Bond Rupture Process Research Articles

Covalent bond force profile and cleavage in a single polymer chain

We present here the measurement of the single-polymer entropic elasticity and the single covalent bond force profile, probed with two types of atomic force microscopes (AFM) on a synthetic polymer molecule: polymethacrylic acid in water. The conventional AFM allowed us to distinguish two types of interactions present in this system when doing force spectroscopic measurements: the first interaction is associated with adsorption sites of the polymer chains onto a bare gold surface, the second interaction is directly correlated to the rupture process of a single covalent bond. All these bridging interactions allowed us to stretch the single polymer chain and to determine the various factors playing a role in the elasticity of these molecules. To obtain a closer insight into the bond rupture process, we moved to a force sensor stable in position when measuring attractive forces. By optimizing the polymer length so as to fulfill the elastic stability conditions, we were able for the first time to map out the entire force profile associated with the cleavage of a single covalent bond. Experimental data coupled with molecular quantum mechanical calculations strongly suggest that the breaking bond is located at one end of the polymer chain.

Thermal decomposition behavior of main-chain thermotropic liquid crystalline polymers, Vectra A-950, B-950, and Xydar SRT-900

We investigated the thermal decomposition behavior of three commercially available liquid crystalline polymers (LCPs), Vectra A950, Vectra B950, and Xydar SRT-900. The apparent activation energies (Ea) associated with the thermal degradation processes were determined by the Ozawa and Kissinger methods, using data from dynamic thermogravimetric analysis (TGA) experiments. The magnitudes of the Ea for these LCPs follow the order: Xydar > Vectra A > Vectra B in both air and N2 environments. The stability of the samples at the beginning of the degradation processes follows the same order. This order may result from the kink naphthoyl units in Vectra A and a relatively weak bond dissociation energy of C-N in Vectra B. However, at 560°C the weight loss values of these three LCPs in N2 become close (around 37%). After 600°C, the stability order surprisingly changes to Vectra B950 > Vectra A950 > Xydar SRT-900. This suggests that the more stable the sample is at the beginning, the less stable the corresponding residue is. Fourier Transform Infra-red (FTIR) spectra imply that random chain scission and hydrogen abstraction are the degradation mechanisms in N2 atmosphere and the bands of CO stretching for all 3 LCPs decrease after 560°C, indicating the finish of the ester bond rupture process. Further increasing temperature mainly results in carbonization. For all three LCPs, CO2 is the dominant degradation product during the entire testing periods in both N2 and air environments and the change of CO2 amount is consistent with the degradation rate. Residues after TGA experiments in N2 were analyzed and found to have a relatively high percentage of oxygen element, indicating the formations of ether and ketone structures during the thermal degradation of these three LCPs. Forty-seven percent of the nitrogen element remaining in the case of Vectra B950 indicates the formation of the structures containing nitrogen. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2195–2207, 1999

Viscoelastic effects in cutting of elastomers by a sharp object

The cutting behavior of elastomers by a sharp object was investigated using various elastomers such as acrylonitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), and natural rubber (NR). The effects of crosslinking density, cutting rate, and temperature on the cutting energy of elastomers were investigated. The cutting behavior of swollen elastomers was also investigated. It was found that the cutting energy increased as the molecular weight between crosslinks increased. It was also found that the cutting energies of various elastomers did not yield a single line. Moreover, even in the threshold condition of cutting process, the cutting energy was much higher than the threshold fracture energy. These results suggest that the cutting behavior cannot be explained by only a C-C bond rupture process, but it includes other energy dissipation processes. The curves for cutting energies obtained at different cutting rates and temperatures were well superimposed on a single master curve when they were shifted using the WLF (Williams, Landel, and Ferry) equation. Therefore, it is supposed that the cutting of elastomers by a sharp object includes viscoelastic energy dissipation process and is the viscoelastic behavior. It was also found that the variation of cutting energy over a considerable range of effective rates was smaller than that of the tear energy. It is attributed to the fact that the change of the crack tip diameter, i.e., roughening or reduction, was restricted by the diameter of razor blade.

Semiconductor bond rupture phenomena and surface properties

The complexities of semiconductor bond rupture processes and cleavage surfaces are becoming clarified by recent studies. The high energies released during bond breaking cause a variety of phenomena, including generation of voltages and currents, and emission of light of various wavelengths and durations. Electrical effects during cleavage of Si include generation of voltage pulses up to about 400 mV, with currents up to about 5 mA. If voltages are applied during cleavage, the duration of the luminescence is affected in a systematic way. Low temperature cleavage experiments show that the surface is at an elevated temperature for a short period. There is persistent evidence for the finding first reported many years ago, that crack healing can take place, which is incompatible with cleaved surface structure models featuring irreversible changes.

A Chemical Kinetics Model for Glass Fracture

We utilize a chemical‐kinetics‐based model to describe the rate of crack extension in vitreous silica as a function of the applied stress and the presence of reactive species. Our approach builds upon previous fracture models that treat the atomic bond rupture process at the crack tip as a stressenhanced hydrolysis reaction. We derive the stress dependence for siloxane hydrolysis from measurements of hydrolysis rates for strained silicate ring structures. The stress dependence determined for siloxane hydrolysis yields an activation volume of 2.0 cm3/mol, which is in good agreement with the stress dependence determined for silicate glass fracture. This result supports previous fracture models that are based on absolute reaction rate theory and predicts an exponential dependence of crack extension rate on applied stress intensity.

Slow crack growth in glass in combined mode I and mode II loading

Slow crack growth in soda-lime glass under combined mode I and mode II loading was investigated in precracked disk specimens in which pure mode I, pure mode II, and various combinations of mode I and mode II were achieved by loading in diametral compression at selected angles with respect to symmetric radial cracks. It is shown that slow crack growth under these conditions can be described by a simple exponential relationship with elastic strain energy release rate as the effective crack-driving force parameter. It is possible to interpret this equation in terms of theoretical models that treat subcritical crack growth as a thermally activated bond-rupture process with an activation energy dependent on the environment, and the elastic energy release rate as the crack-driving force parameter.

ChemInform Abstract: Single and Double Photoionization Processes in Naphthalene Between 8 and 35 eV

The photostability of C{sub 10}D{sub 8}{sup +} and C{sub 10}D{sub 8}{sup 2+} is investigated between 8 and 35 eV by threshold photoelectron-photoion coincidence (T-PEPICO) measurements using monochromatized synchrotron radiation as excitation source for naphthalene. MNDO calculations were used to interpret photoelectron spectra. The threshold photoelectron spectrum shows evidence of extensive autoionization, considered to be mainly to high vibrational levels of the X{sup 2} A{sub u} state of C{sub 10}D{sub 8}{sup +}. The possibility of a kinetic isotope effect on dissociation is discussed. Fragmentation processes, thresholds, and cutoffs are interpreted. The dissociation rates of the process C{sub 10}D{sub 8}{sup +} {yields} C{sub 8}D{sub 6}{sup +} + C{sub 2}D{sub 2} are measured between 16 and 17.7 eV. The results of an RRKM calculation are fitted to the experimental results to give an activation energy of 3.50 eV and activation entropy {Delta}S{sup {double dagger}} = 14.1 J/mol{center dot}K at 1000 K. A mechanistic model of this dissociation is propose din terms of two successive C-C bond rupture processes. Appearance potentials determined for the dications C{sub 10}D{sub 8}{sup 2+} (21.5 {plus minus} 0.5 eV) and C{sub 8}D{sub 6}{sup 2+} (23.5 {plus minus} 0.5 eV) are about 1 eV lower than electron impact values. Applicationmore » of the results to interstellar polycyclic aromatic hydrocarbons is briefly discussed. Further work with improved mass resolution is proposed.« less

Single and double photoionization processes in naphthalene between 8 and 35 eV

The photostability of C{sub 10}D{sub 8}{sup +} and C{sub 10}D{sub 8}{sup 2+} is investigated between 8 and 35 eV by threshold photoelectron-photoion coincidence (T-PEPICO) measurements using monochromatized synchrotron radiation as excitation source for naphthalene. MNDO calculations were used to interpret photoelectron spectra. The threshold photoelectron spectrum shows evidence of extensive autoionization, considered to be mainly to high vibrational levels of the X{sup 2} A{sub u} state of C{sub 10}D{sub 8}{sup +}. The possibility of a kinetic isotope effect on dissociation is discussed. Fragmentation processes, thresholds, and cutoffs are interpreted. The dissociation rates of the process C{sub 10}D{sub 8}{sup +} {yields} C{sub 8}D{sub 6}{sup +} + C{sub 2}D{sub 2} are measured between 16 and 17.7 eV. The results of an RRKM calculation are fitted to the experimental results to give an activation energy of 3.50 eV and activation entropy {Delta}S{sup {double dagger}} = 14.1 J/mol{center dot}K at 1000 K. A mechanistic model of this dissociation is propose din terms of two successive C-C bond rupture processes. Appearance potentials determined for the dications C{sub 10}D{sub 8}{sup 2+} (21.5 {plus minus} 0.5 eV) and C{sub 8}D{sub 6}{sup 2+} (23.5 {plus minus} 0.5 eV) are about 1 eV lower than electron impact values. Applicationmore » of the results to interstellar polycyclic aromatic hydrocarbons is briefly discussed. Further work with improved mass resolution is proposed.« less

One- and two-photon photodissociation of Fe(CO)5 at 248 nm. Application of an accurate method for calculating angle resolved velocity distributions for multiple sequential bond rupture processes

A method is presented for computing the effective center-of-mass velocity distribution for photofragments produced by sequential bond ruptures using Fourier transforms. The method has the advantage that energy and linear momentum conservation are correctly accounted for while remaining computationally feasible. To illustrate the method, the one- and two-photon photodissociation of Fe(CO)5 at 248 nm has been experimentally investigated using the crossed laser-molecular beam method and the measured velocity distributions compared to the prediction of various statistical models for the photodissociation process calculated by the Fourier transform method. The strength of the Fourier transform method is illustrated by the two-photon channel which involves five sequential bond ruptures. The main conclusion regarding the photodissociation mechanism is that a modified form of the separate statistical ensemble theory developed by Wittig and co-workers can quantitatively explain the observed velocity distributions for the one-photon process. Fluxional interchange of the CO ligands is shown not to be important on the CO elimination time scale. The two-photon products have a faster than statistical translational energy distribution which is rationalized by both a dynamical constraint on CO rotational excitation and on the change in orbital– and spin–coupling configuration of the Fe atom as the last two CO ligands are removed.

Cyclopentadienyl iron xanthate complexes. Crystal and molecular structure and electrochemical reduction of η-C 5R 5Fe(CO) 2(η 1-SC(S)OEt) (RH or CH 3)

Complexes η-C 5R 5Fe(CO) 2(η 1-SC(S)OEt) (RH, (1) and RCH 3 (2)) have been analysed by X-ray diffraction techniques. 1: P2 12 12 1 11.3560(5), 10.8595(4), 10.1158(3) Å, Z=4; 1023 observed reflexions, R and R w 0.069 and 0.073. 2: Pbca, 15.6907(11), 15.4566(13), 14.3083(11) Å, Z=8; 2271 observed reflexions, R and R w being 0.071, 0.073. The coordination is quite similar for both compounds with the xanthate monodentate ligand almost perpendicular to the ring planes and a relative twist, one from each other of about 10°. The reductive electrochemistry of both complexes has been examined by cyclic voltammetry and coulometry. In a carbon electrode the first-one electron reduction step can be ascribed to the formation of corresponding carbonyl dimers. In a mercury electrode, the first reduction step of 1 leads to a bond rupture process with formation of a mercury compound [CpFe(CO) 2] 2Hg and further reduction to the anion CpFe(CO) 2 −. However, the behaviour of the pentamethylcyclopentadienyl complex (2) is quite different, and it is reduced in a three step process.

Electrochemical reduction of the dithiophosphate complexes Cp ★Fe(CO) 2[η 1-SP(S)(OR) 2], (Cp ★ = η-C 5H 5 or η-C 5Me 5)

The reductive electrochemistry of compounds of the type Cp ★Fe(CO) 2L (Cp ★ = η-C 5H 5, η-C 5Me 5; L = SP(S)(OEt) 2, SP(S)(O iPr) 2) has been examined by polarography, cylic voltammetry and coulometry. The first one-electron reduction step leads to a bond rupture process with formation of a mercury compound, [Cp ★Fe(CO) 2] 2Hg, at a mercury electrode and the corresponding dimer species at a platinum electrode. The second reduction step corresponds to the reduction of the dimer [Cp ★Fe(CO) 2] 2, except in the polarographic reduction of pentamethylcyclopentadienyl compounds.

Electrochemistry of Fe(CO) 2(η 5-C 5H 5)X series. II. Mechanism of reduction for X = Cl, Br, I, SnCl 3, GeCl 3

Electrochemical reduction of Fe(CO) 2(η 5-C 5H 5)X (X = Cl, Br, I, SnCl 3, GeCl 3) proceeds in two one-electron steps. The primary products of the first reduction, the E 1 2 of which depends slightly upon X, are the anion X − and the kryptoradical Fe(CO) 2(η 5 -C 5H 5) •, from which, in a follow-up chemical step, [Fe(CO) 2(η 5-C 5H 5)] 2Hg is formed. The second reduction step at −1.54 V corresponds essentially to the reduction of the dimer, [Fe(CO) 2(η 5-C 5H 5)] 2, which is formed in a complicated sequence of chemical reactions coupled with the electrode reaction. The detailed analysis of the electrode process and comparison with homogeneous redox reactions leads to the conclusion that stability of the anion X − in solution is the most decisive factor governing the concerted electron transfer and bond rupture process taking place in the course of the first reduction step. The detailed mechanism of the reduction of the dimer, [Fe(CO) 2(η 5-C 5H 5)] 2, is elucidated and shown to be identical with that of other Fe(CO) 2(η 5-C 5H 5)X compounds.

NMR observation of mixed complex formation and stereochemical non-rigidity in cobalt(II) complexes

Contact shift measurements of aqueous solutions containing a 1:1:1 moi ratio of Co(II): dien:asp (dien = NH 2CH 2CH 2NHCH 2CH 2NH 2, Asp = NH 2CH(CH 2CO 2H)CO 2H) indicate that at pD's above 7.5 the predominant species (>97%) in aqueous solution is Co(dien)(Asp)(D 20). This represents approximately a 4 Kcai mol −1 preference for the heterogenous complex. At lower pH's protonation of the dien remits in lots of dien and production of a significant amount of Co(Asp)(D 20) 3 andCo(Asp) −2 2. The mixed ligand complex is stereochemically non-rigid at 34°C with the asp portion undergoing rapid interconversion between isomers which differ in the carboxylate that is coordinated to the metal. At −14°C, this interconversion is slow on the nmr time scale. At 100°C, a bond rupture process becomes rapid which results in inversion of the dlen nitrogens and stereochemical nonrigidity of the allen side. The complex Co(dien)(D 2) 2+ is found to exist solution as approximately 67% meridonal and 33% facial form which are in rapid equilibrium with each other at 34 °C but slowly interconvert at −5°C. This process is found to involve breakage of a cobalt nitrogen bond.

The decomposition kinetics of chemically activated methyl‐d1‐methylsilane‐d2 and ethylsilane‐d3

AbstractThe decomposition kinetics of chemically activated methyl‐d1‐methylsilane‐d2 (DMS‐d) and ethylsilane‐d3 (ES‐d) from the Si‐D and C‐H insertion reactions of CH2 (1A1) with methylsilane‐d3 have been studied. The total rate constants for decomposition of chemically activated DMS‐d3 and ES‐d3 have been measured. The individual rate constants for molecular elimination of CH3D, CH2D2, and D2 from DMS‐d and for molecular elimination of CH3CH2D and D2 from ES‐d have been measured. All of the above rate constants exhibit the expected kinetic isotope effect when compared to those found previously in the undeuterated system. RRKM theory calculations of the rate constants for the expected C‐Si and Si‐D bond rupture processes, based on energetics and activated comple× models deduced previously for the undeuterated system, were carried out. In the case of DMS‐d the RRKM theory calculations of rate constants for the bond rupture processes combined with experimental rate constants for the molecular elimination processes gave a total rate constant for decomposition in agreement with the measured value. The results of a high‐pressure study of the CH3D/CH2D2 ratio from chemically activated DMS‐d3 decomposition were consistent with complete randomization of internal energy up to a pressure of 4 atmospheres (lifetime of ∼1.7 × 10×11 sec). This is not an unexpected result in light of earlier work.

Gas phase radiation chemistry of pentafluoroethyl iodine

The gamma radiolysis of gaseous C 2F 5I was studied at 50 torr (6.6 × 10 3 Pa) and 24°C, both pure and with added HI. In all, 17 products were formed in the radiolysis. For the pure system the major radiolytic products and their initial G values are I 2 0.91; C 2F 4, 0.78; C 3F 8, 0.15; n-C 4F 10, 0.42; CF 3I, 0.18; CF 2ICF 2I, 0.11; and CF 3CFI 2, 0.052. It was observed that the addition of approximately 5% HI sharply increased the G values of I 2 (from 0.91 to 2.84) and the initial G value of CF 3I (from 0.18 to 2.50), while decreasing other product yields by 50–100%. In addition, CF 3H, C 2F 5H, CF 2IH and H 2 were also formed in the HI-added system. Results are discussed in terms of ionic fragmentation and ion-molecule chemistry of C 2F 5I, as observed in a parallel investigation, as well as postulated bond-rupture processes of neutral excited species and subsequent displacement and combination reactions.

Kinetics of the thermal decomposition of CF3CHCL2 in a single‐pulse shock tube

AbstractThe pyrolysis of 2,2‐dichloro‐1,1,1‐trifluoroethane was studied over the temperature range of 1120–1260°K at total reflected shock pressures from ∼2800 to 3100 torr. Below 1260°K, the decomposition leads to three reaction products which were identified as CF2CFCl, CF2CFH, and CF3CCl3. The results are interpreted in terms of a parallel CCl bond rupture process which becomes competitive with the molecular HCI elimination. The rate constant for the α,α‐elimination process has been deduced to be It was also possible to obtain the overall rate constant for the formation of CF2CFH, which is given by Some evidence for hydrogen fluoride elimination was found at temperatures above 1260°K. However, at these higher temperatures CC bond scission also occurs and the kinetics of the system become untractable.

Effects of Ionizing Radiation on Two Gelatin Fractions II: Studies on Viscosity, Sedimentation Velocity, and Molecular Weight Effects

The effects of subdenaturant levels of irradiation on two fractions, F-I and F-II, isolated from commercial pigskin gelatin, were investigated through viscosity and sedimentation studies in 2M potassium thiocyanate solutions. The source of irradiation was a 3 Mev Van de Graaff accelerator. When used as controls, F-l and F-II were found to have intrinsic viscosity values of 0.675 and 0.550, respectively, while their respective average molecular weights were determined to be 173,000 and 86,000. After subjection to variable irradiation dose, the log of intrinsic viscosity values for F-I were directly proportional to the log of absorbed dose while an inverse proportionality occurred with F-II. Determination of Svedberg constants showed a similar pattern prevailed in the respective fractions when observed under sedimentation velocity conditions. Data obtained indicate random coil configuration was substantially undisturbed as radiation was applied, but bond rupture and linkage processes did occur. The effects of subdenaturant levels of irradiation on two fractions, F-I and F-II, isolated from commercial pigskin gelatin, were investigated through viscosity and sedimentation studies in 2M potassium thiocyanate solutions. The source of irradiation was a 3 Mev Van de Graaff accelerator. When used as controls, F-l and F-II were found to have intrinsic viscosity values of 0.675 and 0.550, respectively, while their respective average molecular weights were determined to be 173,000 and 86,000. After subjection to variable irradiation dose, the log of intrinsic viscosity values for F-I were directly proportional to the log of absorbed dose while an inverse proportionality occurred with F-II. Determination of Svedberg constants showed a similar pattern prevailed in the respective fractions when observed under sedimentation velocity conditions. Data obtained indicate random coil configuration was substantially undisturbed as radiation was applied, but bond rupture and linkage processes did occur.

Papers to be published in future issues

The effects of subdenaturant levels of irradiation on two fractions, F-I and F-II, isolated from commercial pigskin gelatin, were investigated through viscosity and sedimentation studies in 2M potassium thiocyanate solutions. The source of irradiation was a 3 Mev Van de Graaff accelerator. When used as controls, F-l and F-II were found to have intrinsic viscosity values of 0.675 and 0.550, respectively, while their respective average molecular weights were determined to be 173,000 and 86,000. After subjection to variable irradiation dose, the log of intrinsic viscosity values for F-I were directly proportional to the log of absorbed dose while an inverse proportionality occurred with F-II. Determination of Svedberg constants showed a similar pattern prevailed in the respective fractions when observed under sedimentation velocity conditions. Data obtained indicate random coil configuration was substantially undisturbed as radiation was applied, but bond rupture and linkage processes did occur.

CARBON-13 KINETIC ISOTOPE EFFECTS IN THE SOLVOLYSIS OF 1-BROMO-1-PHENYLETHANE

Carbon-13 kinetic isotope effects have been measured for the solvolysis in methanol and ethanol of 1-bromo-1-phenylethane of natural isotopic abundance. Methanolysis of the bromide at 25 °C gave a kinetic isotope effect (k12/k13) of 1.0065 ± 0.0006, and ethanolysis at 45 °C gave 1.0064 ± 0.0010. These surprisingly low effects for a bond-rupture process have been interpreted in terms of a model for the transition state in which the bonding of the isotopic. carbon is strengthened by conjugation of the electron deficient center with the ring. The results are considered to provide support for the mass fragment model for evaluation of the effective mass term of the Bigeleisen expression for the theoretical calculation of kinetic isotope effects.