2- Methyltetrahydrofuran glass

The results of low-temperature matrix-isolation studies with 6 [41a] are quite consistent with the photochemical formation of cyclo-Cif, via 1,2-diketene intermediates [59] and subsequent loss of six CO molecules. When 6 was irradiated at A > 338 nm in a glass of 1,2-dichloroethane at 15 K, the strong cyclobut-3-ene-1,2-dione C=0 band at 1792 cm in the FT-IR spectrum disappeared quickly and a strong new band at 2115 cm appeared, which was assigned to 1,2-diketene substructures. Irradiation at A > 280 nm led to a gradual decrease in the intensity of the ketene absorption at 2115 cm and to the appearance of a weak new band at 2138 cm which was assigned to the CO molecules extruded photo-chemically from the 1,2-diketene intermediates. Attempts to isolate cyclo-Cig preparatively by flash vacuum pyrolysis of 6 or low-temperature photolysis of 6 in 2-methyltetrahydrofuran in NMR tubes at liquid-nitrogen temperature have not been successful. 

FIGURE 25. Mechanism of formation of radical anions M and cations M in frozen glasses of methyltetrahydrofuran (top) and alkyl chlorides (bottom), respectively... 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet (3BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to 3BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

Irradiated 2-methyltetrahydrofuran glass gives an ESR signal as shown in Fig. 4, which is composed of a central sharp spectrum and a broad seven-line spectrum. The former has the width of 4.5 G and is bleached out by visible lights. This spectrum is due to the trapped electrons in the glass. 

Fig. 4. ESR spectra of pure 2-methyltetrahydrofuran glass irradiated by y-rays to a dose of 1.5 x 10s rad at 77° K. Solid line, before photobleaching dotted line,...

Consider a small amount of monomer added to the glasses the reaction of monomer with the electrons and that with the cation radicals are thought to prevail in the glasses of 2-methyltetrahydrofuran and n-butylchloride, respectively. In the 3-methylpentane glass, whether the anionic reaction and/or the cationic one occurs depends on the nature of the monomer. 

From the viewpoint of the glass matrix, 2-methyltetrahydrofuran is useful to study the anionic reactions of solute monomers, while in n-butylchloride the cationic reactions are studied selectively. Such a selection of glass matrices was made in the study of radiation-formed ionic species by optical absorption measurements (24, 25). 

In order to examine the new ESR spectrum, the glass containing a larger amount of nitroethylene (28 mole-%) was irradiated, which gives a signal shape as shown in Fig. 6b. When the temperature is raised, the seven-line spectrum due to the free radicals formed from 2-methyltetrahydrofuran disappears, leaving the spectrum of present interest as shown in Fig. 6c. The spectrum has the hyperfine structure due to three... 

Table 1. Effect of bleaching samples with visible light on the polymerization of nitroethylene in 2-methyltetrahydrofuran glass irradiated to a dose of 2 X 70 rad at 77°...

One the other hand, short-lived intermediates formed from styrene by radiations were studied by the pulse radiolysis technique by Metz et al. (43). They observed the anion radicals of styrene as an optical absorption band with the maximum at 370 mp, but could not find cationic intermediates. Shida and Hillma irradiated the 2-methyltetrahydrofuran glass and butylchloride glass, both containing styrene, and observed the absorption bands due to added styrene at 410 mp and 350 mp, respectively. The former band was assumed to be due to the anion-radicals and the latter to the cation radicals (44). 

In the presence of a small amount of styrene in the irradiated 2-methyltetrahydrofuran glass, the observed signal shape is as shown in Fig. 8 (solid line). It is the superposition of both the seven-line spectrum,... 

Styrene in the irradiated glass matrix of an equimolar mixture of 2-methyltetrahydrofuran and n-butylchloride gives no spectrum. The absence of any spectrum due to the added styrene is a result of the fact that both the electrons and positive charges are captured and stabilized by the n-butylchloride and 2-methyltetrahydrofuran molecules, respectively, and they are unable to react with the styrene solute. This observation implies that the spectrum in the 2-methyltetrahydrofuran glass and that in the n-butylchloride cannot be attributed to the same intermediates, though their shapes are similar except for the difference in the width. 

These two processes are so efficient that the yield of the formation of the ion radicals saturates in respect to the concentration of added styrene in the glasses, even at the low concentration of 101 1 mole-%. In this saturation region, the G value for the formation of the ion radicals is 1.3 in the 2-methyltetrahydrofuran glass and 0.86 in the n-butylchloride glass. 

When the 2-methyltetrahydrofuran and n-butylchloride glasses with about 10 mole-% of styrene in them are irradiated to the dose of 1 X 107 rad, polystyrene is obtained only from the latter glass, though the polymer yield is low (1 2%). This indicates that radiation-induced polymerization occurs in the glass where the cation radicals are formed by the radiation. The experimental results of the polymerization in glass matrices are consistent with the previous report that the radiation-... 

Ionic processes of monomers, nitroethylene, n-butylvinylether and styrene, in organic glass matrices of 2-methyltetrahydrofuran, 3-methyl-pentane and n-butylchloride irradiated by y-rays at 77° K, are studied by observing the electron spin resonance spectra of trapped electrons and ion radicals formed from the solute monomers. The primary ionic intermediates are the trapped electrons and their counterpart, cation radicals of matrix molecules. However, in 2-methyltetrahydrofuran glass, the anionic processes of solute monomers resulting from the trapped electrons proceed selectively. On the contrary, only the cationic processes proceed selectively in n-butylchloride glass. Both processes are able to occur in 3-methylpentane glass. 

A recent study of the radiation-induced polymerization of nitro-ethylene (38) indicates that the predominant propagating species in this monomer is an anion. This is based on scavenger studies utilizing HBr, substantiated by a study of the electron-trapping capabilities of this monomer in irradiated 2-methyltetrahydrofuran glass (38). 

Controversy arose almost from the start in this story of the ground state of TME. Dowd, in 1986, prepared TME by photolysis of 4,5-bis(methylene)-3,4,5,6-tetra-hydropyridazine 35 in a methyltetrahydrofuran glass at 10 K. The ESR spectrum... 

Most of the studies concerning the formation and decay mechanisms of (RS.-. SR)- radical anions have been performed on aliphatic disulfides in aqueous solutions. Since the solubility of aromatic disulfides (ArS-SAr) is very poor in water, their radical anions (ArS.-. SAr)- can be easily investigated in organic solvents. A dissociation mechanism of the S-S bond in the a,a -dinapthyl disulfide radical anion (NpS.. SNp)" in dimethylformamide (DMF) solutions was investigated by pulse radiolysis and in methyltetrahydrofuran (MTHF) rigid glasses by y-radiolysis. ... 

Wasielewski and coworkers have studied a series of molecules in which porphyrins are linked to quinones and similar electron acceptors, and determined that when a solvent such as 2-methyltetrahydrofuran freezes, about 0.8 eV of driving force is lost [116]. These researchers have prepared dyads with a driving force for photoinduced electron transfer greater than this value in polar solvents, and found that they are capable of displaying photoinduced electron transfer even in frozen organic glasses. This, in turn, has opened the door to low-temperature EPR studies of the charge-separated states in various donor-acceptor systems and their decay pathways [66, 99, 117]. 

Unlike the situation with the quinone- and imide-based acceptors, photoinduced electron transfer is also observed in frozen organic glasses down to at least 8 K. For example, excitation of the porphyrin moiety of 43 in a 2-methyltetrahydrofuran glass at 77 K is followed by appearance of the C +-P-C6o charge-separated state, which is formed with a quantum yield of 0.10. The rise time for C -P-Ceo is 770 ps, which is presumably the reciprocal of ( 4 -I- kq) at this temperature. 

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