Dissociation first

Second-order effects include experiments designed to clock chemical reactions, pioneered by Zewail and coworkers [25]. The experiments are shown schematically in figure Al.6.10. An initial 100-150 fs pulse moves population from the bound ground state to the dissociative first excited state in ICN. A second pulse, time delayed from the first then moves population from the first excited state to the second excited state, which is also dissociative. By noting the frequency of light absorbed from tlie second pulse, Zewail can estimate the distance between the two excited-state surfaces and thus infer the motion of the initially prepared wavepacket on the first excited state (figure Al.6.10 ). 

In order to investigate whether COj reacts in a concerted way with surface carbon or whether it dissociates first to CO and adsorbed oxygen and the adsorbed oxygen reacts, infrared spectroscopy, pulse reactor studies and XANES measuremerrts were used. The i.r. spectrum of a prereduced (Ihour at 675K in S /oHj/Nj) Pt/ZrOj catalyst in contact with CO2 at 775K is shown in Fig 6 The spectrum shows the presence of linearly bound CO on Pt at 2053 cm [15]. Additionally, bands of carbonate type species appeared in the region between 1375 and 1540 cm . Over pure supports (in the absence of Pt) the CO band was not seen, but peaks in the carbonate region were observed... 

It is immaterial for this explanation whether it is assumed that the salt itself is converted or that dissociation first occurs. 

In most instances, two reacting molecules do not react directly as H2 and I2 do rather one molecule dissociates first to form radicals. These radicals then initiate a chain of steps. Interestingly, this procedure occurs in the reaction of H2 with another halogen, Br2. Experimentally, Bodenstein [12] found that the rate of formation of HBr obeys the expression... 

A dissociative first step is anticipated exclusively in photolysis of the endo-cyclic enamide 181 (compare also 80 -a- 85 and 173 -> 174). The reaction is formulated through a ketene intermediate 182. The enamine 183 formed then undergoes hydrolysis to give the oxocarboxylic acid derivative 184 as the product. 

It is suggested that substances such as potassium cliloraurate are dissociated first by ammonia into simple halides, for the alkali clilor-aurates and bromaurates behave towards ammonia like the auric salts.2 Auric bromide, obtained by dissolving precipitated gold in bromine, also absorbs ammonia, the amount depending on the temperature. 

Either of the two protons might dissociate first as the pH is raised (Eq. 6-75). However, the two microscopic dissociation constants pK x and pKB are distinctly different. The result is that at 25°C in the neutral (monoprotonated) form 80% of the molecules carry a proton on the N, while the other 20% are protonated on the less basic - O. Notice that the subscripts a and b used in this discussion do not refer to acidic and basic but to the individual dissociation steps shown in Eq. 6-75. Microscopic constants will always be indicated with asterisks in this discussion. 

Chapter 8 gives the evidence that plugs dissociate first at the pipewall, thus becoming a projectile in a pipeline with substantial momentum, relative to the gas phase. Lysne (1995, p. 78) lists three such incidences in which hydrate projectiles erupted from pipelines at elbows and caused the loss of three lives and over US 7 million in capital costs. Where possible, depressurization from both sides of a plug is recommended. 

Bocarsly, Pfennig and co workers reported interesting multi electron photoreac tions for the trimeric M" Ptlv M" complexes 25a-c.212 215 In this system, a single photon excitation into the intervalence charge transfer band results dissociation of the trimer into [Pt(NH3)4]2+ and two equivalents of a M111. The initial photoexcited complex is though to dissociate first to a Mm complex and Ptni-Mn intermediate. The latter dimer subsequently undergoes a thermal electron transfer reaction to yield the final products. 

Zhao and Rice have calculated the fragmentation rate constant k by fitting the numerical simulation data from the time at which dissociation first starts to the time when... 

Interaction of the TATA-box-binding protein (TBP) with promoter DNA is rather inefficient and appears to be the rate-limiting step for the start of transcription. TBP must actually dissociate first from the TFIID complex before it can bind to the TATA-box DNA. Dissociation of TBP is facilitated by the dimeric structure of TFIID, when it is not bound to DNA, and by the interaction of TBP with TFIIA. 

Suppose a and h gram-molecules respectively of two weak acids Z H and Z2H, which therefore obey Ostwald s law of dissociation (first simplification) and to them added c gram-molecules of a strong baseMOH, insufficient to saturate both acids. Let the volume of the entire solution be V htres, and as second simplification let it be assumed that the salts formed are completely dissociated. Without regard to dissociation we have... 

This proposed process may explain the hydrogen-assisted NO dissociation first proposed by Hecker and Bell (89). Investigations of Rh singlecrystal surfaces have also indicated that the presence of hydrogen may have a beneficial effect on NO dissociation (21, 90). It was also proposed that this alternative process for NO dissociation may be fast under conditions of slow direct decomposition because of the absence of free adjacent sites needed for NO dissociation (87). It was argued that a high coverage of... 

There has long been dispute as to whether reduction processes such as in electroplating require the complex ion to dissociate first. Dissociation undoubtedly occurs in some cases, but Dwyer provided the most direct proof that oxidation and reduction processes can occur without dissociation 15). The optically active complex ion [Ru(dipy)3] (dipy = dipyridyl) was oxidized to [Ru(dipy)3] " with Ce(IV) and then reduced back to the original complex ion with FeS04 without loss in rotation. 

Either of fhe two protons might dissociate first as the pH is raised (Eq. 6-75). However, the two microscopic dissociation constants pR dis-... 

These types of mechanisms can be applied to most linear polymers. During the initiation reaction, the weaker bonds usually tend to dissociate first. It was noticed, for example, that the bonds (not including the bond to an sp carbon) of a carbon atom in a position to the double bond (the allyl carbon) are weaker than other C-C or C-H bonds. Therefore, the polymer containing an allyl carbon will be more likely to be involved in an initiation reaction. However, other reactions are not excluded in the free radical formation. 

In the discussion of the example chosen above, not all the possibilities were considered. For example, during the propagation process the formation of smaller molecules from the free radical chains were shown to take place with the dissociation of the weaker bonds, which are expected to dissociate first. This was also shown for the free radical formation. The strength of the bond being broken is commonly unknown, but it can be derived from tabulated heats of formation as shown in Section 3.1. Besides the weaker bonds, other bonds can also be dissociated, most commonly when there are small differences between the bond dissociation energies. 

Regular crystalline insulin naturally self-associates into a hexam-eric (six insulin molecules) stmcture when injected subcutaneously. Before absorption through a blood capillary can occur, the hexamer must dissociate first to dimers, and then to monomers. This principle is the premise for additives such as protamine and zinc described below, and modification of amino acids for insulin analogs. Lispro, aspart, and glulisine insulins dissociate rapidly to monomers, thus absorption... 

Two types of reaction schemes are usually considered. Below we will use the notation of Cleland for denoting reactants and products. In sequential mechanism both substrates bind to the enzyme before any product is released. Such mechanism can be ordered with an obligatory order of addition of substrates and release of products. Substrate A binds E first, giving an EA complex. Then B binds to EA, giving a ternary complex (EAB). A and B are converted to products P and Q on (EAB), giving an (EPQ) complex. P dissociates first from (EPQ), then Q from EQ, regenerating E. 

Unsaturated compounds such as CO can be formally inserted into a metal-heteroatom bond such as hydroxide, alkoxide, amide, and sulhde. However, it presents a difficult problem to determine whether the overall insertion process proceeds through a migratory insertion process or by an alternative process. For example, the anionic ligand such as OH and alkoxides may dissociate first to provide a vacant coordinahon site for the CO ligand that is subsequently attacked by the anion to give the same product as that obtained by the migratory insertion of the anionic ligand on the coordinated CO. 

Iron has also been reacted with equilibrated gas mixtures in the CO-CO2-SO2-N2 system and it was found that gas mixtures with the same SO2 partial pressure reacted at the same rates for a given temperature. This result confirms Hatley and Birks assumption that the SO2 species reacts directly with the metal it does not dissociate first to provide O2 and S2 at the metal surface as the reactive species. The direct reaction of the SO2 molecule with the metal is implied in Figure 7.8 and has been demonstrated subsequently by several authors. ... 

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