to decrease in the order L: PMe 2Ph≫4-Xpy∼BzIm>H 2O, consistent the order of trans influence of L. For [M(TTP)(CH 3)] (M=Co, Rh, Ir, Ga, or In) the M–C force constant was found to decrease in the orders Ir>Rh>Co and Ga>In, consistent with the trends of metal–carbon bond strength for these metals. For trans-[Rh(bpb)(CH 3)(4-Xpy)] and trans-[Rh(TTP)(CH 3)(4-Xpy)], the ν(Rh–C) were found to be not very sensitive to the nature of X, suggesting that the electronic factors of the axial pyridine ligand do not have a significant effect on the Rh–C bonds for these rhodium alkyl complexes.

Two-Step Methane Conversion to Higher Hydrocarbons: Comment on the Relevance of Metal–Carbon Bond Strength

Herein, data available in the literature for nonoxidative, two-step CH4 homologation over SiO2-supported transition metals are analyzed according to the principle of Sabatier. Volcano plots are constructed which provide empirical evidence to support the hypothesis that the metal–carbon bond strength is pivotal to determine catalyst efficiency, chain-growth probability, and selectivity to C+2 hydrocarbons.

Photolysis Primary Products of Alkylcobaloximes Controlled by the Cobalt−Carbon Bond Strength

Anaerobic photolysis of several methyl- and benzylpyridinecobaloximes was studied by electron spin resonance spectroscopy in aprotic solvents, at three temperatures of irradiation and with use of visible and UV light sources. Homolysis of the cobalt−carbon bond is photochemically induced when irradiation with visible light is performed at room temperature, giving rise to Co(II) five-coordinate species and organic radicals. Irradiation with visible light of fluid (250 K) or frozen (77 K) solutions does not cause homolysis of the cobalt−carbon bond, in contrast with the results reported for other alkylpyridinecobaloximes. Homolysis of the cobalt−carbon bond is achieved at all temperatures if a UV light source is used, and the results are rationalized in terms of the values of the cobalt−carbon bond dissociation energies. The results obtained at 250 K for a series of isotopically substituted methylpyridinecobaloximes allowed the definitive assignment of the spectrum obtained upon photolysis at 250 K to a cobalt(III) nitroxide radical and not to a Co(II) center as has been proposed earlier.

Ab Initio Molecular Orbital Study of Organometallic Complexes Containing Benzo[b]thiophene

Ab initio molecular orbital calculations have been carried out on benzo[b]thiophene (BT), 2-methylbenzo[b]thiophene (2-MeBT), 3-methylbenzo[b]thiophene (3-MeBT), and a number of organometallic complexes containing these thiophenic moieties in an attempt to understand the usual preference for insertion of metal fragments into the sulfur−carbon(vinyl) bond of BT and the recent observations of insertion into the sulfur−carbon(aryl) bond. The results indicate that insertion selectivity is governed by several factors, including the orbital structure of BT, steric interactions between the metal and the heterocycle, and the metal−carbon bond strengths of the inserted products. In most cases, all three factors support the experimentally observed preference for insertion into the S−C(vinyl) bond. Subtle structural changes, however, can lead to S−C(aryl) metal-inserted products. For instance, steric factors inhibiting insertion of metal fragments into the sulfur−carbon(aryl) bond of BT can be overcome by “tied back...