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Radicals in the gas phase olefin ozonolysis: ESR-matrix spectroscopy
V~irrruc 67. number 1
CIIEMIC_~L t’IfYSICS LETI-LRS
R-IDICALS IN THE GAS PHASE OLEFIN
OZONOLYSIS:
ESR-MATRIX
1 No\cmber 1979
SPECTROSCOPY
Receired 2-I Jul) 1979
B_r using a combination of 3 microreactor A irh m.ttri\-ESR spectroscopy the gas phase ozonol~ sis of ethq Iene and cisbutene 0.1s imestig-ated. For residence times in the order of milliseconds the existence of radicals WZISproten. The ESR signal nuy m;tinlv be attributed to ox_v- and perov -mdic&.
Ry this Ietter we wish to report the direct detection of radktls produced in the g3.s phase ozonolysis of ethylene and cis-butene. The esistence of radicals in the mechanism of the gas ph3se reaction of ozone with olefins has been postulated by msny nuthors [ I- I?]_ Criegee assumed formation of 3 zwitterion [ 121. but recent qusntum chemical &culations in the ethylene case have shown that the zwitterion is Iess stable than the eIectromeric biradical 3nd the latter agsin less stable than the isomeric dioxyrane [4]_ The formation of OH radicrtls ~3s proven by experiments of Pitts et al_ [2] _ The same authors inferred the production of CH3C0 in the ethyiene ozonolysis and of HCO, C,Hs, CA;CH,O, CH,O, in the cis-butene OzonoIysis [3] _Observrttion of compIex radic& of the type postulated by the O’NeiI-BIumen-‘c-_c: stem mec!ianism [IO], nameIy / \ 0
_O
1~3s not been reported so far_ ‘oWe should like to discuss the results of 311 attempt to prove the existence of polyatomic radicals by the combination of the microreactor with mstris-ESR spectroscopy (MR-ESR). The use of the reactor cotnbined with either infrared matrix or microwsve spectroscopy has proven to be an efficient means for the detection of complex reaction species in the gas phase olefii ozonolysk [4-7]_ Oxygen (Linde, quahty 4.8) was passed through a Fisher ozonizator and the ozone was adsorbed on silicagel at -78°C. After pumping off the excess oxy-
gerr the sdieagel WIS allowed to warm up slowly by using m argon current @lesser-Griesheim, quality 6.0) 3s diIuent_ Mixtures of ethylene (hiatheson Research Cmde 2.8) or cis-butene (FIuk3, purum 2.0) .md 3rgon were prep3red and the reactants were passed through the microreactor which w3s attached to 8 high vacuum cell_ The reaction mixture leaving the microreactor was then condensed on a sample holder kept at cryogenic tempefature (8 K). The ESR spectrn were recorded by an ESR spectrometer of our own design equipped with a Varian E-line microwave bridge, self-Iocking NMR magnetometer, and 3 homebuilt Iiquid helium cryostat. The spectra were electronically digitalized for off-line analysis by means of a Wang computer_ Fig. ! gives 3 schematic view of the set-up used to genemte the molecular beam from the microreactor that was fed with mivtures of ozone and oiefin with argon_ In table 1, the relevant operating conditions used in these MR-ESR esperiments are collected. Typical MR-ESR spectra are shown by figs. 2a2d. These data may be commented upon as follows: (i) Pure O3 : Ar matrices give an isotropic ESR signal consisting of three lines of equal intensity with g = 2.0033 and a = 4.5 G (fig. 23,Ieft)_ The spectrum may be assigned to NO,_ This radical was studied in the single crystal system NO, I KNO, generated by y-irradiation [I31 _Isotropy and line width (=0-S G) indicate that the radical NO, experiences a fast re173
nctic const3nts (t3bIc 2) wxe estimated by me311sof tile model contour Maculations publislled b) Srxrl et A_ [I-l]_ A compxison wit11hnown spectra of 3Ikoxy, 3lkyLperoxy [ t 5- 171. hydropcro.\_v and t1ydroxyI r3dic3ls [ 18.191 suggests tlmt tI1e signal should be 3ttributed to radic3ts of tile ROO and RO type. For most species of tI1issort. tI1eJ’iu, and fzlsDvalues 3re in tile rttngc of our observations_ (iv) MR-ESR experiments wit11 cis-butcne yield tl1e spwtr3 slunvn in figs_ 2c 311d2d_ T11e forn1er csn be observed wI1en using rextion mhtures wit113 smaller Mfrl ratio (re3ctant concentration [OS] = =Z-I_7 X IO-’ n101/dm3). TI1e n13in fe3ture [C,II,l of tI1issign31(fig. 3~) &ouId 3Fhi be rtttributed to oq- 3nd peroxy-type radic3k Furthermore, the cI13rxteristic pattern of CtIj is cIe3rIy recognirable. 1 l1is 3ttribution is supported by 11x _r~ 3nd a c3lucs derived f-roni tllf iine shape of this signrtl. At Iower concentr3tion (I _-ZX Iowa moI/dm’) tI1e ESR spectrur113ppews to consist of tile sigrwl 01, rtl1dSO, and 3 weak doublet wit113 splitting ofapproxim3teIy 130 5 3 G besides tI1e strong spectrum attribut3bIe to the osy zmd pero.xy radi&s_ TI1e doublet should tnost probtibly be assigned to the formy radical lIC0 [20]_ The latter ~3s detected earlier in the grrs pIxt.se ozonolysis of cis-butene by
,A5 -
- -
Fig_ 1_~lic~orhctur-mrttri\-ESR set&p_ 1: SampIe 1101&x_ 2: T&on inset. 3: Rr;tc:or space. -$: Xo~zIe_ 5: Cookrfhawr. process at Kquid It&urn tcmperrrture. A possible source of ti~is intermediate could be X0 contaminatIo11 in tI1e order of nia@tude of scLer3I ppm in the oxygen used_ (ii) The system cis-butene : AK 4 ields 3 ve;)- weak ESR dgn3I (ccf. fig_ 3;t, ri$t) of unknown origin_ The signal does not stem from tile s3mpIe 11oIder (sintrrcd BeO) 3s has been proven by controi experiments. (iii) TI1e products of tI1e g3s pl1rue reaction of etI1yIene with ozone 3f P residence time of 0.1 ms give the ESR signal &own in fig_ 2b_ TI1is spectrunr orientation
should be classified as a typical powder
Atkinson
in the gas phase ozonoiysis
Experiment no_
o~onoksis -
microreactor
Olefm
intermediates
in this reaction mechanism
and confirms
tI1e usefulness of tI1e microreactor-matriu-ESR te&nique in investig3ting complex g3s pI13se reactions_
operating conditions -------
Reactor temperature ea
3re produced
of oIefms_ This n13y be
considered 3s the first direct evidence for radical
pattern of
ZxialIy symmetrial species and appears to arise from more than one mdic31, From tile line &ape tI1e mag
Table L Gas phaseoIefin
et aI_ [31_
Tiw present report ~110~stI13t radiak
___----
Partial pressures in the reactor (Torr) AK
olefm ---_-__
Residence time al (ms)
Deposition time (min)
03
I 2
cis-butene
22 22
510.3 5125
0 27.5
21.7 0
-
-
3 4 5
ethyIeie cis-butene cis-butene
23 23 22
525.4 534.6 5520
14.8 8-7 25
14.8 8.7 2-S
0.2 0.2 0.2
120 I5 23
=) caIeulated_
174
53
Vdume
67. number 1
CHEMICAL
PHYSICS
LE-ITERS
I Notember
1979
I Sc~rernbcr
176
1979
VoIume
67. number 1
CHE.\fICAL
PHYSICS
Tile authors wisil to thank the Swiss National Foundation (Project no. 2_033-0.78,2.5 19-0.76) .ts well as Messrs. Smdoz AC, Basle, for the financisi support of this work_ Furthermore, they wish to thmk the staff of their workshop, in particular hlr_ E_ Peyer. for their vaiusble technical assistance_
References [ 11 R. Giqee ahd G. Weiner, Justus Liebgs Arm. Chem. 546 (1949). [2] B J. Finlayson and J N. Pitts Jr.. Chcm Phys. Letters 12 (1971)195_ [3] R. Atkinson. B-l. FinIa) son and J.N. Pitts Jr.. J. Am. Chrm. Sot. 95 (1973) 7592: 96 (197-t) 5356. [41 T-K_ Ha. H_ Kiihnc. S. Vaccani and 1is.H. Ciinrhard. Chcm. Phys. Letters 1-I (1974) 171. I.51 Ii. Kiihnc, S. \hccani. T Ki. t1.1 .md Iis ti. Gtinrhad. Chem. Phis. Letters 3S (1976) 449_ [6] II. Kuhne and HKII. Giiutlurd, J_ Phys. Chem. 80 (1976) 113s. [ 71 S_ Vacctni, A. Bauder and I-is H. Gunzhnrd, Chem. Phys. Letters 35 (1975) 457.
LETTERS
1 November
1979
[S] A.D. Hansen. R. Atkinson and J N. Pats Jr., J. Photothem. 7 (I 977) 379_ [9] A.D. Hansen and J.N. Pitts Jr., Chem. Phys. Letters 35 (1975) 569. [IO] E.11. O’NeaI and C_ Blumstein, Intern. J. Chem. Kinetics 5 (1973) 397_ [ 1 I ] S-F. Toby, S. Toby and E.H. O’Nrd, Intern. J. Chem. Kinetics 8 (1976) 25. [ 121 R. Criesee, Advan. Chem. Series :! I (1959) 133. [ 131 J- Cunningham, J-A. McMiIIan, B. Smslier and E. Yasairis, Phys. Chem. Sot_ 23 (1962) 167. [ 141 J-W. SearI, R.C. Smith and S-l. Wyxd, Proc. Phys. Sot. (London) 74 (1959) 491. i IS] H. Fischer, K.H. Heke~e and P. WendGrfl,J. Polymer Sci. A I (1963) 2109. [16] J-F. Gnbson. S1.C.R. Symons and XC. Townsend, J. Chem_ Sot. (1959) 69. [ 171 G-W_ Easstland and hl.C.R_ Symons, J_ Phys_ Chem. 81 (1977) 1501. [IS) R-C_ Catton and MC-R_ Symons, J. Chem_ Sot. A (1969) 1393. [ 191 J-A_ Brlvati, M.C.R. Symons. D-IA. TinIing, H-W_ Wardale and D-0. WiIIiams, Tr.ms. Faraday Sot 63 (1967) 2111 [30] J-A. Brirati, N. Keen and M.C.R. S> mans. J. Chem. Sot. (1962) 237.
177
CIIEMIC_~L t’IfYSICS LETI-LRS
R-IDICALS IN THE GAS PHASE OLEFIN
OZONOLYSIS:
ESR-MATRIX
1 No\cmber 1979
SPECTROSCOPY
Receired 2-I Jul) 1979
B_r using a combination of 3 microreactor A irh m.ttri\-ESR spectroscopy the gas phase ozonol~ sis of ethq Iene and cisbutene 0.1s imestig-ated. For residence times in the order of milliseconds the existence of radicals WZISproten. The ESR signal nuy m;tinlv be attributed to ox_v- and perov -mdic&.
Ry this Ietter we wish to report the direct detection of radktls produced in the g3.s phase ozonolysis of ethylene and cis-butene. The esistence of radicals in the mechanism of the gas ph3se reaction of ozone with olefins has been postulated by msny nuthors [ I- I?]_ Criegee assumed formation of 3 zwitterion [ 121. but recent qusntum chemical &culations in the ethylene case have shown that the zwitterion is Iess stable than the eIectromeric biradical 3nd the latter agsin less stable than the isomeric dioxyrane [4]_ The formation of OH radicrtls ~3s proven by experiments of Pitts et al_ [2] _ The same authors inferred the production of CH3C0 in the ethyiene ozonolysis and of HCO, C,Hs, CA;CH,O, CH,O, in the cis-butene OzonoIysis [3] _Observrttion of compIex radic& of the type postulated by the O’NeiI-BIumen-‘c-_c: stem mec!ianism [IO], nameIy / \ 0
_O
1~3s not been reported so far_ ‘oWe should like to discuss the results of 311 attempt to prove the existence of polyatomic radicals by the combination of the microreactor with mstris-ESR spectroscopy (MR-ESR). The use of the reactor cotnbined with either infrared matrix or microwsve spectroscopy has proven to be an efficient means for the detection of complex reaction species in the gas phase olefii ozonolysk [4-7]_ Oxygen (Linde, quahty 4.8) was passed through a Fisher ozonizator and the ozone was adsorbed on silicagel at -78°C. After pumping off the excess oxy-
gerr the sdieagel WIS allowed to warm up slowly by using m argon current @lesser-Griesheim, quality 6.0) 3s diIuent_ Mixtures of ethylene (hiatheson Research Cmde 2.8) or cis-butene (FIuk3, purum 2.0) .md 3rgon were prep3red and the reactants were passed through the microreactor which w3s attached to 8 high vacuum cell_ The reaction mixture leaving the microreactor was then condensed on a sample holder kept at cryogenic tempefature (8 K). The ESR spectrn were recorded by an ESR spectrometer of our own design equipped with a Varian E-line microwave bridge, self-Iocking NMR magnetometer, and 3 homebuilt Iiquid helium cryostat. The spectra were electronically digitalized for off-line analysis by means of a Wang computer_ Fig. ! gives 3 schematic view of the set-up used to genemte the molecular beam from the microreactor that was fed with mivtures of ozone and oiefin with argon_ In table 1, the relevant operating conditions used in these MR-ESR esperiments are collected. Typical MR-ESR spectra are shown by figs. 2a2d. These data may be commented upon as follows: (i) Pure O3 : Ar matrices give an isotropic ESR signal consisting of three lines of equal intensity with g = 2.0033 and a = 4.5 G (fig. 23,Ieft)_ The spectrum may be assigned to NO,_ This radical was studied in the single crystal system NO, I KNO, generated by y-irradiation [I31 _Isotropy and line width (=0-S G) indicate that the radical NO, experiences a fast re173
nctic const3nts (t3bIc 2) wxe estimated by me311sof tile model contour Maculations publislled b) Srxrl et A_ [I-l]_ A compxison wit11hnown spectra of 3Ikoxy, 3lkyLperoxy [ t 5- 171. hydropcro.\_v and t1ydroxyI r3dic3ls [ 18.191 suggests tlmt tI1e signal should be 3ttributed to radic3ts of tile ROO and RO type. For most species of tI1issort. tI1eJ’iu, and fzlsDvalues 3re in tile rttngc of our observations_ (iv) MR-ESR experiments wit11 cis-butcne yield tl1e spwtr3 slunvn in figs_ 2c 311d2d_ T11e forn1er csn be observed wI1en using rextion mhtures wit113 smaller Mfrl ratio (re3ctant concentration [OS] = =Z-I_7 X IO-’ n101/dm3). TI1e n13in fe3ture [C,II,l of tI1issign31(fig. 3~) &ouId 3Fhi be rtttributed to oq- 3nd peroxy-type radic3k Furthermore, the cI13rxteristic pattern of CtIj is cIe3rIy recognirable. 1 l1is 3ttribution is supported by 11x _r~ 3nd a c3lucs derived f-roni tllf iine shape of this signrtl. At Iower concentr3tion (I _-ZX Iowa moI/dm’) tI1e ESR spectrur113ppews to consist of tile sigrwl 01, rtl1dSO, and 3 weak doublet wit113 splitting ofapproxim3teIy 130 5 3 G besides tI1e strong spectrum attribut3bIe to the osy zmd pero.xy radi&s_ TI1e doublet should tnost probtibly be assigned to the formy radical lIC0 [20]_ The latter ~3s detected earlier in the grrs pIxt.se ozonolysis of cis-butene by
,A5 -
- -
Fig_ 1_~lic~orhctur-mrttri\-ESR set&p_ 1: SampIe 1101&x_ 2: T&on inset. 3: Rr;tc:or space. -$: Xo~zIe_ 5: Cookrfhawr. process at Kquid It&urn tcmperrrture. A possible source of ti~is intermediate could be X0 contaminatIo11 in tI1e order of nia@tude of scLer3I ppm in the oxygen used_ (ii) The system cis-butene : AK 4 ields 3 ve;)- weak ESR dgn3I (ccf. fig_ 3;t, ri$t) of unknown origin_ The signal does not stem from tile s3mpIe 11oIder (sintrrcd BeO) 3s has been proven by controi experiments. (iii) TI1e products of tI1e g3s pl1rue reaction of etI1yIene with ozone 3f P residence time of 0.1 ms give the ESR signal &own in fig_ 2b_ TI1is spectrunr orientation
should be classified as a typical powder
Atkinson
in the gas phase ozonoiysis
Experiment no_
o~onoksis -
microreactor
Olefm
intermediates
in this reaction mechanism
and confirms
tI1e usefulness of tI1e microreactor-matriu-ESR te&nique in investig3ting complex g3s pI13se reactions_
operating conditions -------
Reactor temperature ea
3re produced
of oIefms_ This n13y be
considered 3s the first direct evidence for radical
pattern of
ZxialIy symmetrial species and appears to arise from more than one mdic31, From tile line &ape tI1e mag
Table L Gas phaseoIefin
et aI_ [31_
Tiw present report ~110~stI13t radiak
___----
Partial pressures in the reactor (Torr) AK
olefm ---_-__
Residence time al (ms)
Deposition time (min)
03
I 2
cis-butene
22 22
510.3 5125
0 27.5
21.7 0
-
-
3 4 5
ethyIeie cis-butene cis-butene
23 23 22
525.4 534.6 5520
14.8 8-7 25
14.8 8.7 2-S
0.2 0.2 0.2
120 I5 23
=) caIeulated_
174
53
Vdume
67. number 1
CHEMICAL
PHYSICS
LE-ITERS
I Notember
1979
I Sc~rernbcr
176
1979
VoIume
67. number 1
CHE.\fICAL
PHYSICS
Tile authors wisil to thank the Swiss National Foundation (Project no. 2_033-0.78,2.5 19-0.76) .ts well as Messrs. Smdoz AC, Basle, for the financisi support of this work_ Furthermore, they wish to thmk the staff of their workshop, in particular hlr_ E_ Peyer. for their vaiusble technical assistance_
References [ 11 R. Giqee ahd G. Weiner, Justus Liebgs Arm. Chem. 546 (1949). [2] B J. Finlayson and J N. Pitts Jr.. Chcm Phys. Letters 12 (1971)195_ [3] R. Atkinson. B-l. FinIa) son and J.N. Pitts Jr.. J. Am. Chrm. Sot. 95 (1973) 7592: 96 (197-t) 5356. [41 T-K_ Ha. H_ Kiihnc. S. Vaccani and 1is.H. Ciinrhard. Chcm. Phys. Letters 1-I (1974) 171. I.51 Ii. Kiihnc, S. \hccani. T Ki. t1.1 .md Iis ti. Gtinrhad. Chem. Phis. Letters 3S (1976) 449_ [6] II. Kuhne and HKII. Giiutlurd, J_ Phys. Chem. 80 (1976) 113s. [ 71 S_ Vacctni, A. Bauder and I-is H. Gunzhnrd, Chem. Phys. Letters 35 (1975) 457.
LETTERS
1 November
1979
[S] A.D. Hansen. R. Atkinson and J N. Pats Jr., J. Photothem. 7 (I 977) 379_ [9] A.D. Hansen and J.N. Pitts Jr., Chem. Phys. Letters 35 (1975) 569. [IO] E.11. O’NeaI and C_ Blumstein, Intern. J. Chem. Kinetics 5 (1973) 397_ [ 1 I ] S-F. Toby, S. Toby and E.H. O’Nrd, Intern. J. Chem. Kinetics 8 (1976) 25. [ 121 R. Criesee, Advan. Chem. Series :! I (1959) 133. [ 131 J- Cunningham, J-A. McMiIIan, B. Smslier and E. Yasairis, Phys. Chem. Sot_ 23 (1962) 167. [ 141 J-W. SearI, R.C. Smith and S-l. Wyxd, Proc. Phys. Sot. (London) 74 (1959) 491. i IS] H. Fischer, K.H. Heke~e and P. WendGrfl,J. Polymer Sci. A I (1963) 2109. [16] J-F. Gnbson. S1.C.R. Symons and XC. Townsend, J. Chem_ Sot. (1959) 69. [ 171 G-W_ Easstland and hl.C.R_ Symons, J_ Phys_ Chem. 81 (1977) 1501. [IS) R-C_ Catton and MC-R_ Symons, J. Chem_ Sot. A (1969) 1393. [ 191 J-A_ Brlvati, M.C.R. Symons. D-IA. TinIing, H-W_ Wardale and D-0. WiIIiams, Tr.ms. Faraday Sot 63 (1967) 2111 [30] J-A. Brirati, N. Keen and M.C.R. S> mans. J. Chem. Sot. (1962) 237.
177