A2 system

Fig. 3. The possible design of systems for PET across membranes (a) — electron tunneling in S // — // A system (b) — carrier-mediated transport in S // A, // A2 system (c) — sensitizer-mediated transport in D // S // A system...

The possibility of using vesicle membranes in such a way has been demonstrated in systems of the type D + S + Ax // — // A2 (i.e. systems in which the inner volume of the vesicle contains the donor D, photosensitizer S and primary acceptor A, no special carrier is inserted into the membrane and the outer volume of the vesicle contains the ultimate acceptor A2). Systems 27 and 28 of Table 1 belong... 

Hence one finds two overlapping pseudotriplets (SH = 3.41 and 3.44) for the NCH2 group which appears only once and two overlapping quartets (SH = 3.22 and 3.25) for the NC//2 groups which appear twice. Since the shift differences of the CH2 protons are so small, the expected AH systems of the coupling partners approximate to A2 systems at 200 MHz thus one observes only the central multiplet signals of these AB systems. 

Table 10 1H NMR data for six-membered heterocyclic rings with one endocyclic double bond (A) A2-Systems...

We shall illustrate this process as it applies to the [2 - -2] cycloaddition process. This treatment will be general, making no distinctions between [ji2] and [a2] systems. We assume that the ligand transformation, however, is suprafacial, i.e., [ , i2J. To illustrate the role of coordinate bond-... 

For 60 we observe an AB- rons system, which is very close to an A2-system, corresponding to the protons at C-7 and C-8, while for 61, these same protons form an A2 system and appear as a single signal. Replacement of the ester group by an alcohol function makes the protons almost indistinguishable. The irradiation of the alcohols is performed in a quartz vessel by means of a 400-watt medium pressure lamp in solution in diethyl ether. 

Mixtures of ozone (4 to 6 mm. of Hg pressure) and ammonia, hydrogen, hydrogen chloride, methane, or water (up to 150 mm. of Hg pressure) were flashed (7). In each system the spectrum of the OH radical was observed in the 0,0 1,0 1,1 2,1 2,2 and 3,2 bands of the A2S+ — system. Transitions from vibrationally excited levels were visible for 15 jujsec. and those from the zeroth level for up to 75 /jisec. In addition, the spectra of NH, O2, and CIO were recorded, the latter persisting for 1 msec. The only spectrum visible after this time was that of formaldehyde. 

The triazolines resulting from the various reaction schemes are predominantly A2-compounds the cycloaddition reactions of azomethinimines and nitrile ylides lead to A3-triazolines. The amidrazones are unique a A3- or A2-system results, depending on the presence or absence of N-2 substitution (84KGS1415). The rare A -compounds are presumed to result from the nonregioselective addition of diazomethane to the imine double bond (72LA9) or by fluorination of 3,6-diamino-s-tetrazine (67USP3326889 70USP3515603). 

Honn KV. Inhibition of tumor cell metastasis by modulation of the vascular prostacyclin/ thromboxane A2 system. Clin Exp Metastasis. 1 (1983) 103-114. 

As described in Section 2.2, chiral supramolecular assemblies can render pairs of methylene protons diastereotopic and that this is true even if the complex exists as a racemic mixture, provided the rate of exchange is slow. Figure 11 illustrates the process of exchange between enantiomeric forms of helicate 16 leading to the coalescence of the AB pattern to an A2 system. ... 

Fig. 3 - Instantaneous local densities as a function of time. The curves a and b refer to the L1-L2 interface, while c corresponds to A1-A2 system. They have been calculated in the same slab (Z AZ/2) with a thickness (AZ) ec[ual to 3.4 A. The dashed lines indicate the average values for a and b, and the expected average for c.

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