ESR evidence for the formation of the trimethylene padical cation -CH2CH2CH+ from cyclopropane

ESR evidence for the formation of the trimethylene padical cation -CH2CH2CH+ from cyclopropane


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VoIumc 112, number 1


23 November 1981



Xue-Zhi QIN and Ffrancon WILLIAMS Depurrmmr of Ctzenrisrry, University of Temessee. Knoxville. Tezmessee 37996-1600.


Received 4 September 1984

Solid-state ESR studies show that the cyclopropane radical cation undergoes ring opening in the CFC12CFzQ mat&i above 80 I(. The two terminal CH, groups in the derived trimethylene radical cation are perpendicular to each other such that the electron spin is confined to one end of the molecule. the radical centre adoptins a bisected conformation very similar to that of the n-propyl radical.

1. Introduction A recent ESR study [ 1] has reported that the radical cation formed radiolytically [2] from ethylene oxide in the solid state has a C...C ring-opened structure simitar to that of the isoelectronic alIyI radical, a conclusion which has since received strong support from measurements of the eIectronic absorption spectra of radical cations similarly derived from substituted ethylene oxides [3] _ Earlier mass spectrometry exper-

iments [4-6] had in fact susested that the ethylene oxide radical cation undergoes this ring-opening isome&&ion_ Moreover,very recent high-level ab initio calculations [7-9] have shown that the planar symmetric (Cz,,) structure of the ring-opened radical cation lies 19.6 kcal mol-l below its ring-closed isomer [S] and that the barrier to the isomerization is probably less than 3.7 kcal mol-l 191. In contrast to the findings for ethylene oxide, the evidence for a simple C...C ring-opened C3$ species (i.e. [email protected] from cyclopropane in the gas phase is ambiguous. However, it has been established [lo] that two structurally distinct C,$ isomers are produced from cyclopropane above the ionization threshold of 9.93 eV, one being the propylene cation whose relative abundance is very small just above threshold but increases at higher electron and photon energies [ Ill_ These two isomers differ &kingly in their reactivity with amm&ia [11-l 31, The propylene 0 009-2614/84/S 03.00 OElsevie;Sciende Publishers B-V_ bob-Ho~and Physics Publishing Division)

cation reacting exclusively by proton transfer [ 1 1] whereas the non-olefiiic C,Hz undergoes two very selective reactions 112,131, one of which involves the transfer of CH$ to the base. It is interesting that this Iatter reaction type is considered to be diagnostic for the ring-opened ethylene oxide radical cation 14,5,14]. These gas-phase results can therefore be summarized by saying that while C3g produced with low

internal energy from cydopropane has a different structure from its propylene cation Isomer, the question of whether it retains she cyclopropane structure or becomesa trimethylene radical cation p&r to reaction with ammonia has not been resolved II 3] _ Turning to solid-state ESR studies, the radical cations of cyclopropane [is] and propylene [16,17] have already been reported, these species having unisomerized ring-closed and olefii structures only slightly distorted from those oftheir parent molecules. It was therefore of interest to search for the possible third member of this triad, namely the ring-opened -CH2CH2CG isomer, and here we report conclusive evidence for this species.

2. Experimental Cyclopropane (99%) and cyclopropane+ (98%) were obtained from Matheson and Merck Sharp and Dohme isotopes, respectively. Propylene was supplied 79

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23 November 1984


:~y the PIliIIips Petroleum Co. (Lot 1202) and CFCIzCF,CI (99%) was obtained from Matheson as freon-l 13: The methods of sample preparation, radical cation generation,and ESR examination have been described [ 181.


3. Results and discussion



Comparative studies of several freon matrices in this laboratory suggest that the use of CFCl,CF,CI facilitatesa simple bond dissociation process for certain radical cations [ 19,20]_ Thus, the thermal dissociation

of t-BuSnMe; to the tert-butyl radical occurs at 1 lo120 K in this matrix whereas the same reaction does not take place in CFCI, and CF,CCI,, even at higher temperatures (~150 K) near the softening points of those matrices [19] _Similarly, it was found recently that the ring-closed form of the 1,1,2,Ztetramethylcyclopropane radical cation undergoes an irreversible monorotatory ring opening above 110 Kin CFC13CF7CI to give an orthogonal structure, although this reaction was not observed in other freons up to at least 145 K [20] _In view of this latter result, it was natural to use CFCIICF,Cl for the present study. The ESR spectrum shown in fig. la at 80 K consists of a broad unresolved singlet with ag factor of 2.0042 and an overall width of about 100 G. This spectrum is identical to that reported in a previous study and assigned to the ring-closed cyclopropane cation [ lS]_ II was noted in this earlier work [IS] that cycloC31-$ in CFCI,CF,CI decomposes at =I00 K and does not give a narrower line like that produced in SF6 and CFCI, by a dynamic Jahn-Teller effect. The nature of this decomposition, however, was not spccilied [ IS], and no additional information on this topic is given in a more recent comprehensive paper on the structure and reactions of cycloalkane radical cations [?I] _ As shown in fig. I. we find an irreversible change in the ESR spectrum on annealing cyc10-C~~ in CFCI,CF,CI from 80 to S4 K for a few minutes. The spectrum m-measured at SO K (fig. Ic) is no longer a broad singIet but possesses some hyperfine structure which is,however, not well resolved below 100 K. Fig. 2 shows that at 10s K, the spectrum resolves into a clear triplet of 1:2: I triplets, this change in resolution being completely reversible between SO and 108 K. Unfor80



Fig. 1. First-derivative ESR spectra of 1 mole% of cyclopropane in CFCl2CF~CI after 7 irradiation at 77 I; for a dose of 1 hlrad. The spectra show the irreversible effect of thermal annealing and were recorded (a) at 80 K, (b) at 84 K, and (c) on subsequent retooling to 80 K.

tunately, the temperature could not be raised above 110 K without inducing further irreversible changes including the decay of this well-resolved spectrum and the growth of strong signals, presumably from matrix radicals. However, using the rapid-scan accessory during fast pulse annealing, it was observed on the oscihoscope screen that the lines belonging to the central triplet intensified relative to those of the two outer triplets as compared to the spectrum In fig. 2b. Consequently, the spectrum of the radical which grows in at 84 K is analysed as a 1:2:1 trip!et of 1:3: 1 triplets resulting from hyperfme interaction with two sets of two equivalent hydrogens. A simulation spectrum using the ESR parameters given in table 1 is shown in fig. 2c. Confirmation of the above change was sought in a


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Fe. 3. Comparison of (a) the first-derivative ESR spectrum of tbe trimethylene radical cation recorded at 110 K and (b) the simulated spectrum obtained using the ESR parameters in tabIe L Signals from estraneous radicals in (a) are marked by asterisks.

Fig. 2. First-derivative ESR spectra of the trimethylene& radical cation recorded {a) at 100 Ic and (b) at 108 Z;. The spectrum (c) was simulated using the ESR parameters given in table 1.

Table 1 ESR parameters for the ~~~~ylene

parallel study using cyclopropaned6 _ In this case the irreversible change occurred at about 92 K and the spectrum became reasonably well resolved at 110 K (fig, 3a). Using the previously derived ESR parameters for the protiated radical as the input data, the spectrum of the corresponding deuteriated species was simulated according to the related parameters in table 1 and is sl~ownin fig_3b_The excellent fit to the &xperimental spectrum verifies that the radical which grows in originates from cyclopropaued6 and possesses the same

radical cation and the IZ-propyl and ally1 radicals



7’ WI

g factor

Hyperfme coupling (G)




108 110 165 93 108 153

30028 2.0028 2.00287 2.00260 2.0025 2.00254

o(‘H& = 30.2; a(2Ha = 22.4 iz(2Dp) = 4_64a);a(2DJ = 3.44a) n(2Hfl) = 31_17;a(2H&) = 22_13 n(2Hp) = 33.2;n(2H,) = 22.08 a(4H) = 14.7b);~(lH) = 4.1 a(2Hexo) = 14.83;O(2Hendo) = 13.93;nflH)

this work this work


C3Ha CFCf2CF2C1 c-CsH6

f%l =4-06

V41 this work I241

a) These parameters were obtained by dividing the corresponding proton hyperfime couplings by 6.514 and then used to obtain the simulation spectrum in fig. 3b. W Hyperfme couplings to the eso- and endo-hydrogens could not be differentiated because of low spectral resolution. ~.



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structure as that similarly produced from cyciopropane itself. It can easily be proved that the radical derived frcm the ring-closed cyclopropane radical cation is rhe trimethylene radical cation. First, it is useful to note that the SOMO (structure 1) of the Jahn-Teller

1 (Czv) distorted cycle-C3G species [ 151 is bonding between the equivalent carbons of the isosceles triangle so that this bond is expected to weaken and elongate in the radical carion [22,23]. This structure was said IO be topographically equivalent to the trimethylene radical cation 1221 but we shall see that the latter species is in fact produced from the distorted cyclopropane cation by the rotation of one of the CH, groups at the weakened bond. The structural proof of the derived radical is given by the near identity of its hyperfine parameters to those of the n-propyl radical (table 1) as well as other -CH,CH,X _ _ radicals [24,25], and therfore the conformation at the radical centre must be very similar, as depicted in structure 3. This bisected conformation

“\ H

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30.2G -


Hence the trimethylene radical cation possesses a twisted orthogonal structure (2) although a slow torsional oscillation about the C&-C0 bond is quite possible given that the experimental line intensities may be indicative of a slight inequivalence in the two /3-hydrogens couplings below 1 IO K.

INDO calculations [27] confirmed that for an orthogonal trimethylene radical cation geometry, rhe unpaired electron occupies the 3~ orbital in the rotated CH? group as shown in 2 with the usual hyperconjugative admixture into the B-CH, group. It is interesting t_onote that this level of theory also correctly predicted the opposite (i.e. eclipsed) conformation for the orthogonal 1,1,3,3-tetramethyl-trimethylene radical cation produced by ring opening of the 1,1,2,2tetramethylcyclopropane radical cation [20], the spin density therefore remaining in the unrotated CMe, group in this case. A theoretical study using high-level ab initio calculations is obviously desirable, however, to provide a more rigorous comparison of experiment and theory for these small open-shell molecules. It remained for us to check the ESR spectrum of the propylene radical cation in the CFCIzCF2CI matrix, since this combination has not been reported [I 6,171. The spectrum generated from a propylene solution was only poorly resolved, however, between 77 and 90 K, and no definite conclusion could be reached as to identity of the original radicals present after irradiation at 77 IS. Above 108 K, the spectrum was clearly resolved into a quintet of doublets with couplings characteristic of the ally1 radical (table l)_ Presumably, propylene radical cations in this matrix undergo proton transfer to give the ally1 radical. Since no ally1 radicals were observed in the cyclopropane experiments at 108 K, it seems reasonable to conclude that the propylene radical cation is not a product of the ring opening of cyc1o-Q~. This conclusion implies that the trimethylene radical cation does not 2

isomerize by a 1,2-hydrogen shift to give the propylene radical cation below 10s K.

can only be achieved from the ring-closed cation by rhe aforementioned CH, group rotating through 90”. Had both CH2 groups rotated to produce a Czv edgetoedgc structure [36] similar to that of the ethylene oxide radical cation [13,7-91, the electron spin would be distributed in an A? SOMO made up ofp, orbitals on both of the terminal carbon atoms [26], and therefore this possibility is excluded by the ESR results. 52

4. Concluding


Although the trimethylene radical cation has previously been considered as a possible alternative structure for the cyclopropane radical cation in gas-phase ion-molecule reactions, its existence as a separate and

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distinct species does not seem to have been rccognised-

WJ. Bouma, D. Poppinger, S. Saeb& J-K. MacLeod and L. Radom. Chem. Phys. Letters 104 (1984) 198. T. Clark, J. Chem. Sot. Chem. Comm. (1984) 666. S.G. Lias and P_ Ausioos, Ion-molecule reactions. Their role in radiation chemistry @merican Chemical Society, Washington, 1975) ch. 2, p_ 30. L.W. Sieck. R. Gorden Jr_ and P_ Ausloos, J_ Am_ Chem. sot. 94 (1972) 7157. M-L. Gross and F-W. &Lafferty. J. Am. Chem_ Sot. 93 (1971) 1267. h5.L. Gross. J. Am. Chem_ Sot. 94 (1972) 3744. WJ. Bouma, 3-K. MacLeod and L. Radom, Advan. Mass


showing that this ring-opened species can be formed from the cyclopropane radical cation in the solid state without isomerizing immediately to the propyikne radical cation, the present work has established the existence of a third [email protected] isomer characterized by ESR

as a twisted orthogonal XH2CH2CHz species.

Acknowledgement by the Division of Chemical Sciences, Office of Basic Energy Sciences,

Spectrom. 8 (1980) 178. hf. Irvas&i, IL Toriyama and K. Nunome, J. Chem. Sot.

This work was supported

US Department of Energy (document No. DOE/ER/ 02968/156).

23 November 1984

Chem. Commun. (1983) 203. K. Toriyama, K. Nunome and M. Iwasaki, Chem. Phys. Letters 107 (1984) 86;J. Chem. Phys. 77 (1982) 5891. 1171 M. Shiotani, Y. Nagata and J. Sohma, 3. Phys. Chem.

88 (1984)). to be pubfished. L.D. Snow and F. WiWms, Chem. Phys. Letters 100 (1983) 198. f IS] B.W. W&her, Ph.D. Thesis, University of Tennessee (1984). [XI] X.-Z. Qin. L.D. Snow and F. Williams, J. Am. Chem. Sot. 106 (1984), to be published.


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1271 J.A. Pople, D-I.. Bcueridge and P-A. Dobosh, J_ Chem.. Phys_ 17 (1967) 2026.