Irradiation molecular crystals

As we have seen, the expressions for the rate constant obtained for different models describing the lattice vibrations of a solid are considerably different. At the same time in a real situation the reaction rate is affected by different vibration types. In low-temperature solid-state chemical reactions one of the reactants, as a rule, differs significantly from the molecules of the medium in mass and in the value of interaction with the medium. Consequently, an active particle involved in reaction behaves as a point defect (in terms of its effect on the spectrum and vibration dynamics of a crystal lattice). Such a situation occurs, for instance, in irradiated molecular crystals where radicals (defects) are formed due to irradiation. Since a defect is one of the reactants and thus lattice regularity breakdown is within the reaction zone, the defect of a solid should be accounted for even in cases where the total number of radiation (or other) defects is small. 

The diffusion of H and D atoms in the molecular crystals of hydrogen isotopes was explored with the EPR method. The atoms were generated by y-irradiation of crystals or by photolysis of a dopant. In the H2 crystals the initial concentration of the hydrogen atoms 4x 10 mol/cm is halved during 10 s at 4.2 K as well as at 1.9 K [Miyazaki et al. 1984 Itskovskii et al. 1986]. The bimolecular recombination (with rate constant /ch = 82cm mol s ) is limited by diffusion, where, because of the low concentration of H atoms, each encounter of the recombinating partners is preceded by 10 -10 hops between adjacent sites. 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

Figure 4. Examples of low-temperature limit of rate constant of solid-state chamical reactions obtained in different laboratories of the USSR, United States, Canada, and Japan (1) formaldehyde polymerization chain growth (USSR, 1973 [56]) (2) reduction of coordination Fe-CO bond in heme group of mioglobin broken by laser pulse (United States, 1975 [65]) (3) H-atom transfer between neighboring radical pairs in y-irradiated dimethylglyoxime crystal (Japan, 1977, [72], (4, 5) H-atom abstraction by methyl radicals from neighboring molecules of glassy methanol matrix (4) and ethanol matrix (5) (Canada, United States, 1977 [11, 78]) (6) H-atom transfer under sterically hampered isomerization of aryl radicals (United States, 1978 [73]) (7) C-C bond formation in cyclopentadienyl biradicals (United States, 1979 [111]) (8) chain hydrobromination of ethylene (USSR, 1978 [119]) (9) chain chlorination of ethylene (USSR, 1986 [122]) (10) organic radical chlorination by molecular chlorine (USSR, 1980 [124,125]) (11) photochemical transfer of H atoms in doped monocrystals of fluorene (B. Prass, Y. P. Colpa, and D. Stehlik, J. Chem. Phys., in press.).

Energy transfer in molecular crystals seems to be a well-established phenomenon. Irradiated crystals of anthracene containing only a trace of naphthacene show the characteristic green fluorescence of naphthacene rather than the violet of the primary constituent. If the material is dissolved in benzene, the anthracene fluorescence predominates. 

As might be expected, the X-ray crystal structure of triphenylphosphorany-lidene-2-propanone (10) reveals the formation of a web of intra- and inter-molecular hydrogen bonds. An EPR/ENDOR study of X-irradiated single crystals of (10) shows that upon radiolysis the molecules undergo a drastic reorientation about the ylidic P-C bond as radical cation (11) is formed. ... 

Decarbonylation and Decarboxylation - The photochemical reactions between carboxylic acids such as formic and trifluoroacetic acid and silica surfaces have been studied. The irradiation of diphenylacetic acid in acetonitrile with acridine results in the decarboxylation and the formation of the adduct (102). When the two achiral compounds, acridine and diphenylacetic acid, are cocrystallized a chiral two component molecular crystal is obtained in which the components are present in a 1 1 ratio. Both (—)- and (+)- crystals can be obtained. This crystalline material is photochemically reactive and irradiation brings about decarboxylation and formation of the adduct (102) with an ee of 35%. This compound is accompanied by a low yield of (103) which is formed exclusively on irradiation of the reactants in acetonitrile solution.Photochemical decarboxylation can be brought about by the irradiation of benzilate (104)/ methyl viologen pairs. ... 

X-ray irradiation, hence we can determine the crystallographic structures sufficiently well at room temperature. Because the crystal structure analysis has fundamental importance for any quantitative analyses, accumulation of the structural data is necessary in the search for practical applications of carotenoid molecular crystals. In this section, growth and structures ofall-/ra j-]8-carotene single crystals are introduced. 

Crystallization enthalpies of the irradiated blends were lower than predicted by the additivity rule. The crosslinking of the amorphous domains lowered the melting enthalpies. Irradiated blends showed separated crystallizations, at their characteristic temperatures of crystallization, independent of composition. On irradiation, the crystallization temperature of PE did not change, whereas that of UHMWPE slightly decreased. Separate irradiation of UHMWPE and PE favored molecular scission over the crosslinking, whereas irradiation of the blends favored crosslinking. The melting temperature, T, increased linearly with... 

The fundamental observable of exciton mobility and energy conduction in molecular crystals is sensitised fluorescence (compare Fig. 6.20). One irradiates a host crystal H, for example anthracene, which has a very low doping or impurity concentration of a guest G, for example tetracene, with light that can be absorbed by the host... 

Ozawa et al. have performed crystal structural analyses on these dinuclear copper complexes in the excited state while irradiating the crystal with CW laser [6]. In their experiment, the interatomic distance between the two copper atoms is almost the same, whereas that between the two bromine atoms is shortened in the excited state. The structural change is reported to be about 10 %. In the excited state, the electron in the copper 3d orbital slightly moved outside from the Cu2Br2 unit plane due to MLCT. Thus, the intermolecular distance between the two bromine atoms is shortened due to the expansion of the two copper atomic orbitals. Luminescence can be explained by the electron in the ligand molecular orbital dropping into the copper 3d and bromine 4p orbitals. 

Laser-rf spectroscopy was also performed in molecular crystals. Dinse and collaborators pumped nuclear spins of N in phenazine by exciting a spin-forbidden phononless singlet-triplet transition with a single-mode cw dye laser. The crystal, which was immersed in liquid helium, could be irradiated with an rf field in the range of 1-40 MHz and exposed to a static magnetic field of up to 60 G. A static field of 50 G was applied in order... 

Molecular Crystals Fullerites. Organic crystals are usually prone to ionization damage and decompose very rapidly under electron irradiation they can thus be studied for only a short time (a few. seconds) and only with a very low electron beam intensity. Transmission electron microscopy has, therefore, seldom been applied to organic crystals. However, the all-carbon molecules Qo, C70. etc., (fullerenes) discovered at the end of the 1980s resist electron radiation fairly well. Early structural studies on the crystalline phases of ftil-lerenes (fullerites) were performed mainly by electron microscopy because only small quantities of sufficiently pure material were available. At room... 

Antonov, V. S., Letokhov, V. S., and Shibanov, A. N. (1980o). Formation of molecular ions upon irradiation of molecular crystal with a UV laser radiation. Journal of Experimental and Theoretical Physics Letters, 31, 441-444. 

A possible way to induce selectivity in the photodecarboxylation process could be through photosensitized reactions in the soHd state. In fact, when a two-component molecular crystal of phenanthridine and 3-indoleacetic acid is irradiated at low temperature (-70°C), 3-methyHndole is formed in high yield as the sole product by contrast, when the same reaction is carried out in acetonitrile solution, four products are obtained.Furthermore, irradiation of two-component molecular crystals of arylalkyl carboxylic acids with stoichiometric amounts of electron acceptor causes decarboxylative condensation between the two components with important selectivities. " Thus, irradiation of (S)-naproxen in a chiral crystal with 1,2,4,5-tetracyanobenzene produces a decarboxylated condensation product retaining the initial chirality." Photolysis of an enantiomorphous bimolecular crystal of acridine with the R or S enantiomer of 2-phenylpropionic acid causes stereoselective condensation to give three optically active products. An absolute asymmetric synthesis has also been achieved by the enantioselective decarboxylative condensation of a chiral molecular crystal formed from achiral diphenylacetic acid and acridine (Scheme 9). ... 

Solid-state irradiation of two component molecular crystals of thienylacetic acids with aza aromatic compounds (acridine and phenanthridine) ° results in photodecarboxylation and gives decarboxylated and condensation products. Two-component molecular crystals of the above azo aromatic compounds with 3-indolepropionic acid and 1-naphthylacetic acid, upon solid-state irradiation, give radical intermediates via electron transfer and ultimately afford decarboxylated compounds in near quantitative yield. Irradiation of crystalline charge-transfer complexes of 3-indoleacetic acid and 2-naphthylacetic acid with 1,2,4,5-tetracyanobenzene gives methylnaphthalene (decarboxylation) and naphthyl(2,4,5-tricy-ano)methane (dehydrocyanating condensation) in the solid state. 

D.-S. Seo and C.-H. Lee, Investigation of liquid crystal ahgmnent and pretilt angle generation in the cell with linearly polarized UV light irradiation on polymer surface. Molecular Crystals and Liquid Crystals 329, 255 (1999). 

Transformations in the Solid State. From a practical standpoint, the most important soHd-state transformation of PB involves the irreversible conversion of its metastable form II developed during melt crystallization into the stable form I. This transformation is affected by the polymer molecular weight and tacticity as well as by temperature, pressure, mechanical stress, and the presence of impurities and additives (38,39). At room temperature, half-times of the transformation range between 4 and 45 h with an average half-time of 22—25 h (39). The process can be significantly accelerated by annealing articles made of PB at temperatures below 90°C, by ultrasonic or y-ray irradiation, and by utilizing various additives. Conversion of... 

It should be emphasized that, in all the topochemical photoreactions without exception, an apparent reaction rate at the initial stage increases with increase in the irradiation temperature, as long as the temperature is sufficiently low to maintain the molecular orientation in the crystal. 

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