Bi-stability

Since the first report of oscillation in 1965 (159), a variety of other nonlinear kinetic phenomena have been observed in this reaction, such as bi-stability, bi-rhythmicity, complex oscillations, quasi-periodicity, stochastic resonance, period-adding and period-doubling to chaos. Recently, the details and sub-systems of the PO reaction were surveyed and a critical assessment of earlier experiments was given by Scheeline and co-workers (160). This reaction is beyond the scope of this chapter and therefore, the mechanistic details will not be discussed here. Nevertheless, it is worthwhile to mention that many studies were designed to explore non-linear autoxidation phenomena in less complicated systems with an ultimate goal of understanding the PO reaction better. 

The main features of the copper catalyzed autoxidation of ascorbic acid were summarized in detail in Section III. Recently, Strizhak and coworkers demonstrated that in a continuously stirred tank reactor (CSTR) as well as in a batch reactor, the reaction shows various non-linear phenomena, such as bi-stability, oscillations and stochastic resonance (161). The results from the batch experiments can be suitably illustrated with a two-dimensional parameter diagram shown in Pig. 5. 

At high [Cu(II)] and low [H2A] initial concentrations, the Pt electrode potential, used to follow the chemical process, increased monotonously. When both species were present at high initial concentrations, a monotonous decrease was observed. Various non-monotonic transient regimes were found at approximate initial concentrations of [Cu(II)] 10-4 M and [H2A] 10-4 M. Thus, the batch experiments properly illustrate that the system is sensitive to variations of the initial concentrations of ascorbic acid and copper(II) ion, and the observations can be indicative of a transient bi-stability. 

In Fig. 6, separate regions of bi-stability, oscillations and single stable steady-states can be noticed. This cross-shaped phase diagram is common for many non-linear chemical systems containing autocatalytic steps, and this was used as an argument to suggest that the Cu(II) ion catalyzed autoxidation of the ascorbic acid is also autocatalytic. The... 

Another effect of the nanometer size in reaction kinetics of catalytic reaction is the disappearance of the bi-stability in the CO oxidation as recently evidenced by molecular beam experiments on supported model catalysts [26]. 

The extent of new and insightful knowledge regarding metal complex photophysics that can now be derived from a diverse variety of time-resolved pump-probe spectroscopic techniques is illustrated by recent examples in the field of spin-state crossover complexes. This is especially so in the solution state/ but also in the solid, crystalline state straddling several time domains, from the steady-state to femtoseconds. Examples are discussed in Section 4 below on Molecular bi-stability in solution and the solid state . First however we look at recent examples where Raman spectroscopy in both steady-state and time-resolved modes has been applied to the investigation of metal-centred species of bioinorganic and catalytic interest. 

A DFT-based computational study has established the thermal and photochemical isomerisation mechanisms from the terminal P-bound phosphinidene oxide complex [Ru(tpy)(bpy)(POPh)] to the corresponding O-bound, [Ru(tpy)(bpy)(OPPh)] . Thermal isomerisation was found to be both kinetically and thermodynamically unfavourable, while photoisomerisation can readily take place by either adiabatic or non-adiabatic pathways. The different absorption spectra of the two isomers and the bi-stability of the system make this complex a good candidate for photochromism. ... 

The encapsulation process for ChLC is mainly attributed to its transport and optical properties [15]. Firstly, since viscosity of pure ChLC is close to that of water, its fluidity prevents ChLC from being coated on flexible substrates. Secondly, when a cholesteric liquid crystal is pressed, the flow generated inside makes the displayed image erase. Therefore, droplet dispersions by encapsulation act as a protector for its bi-stability and optical properties. The additional advantage is that encapsulated cholesteric liquid crystals are self-sealing the materials confined to the droplets cannot flow through an interface of the droplets. 

Bi-stability refers to two different steady-state results with the same set of parameters but different initial conditions. 

Interestingly, the allylation of a stabilized carbon nucleophile has been found to be reversible. Complete isomerization of dimethyl methylmalonate, involving bis-allylic C—C bond cleavage, from a secondary carbon 38 to a primary carbon 39 was observed by treatment with a Pd catalyst for 24 h. The C—C bond cleavage of a monoaliylic system proceeds slowly[40]. 

Typically, soHd stabilizers utilize natural saturated fatty acid ligands with chain lengths of Cg—C g. Ziac stearate [557-05-1/, ziac neodecanoate [27253-29-8] calcium stearate [1592-23-0] barium stearate [6865-35-6] and cadmium laurate [2605-44-9] are some examples. To complete the package, the soHd products also contain other soHd additives such as polyols, antioxidants, and lubricants. Liquid stabilizers can make use of metal soaps of oleic acid, tall oil acids, 2-ethyl-hexanoic acid, octylphenol, and nonylphenol. Barium bis(nonylphenate) [41157-58-8] ziac 2-ethyIhexanoate [136-53-8], cadmium 2-ethyIhexanoate [2420-98-6], and overbased barium tallate [68855-79-8] are normally used ia the Hquid formulations along with solubilizers such as plasticizers, phosphites, and/or epoxidized oils. The majority of the Hquid barium—cadmium formulations rely on barium nonylphenate as the source of that metal. There are even some mixed metal stabilizers suppHed as pastes. The U.S. FDA approved calcium—zinc stabilizers are good examples because they contain a mixture of calcium stearate and ziac stearate suspended ia epoxidized soya oil. Table 4 shows examples of typical mixed metal stabilizers. 

A.ntioxidants. PhenoHc antioxidants, added at about 0.1—0.5 phr, are usually chosen from among butylated hydroxytoluene [128-37-0] (BHT), and Nnonylphenol [104-40-5] for Hquid stabilizer formulations and bisphenol A [80-05-7] (2,2-bis-(/)-hydroxyphenyl)propane) for the soHd systems. Low melting thioesters, dilauryl thiodipropionate [123-28-4] (DLTDP) or distearyl thiodipropionate [693-36-7] (DSTDP) are commonly added along with the phenoHcs to enhance their antioxidant performance. Usually a 3 1 ratio of thiodipropionate to phenoHc antioxidant provides the desired protection. Most mixed metal stabilizer products contain the antioxidant iagredient. 

Methylphenol is converted to 6-/ f2 -butyl-2-methylphenol [2219-82-1] by alkylation with isobutylene under aluminum catalysis. A number of phenoHc anti-oxidants used to stabilize mbber and plastics against thermal oxidative degradation are based on this compound. The condensation of 6-/ f2 -butyl-2-methylphenol with formaldehyde yields 4,4 -methylenebis(2-methyl-6-/ f2 butylphenol) [96-65-17, reaction with sulfur dichloride yields 4,4 -thiobis(2-methyl-6-/ f2 butylphenol) [96-66-2] and reaction with methyl acrylate under base catalysis yields the corresponding hydrocinnamate. Transesterification of the hydrocinnamate with triethylene glycol yields triethylene glycol-bis[3-(3-/ f2 -butyl-5-methyl-4-hydroxyphenyl)propionate] [36443-68-2] (39). 2-Methylphenol is also a component of cresyHc acids, blends of phenol, cresols, and xylenols. CresyHc acids are used as solvents in a number of coating appHcations (see Table 3). 

The bulk of 4-methylphenol is used in the production of phenoHc antioxidants. The alkylation of 4-methylphenol with isobutylene under acid catalysis yields 2-/ f2 -butyl-4-methylphenol [2409-55-4] and 2,6-di-/ f2 -butyl-4-methylphenol [128-37-0]. The former condenses with formaldehyde under acid catalysis to yield 2,2 -methylene bis(6-/ f2 -butyl-4-methylphenol) [119-47-1], which is widely used in the stabilization of natural and synthetic mbber (43). The reaction of 2-/ l -butyl-4-methylphenol with sulfur dichloride yields 2,2 -thiobis(6-/ l -butyl-4-methylphenol) [90-66-4]. 

Thermal Stability. Dimethyl sulfoxide decomposes slowly at 189°C to a mixture of products that includes methanethiol, formaldehyde, water, bis(methylthio)methane, dimethyl disulfide, dimethyl sulfone, and dimethyl sulfide. The decomposition is accelerated by acids, glycols, or amides (30). This product mixture suggests a sequence in which DMSO initially undergoes a Pummerer reaction to give (methylthio)methano1, which is labile and reacts according to equations 1—3. Disproportionation (eq. 4) also occurs to a small extent ... 

Classic A/-heterocychc ligands, eg, bipyridyl (bipy), terpyridyl, imidazole, pyrazine, phenanthroline, piperazine (including alkyl- and aryl-substituted derivatives), and polypyrazol-l-yl-borates (bis, tris, and tetra), have all been found to coordinate Th(IV) chlorides, perchlorates, and nitrates. The tripodal hydrotris(pyrazolyl)borates, HBPz, have been used to stabilize organometaHic complexes (31). Bis-porphyrin Th(IV) "sandwich" complexes have been... 

These polymers are subjected to high temperatures, ca 300°C, duting extmsion and iajection molding. Processing stabilizers are used to decrease both the change ia viscosity of the polymer melt and the development of color. A phosphite, such as tris(2,4-di-/ f2 -butyiphenyi)phosphite (25) or bis(2,4-di-/ f2 butyiphenyi)pentaerythritol diphosphite [26741-53-7] ia combination with a phenoHc antioxidant such as octadecyl... 

Oligomeric hindered amine light stabilizers are effective thermal antioxidants for polypropylene. Thus 0.1% of A[,Af-bis(2,2,6,6-tetramethyl-4-piperadinyl)-l,6-hexanediamine polymer, with 2,4,6-trichloro-l,3,5-triazine and 2,4,4-trimethyl-2-pentaneainine [70624-18-9] (35) (Fig. 5), protects polypropylene multifilaments against oxidation when exposed at 120°C in a forced-air oven (22) for 47 days. 3,5-Di-/ l -butyl-4-hydroxytoluene [128-37-0] (0.1%) affords protection for only 14 days. 

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