"SELECTIVE HYDROGEN ATOM ABSTRACTION BY HYDROGEN ATOMS IN PHOTOLYSIS AND RADIOLYSIS OF ALKANE MIXTURES AT 77° K" TfftMio Miyuaki. Kuuhira Kinugaws. MMK> Eguchi and Sclma M. L. Gwdas PUBLICAÇÃO IEA 462 JANEIRO/1977 COUR-ARO 70 PlIBL. IEA 462 JANEIRO/1977 COUR ARQ 70 "SELECTIVE HYDROGEN ATOM ABSTRACTION BY HYDROGEN ATOMS IN PHOTOLYSIS AND RADIOLYSIS OF ALKANE MIXTURES AT 77° K" Tetsuo Miyazaki, Kazuhisa Kinugawa, Masao Eguchi and Selma M. L Guedes CENTRO DE OPEPAÇÃO E UTILIZAÇÃO DO REATOR DE PESQUISA- IEAR-1 (Área de Radioquímica) INSTITUTO DE ENERGIA ATÔMICA SÂO PAULO- BRASIL APROVADO PARA PUBLICAÇÃO EM JULHO/1976 CONSELHO DELIBERATIVO Eng? Hélcio Modttto da Cona Eng? Ivano Humbtrt Marcnati Prof. Admar Cttvdlini Prof. Sérgio Ma«cartnha* dt Olivaira Dr. Klaus R«inach Or. Robano D'Utra Vaz SUPERINTENDENTE Prof. Or. Rômulo Ribairo Piaroni INSTITUTO DE ENERGIA ATÔMICA Caixa Poital 11.049 (Pinheiros) Cidade Universitária "Armando dt Salltt Olívaíra" SAO PAULO - BRASIL NOTA: Ette trabalho foi confgrido p«1o autor dípou d< compoíto • <ut redação «tá conforme o original, «m qualquer corrução 0..1 mudança. "SELECTIVE HYDROGEN ATOM ABSTRACTION BY HYDROGEN ATOMS IN PHOTOLYSIS AND RADIOLYSIS OF ALKANE MIXTURES AT 77°K" Tetsuo Miyazafci* Kajuhisa Ktnugawa". Masao Eguchi* * and Stlma M. L Guedes* * ABSTRACT Selective hyd-ogen atom abstraction reaction by H atonti **wch er« p-oduced at 77 K try radtotv*t* of etkene 1- photolysis of hydrogen het-de» has been founo in sobutane. 2 2.3,3 tetramethyibutane(TMB>, cyclopropane matrices betides neopemane ma'nx The selective hyd'ogen atom abstraction "«acton «n neopentane isobutane mixture is affected by the difference of k netic ene-g es of H atoms The 'eact.on accu s more favorably with decreasing the kinetic energy of H atoms Competitive reaction be-ween c-C^H,,, and Hi for H atoms has been studied in the radiolyiis and photolyss of neoC.Hi; H! m<*tu>e at 77 K The 'ate constants of these reactions in neopentane mat/ix are quite di'te'ent from those of the-mal H atom eacton, but vm lar to those of hot H atom reaction Importance of the selecr-ve hyd'ogen atom 'bseacticn reaction by H atoms <s pointed out in the radical formation tn the radtolysis of purc TMB at 77 K INTRODUCTION We can easily investigate solid state reactions at 77 K by irradiation with ultraviolet light or 7-rays Recently quite interesting phenomena have been found in the radtolysis of neopentane-alkane mixture in the solid phase at 77 K'14' When neopentane containing a small amount of alkane is 7 irradiated at 77 K, alkyi radicals from solute alkanes rather than neopentyl radical are selectively formed Since the formation of the solute alky I radical is not affected by the addition of an electron scavenger or a ho'e scavenger, it was concluded that the alkyi radical :s formed by the nonionic process' 10V The following observations obtained previously showed that the solute alkyl radical is not formed by the excitation transfer from the irradiated neopentane to the solute, but by the selective hydrogen atom abstraction from the solute by H atoms1131 (1) The mechanism of excitation transfer expects the formation of D; in the radio lysis of neo-C.H, 2 >C*H9D at 77 K Though HD is 3 formed remarkably «n this system, D; is not formed at all" ' I2> When neo-CsH| j-c-CjH, 2 U mol%) mixture is irradiated at 77 K by 7 rays, cyclohexyl radicals are mainly formed. When propyler.e is added to the mixture, the formation of the cyclohexyl radicals 1$ suppressed, while C3H7 radicals are produced complementarity by the addition of H atoms to propylene191 (3) H atoms, which are produced by the photolysis of hydrogen iodide in neopentane contam-ng a small amount of alkane, react selectively with the solute alkane at 77 K(1327) The selective hydrogen atom abstraction reaction by H atoms has proposed new problems in a reaction kinetics in the sol'd phase at 77 K When H atoms are hot, we must assume that they migrate a long distance at 77 K through the neopentane crystal without losing their kinetic energies When On leave of absence f-am the Nagoya Un.vernty Japan Department of Synthetic Chemistry, Faculty of Eng<necir>g. Nagoya University Crnku>a-ku, Nagoya 464, Japan 'Centro de Operação e UM lacão do Reator de PesquiW - IEAR 1, Area de Radioqumica, Instituto !e Energia Atômica, São Paulo. SP H atoms are thermal, we must assume that the activation energy for hydrogen atom abstraction is nearly zero in the neopentane matrix at 77 K, though the value amounts to 7 — 10 Kcal/mol in the gas phase*8 25i The research described herein was undertaken »n order to obtain further informations about the nature of the react've H atoms The previous studies of the selective hydrogen atom abstraction have been limited only to neopentane matrix and to the reaction of H atoms In the presert paper the study will be extended to other alkane matrices, such as isobutane. 2 2.3,3, tetramethy I butane (TMB) and cyclopropane, and to the reaction of H and D atoms. The formation of 'adicats >n the radiolysu of pure TMB and TMB alkane mixture is discussed here in terms of the selective hydrogen atom abstraction by H atoms EXPERIMENTAL Neopentane, isobutane, and propane were more than 99.9 mol%. Cyclopropane and ethane were more than 99.0 and 99.7% respectively. Spectrograde cyclohexane was passed through a l-m column packed with silica gel and then distilled on a vacuum line before use. 2,2,3,3,-Tetramethylbutane (TMB) was of a high purity and purified by sublimation on a vacuum line before use To prepare hydrogen halide (HX), such as HI, HBr. DBr. and HCI, the aqueous solution of the hydrogen halide was passed through P2O<; and subjected to several trap to-trap sublimation on the vacuum line UV illumination was provided by Toshiba medium-pressure or low pressure mercury lamps. 7 Irradiation was done by Co 60 at a dose rate of 4 2 x 101 * eV/g hr The free radicals were measured at 77 K with a JES-3BX esr spectrometer RESULTS Table I shows the yields of radical by the 'eaction of hydrogen (or deuterium) atom in the radiolysis or photolysis of alkane mixtures in the sol'd phase at 77 K t-C«Hv radical is mainly formed in the photolysis of neo-CiH, ? for TMB) i C4Hl0-hydrogen halide(HX), while a small amount of iC«H9 radical is also produced. Since the ratio of amounts of iC4H9 radicals to those of t-C4H9 radicals increases with increasing the illumination time of ultraviolet light, iC4H9 radical is formed by the 71 isomenzation of t-C4H3 radical which absorbs ultraviolet light' . The amounts of C«H, radicals represent a sum of tC4Hy and iC4H, radicals, and are measured by double integration of the first-Henvative esr signal of the radicals Since only C4H9 radicals are formed selectively in the photolysis of neoC«,H,, (or TMB) i-C,H,0 HX, overlapping of the spectra of the solute radical and the neopentyl radical does not occur. neoC5H,,, C3H7, i-C4H9, and CjH, radicals do not isomerize in the illumination of ultraviolet light Therefore the relative yields of these radicals are obtained from the height of peaks in the esr spectrum The yield of radical in the photolysis of HI is much higher than that in the photolysis of HBr. DBt, or HCI This is because the absorbance of the ultraviolet light by HI is stronger than that by other hydrogen halide The ratio of the yield of C4H9 radical in the photolysis of neo-CjH, ,i-C4Hi0-HX to that of neo-C,H,, radical in the photolysis of neo-CsH, j-HX is plotted in figure 1 against the kinetic energy of H atom. Plots, indicated by a, b, and c, represent the reaction: of H atoms formed by the photolysis of HCI, HBr, and Ml respectively The photolysis was done with light of 253 7 nm from low pressure mercury lamp. The 253.7 nm radiation corresponds to 112.6 Kcal/mol, while the bond strengths of HCI, HBr, and HI are 103.1, 87.6, 71 3 Kcal/mol respectively To conserve momentum essentially all of the energy from the primary process in excess of that used in bond rupfjre must appear as translational unergy of the H atom, if halogen atoms produced by photolysis are in ground states. When HI if. decomposed by 253 7 nm radiation, both a ground state I atom and an excited I atom are produced1121. Therefore the energies of H atom are 41 or 20 Kcal/mol. Only the maximum energy of the H atom is shown in figure 1. d,e, and f represent the reactions due to H atoms produced by the photolysis of HI witi a medium-pressure mercury lamp through Toshiba ultraviolet filters, such as UV31, UV29, and UV27 filters, respectively. When UV31 filter is used, the photolysis is caused by the light longer than 305 nm. The yields of radicals in the photolysis with UV31 filter are much higher than those in the photolysis with glass f Iter which cuts off the light shorter than 330 nm. Therefore the photolysis with UV31 filter is caused mainly by the light from 330 nm to 305 ran. It is determined by the similar fashion that the photolysis with UV29 or UV27 filters is caused mainly by the light from 330 to 280 nm, or the light from 2S0 to 255 nm, respectively.