Back transfer

Consider a lean phase, j, which is in intimate contact with a rich phase, i, in a closed vessel in order to transfer a certain solute. The solute diffuses from the rich phase to the lean phase. Meanwhile, a fraction of the diffused solute back-transfers to the rich phase. Initially, die rate of rich-to-lean solute transfer surpasses that of lean to rich leading to a net transfer of the solute from the rich phase to the lean phase. However, as the concentration of the solute in the rich phase increases. 

Cross-diagonal peaks (o>i = Cross-diagonal peaks (oj. = w,) Back-transfer peaks Diagonal peaks... 

Figure 6.4 Three-dimensional spectrum of a three-spin system showing peak types appearing in a three-dimensional space. Three diagonal peaks, six (wi = Wj) and six (wj = w,) cross-signal peaks, six back-transfer peaks, and six cross-peaks are present in the cube, (a) The cubes (b-d) represent three planes in which crossdiagonal peaks and the back-transfer peaks appear on their respective (atj = 0)2), u>2 = cof), and ( >i = Wj) planes. (Reprinted from J. Mag. Reson. 84, C. Griesinger, et al., 14, copyright (1989), with permission from Academic Press, Inc.)...

A 3D spectrum of a three-spin subsystem in which all the nuclei are coupled to one another, such as C(H/i)(Hb)-C(Hc), will lead to 27 peaks, comprising six cross-peaks, 12 cross-diagonal peaks (six at o) — o>2 and the other six at 0)2 = (O3), six back-transfer peaks, and three diagonal peaks. However, in the case of a linear three-spin network (e.g., CH -CHg-CHc), the number of peaks will depend on whether two equal (e.g., COSY-COSY or NOESY-NOESY) or unequal (e.g., COSY-NOESY) mixing processes are... 

So a simplified 3D spectrum is obtained having eight peaks, i.e., one crosspeak, four cross-diagonal peaks (two at V] = V2 and two at V2 = vj), three diagonal peaks, and no back-transfer peaks (Fig. 6.5b). 

The generally accepted essential features for a catalyzed hydrogen transfer reaction are quite severe (a) the DH2 molecule must bind to a metal transfer hydrogen to it and must be released from the metal environment before back-transfer takes place (b) the A molecule must be stable to hydrogen abstraction under the reaction conditions employed. 

Figure 8. Li + H2 Ground-state population as a function of time for a representative initial basis function (solid line) and the average over 25 (different) initial basis functions sampled (using a quasi-classical Monte Carlo procedure) from the Lit2/j) + H2(v — 0, j — 0) initial state at an impact parameter of 2 bohr. Individual nonadiabatic events for each basis function are completed in less than a femtosecond (solid line) and due to the sloped nature of the conical intersection (see Fig. 7), there is considerable up-funneling (i.e., back-transfer) of population from the ground to the excited electronic state. (Figure adapted from Ref. 140.)...

Figure 5.18 Weak ooh->- obh back-transfer involving (left) H-bonded and (right) free B—H antibonds of the borohydride ion. Note that the nodal plane of the <7BH(b) precludes significant net overlap with ctQh in the L-shaped geometry.

The up-conversion efficiency is low and varies with the concentration of the activator and sensitisor ions. A maximum efficiency is observed with concentrations of about 1-3% of the active center. Above this value increasing back transfer from Er3+ to Yb3+ and increasing interactions between both lanthanide ions, leading to cluster formation and Yb3+-Yb3+ energy transfer, limits the efficiency. 

The first part, before the t period, of the experiment is identical to the HN(CO)CA-TROSY scheme. This step chooses solely the sequential pathway in an HN(CO)CA-TROSY manner. The chemical shift of the 13C nucleus is recorded during the t evolution period. The back-transfer route is, however, quite different. We transfer the desired coherence from 13C directly back to the 15N nucleus and remove the second 13C -> 13C INEPT step found in HN(CO)CA-TROSY and replace it with the HNCA like back-transfer step. The antiphase 2./N<> coupling then refocuses simultaneously with VNc during the 13C 15N back-INEPT step. Thus, the HN(CO)CANH-TROSY... 

Let us begin with the one-mode electron-transfer system. Model IVa, which still exhibits relatively simple oscillatory population dynamics [205]. SimUar to what is found in Fig. 5 for the mean-field description, the SH results shown in Fig. 13 are seen to qualitatively reproduce both diabatic and adiabatic populations, at least for short times. A closer inspection shows that the SH results underestimate the back transfer of the adiabatic population at t 50 and 80 fs. This is because the back reaction would require energetically forbidden electronic transitions which are not possible in the SH algorithm. Figure 13 also shows the SH results for the electronic coherence which are found to... 

Due to the large-level density of the lower-lying adiabatic electronic state, the chances of a back transfer of the adiabatic population are quite small for a multidimensional molecular system. To a good approximation, one may therefore assume that subsequent to an electronic transition a random walker will stay on the lower adiabatic potential-energy surface [175]. This observation suggests a physically appealing computational scheme to calculate the time evolution of the system for longer times. First, the initial decay of the adiabatic population is calculated within the QCL approach up to a time to, when the... 

Salt type and concentration For back-extraction, increases in pH are not enough to strip the protein out from reverse micelles this is also due to the size exclusion elfect resulting from a decrease in the reverse micelle size [31,32]. This means that high salt concentration and salts that form small reverse micelles favor back transfer. Most of the work reported in the literature used KCl solution, normally 1.0 mol dm KCl coupled with a pH around 7.5. Marcozzi et al. [23] also showed that the back transfer efficiency of a-chymotrypsin depends on the salt type and concentration used in the forward transfer. 

For the scale-up of reverse micelle extractions, it is important to know which factors determine the mass transfer rate to or from the reverse micelle phase. So far most work has concentrated on the kinetics of solubilization of water molecules [34,35], protons [36], metal ions [20,35,37,38 0], amino acids [41], and proteins [8,35,42,43]. There are two separate processes forward transfer, which is transfer of solute from the aqueous to the reverse micelle phase, and back transfer, which is the antithesis of the first one. 

Depending on the solvent polarity and redox potentials of a donor and an acceptor, the ions resulting from electron transfer may remain associated either as a contact IRP or as a solvent-separated IRP. In the contact pair, back electron transfer can take place. For such electron back-transfers, the solvent reorganization energy is less than 5% of the total reorganization energy (Serpa and Arnaut 2000). 

The main features of the chemiluminescence mechanism are exemplarily illustrated in Scheme 11 for the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide in the presence of imidazole (IMI-H) as base catalyst and the chemiluminescent activators (ACT) anthracene, 9,10-diphenylanthracene, 2,5-diphenyloxazole, perylene and rubrene. In this mechanism, the replacement of the phenolic substituents in TCPO by IMI-H constitutes the slow step, whereas the nucleophilic attack of hydrogen peroxide on the intermediary l,l -oxalyl diimidazole (ODI) is fast. This rate difference is manifested by a two-exponential behavior of the chemiluminescence kinetics. The observed dependence of the chemiexcitation yield on the electrochemical characteristics of the activator has been rationalized in terms of the intermolecular CIEEL mechanism (Scheme 12), in which the free-energy balance for the electron back-transfer (BET) determines whether the singlet-excited activator, the species responsible for the light emission, is formed ... 

The lifetime of the triplet excited molecule with energy 1 eV below the Si energy is probably extremely short the triplet excitations are suggested to take place to dissociative states [90]. Such conclusion can be obtained from the biphotonic sensitization experiments mentioned in connection with the energy of the triplet states (Sec. 2.1) if the decomposition had not been fast enough (t < 10 sec) due to the back transfer of energy, the sensitization could not have been observed [29]. 

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