Impurity molecules

Ambrose W P and Moerner W E 1991 Fluorescence spectroscopy and spectral diffusion of single impurity molecules in a crystal Nature 349 225-7... 

Basche T and Moerner W E 1992 Optical modification of a single impurity molecule in a solid Nature 355 335-7... 

Moerner W E 1994 Examining nanoenvironments in solids on the scale of a single, isolated impurity molecule Science 265 46-53... 

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

Water-soluble polymeric dyes have been prepared from water-insoluble chromophores, viz., anthraquinone derivatives. Unreacted chromophore and its simple derivatives, which are all water-insoluble, remain in solution due to solubilization by the polymeric dye. A method has been developed to separate and quantitate the polymeric dye and these hydrophobic impurities using Sephadex column packing. The solvent developed has the property of debinding the impiirities from the polymer, and further allows a separation of the imp irities into discrete species. This latter separation is based on the functional groups on the impurity molecules, having a different interaction with the Sephadex surface in the presence of this solvent. The polymer elutes at the void volume... 

It has been suggested that trapping of this delocalized excitation by low-energy impurity molecules could play an important role in the photophysics of chloroplasts. After trapping the excitation energy, it could be transferred to a reactant or an electron transfer reaction could occur/351... 

Activated charcoal is ultrafinely divided carbon with lots of places to suck up big, huge, polar, colored impurity molecules. Unfortunately, if you use too much, it ll suck up your product And, if your product was white, or yellow, it ll have a funny gray color from the excess charcoal. Sometimes, the impurities are untouched and only the product gets absorbed. Again, it s a matter of trial and error. Try not to use too much. Suppose you ve got a hot solution of some solid, and the solution is highly colored. Well,... 

Because Si — Ti absorption has a very small molar absorption coefficient, we would expect (because of the inverse relation between 8 and T0) the Ti state to have a much greater luminescent lifetime than the same molecules in the Si state. As a result of this longer lifetime, the Ti state is particularly susceptible to quenching, such that phosphorescence in fluid solution is not readily observed as the Ti state is quenched before emission can occur. This quenching in solution involves the diffusion together of either two Ti molecules or the Ti molecule and a dissolved oxygen molecule or some impurity molecule. In order to observe phosphorescence it is necessary to reduce or prevent the diffusion processes. The techniques most often used are ... 

Crystals grow by addition of molecules from the surroundings to the exposed faces of the crystal. When there are present in the surroundings not only substrate molecules but also impurity molecules that are stereochemically similar to the substrate, these latter molecules may also add to the surfaces. The result will be a retardation of crystal growth (267). Furthermore, since the energy of the attachment of the impurity will differ from face to face, a modification of the crystal morphology may also result (268,269). 

Cells can be made in which the cathode-anode distance is only 10-3 cm. Such cells have the advantage that the total impurity present is veiy small and may not be enough to cover more than 0.1% of the electrode surface if they were all adsorbed. Thus, suppose the impurity concentration were 10-6 mol liter-1 or 10-9 mol cc 1 or 10 12 mol in the cell Because an electrode surface can cany (at most) about 10-9 mol cm-2, the maximum fraction of the surface covered with impurity molecules is 0.1%. Does work with thin-layer cells eliminate the inpurity problem in electrode kinetics It improves it. However, active sites on catalysts may occupy less than 0.1% of an electrode and preferentially attract newly arriving impurities, so that even thin-layer cells may not entirely avoid the impurity difficulty,32 particularly if the electrode reaction concerned (as with most) involves adsorbed intermediates and electrocatalysis. 

It has been suggested that one reason for the nonreproducibility is the high triplet energy of phenanthrene which makes its triplet susceptible to quenching by far more kinds of impurity molecule than, say, that of anthracene. However, this is not the whole explanation because naphthalene also has a high-lying triplet but seems to be less susceptible to quenching. 

A sample of water that is 99.9999 percent pure contains 0.0001 percent impurities. Consider from Chapter 1 that a glass of water contains on the order of a trillion trillion (1 X 1024) molecules. If0.0001 percent of these molecules were the molecules of some impurity, about how many impurity molecules would this be ... 

Find the number of water molecules in the glass and then compare them to the number of impurity molecules. According to exercise 63, there are a trillion trillion water molecules in a glass of water. If this water is 99.9999 percent pure, then it also contains a million trillion impurity molecules. A trillion trillion is a million times more than a million trillion, therefore, there are a million times more water molecules than there are impurity molecules. In other words, for every million water molecules, there is only one impurity molecule. Thus, in a sample of water that is 99.9999 percent pure, the number of water molecules far exceeds the number of impurity molecules, even though there are trillions of each. 

This small current is normally quite steady, and may be about 10 nanoamperes. If, however, certain impurities are added to the gas stream, the impurity molecules may capture some of the free electrons before they can be extracted by the electrode. The drop in the number of electrons extracted causes a drop in the electrode current this drop in current signals the presence of the impurity... 

The role of subexcitation electrons is most important when the irradiated medium contains small amounts of impurity molecules the excitation energy ha) 0j (or the ionization potential I ) of which is below h(o0l. Such additive molecules can be excited or ionized by the subexcitation electrons the energy of which is between h(o 0j and fuom, and, consequently, the relative fraction of energy absorbed by an additive will be different from what it should be if the distribution of absorbed energy were solely determined by the relative fraction of valence electrons of each component of the mixture.213 214 According to estimates of Ref. 215, this effect is observed when the molar concentration of the additive is of the order of 0.1%. This selective absorption with ionization of additives has been first pointed out by Platzman as an explanation for the increase in the total ionization produced by alpha particles in helium after small amounts of Ar, C02, Kr, or Xe were added (the so-called Jesse effect).216... 

We now consider hydrogen transfer reactions between the excited impurity molecules and the neighboring host molecules in crystals. Prass et al. [1988, 1989] and Steidl et al. [1988] studied the abstraction of an hydrogen atom from fluorene by an impurity acridine molecule in its lowest triplet state. The fluorene molecule is oriented in a favorable position for the transfer (Figure 6.18). The radical pair thus formed is deactivated by the reverse transition. H atom abstraction by acridine molecules competes with the radiative deactivation (phosphorescence) of the 3T state, and the temperature dependence of transfer rate constant is inferred from the kinetic measurements in the range 33-143 K. Below 72 K, k(T) is described by Eq. (2.30) with n = 1, while at T>70K the Arrhenius law holds with the apparent activation energy of 0.33 kcal/mol (120 cm-1). The value of a corresponds to the thermal excitation of the symmetric vibration that is observed in the Raman spectrum of the host crystal. The shift in its frequency after deuteration shows that this is a libration i.e., the tunneling is enhanced by hindered molecular rotation in crystal. 

The H(D) atom abstraction rate constants in durene crystals by the impurity molecules quinoline, isoquinoline, quinoxaline, and quinozaline in their excited triplet state were measured by Hoshi et al. [1990] using the phosphorescence method described above. The transfer occurs in the fragment CH N formed by a methyl group of durene and a nitrogen atom of the impurity molecule. In the interval 300-100 K the activation energy drops from 3.5 kcal/mol to 1.6 kcal/mol. Deuteration reduces the... 

Hydrogen atom transfer from anthracene, excited into its lowest excited singlet state, to anthraquinone impurity molecules creates a radical pair that strongly quenches the fluorescence from anthracene crystals. The reverse transfer rate constant, found from measurements of fluorescence intensity and its characteristic lifetime at different moments after the creation of the radical pair, varies from 106 to 10s s 1 in the range 110-65 K, kc = 4 x 104 s 1, TC = 60K. The kc values drops to 102 s 1 in the deuteroanthracene crystal [Lavrushko and Benderskii, 1978]. 

At higher temperatures, the transfer rate of the impurity molecule between sites is enhanced by phonon emission or absorption this can be viewed as a phonon-dressed state transfer. When these off-diagonal processes are included in the analysis, the transition probability can be broken up into phonon processes of different order. One-phonon processes do not contribute, because momentum is not conserved in these events. Among the two-phonon processes, only Raman events contribute. These lead to a I7 dependence of the transition probability (see, for example, Flynn and Stoneham [1970]). This regime takes place when kBT... 

Recent work (Lupinski et al., 1967 Hadek et al., 1971 Barlow et al., 1976) has shown that the maximum conductivity depends critically upon the precise composition of the complex salts and is essentially controlled by the neutral impurity molecules. Just as radical anion salts are based on strong electron acceptors such as TCNQ, radical cation salts based on strong electron donors such as TTF are also known. A listing of the more important radical ion salts is included in Table 3. As a rule these salts give rise to an intense esr signal with their spin concentrations reaching 1 to 1.5 spins per DA pair. 

Perhaps the simplest optically controlled switches are single molecules embedded in a solid host matrix. These systems consist of an amorphous, polycrystalline, or crystalline film doped with dilute concentrations of impurity molecules. The most commonly used dopant molecules are fused polycyclic aromatic hydrocarbons and porphyrins. In addition to facile sample preparation, these planar molecules absorb in the visible to near IR regions of the spectrum, possess large extinction coefficients in both the ground and excited states, and have high fluorescence quantum yields. 

Enzymes can be poisoned if an impurity molecule blocks up the active site of the enzyme. Some enzymes can be used in treatments to prevent unwanted reactions occurring, like the production of bacteria. 

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