Reaction time window

The rate of hydrolysis depends strongly on the nature of all three building blocks, i.e. the leaving group, the coordinated arene, and the chelate, and can be varied over several orders of magnitude, opening a time-window of activation. A detailed study of the aquation and the reverse, anation reactions of three [Ru(rj6-arene)Cl(en)](PF6) complexes (arene — bip (10), dha (11), and tha (12)) showed that the rates of aquation... 

The major reasons for using intrinsic fluorescence and phosphorescence to study conformation are that these spectroscopies are extremely sensitive, they provide many specific parameters to correlate with physical structure, and they cover a wide time range, from picoseconds to seconds, which allows the study of a variety of different processes. The time scale of tyrosine fluorescence extends from picoseconds to a few nanoseconds, which is a good time window to obtain information about rotational diffusion, intermolecular association reactions, and conformational relaxation in the presence and absence of cofactors and substrates. Moreover, the time dependence of the fluorescence intensity and anisotropy decay can be used to test predictions from molecular dynamics.(167) In using tyrosine to study the dynamics of protein structure, it is particularly important that we begin to understand the basis for the anisotropy decay of tyrosine in terms of the potential motions of the phenol ring.(221) For example, the frequency of flips about the C -C bond of tyrosine appears to cover a time range from milliseconds to nanoseconds.(222)... 

In a typical experiment, the appropriate IL (2.0 ml), the iridium complex 11 (3 X 10 mmol) and the substrate 8 (11 8 = 500 1) were loaded under argon in a window-equipped stainless steel autoclave (V = 12 ml). The reactor was then pressurised with H2 and the desired amount of CO2, followed by heating under stirring to 40 °C for a standard reaction time of 22 h. The products were collected for GC and HPLC analysis by extraction of the IL phase with hexane after cooling and venting, or alternatively isolated by CO2 extraction. Representative results are summarised in Table 3. 

Varieties of primary and secondary alcohols are selectively oxidized to aldehyde or carbonyl compounds in moderate to excellent yields as summarized in Table 3. As can be seen, /(-substituted benzyl alcohols (e.g., -Cl, -CH3, -OCH3, and -NO2) yielded > 90% of product conversion in 3-4 h of reaction time with TOP in the range of 84-155 h (entries 2-5, Table 3), Heterocyclic alcohols with sulfur- and nitrogen-containing compoimds are found to show the best catalytic yield with TOP of 1517 and 902 h for (pyrindin-2-yl)methanol and (thiophene-2-yl) methanol, respectively (entries 9 and 10, Table 3). Some of aliphatic primary alcohols (long chain alcohols) and secondary alcohols (cyclohexanol, its methyl substituted derivatives and norboman-2-ol) are also selectively oxidized by the membrane catalyst (entries 11-14 and 15-17, Table 3) with TOP values in the window of 8-... 

The unimolecular reactions observed in the bare pentanal molecule occur almost uniformly over the whole timescale of 100 ps. In contrast, the same unimolecular reactions in the pentanal cluster are much more pronounced in the beginning of the simulation time window. This suggests that unimolecular reactions can only prevail if the excited molecule is optimally oriented at the beginning of the simulation if it is not, cross-reactions become more probable. 

In general, the results of the calculations establish that it is possible to guide the reaction to preferentially form one or the other product with high yield. Note that, unlike the original Tannor-Rice pump-dump scheme, in which the pulse sequences that favor the different products have different temporal separations, the complex optimal pulses occupy about the same time window. Indeed, the optimal pulse shape that generates one product is very crudely like a two-pulse sequence, which suggests that the mechanism of the enhancement of product formation in this case is that the time delay between the pulses is such that the wavepacket on the excited-state... 

In addition to intermolecular reactions, C-glycosides can also be synthesized by intramolecular sequences. A radical cyclization is a very fast reaction, in particular 5-exo-trig cyclizations. Thus, intermediate anomeric radicals have only a short time window of reactivity before undergoing the desired cyclization. 

Prior to this discussion, we would like to refer to a qualitative introduction given by Bard and Faulkner (ref. 21, Sect. 11.1.2 and 11.2.3). In a few pages they give a clear indication of the effect of the chemical reaction on the several characteristic electrochemical quantities (e.g. half-wave potential, limiting current, etc.). In addition, it is argued that a chemical rate constant, ft,-, is measurable by a given technique if its reciprocal value, 1/fc, falls within the experimental time range accessible for the technique (the so-called time window ). 

Nuclei provide a large number of spectroscopic probes for the investigation of solid state reaction kinetics. At the same time these probes allow us to look into the atomic dynamics under in-situ conditions. However, the experimental and theoretical methods needed to obtain relevant results in chemical kinetics, and particularly in atomic dynamics, are rather laborious. Due to characteristic hyperfine interactions, nuclear spectroscopies can, in principle, identify atomic particles and furthermore distinguish between different SE s of the same chemical component on different lattice sites. In addition to the analytical aspect of these techniques, nuclear spectroscopy informs about the microscopic motion of the nuclear probes. In Table 16-2 the time windows for the different methods are outlined. 

When taking samples for the determination of enantiomeric purity in a kinetic resolution, it is mandatory to choose the proper time window, e. g., when conversion is between 20% and 80%. If the process of directed evolution leads to enzyme variants differing widely in activity, mass screening needs to be time-resolved, i.e., several samples of a given reaction should be taken as a function of time. 

Fig. 9.11 In situ ATR FTIR spectra of the silsesquioxane reaction mixture as a function of the reaction time for a window of 670-1000 cmf1. Analytical parameters were IR analysis (670-4000 crrT1) was performed for 10 h with the acquisition of a spectrum every 2 min, 16 scans per spectrum and a resolution of 4 cmf1.

Reaction Dynamics in Femtosecond and Microsecond Time Windows ... 

The excess energy within the cluster ion owing to the above exothermic reaction and the relaxation around the newly formed ion, as well as possible further multiphoton excitations, contributes to heating of the cluster ions and concomitant evaporative dissociation. As the cluster ions cool evaporatively, the dissociation extends to longer times, and the present study is directed to an investigation of the metastable dissociation processes of cluster ions (NH3) H+ in the field-free region (where the time window is about one to a few tens of microseconds). In the case of the ammonia cluster ion system, the dissociation process can be expressed as... 

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