Stopcock molecules

The heads of the stopcock molecules must be large enough so that they cannot enter the channels. They should fulfill the stability criteria imposed by a specific application and it should be possible to functionalize them in order to tune the properties of the surface (e.g., the wetting ability, refractive index matching, and... 

It is a challenge to couple the antenna systems to a device (e.g., a semiconductor). It has already been shown that it is possible to prepare organized zeolite monolayers on flat surfaces [78, 79]. For coupling, the interface between the chromophore loaded zeolite L antenna systems and the semiconductor becomes very important. Stopcock molecules are expected to function as a bridge between the chromophores in the zeolite L channels and the device surface, which will open a whole new exciting research area. The first successful experiments with slopcock modified crystals have been reported recently [80]. 

Figure 20 (a) Typical shape of a stopcock molecule located at the end of a zeolite channel, (b) Fluorescent molecules which have already been inserted in zeolite L are modified with an inert head in order to build stopcock molecules with fluorescent tails, (c) Examples of molecules which can be used as stopcocks with a fluorescent head. 

The heads of the stopcock molecules are too large to be able to enter the channels. Typical functionalized groups such as bucky balls [34], chelating centers [35], and others can be used. Some examples are given in Fig. 21. Similarly to the labels, heads can be reactive or nonreactive. Reactive heads may have arms , which can interact with each other to form a monolayer polymer or bind to the zeolite external surface. 

In some cases, it is desirable to add a spacer which elongates the stopcock molecule so that the length of the tail can be controlled. This can be a tool to improve the solubility of the whole molecule. Polar groups might help to bind the molecules more strongly inside of the zeolite channels. Spacers which are sufficiently flexible so that they can bend the tail into the zeolite channels include, for example, aliphatic chains, polyethers, or amides. 

First experimental results on dye-loaded zeolite L systems modified with commercial stopcock molecules on the external surface show that electronic excitation energy can be transferred from molecules inside the channels to the stopcocks and vice versa and that the stopcocks prefer to adsorb on the cylinder base instead of the coat [42]. 

Fig. 4. a) Schematic representation of zeolite with donor molecule modified with acceptor stopcocks and b) magnification showing details of zeolite channel and the shape of the stopcock molecule. Ref. 25b... 

Now let s consider a process a bit closer to chemistry (Figure 17.2, p. 453). Two different gases, let us say H2 and N2, are originally contained in different glass bulbs, separated by a stopcock. When the stopcock is opened, the two different kinds of molecules distribute themselves evenly between the two bulbs. Eventually, half of the H2 molecules will end up in... 

Positively charged stopcocks can be plugged in the zeolite channels by ion exchange, whereas neutral stopcocks can be added by dehydration of the zeolite channels and adsorption from a nonaqueous solution or from the gas phase. The zeolite s external surface consists of a coat and a base. These two surfaces differ in a number of properties so that the interactions can be tuned. For MFI- and FAU-type zeolites, as an example, it was reported that guest molecules bind to the holes on the external surface much more strongly than on the framework between the holes [38,39]. 

Figure 1.19. Le Typical shape of a head-tail molecule acting as an injector-, acceptor-, or Simply as a stopcock. The positive charge is desirable but not necessary for anionic framework hosts. Reactive groups may help to fix the stopcock. Middle Fluorescent molecules that already have been inserted in zeolite L could be modified with an inert head. Right Examples of molecules that could be used as stopcocks with a fluorescent head.

Their system included stopcocks, mercury columns, cold traps, and portions of glass that could not be baked. Nevertheless, their films were probably free from serious contamination. Any residual gas molecules which entered their testing tube would first strike thinner portions of the deposited metal film, where they could be adsorbed. Furthermore, even... 

A vacuum flow apparatus was used in experiments on hydroxyl reactions (Fig, I). The free hydroxyl, together with other active species and nondisssociated water molecules, was pumped out of the high-voltage discharge zone into the reaction vessel, through a nozzle. The substance studied was introduced into the reaction vessel from a flask of prefixed volume, through a stopcock valve. The reaction vessel was heated by means of an electric furnace. The temperature constancy was checked by means of thermocouples at various sites in the reaction vessel. 

Consider the reaction A + B AB. The vessel on the right contains an equilibrium mixture of A molecules (red spheres), B molecules (blue spheres), and AB molecules. If the stopcock is opened and the contents of the two vessels are allowed to mix, will the reaction go in the forward or reverse direction Explain. 

We have defined a spontaneous process as one that proceeds on its own without any external influence (Section 8.13). The reverse of a spontaneous process is always nonspontaneous and takes place only in the presence of some continuous external influence. Consider, for example, the expansion of a gas into a vacuum. When the stopcock in the apparatus shown in Figure 17.1 is opened, the gas in bulb A expands spontaneously into the evacuated bulb B until the gas pressure in the two bulbs is the same. The reverse process, migration of all the gas molecules into one bulb, does not occur spontaneously. To compress a gas from a larger to a smaller volume, we would have to push on the gas with a piston. 

At the beginning of the experiment, chlorine and nitrogen molecules occupy separate bulbs. But when the stopcock is opened, the gas molecules move back and forth between the two bulbs and become thoroughly mixed. Refer to Table C-1 In Appendix C for a key to atom color conventions. 

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