The photolysis of methyl disulfide vapor in the pressure range 2-25 Torr a t wavelengths between 2 300 and 2 800 and a t 2 288 A has been examined and the effect of temperature, pressure, added inert gases, ethyl disulfide, and nitric oxide determined.The primary process is a direct production of two CH3S radicals which have excess energy and which can be observed as methyl thionitrite when NO is present during the decomposition. When pure disulfide is photolyzed the inajor observable product is methanethiol, although this material accounts for only a small fraction of the primarily produced thiyl radicals whose principal fate is recombination in a substrate-reforming reaction producing excited disulfide molecules. The latter species are deactivated by added gases, or by the substrate itself. The mode of mercaptan formation is by abstraction of H atoms from the substrate by excited CH3S radicals with a n apparent activation energy of 1.5 kcal.

DINESH GUPTA and ARTHUR R. KNIGHT. Can. J. Chem. 58, 1350Chem. 58, (1980. The liquid phase photolysis of some 35 liquid alkyl disulfide mixtures has been studied. The corresponding unsymmetrical disulfide is the only significant product of the reaction and the system attains a photo-equilibrium state at longer exposure times. Product formation is completely eliminated by the addition of nitric oxide.Quantum yields have been redetermined for the methyl disulfide -ethyl disulfide system using four different light sources. Initial rates have been measured for each of the 35 combinations and have been shown to be a function of total molecular weight of the disulfide mixture and to be strongly influenced by steric factors. These effects are most pronounced for mixtures wherein the exchange reaction between thiyl radical and a symmetrical disulfide is important as indicated by quantum yields above unity. Combinations of isodisulfides give rise to anomalously high rates.DINESH GUPTA et ARTHUR R. KNIGHT. Can. J. Chem. 58, 1350Chem. 58, (1980. On a etudie la photolyse en phase liquide de 35 melanges de disulfures d'alkyles a I'etat liquide. Le disulfure asymetrique correspondant est le seul produit important de la reaction et le systeme atteint un etat d'equilibre photochimique a des temps d'exposition plus longs. L'addition d'oxyde nitrique tlimine completement la formation des produits.On a determine de nouveau les rendements quantiques du systeme disulfure de methyle -disulfure d'ethyle en utilisant quatre sources differentes de lumiere. On a mesure les vitesses initiales pour chacune des 35 combinaisons; on a montre qu'elles dependent du poids moleculaire total du melange de disulfure et que des facteurs steriques les influencent fortement. Ces effets sont plus prononces dans les cas de melanges ou la reaction d'echange entre le radical thiyle et le disulfure symetrique est importante tel qu'indique par les rendements quantiques du systeme deja mentionnk. Les combinaisons d'isodisulfures conduisent a des vitesses anormalement grandes.[Traduit par le journal]

X study has been ~n a d e of the decomposition of methanol vapor by I-Ig O(VJ1) atollis, ut~der static conditions, a t 25' C. A system constr~icted entirely of quartz was c~i~ployed to eliminate the darli rcactiori between methanol vapor and pyrex glass to for111 trimethyl borate. Etiiphasis it1 this investigation was placed ou the determin;itioli of the nature of the primary processes operative, and to this end, the reaction was studied ill some detail in the presence of nitric oxide.For pure methanol, the major primary proclucts were fou~id to be hydrogen and ethylene glycol. The primary quantum yield for hydrogen was 0.46. Millor products of primary origiri were formaldehyde (0.01), ethane (0.005), and dimethyl ether (O.OOB), with the numbers in parentheses showing the primary quantum yielcls.In the presence of nitric oside, thc major products became methyl nitrite, riitro~is oxide, a~i d water. The q u a n t u~n yield of methyl nitrite rose steadily with increasing NO concentrations. For a 1 :I C H 3 0 H : NO inisture, the primary quantunl yield for methyl nitrite was found to be 0.56. The behavior of the reaction in the presence of NO establishes fairly unec~uivocally that the ~iiajor primary process invol\~es 0-1-1 b o~l d scission to form methoxy radicals ancl H atoms. These primary radicals t1ie11 form hydrosy~netliyl radicals by abstraction reactions with the substrate. The ethylene glycol product arises by reconibi~iation of the secondary hydroxymethyl radicals. These conclusions were f~rrther substantiatecl by mass spectrometric alialysis of the products of the decomposition of CDsOFI. It w;ls shown that CD:rOIl is ;l significant product of the reaction. ISTIZODUCTIOKThe reaction of methanol vapor with I-Ig Ci(3PI) atoms has been studied previously (1, 2), aiid in detail by Phibbs and Dar\vent (3) from 25-500'. From the fact that 1-12 (+ = 0.43) and I-IOCI-I2CI-1201-I were the inajor products a t 25', the). concluded (3) quite reasonably that the major primary process was:The observed products could also be explained by the two-step sequence:For both [I] ancl [S] the [-I atoms for~ned are nssunied to attack the methyl group of the substrate:I'hibbs and Darwe~it rejectecl [2] as a n important PI-inlary process since they found that CI-IBOD substrate produced little IID. I-Iowever, the results of a number of inore recent studies have indicated that the methanol reaction should be re-exainined. Thus Tal

Articles you may be interested inThe spectroscopy and single vibronic level fluorescence quantum yields of jet cooled transstilbene and its van der Waals complexes J. Chem. Phys. 78, 5493 (1983); 10.1063/1.445477Spectroscopic studies on naphthalene in the vapor phase. II. Fluorescence quantum yields from single vibronic levels in the first excited singlet state-behavior of higher excited singlet statesThe quantum yields of fluorescence from the second excited singlet state of thiophosgene vapor have been measured as a function of pressure for eight S,<-So transitions, and parameters for the decay of the levels pumped have been calculated. The results indicate that reassignment of the origin of the S,<-So band system to 35112 from 36007 cm-1 is necessary. Quantum yields of fluorescence from the lowest levels in S, are extremely high (0.50 to 1.0) and are indicative of a very slow radiationless decay rate. Weakening of fluorescence from higher S, vibrational levels is most likely caused by predissociation.

The photolysis of ethanethiol vapor in the pressure range 10 to 350 Torr and the effect of % decomposition, wavelength, and of added inert gas and ethylene on the rate of formation of the products has been determined. In addition, the effect of temperature on the photolysis of methanethiol-ethylene -inert gas and ethanethiol -ethylene -inert gas systems has been studied.For the pure substrate, the primary process is S-H bond cleavage and the hydrogen atom produced abstracts the sulfyhydryl hydrogen of the substrate. The quantum yield of molecular hydrogen formation is unity, independent of pressure. C,H5S radicals, formed either in the primary process or as a result of abstractive attack by H atoms or radicals on the substrate, combine to form an excited disulfide molecule.The major condensable product, C,H5SSC,H,, arises via collisional deactivation of this excited species which can also sensitize the decomposition of the substrate to give C2H4, C2H6, and H,S, the other products observed. With added inert gas the rates of formation of the mlnor products decrease whereas hydrogen and disulfide remain unchanged.With added ethylene, the major products are H,, C2H5SSC2F15, and C2H,SCZH,, the addition product. Diethyl sulfide is forrned in a chain reaction by the attack of C2H5S on the olefinic double bond to form the con~posite radical, followed by H abstraction from the substrate. The hydrogen rate decreases due to the addition of H atoms to ethylene. Thc rate constant ratio, kzzlk4, for the reactions [22] and [4], respectively,is pressure dependent, indicative of the formation of hot hydrogen atoms in the primary process of the photolysis of methanethiol and ethanethiol. The Arrhenius parameters A,Z/A4 and E,, -E, for the methanethiol-ethylene and ethanetl~iol-ethylene systems have been determined. Kinetic treatments of the proposed mechanisms have been undertaken.