Crystallization of naphthalene

Exercise 27-13 The esr spectrum shown in Figure 27-18 is a first-derivative curve of the absorption of a radical produced by x irradiation of 1,3,5-cycloheptatriene present as an impurity in crystals of naphthalene. Sketch this spectrum as it would look as an absorption spectrum and show the structure of the radical to which it corresponds. Show how at least one isomeric structure for the radical can be eliminated by the observed character of the spectrum. 

In the intermediate domain of values for the parameters, an exact solution requires the specific inspection of each configuration of the system. It is obvious that such an exact theoretical analysis is impossible, and that it is necessary to dispose of credible procedures for numerical simulation as probes to test the validity of the various inevitable approximations. We summarize, in Section IV.B.l below, the mean-field theories currently used for random binary alloys, and we establish the formalism for them in order to discuss better approximations to the experimental observations. In Section IV.B.2, we apply these theories to the physical systems of our interest 2D excitons in layered crystals, with examples of triplet excitons in the well-known binary system of an isotopically mixed crystal of naphthalene, currently denoted as Nds-Nha. After discussing the drawbacks of treating short-range coulombic excitons in the mean-field scheme at all concentrations (in contrast with the retarded interactions discussed in Section IV.A, which are perfectly adapted to the mean-field treatment), we propose a theory for treating all concentrations, in the scheme of the molecular CPA (MCPA) method using a cell... 

FIGURE 5.11. Refractive indices and molecular stacking in a crystal of naphthalene, showing how the largest refractive index corresponds to the direction of the greatest density of atoms. The edges of the unit cell are shown. 

Application of the Theory to the Mixed Crystals of Naphthalene and Deuteronaphthalene. 

Bobrov et find for ethylene on silver films in UHV that an exposure of 1 L leads to a monolayer, based on a kinetic theory estimate, assuming a sticking factor of 1. They see Raman signals already at exposures of 0.03 L which saturate at 2 L. By comparison to the scattering from a crystal of naphthalene ( ) they estimate the enhancement as lO -lO (no further details are given). 

Consider, as an example, crystals of naphthalene type, C%h being the factor group and C the site group. Their characters are collected in Tables 2.1 and 2.2. 

As an example we consider the case of the naphthalene crystal, G ft being its space group. The point group C-2h (see Table 2.1) has only one-dimensional representations. Since any subgroup of the group C h can also have only onedimensional representations, it is clear that in crystals of naphthalene type the compulsory degeneracy for excitonic states inside the first Brillouin zone is not possible. 

An example of a quantitatively-analysed experimental result for these constants is shown in Fig. 7.29 in mixed crystals of naphthalene-dg 0.1% quinoxaline, the ESR transition T. To for the field direction Bo Xquinoxaiine and at a temperature T = 1.8 K is an absorption signal in the stationary state (Fig. 7.29a), while the transition I To) T-) in the stationary state exhibits stimulated emission of microwaves (Fig. 7.29b). After the end of the UV excitation at t = 0, the absorption line temporarily becomes an emission tine and vice versa. The interpretation of these results is simple (Fig. 7.29d) due to the negligible spin-lattice relaxation at T= 1.8 K, the three Zeeman components decay after the end of the U V excitation independently of one another, each with its own lifetime tj = into the So ground state. Since the difference of the populations of the three states is directly proportional to the intensity of the ESR signals, their time dependence can be used to determine the individual lifetimes of the Zeeman components involved. In the case of the particular orientation Boll, the state is To) = IT ), and one obtains directly from the measurements, e.g. the decay constant feo = kx and thus the lifetime of the zero-field constant Tx) of quinoxaline. 

The main aim of naphthalene crystallization is the separation of thianaphthene, which, with a boiling point of 219.9 °C, boils just 1.9 °C higher than naphthalene. However, the large difference in melting points (48 °C) facilitates separation to be carried out by crystallization, although this is complicated by the formation of mixed crystals of naphthalene and thianaphthene, rendering it necessary to carry out the process in several stages. 

Gorle S, Smirnova 1, Dragan M, Dragan S, Arlt W (2008) Crystallization of naphthalene in silica aerogels from supercritical CO2. J Supercrit Fluids 44 78... 

A portion of a crystal of naphthalene, showing molecules CjoH. ... 

The simple Htiekel energies, occupation numbers, and coefficients for MOs in naphthalene (VI) are listed in Table P12-25. Assume that a single crystal of naphthalene is oriented so that each molecule is aligned with respect to an external eoordinate system (VI). 

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