Glass 3-methylpentane

Site-selection spectroscopy Maximum selectivity in frozen solutions or vapor-deposited matrices is achieved by using exciting light whose bandwidth (0.01-0.1 cm-1) is less than that of the inhomogeneously broadened absorption band. Lasers are optimal in this respect. The spectral bandwidths can then be minimized by selective excitation only of those fluorophores that are located in very similar matrix sites. The temperature should be very low (5 K or less). The techniques based on this principle are called in the literature site-selection spectroscopy, fluorescence line narrowing or energy-selection spectroscopy. The solvent (3-methylpentane, ethanol-methanol mixtures, EPA (mixture of ethanol, isopentane and diethyl ether)) should form a clear glass in order to avoid distortion of the spectrum by scatter from cracks. 

Early pulse radiolysis studies of alkanes at room temperature showed that the solvated electron absorption begins around 1 pm and increases with increasing wavelength to 1.6 pm for -hexane, cyclohexane, and 2-methylbutane [77]. More complete spectra for three liquid alkanes are shown in Fig. 4. The spectrum for methylcyclohexane at 295 K extends to 4 pm and shows a peak at 3.25 pm [78]. At the maximum, the extinction coefScient is 2.8 x 10 cm The spectrum for 3-methyloctane at 127 K, shown in Fig. 4, peaks around 2 pm. The peak for methylcyclohexane is also at 2 pm at lower temperature. Recently, the absorption spectra of solvated electrons in 2-methylpentane, 3-methylpentane, cA-decalin, and methylcyclohexane glasses have been measured accurately at 77 K [80]. For these alkanes, the maxima occur at 1.8 pm, where the extinction coefScient is 2.7 x 10 cm. ... 

The energy of a single photon is obviously insufficient to ionize an organic compound. As early as the nineteen forties (3, 4), however, it -was observed that Wurster blue cation radical is produced by photoirradiation of 3-methylpentane glass containing N,N-tetramethyl p-phenylenediamine (TMPD) at 77° K. The recent detailed study of this system by electric conductivity measurement (5, 6) and electronic spectroscopy (7) provided conclusive evidence that the ionization is brought about via excitation to the triplet state followed by successive photoabsorption at the triplet state. This mechanism is supported by the facts that the life-time of the photochemical intermediate is identical with that of phosphorescence and the formation of Wurster blue, and that phosphorescence is inhibited in the presence of triplet scavengers. 

In addition, the glass matrix has an essential merit in comparison with the solvent which crystallizes at low temperature. For example, Smith et al. irradiated several olefins at 77° K and examined their ESR spectra, and they found that the electrons were trapped in the frozen state of glass but never in the crystalline state (9). This is also the case with 3-methylpentane (70), and other compounds such as alcohols and ethers. This fact may imply that the radiation-formed ionic intermediates are much more stable in the glass matrix than in the crystalline matrix, though the reason has not yet been confirmed. 

Although the trapped electrons in 3-methylpentane glass irradiated at 77° K were studied by optical absorption measurements and reported on in 1966 by Guarino and Hamill (72), the first observation of their ESR spectrum was reported by Tsuji and the present authors in 1967 (13). The observation was difficult because of the very rapid disappearance of the trapped electrons at 77° K. The assignment of the spectrum was nicely reconfirmed by Shirom et al (14). 

Fig. 1. ESR spectra of pure 3-methylpentane glass irradiated by y-rays to a dose of 3 x 10s rad at 77° K. Solid line, 6 min after irradiation dotted line, after photo-bleaching by room lights (Ref. 11)...

The ionization potential of 2-methylpentene-l ( 9 eV) is lower than that of 3-methylpentane (10.30 eV). Therefore, the added 2-methylpentene-l is thought to trap the positive charges and to prevent their migration in the glass (15). This means that the fast disappearance of the trapped electrons is independent of the migration of the positive charges,... 

Fig. 3. Dependence of the concentration of the trapped electrons in 3-methylpentane glass, irradiated by y-rays at 77° K to a dose of 1.5 x 10 rad, on the concentration of 2-methylpentene-l added to the glass. , Fast disappearing electrons o, slowly disappearing electrons (Ref. 11)...

The first-order decays imply that the electrons recombine with some cations by the so-called spur reaction the electrons recombine always with their counterpart cations. The increase of the G value (see Fig. 3) shows that the added 2-methylpentene-l provides the sites for the stable electron trapping as well as stabilizing the positive charges in the glass. As far as the present authors know, the evidence for the cation radicals of 3-methylpentane has not been obtained by ESR. 

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. 

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. 

West and coworkers photolysed a series of hindered azidosilanes 707-710 in a 3-methylpentane (3-MP) glass at 77 K and in solution at low temperatures317. The products were analysed by UV-Vis, GC/MS and NMR spectroscopy. 

Divinyl-tetramethyldisilane similarly rearranged to the analogous silene CH2=CHMeSi=CHCH2SiMe3, which showed a UV absorption maximum at 336 nm in 3-MP (3-methylpentane) glass at 77 K46,47. 

When di(t-butyl)silylene 321, generated in a 3-methylpentane glass at 77 K or in an argon matrix at 10 K by photolysis of a precursor bis-azide, was irradiated with 500-nm light, intramolecular C—H insertion occurred yielding the silacyclopropane 322 (equation 26)161. 

At liquid nitrogen temperature, 77 K, matrix isolation in hydrocarbons is successful for many silylenes because they are singlet species and so do not abstract hydrogen from C—H bonds, as would be expected for triplet molecules. The usual hydrocarbon is 3-methylpentane (3-MP) which forms a rigid glass at 77 K, but sometimes mixtures of hydrocarbons are used which are softer at this temperature, to allow some mobility of the silylene in the matrix. In a few cases, silylenes have also been identified from transient spectra obtained in flash photolysis experiments. 

Extended photolysis of Mu2(CO)io in 3-methylpentane glass gives rise to Mn2(CO)s resulting from double CO-loss and an isomer of Mu2(CO)9 with a semibridging carbonyl. Dft calculations have been used to probe possible structures of the Mu2(CO)8 species. ... 

Other types of sila systems also give di-n-methane products. In the case of (19), irradiation at 10 K in an argon matrix or at 77 K in a 3-methylpentane glass affords the di-n-methane product (20), which on further irradiation or upon heating reverts to the starting material (equation 21). Photolysis of (19) at room temperature gives instead retro Diels-Alder fragmentation to tetramethyldisilene, whose intermediacy was established by means of [4 + 2] cycloaddition with 2,3-dimethyl-1,3-butadiene. ... 

A solution of l-(3,3-dimethyl-2-phenylcycloprop-l-enyl)-4-methylpentane (188 mg) and thioxanthone (19 mg) in benzene (200mL) was irradiated for 15 min with a 450-W Hanovia lamp with a Pyrex glass filter sleeve. The solvent was removed under reduced pressure and the residue was subjected to column chromatography (silica gel, hexane). The major fraction was the title compound yield 117 mg (63%). 

---