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Kinetics of methylene addition tocis- andtrans-but-2-ene. Further evidence for the energy separation between triplet and singlet methylene

 

作者: Henry M. Frey,  

 

期刊: Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases  (RSC Available online 1977)
卷期: Volume 73, issue 1  

页码: 164-170

 

ISSN:0300-9599

 

年代: 1977

 

DOI:10.1039/F19777300164

 

出版商: RSC

 

数据来源: RSC

 

摘要:

Kinetics of Methylene Addition to cis- and trans-But-2-eneFurther Evidence for the Energy Separation between Triplet and Singlet MtlthyleneBY HENRY M. FREY* AND GORDON J. KENNEDYChemistry Department, The University of Reading, Whiteknights,Reading, Berkshire, RG6 2ADReceived 3rd June, 1976The reactions of triplet and singlet methylene with cis- and trans-but-2-ene have been studiedover the temperature range 350-473 K. The results yield a value of (36.5& 3.2) kJ mol-' for theenergy separation between singlet and triplet methylene, and provide further confirmation of theassumption that singlet methylene reactions with hydrocarbons proceed with activation energiesclose to zero. Previous evidence for the similar reactivities of triplet methylene and the methylradical receives additional support.Recent theoretical calculations of the energy separation between the triplet (3B1)and the singlet (lA,) states of methylene lead to a value of (4658) kJ 11101-l.Areinterpretation of earlier experimental data yielded a lower limit of 34 kJ mol-Ifor this separation, in sharp contrast to the much smaller values of 4-10 kJ 11101-1derived earlier.3 In a preliminary report of this work a value of 38 kJ mol-' wasobtained. The additional studies reported here provide further confirmation of thisvalue.EXPERIMENTALMATERIALSKeten was prepared by the pyrolysis of acetic anhydride and purified by several trap-to-trap distillations from -130 to -160°C. It was stored as a gas at pressures below100 Torr in blackened glass vessels.It was degassed before each run at - 196°C.Nitrogen (B.O.C. oxygen free grade) was used without purification.Cis-but-Zene and trans-but-Zene (Matheson Instrument Grade) were better than 99 %pure and were used without further purification.APPARATUSA conventional high vacuum line was used for all gas handling. Teflon-glass stopcockswere used throughout to minimise absorption problems.All photolyses were carried out in a Pyrex reaction cell using the unfiltered output of anOsram HBO 200 W super pressure mercury lamp. The cell was thermostatted in analuminium furnace. Temperature variations over the cell surface were less than +_ 1°C fromthe mean temperature of the cell. Temperature measurements were made using chromel-alumel thermocouples.ANALYSISAll quantitative analyses were by gas chromatography, using a Perkin Elmer 452 gaschromatograph with a flame ionisation detector.A 100m PPG capillary column at 0°Cwas used. The retention times of all the products of interest were determined from puresamples. The detector response to the products, all C5 hydrocarbons, was assumed to beequal. The product ratios were determined by peak height measurements, corrected forretention times.16H . M. FREY AND G . J . KENNEDY 165PROCEDUREPhotolysis mixtures consisted of keten, cis- or trans-but-2-ene and nitrogen in the ratio1 : 2 : 30. Total pressures were in the range 600-700 Torr. A few experiments were carriedout using a ratio 1 : 2 : 200. The mixtures were photolysed and the non-condensable gasesremoved at - 196°C.Most runs yielded sufficient products for two analyses ; for all runsreported, the product ratios for duplicate analyses were reproducible to better than _f5 %.The product ratios were reproducible to & 10 % from separate runs under identical conditions.RESULTSThe following reaction scheme represents the important processes occurring :A vCH2CO --+ CH2 + CO3CH, + M + lCH2 + M3CH2 + cis-but-2-ene -+ cis- + trans-dimethyl cyclopropane(CB2) (DMCP)'CHz + CB2 + cis-DMCPlCH2 + CB2 -+ 2-methylbut-2-ene (2MB2)lCH2 + CB2 --+ cis-pent-2-ene (CP2)3CH2 + CB2 -+ CH3- + (butenyl).3CH2 + trans-but-2-ene (TB2) --+ cis- + trans-DMCP' CHz + TB2 --+ trans-DMCPlCHz +TB2 -+ 2MB2'CH, + TB2 -+ trans-pent-2-ene (TP2)3CH2 +TB2 -+ CH3- + (butenyl).CH3= + (butenyl).--+ 3-methylbut-1-ene (3MBl)CH3- + (butenyl). + CP2CH3- + (butenyl). -+ TP2CH3* + CH3- -+ ethane2(butenyl)* -+ C8 hydrocarbons.On the assumption, discussed below, that reactions (2) and ( - 2 ) maintain anequilibrium ratio of singlet to triplet methylene in the presence of a thirty-fold excessQf inert gas the following relations may be derived :log[(cis-DMCP/trarzs-DMCP) - 0.301 = log(O.43~f~~/A~~) -(AEZ - E3,)/2.303 RT (1)(AE2 -E3,)/2.303 RT (2)andlog[(traizs-DMCP/ciDMCP) - 3.401 = log( 1 .45A4b/A 3b) -where AE, is the energy separation between the two spin states of methylene, E3aand EBb are the energies of activation of reactions (3a) and (3b) respectively and A3a,A3b, A4a and A4, are the Arrhenius pre-exponential factors for reactions (3a), (3b),(4a) and (4b) respectively. In the derivation of relations (1) and (2) the same valuefor the ratio (trans-DMCPlcis-DMCP) has been assumed in reactions (3a) and (3b).The value (3.40) for this isomer ratio has been taken from Montague's study of th166 KINETICS OF METHYLENE ADDITIONmercury photosensitised isomerisation of 3-methylb~t-l-ene.~ Thus the 1.h.s.in eqn(1) and (2) are corrected for the fraction of stereospecific product arising from thetriplet reactions (3a) and (3b) respectively. It has also been assumed that the singletaddition reactions (4a) and (4b) proceed with zero activation energies. Plots of theleft hand sides of eqn (1) and (2) against 1/T are shown in fig.1.103 KITFIG. 1 .-Ratio of geometric isomers of 1,2-dimethyl-cyclopropane as a function of temperature.0, Using CB2 (1.h. axis): A, using TB2 (r.h. axis).Accepting the value of 1.39 for the ratios(TP2/CP2) produced by reactions (9)and (10) the contribution from triplet reaction to the product CP2 in the CB2 systemand to the product TP2 in the TB2 system may be separated from the singlet contri-bution to these products arising from reactions (6a) and (6b) : the two contributions tothese products will be designated s- and t-CP2 and TP2. The triplet contribution tocis-DMCP (CB2 system) and to trans-DMCP (TB2 system) may be separated fromthe yields of these products arising from the singlet reactions (4a) and (4b) : the twocontributions to the total yield of these products may be similarly distinguished.According to the reaction scheme above 2MB2 arises only from the singlet reactions(5a) and (5b).The following relations may then be derived for the CB2 system:~o~(s-CP~/S-C~S-DMCP) = log(A Ga/A4J - (E6a - E,,)/2.303 RT (3)log(2MB2/s-cis-DMCP) = - (Esa- E4a)/2.303 RT. (4)Analogous relations may be derived for the TB2 system.Representative data for the temperature dependence of the ratios 2MB2/s-cis-DMCP and s-CP2/s-cis-DMCP, (CB2 system) and 2MB2ls-trans-DMCP and s-TP2/s-trans-DMCP, (TB2 system) are shown in table 1. The data indicate no systematicvariation of these product ratios with temperature.103 KIT2.302.412.472.522.572.632.742.86TABLE 1.-PRODUCT RATIOS AS A FUNCTION OF TEMPERATURE2MBZ/s-cis-DMCP s-CP2/s-cis-DMCP ~MBZ~S-~~U~S-DMCP S-TPZ~S-~~U~S-DMCP0.380.330.340.300.320.310.3 10.300.740.700.780.710.720.700.690.270.200.230.180.880.840.830.8H .M. FREY A N D G . J . KENNEDY 167Adding t-cis-DMCP to the measured trans-DMCP in the CB2 system gives thetotal yield of reaction (3a); the yield of reaction (3b) in the TB2 system may besimilarly determined. If is is assumed that the activation energies of the radical-radical reactions (8)-(12) are close to zero the following relation may be derived forthe CB2 system :An analogous relation may be derived for the TB2 system. Plots of the left handsides of these equations are shown in fig. 2.log 3MB1 /(t-cis-DMCP+ trans-DMCP) = log(A,,/A,,) - (E,,-E3,)/2.303 RT.( 5 )L103 KITFIG. 2.-Ratio of 3-methylbut-1-ene to t-1,2-dimethylcyclopropane as a function of temperature.In order to evaluate AE, from the values of (AEz - E3,) and (A& - E3J given byeqn (1) and (2) we have previously assumed that E3, and E3b are equal to theexperimentally determined activation energies for methyl radical addition to CB2and TB2 respectively. If the additional assumption is made that A3, and A3b areequal to the A factors for methyl radical addition to the two olefins the evaluation ofthe ratios (A4JA3,) and (A4JASb) on the basis of eqn (1) and (2) permits an evaluationof A4, and A4b as a fraction of the collisional rate.,,The values of the parameters derived from the above analysis are summarised intable 2.0, Using CB2 : A, using TB2.TABLE 2.-ACTIVATION ENERGY DIFFERENCES AND A-FACTOR RATIOSA-factor ratios activation energy differences/kJ mol-1AE,-E3, = 5.3k0.2 ' 9 bA E z - E3b = 3.450.5a AE2 is the energy separation between the two spin states ; if Esa and E3b are assumed to be(30.6 f 2.0) kJ mol-I and (33.8 k 2.0) kJ mol-l respectively, then values of (35.9 k 2.1) kJ mol-I and(37.252.5) kJ mol-l for AE2 result.The mean value of AE2 from the two systems is thus (36.523.2) kJ mol-1 (see text) ; C if values of dm3 mol-1 s-I are assumedfor AJa and A3b respectively, values of 5 . 6 ~ for the ratios of A4a and A4brespectively to the collisional rate result.dm3 mol-I s-' andand 6.8 168 KINETICS OF METHYLENE ADDITIONDISCUSSIONIt is well known that the photolysis of keten (1) at wavelengths shorter than366nm produces methylene in both triplet and singlet states, the latter spin stateiiicseasingly predominating at shorter wavelengths.We assume, on the basis ofC a d s work,l0 that in the presence of a thirtyfold excess of nitrogen the non-equilibrium ratio of the spin states produced by (1) is brought to equilibrium by theinter-system crossing reaction (2) and (-2) and, thus, that the subsequent reactions(3)-(12) involve equilibrium populations of the two spin states. Runs carried out inthe presence of a hundred-fold excess of nitrogen gave product distributions identicalto those reported using a thirtyfold excess, confirming this assumption.Though the products of reactions (3a)-(6a) and (3b)-(6b) are chemically activatedupon formation, there is ample evidence from other work that the half-pressuresfor stabilisation of the hot molecules are 70 Torr or less.Thus at the pressures usedin this work (600-700 Torr), chemically activated decoinpositioiis and isomerisationsof the products of reactions (3n)-(6a) and (3b)-(6b) will not occur to a significant extent.has shown that the isomer ratio (trans-DMCPlcis-DMCP) arisingfrom the ring closure of the biradical initially formed in reactions (3a) and (3b) ispressure-dependent. If it is assumed that the triplet biradical is in conformationalequilibrium at each energy level during collisional deactivation, then the pressuredependence of the isomer ratio will reflect the pressure dependence of the meanenergy level from which the final deactivating collision occurs.On the basis of thisinterpretation the initial excess energy of the biradical is unimportant and the tempera-ture dependence of the ratio is unlikely to be strong. We have thus used the value3.4 for the product ratio (trans-DMCPlcis-DMCP) arising from reactions (3a) and(3b), as determined by Montague for the pressure range of this study.Since it is known that the activation energies for hydrogen abstraction from cis-and trans-but-2-ene by methyl radicals are similar to the activation energies foraddition, on the basis of the similar reactivities of triplet methylene and the methylradical discussed below, reactions (7a) and (7b) are expected to occur at ratescomparable to those of reactions (3a) and (3b) respectively. The presence of ethanein the product mixture is evidence for methyl radical participation and, as notedabove, it is unlikely to be produced in significant amounts by decomposition of thebiradical intermediate of reactions (3a) and (3b) in the pressure range of this study.The addition of 10 % of oxygen to the reactant mixture suppresses completely theproduction of ethane and 3MBl in both the CB2 and the TB2 systems, TP2 in theCB2 system and CP2 in the TB2 system.Since oxygen is known to suppress thereactions of triplet methylene very efficiently this further confirms that the abovescheme represents all the significant reactions occurring in the two systems.We thus derive values of (36+2) and (3712) kJ mol-1 from the CB2 and TB2systems respectively for the energy separation between singlet and triplet methylene.These values are in good agreement both with the results of recent theoreticalcalculations and with the experimental results of Hase et a2.,l2 Simons and Curry l3and Lahmani.14 The key difference between our interpretation of these systems andthe earlier interpretations which led to very much lower values for the energyseparation is our assumption that collisional reactivation of the triplet to the singlet,reaction (2), is significant.Thus, in the presence of a sufficient excess of inert gas,the reactions of an equilibrium population ratio of the two spin states can be studied.That earlier work reveals the presence of insertion products in the presence ofan excess of inert gas sufficient lo, l 6 to deactivate the singlet quantitatively to thetriplet is evidence for the occurrence of reaction (2).That a substantial change inMontaguH . M. FREY ,4ND G . 3 . KENNEDY 169the inert gas/substrate ratio produces no change in the product ratios, as reportedabove, is evidence that an equilibrium ratio of the two spin states is being studicd.The major assumption made in the derivation of the above values is the use ofArrhenius parameters for triplet methylene derived from analogous reactions of themethyl radical. The evidence for the validity of this assumption is less direct.However, on the basis of this assumption we have derived values of the r2iiosA4,/collision rate and A,,/collision rate which are in good agreement with the vaiueof 5 x for the irrsertion of singletmethylene into methane.The values derived for the activation energy differecee(E3,-&J viz., (2.0-)0.7) kJ mol-', for addition to the two olefins and forthedifferences(E7a-E3u) and (E,h-E3b) viz., 5 and 10 kJ mol-1 respectively are of comparablemagnitude to the relative activation energies determined * for the analogous methylradical reactions. The magnitudes of these assumed activation energies for thetriplet are also in general agreement with the results of Carr's BEBOFinally, Lahmani's result of (3 1.4f 3.0) kJ mol-1 for the energy separation is derivedfrom an experimentally determined rate ratio of ( 5 5 2 x 10") for the followingreactionsderived from flash photolysis studies'CHz+C3Hs -+ C4Hio3CH2 + C3Hs + CHS*C3H7*.Precise agreement with our derived value for the energy separation would requirea value of 1.9 x lo5 for this rate ratio.Rowland et aL1* derive a value > lo7 forthis ratio, which is close to the value derived using the experimental Arrheniusparameters for the analogous reaction of the methyl radical with propane.lg A valueof lo7 in Lahmani's calculation yields a value for the energy separation of 44.5 kJmol-I. Thus, pmding a direct determination of the Arrhenius parameters for thetriplet reaction, our present assumption is vindicated by its consistency with theresults of these other studies.We have derived values for the relative rates of the three possible singlet reactionswith the two olefins.Our results indicate no temperature dependence of theserelative rates within the limits of the experimental accuracy. Further, the ratio ofvinylic to methyl C-H bond insertion is close to 0.33 in both olefins, as anticipatedif' insertion proceeds with equal probability in all C-H bonds. Taken in conjunctionwith the agreement of the singlet addition rates with the absolute value of Braun,Bass and Pilling I 7 for methane, these results support the commonly held assumptionthat singlet methylene reactions with hydrocarbons procecd with activation energiesclose to zero.J. F. Harrison, Accounts Chem. Rex, 1974, 7, 378.H. M. Frey, J.C.S. Chem. Comm., 1972, 1024.R. W. Carr, T. W. Eder and M. G. Topor, J. Chem. Plzys., 1970, 53, 4716.H. M. Frey and G. J . Kennedy, J.C.S. Chcin. Comm., 1975, 233.A. D. Jenkins, J. Chem. SOC., 1952, 2563.D. C. Montague, hit. J. Chem. Kinetics, 1973, 5, 513.R. J. Cvetanovic and R. S. Irwin, J. Chem. Phys., 1967, 46, 1694; N. Yokoyania and R. K.Brinton, Canad. J. Chenr., 1969, 47, 2987.P. M. Kelley and W. L. Hase, Chem. Phys. Letters, 1975, 35, 57.H. M. Frey, Proc. Roy. Soc. A , 1959, 251, 575 ; D. C . Montague and F. S. Rowland, J.C.S.Chem. Comm., 1972, 193.' D. C. Montague, J.C.S. Chem. Comm., 1972, 615.l o T. W. Eder and R. W. Carr, J. Chem. Phys., 1970, 53,2258.l 2 W. L. Hase, R. J. Phillips and J. W. Simons, Chem. Phys. Letters, 1971, 12, 161.l 3 J . W. Simons and R. Curry, Chem. Phys. Letters, 1976, 38, 171.l4 F. Lahmani, J. Php. Chem., in press170 KINETICS OF METHYLENE ADDITIONl 5 S . H. Ho and W. A. Noyes, f. Amer. Chern. Soc., 1967, 89, 5091 ; D. F. Ring and B. S.l6 W. Braun, A. M. Bass and M. Pilling, J. Chem. Phys., 1970, 52, 5131.l 7 R. W. Carr, J. Phys. Chem., 1972, 76, 1581.l 8 F. S. Rowland, P. S. T. Lee, D. C . Montague and R. L. Russell, Faraday Disc. Chem. SOC.,l9 W. M. Jackson, J. R. McNesby and B. de B. Derwent, J. Chem. Phys., 1962, 37, 1610.Rabinovitch, Canad. J. Chem., 1968, 46, 2435.1972, 53, 111.(PAPER 6/1052

 

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