ketone in CC1«), and that they have been able to obtain a good fit for the observed variation by means of their calculations. We are grateful to Dr. Kaptein for providing us with a copy of his thesis, which represents a magnificent body of work.(25) Note Added in Proof. Fischer and Lehnig141* have determined the esr parameters of CI2CH• as g = 2.00829 ± 0.00010 and A = 16.79 ± 0.05 G. See H. Fischer and M. Lehnig, Phys. Chem., 75, 3410 1971); this paper reports the work cited in our ref 14a.search, The University of Georgia. The authors gratefully acknowledge illuminating conversations and correspondences with colleagues at the University of Georgia, especially G.

No abstract

The main products of thc photolysis of aromatic azides in matrices of polystyrene, paraffin wax, polyisoprene and some other polymers are primary and secondary amines. The yield of secondary ' A.

The photodecomposition of phenyl azide and or-naphthyl azide can be induced by triplet sensitizers (aromatic ketones) and by singlet sensitizers (aromatic hydrocarbons). From a comparison of transfer rates with sensitizer energies, and from the spectra and the oxidation potentials of the azides, it appears that two different mechanisms may operate concurrently in a i d e sensitization : (1) spin-allowed energy transfer, including transfer to thermally populated non-linear azide ground states, and (2) electron transfer from the ground state of the azide to the excited state of the sensitizer.Mechanism 1 is probably general, mechanism 2 is restricted to aromatic azides.The sensitized decomposition of organic azides is a valuable tool in nitrene chemistry. Triplet sensitization, in particular, opens a route to the direct preparation of triplet nitrenes uncontaminated by the singlet species. This technique has been used to distinguish the reaction products of nitrene singlets and triplet^,^'^ and to establish the spin state of nitrenes generated in photoly~is.~-' Energy transfer to azides also provides a means for estimating their triplet levels, which usually are not accessible by spectroscopy.Lewis and Saunders have investigated the photodecomposition of triarylmethyl azides * and alkyl azides 9 * l o and found that these can be sensitized by donors as low in energy as pyrene (ET = 48 kcal mol-'), i.e., some 20 kcal mol-l below the estimated triplet level of the azide acceptor. This has prompted us to study the photosensitization of aromatic azides. We have measured the efficiency of excitation transfer from both triplet and singlet sensitizers (aromatic ketones and hydrocarbons) to phenyl azide and a-naphthyl azide and we have investigated the effect of sensitizer energy on transfer rates. EXPERIMENTAL MATERIALS Phenyl azide and a-naphthyl azide were supplied by Drs. M. V. Mijovic and H. M.Wagner of this Laboratory and were chromatographed on silica gel before use. The sensitizers were obtained in the purest commercial grade from Eastman Chemicals and from K and K Laboratories, New Jersey. Phenanthrene and anthracene were purified by fractional crystallization followed by room temperature sublimation in a stream of dry nitrogen.Azobenzene supplied by Eastman was used as the authentic sample of this material. 1,l'-Azonaphthalene was prepared from a-naphthylamine by the method of Nietzki and Go1l.l' Both azocompounds were purified by preparative t.1.c. on alumina plates.Eastman Spectrograde benzene was used throughout as solvent. A Z I D E DECOMPOSITIONSolutions of a i d e to M) and sensitizer were exposed to a Hanovia 125 W medium-pressure mercury arc lamp in a merry-go-round apparatus. The 366 nrn mercury line was isolated with a Chance-Pilkington filter 0x1. The quantum flux absorbed by the 1918

The methyl radical attack on H,S has been investigated briefly using azomethane as the radical source. The results indicate that the previous rate-constants for the abstraction reaction are too high and that CF, radicals may react with H,S faster than CH, radicals.Canadian Journal of Chemistry, 47, 689 (1969) The reaction of methyl radicals with hydrogen sulfide has been investigated using the photolysis of azomethane as the source of methyl radicals. Previous workers generated methyl radicals from acetone (1) and acetaldehyde (2, 3) and obtained conflicting results. These earlier data are open to objection on several grounds: in the acetone work (1) the temperature range (50 to 200 "C) included a region (50 to 120 "C) where the Arrhenius relationship for methyl radical attack on acetone is non-linear; no account was taken of this. Furthermore, it was necessary to decompose large amounts of H2S; the assumption that SH radicals reformed H2S is unlikely since hydrogen sulfide was consumed during the reaction, other secondary reactions producing methane could be important. Secondary reactions may also have interfered in the work using acetaidehyde (2, 3) because the aldehyde decomposition is complex in the presence of thiols (4). Although the present work is not conclusive, it indicates that the previous rate-constants are too high.The experimental method was basically as has been described previously (5) although some modification was necessary because ethane and hydrogen sulfide cannot be separated by lowtemperature fractionation. In the bulk of the experiments the ethane content of the C,H6/H2S mixtures was estimated by gas chromatography using an alumina column which retained H2S. Two samples of azomethane were employed; one had been used In prevlous experiments (5) C.P. grade) was distilled through a trap at-80 "C. The purity of the reactants was checked by gas chromatography and mass spectrometry. From 10 experiments between 99 and 171 "C Arrhenius parameters for the reaction were found to be log,, A = 10.7 + 0.4 and E = 2.9 + 0.7 kcal m o l e ' , with log,, Ic = 9.20 at 150 "C. Support for these results is afforded by experiments where ethane was estimated differently; before measurement of the ethane, the C,H6/H2S mixture was treated with KOH pellets after which mass spectrometry indicated the con~plete removal of H2S. Using this technique, log,, k (kin cm3 mole-' s-l) was 8.87 at 147 "C, which can be compared with the corresponding figure of 9.19 by the first method. The agreement is satisfactory considering the rapid rates of reaction and the small amounts of ethane produced. H2S decomposition was limited to between 6 and 10% throughout the work.The present results are compared with previous work and related systems in Table I (all rateconstants are relative to Shepp's rate-constant for methyl radical combination (6)). It is clear that the present rate-constants are lower than previous values by about an order of magnitude at 150 "C. We have already drawn attention to possible sources of error in the ...