Methane from photolysis

We note, however, that even if methane comes from the photolysis of the methyl ester, this does not necessarily mean that all of it arises as a result of first breaking down the PET itself by photolysis. Both the PET of early reports and also the PECT were undoubtedly produced using dimethyl terephthalate (DMT) instead of terephthalic acid and so have methyl ester end groups owing to the well-known incomplete reaction of the DMT. There may be only a small level of these ester groups, but the amount of methane produced was small as well. This, at the very least, causes some potential confusion about the source of all the methane should some of it come from methyl ester photolysis. No such confusion would exist for the ethane should that be coming from photolysis of an ethyl ester. 

Compound 1 (500 mg, 1.39 mmol from photolysis of stilbene and 2-methylmer-capto-benzothiazole) was sealed under vacuum in a thick-walled Pyrex tube (the volume was about 100 mL) and heated to 200 °C for 20 h in the dark. Methan-ethiol (from the [2,l,3]-elimination) and dimethyldisulfide were condensed to a cold trap (-196 °C) at a vacuum line (65 mg, 97%). The residue was separated by preparative TLC on 200 g SiC>2 with benzene to give 340 mg (78%) of 2 (mp 108 °C, methanol) and 96 mg (22%) of 3. 

Trifluoronitrosomethane may be obtained from photolysis of a mixture of trifluoroiodo-methane and nitric oxide [275, 276] or from pyrolysis of trifluoroacetyl nitrite [277, 278] (Figure 8.108). 

The photolysis of dimethyl ether (gas phase) and diethyl ether (liquid phase) was first undertaken by Berthelot and Gaudechon (21c). The main products detected were hydrocarbons (methane from dimethyl ether, ethane from diethyl ether) with some CO and H2. Ethers were later shown also to form dehydrodimers (41). ... 

Methyl-1-phenylcyclopropane (1) was obtained in 33% yield when phenylcyclopropane in tetrahydrofuran was reacted with trimethylsilylmethylpotassium and then with iodo-methane. Photon-induced hydrogen abstraction promoted formation of 7b-phenyl-la,7b-di-hydro-l/f-cyclopropa[a]naphthalen-3(2/f)-one (2) from photolysis of /ra v-l-phenyl-2-(2-oxo-... 

Unlike water, there is no cold trap for methane or H2 on an earthlike planet. Rather, methane and H2 diffuse to the top of the atmosphere and disassociate from photolysis. The H then escapes to space. The remaining carbon makes it back to the surface where it eventually reacts to form more methane. The hydrogen to do this ultimately comes from water. The net reaction is equivalent to disassociating water and having the hydrogen escape to space. 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

Photolysis of Cp2TiAr2 in benzene solution yields titanocene and a variety of aryl products derived both intra- and intermolecularly (293—297). Dimethyl titan ocene photolyzed in hydrocarbons yields methane, but the hydrogen is derived from the other methyl group and from the cyclopentadienyl rings, as demonstrated by deuteration. Photolysis in the presence of diphenylacetylene yields the dimeric titanocycle (28) and a titanomethylation product [65090-11-1]. 

With the addition of CO caused by photochemical oxidation of methane, a significant flux enters the atmosphere annually, but the principal global contributions are terrestrial, anthropogenic and from atmospheric photolysis of methane. 

Hixson<27) prepared the deuterio compound (31) and found that photolysis produced only compound (32) resulting from a di-n--methane rearrangement and not from a hydrogen migration ... 

Upon low conversion direct photolysis the cis isomer (10) gave only the cis isomer (12) and the trans isomer (11) gave only the trans isomer (13). The triplet sensitized reaction of (10) and (11) gave rise only to cis-trans isomerization. Thus the di-ir-methane photorearrangement from the triplet state cannot compete with triplet state deactivation via cis-trans isomerization (Zimmerman has termed this the free rotor effect). Several other examples of regio-specilicity and stereospecificity in di-w-methane photoreactions are as followsa8 a3) ... 

As can be seen from Table 3.1, the Titanian atmosphere contains a relatively large amount of ethane. Laboratory results show that methyl radicals (H3C), which are primary products of methane photolysis, may be present in the upper reaches of the atmosphere ... 

Another extremely reactive form of titanocene, namely black tita-nocene, was discovered by Rausch and Alt in 1974 as the product of the photolysis of Cp2TiMe2 (19) in either aliphatic or aromatic solvents (41). Irradiation of a hexane solution of 19 resulted in the deposition of a dark precipitate with concomitant evolution of essentially only methane. Benzene solutions of this photochemically generated titanocene reacted rapidly with CO to give red solutions from which Cp2Ti(CO)2 (1) could be isolated in 60% yield. Similarly, 1 could be prepared directly under photochemical conditions in similar yield if 19 was irradiated in a CO atmosphere (41,42). 

R.L. Sweany, University of New Orleans I was surprised at seeing your report of a 2D Cu atom being able to abstract a hydrogen atom from methane, but, of course, the copper atom is "hot". I wonder if you see methyl take back its hydrogen atom after photolysis or does the radical pair collapse to give HCuCH3 ... 

A material balance was observed that is consistent with the proposed mechanism within the limits of experimental error. The methane/propane ratio increases from 0.06 at 1 54 torr to 0.11 at 0.54 torr. Considerable uncertainty (approx. 50%) must be attached to these ratios, but the trend is consistent with the higher yield of methane observed by Thrush91 at pressure below 0.1 torr. Fischer and Mains92 question the occurrence of reaction (6) as they could not detect any n-pentane in their reaction products. At the high ethyl radical concentrations obtained in flash photolysis this product would certainly be expected, if a significant concentration of thermal ethyl radicals were present. However, Thrush was unable to detect ethyl radicals spectroscopically under his experimental conditions. Therefore all reactions of ethyl in his system must involve C2H and the extent to which... 

Laser flash photolysis of 46 showed results similar to those obtained for 45. The lifetimes and yields of Z and E photoenols from 46 are comparable to those obtained for 56. Similarly, laser flash photolysis of 47 reveals that the major reactivity pattern of 47 is intramolecular H-atom abstraction to form Z-58 and E-58 even though no products were observed that can be attributed to the formation of photoenol 58. Laser flash photolysis of 47 in methanol showed formation of biradical 57 ( max 330 nm, r = 22ns), which was efficiently quenched with oxygen (Scheme 32). Biradical 57 intersystem crosses to form Z-58 and E-58, which have maximum absorption at 400 nm. Enols Z-58 to E-58 were formed in the approximate ratio of 1 4. Enol Z-58 had a lifetime of 6.5)0,s in methanol, but its lifetime in dichloro-methane was only 110 ns. The measured lifetime of E-58 in methanol was 162)0,s, while it was 44 ms in 2-propanol. Thus, E-58 is considerably shorter-lived than E-56. Furthermore, E-58 is also shorter-lived than the analogous E-59 (Scheme 33), which cannot decay by intramolecular lactonization and has a lifetime of 3.6 ms in methanol. Thus, we proposed that E-58 undergoes solvent-assisted reketonization that is facilitated by the intramolecular H-atom bonding, as shown in Scheme 34. 

Figure 6.1. The Jovian moon lo deep ultraviolet (UV) photolysis of its methane atmosphere proceeds with electron ejection, generating the molecular ion of methane (see color insert). NASA JPL Galileo program image from Voyager 1, http //www.jpl.nasa.gov/galileo/io/...

The fate of the free acyl radical 68 and radical 74 is not known. Most probably it is a constituent of polymer deposits on the wall of the irradiation vessel which hitherto have not been identified more definitely.29 Moreover, the identification of methane and carbon monoxide among the gaseous products of the photolysis of 4-methylphenyl acetate (55) provides evidence for the existence of the acetyl fragment. This intermediate is expected to decarbonylate to give carbon monoxide and a methyl radical, which in turn abstracts hydrogen from the solvent.34... 

---