ESR evidence for the formation of the ring-opened cation CH2OCH2± from ethylene oxide

ESR evidence for the formation of the ring-opened cation CH2OCH2± from ethylene oxide

ESR EVIDENCE 2 September 1983 CHEMICAL PHYSICS LETTERS Volume 100, number 2 FOR THE FORMATION FROM ETHYLENE OF THE RING-OPENED CATION CHZOCH; ...

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ESR EVIDENCE

2 September 1983

CHEMICAL PHYSICS LETTERS

Volume 100, number 2

FOR THE FORMATION

FROM ETHYLENE

OF THE RING-OPENED

CATION CHZOCH;

OXIDE

Larry D. SNOW, Jlh Tzong WANG * and Ffrancon

WILLIAMS

Department of Chemistry, University of Tennessee. Knoxville. Tennessee 37996-l 600. USA Received 28 April 1983; in final form 27 June 1983

ESR results show that the radical cation formed from ethylene oxide in the solid state is the ringapened 2+,xa-trimethylcne cation with a symmetrical (C& planar structure similar to that of the isoelectronic ally1 radical. In contrast. the trimethylene oxide radical cation retains the rins structure of the parent molecule and its ESR parameters are characteristic of an oxygencentred species.

1. Introduction Recent developments have shown that y-irradiation of Freon solutions at 77 K provides a simple general method of generating solute radical cations for ESR studies in the solid state [l-3] _Using this technique, the radical cations of several oxygen-containing organic compounds have been characterized, including those of simple ethers [4,5], acetals [6], aldehydes [7,8] and esters [P] _ As expected in the absence of molecular rearrangements, these cations were all found to be oxygen-centred radicals, the SOMO of either II [4-6,9] or n [7,8] character allowing appreciable spin density to delocalize from the oxygen 2p orbital(s) to the P-hydrogens by means of efficient hyperconjugation. Considering the well-known propensity of certain oxygen-containing radical cations to undergo rearrangement processes in the gas phase [lo], it was clearly also of interest to apply the above technique in au attempt to duplicate the gas-phase isomerization of a suitable cation under condensed-phase conditions_ Indeed, ESR evidence for carbon-centred radicals was found in studies of cations from di-n-propyl ether and higher acyclic ethers [l I] suggesting that H-atom transfer from carbon to oxygen occurs in these cations * Present address: Department of Chemical Engineering, Tatuns Institute ofTechnology, [email protected] North Road3rd section, Taipei, Taiwan Republic of China.

0 009-2614/83/0000-0000/S

03.00 0 1983 North-Holland

under favourable steric conditions. The specificity of such a process is difficult to establish with certainty, however, since the results could also be explained in terms of proton transfer from the cation to a negative ion iu the matrix_ Accordingly, we restricted our

attention to the cations ofsmall ring compounds where hydrogen shifts appear to be largely avoided [4,6] _ Ethylene oxide seemed to be au excellent candidate for the proposed study since recent mass spectrometric and ICR investigations of the gas phase cation from this compound have provided co,“[email protected]~Q evidence for the ring-openedisomerCH,OCH; [12,135_ It is therefore of interest to report ESR evidence which establishes that the cation obtained from ethylene oxide in the solid state is likewise the open Zoxa-trimethylene radical cation, a carbon-centred species isoelectronic with the ally1 radical [ 14]_ Since the radical cation formed from trimethylene oxide under similar conditions is found to have an oxygen-centred structure completely characteristic of the unrearranged ion, it appears that the isomerization of the ethylene oxide cation derives largely from the relief of ring strain.

2. Experimental Ethylene oxide (99.7%) and trimethylene oxide [oxetane] (97%) were supplied by Matheson and the Aldrich Chemical Co., respectively_ Ethylene-l

,2-d?

193

Volume 100. number 2

o.xide (94-9 atom% D) and ethylened4 oxide (9s atom% D) were obtained from Merck, Sharp and Dohme Isotopes. The ethylene oxide gases were condensed at 77 K on a vacuum line and traces of air removed by pumping. The Freon solvents and the basic tcclmiques of sample preparation. y-irradiation, and ESR examination were as described in the following study [S].

3. Results and discussion The ESR spectrum of the radical cation derived from trimethylene aside in CFCI, is shown in fig. 1 _ A satisfactory reconstruction of this spectrum is indicated by the stick diagram consisting of a binomiaf quintet of 1 : 2 : 1 triplets with g-anisotropy. the four observable sub-~onlponents of the quintet pattern resulting from the two sets of overlapping triplets associated with g,,,.,% and g,,,in_ This analysis is supported by the characteristic lineshapes of the non-overlapping triplet components @If = i-1 from gma, and Jf, = --I i-rOSll~,;lir~) ix2the outermost group of lines at hi& field,

1-g. 1. f‘irst-dr’riwtiw ESR spectrum of a 2.5 mule~~ solution oftrimeth~lcne oitdt: in rrichtorofluororlletlians .tt 152 K dfter y-lrradintion (dose. 0.5 hlrzd) at 77 I;. I94

2 Septem~r

CHE!MCAL PHYSICS LETTERS A(2H) = 10.6 G 9

A(4Hi=&.6

A(4H)

=

1983

16.4 G

H

G I

suggesting that grn=, and gmin are the perpendicular and parahel components of an axiaIiy symmetric gtensor. An oxygen-centred structure (I) is therefore indicated. the hyperfine pattern resulting from coupling to the two sets (4p and 2-r) of equivalent hydrogens. Convincing evidence for the assignment of this ring structure (I) to the trimethylene oxide radical cation comes from a comparison of the ESR parameters (table 1) with previous results for ether radical cations of similar structure_ Thus, the average quintet splittin? of 65.6 G almost coincides with the A(4H) value of 65 G for the nearly isostructural tetrahydrofuran rtdiea.l cation [4]_ Also, the ratio of the quintet splitting for the trimethylene oxide radical cation of the A(6H) vahre of 43.0 G for the dimethyl ether radical cation is 1.52. in excellent agreement with the expected ratio of 1.5 (0.75 I 0.5) for the (cos2B) dependence of the @hydrogen coupling on the dihedral angle 8 in the two cations. Finally, the two g components for the trimethylene aside radical cation (table 1) closely resemble the &a~ and &in values of 2.0138 and 2.0045 found for (CH&O+ and may likewise be interpreted [5] in terms of an oxygen-centred SOMO. Turning now to the cation generated from ethylene oxide, a complication was present insofar as the ESR s~ctrunls~io~~ed a curious fine structure which changed reversibly with temperature and differed according to the nature of the Freon solvent. These results were reminiscent of those obtained for the acetaidehyde radical cation and ascribed to a matrix interaction in the following paper [S] _In the present case, however. the substructure persisted up to 150 K when CFC13 ws used as the matrix fvide infra), the spectraI changes becoming irreversible at higher temperatures due to decay of the cation. A study of the cation generated from ethylene o-tide in the CF1,CC13 matrix proved to be more rewarding in terms of removing the matrix interaction at hi& temperature. Thus, as shown in fig:. 2, the ESR spec-

CHEMICALPHYSICSLETTERS

Volume 100, number2

2 September1983

Table 1 ESR parametersfor the radical cations from trimethyleneoxide and ethyleneoxide Radical cation

C

0’

Solvent

CFCls

152

g factor

Hyperfue coupling(G)

g,, = 2.0046

A ,,(4H) = 65.5

g1 = 2.0135

A1(4H) = 65.7 A,,(ZH) 10.5 A1(2H) = 11.1

CHzOCHj CHzOC% CHDOCHD?

CF3Ccl3

132 150 145 140

2.0024 2.0022 2.0022 2.0022

A(4H) A(4H) A(2H) A(2D)

CFC13

143

2.0023

A(2H) = 16.0

CFC13 CF3Ccl3

CFCIB

CHDOCHD: CD20CD;

I

I

Fig. 2. First-derivativeESR spectraof a 2.5 mole% solution of ethylene oxide in 1 .I,l-trichlorotrifluoroethane recorded at 82 K (top) and 143 K (bottom) after +rradiation (dose. 0.5 &ad) at 77 K.

= = = =

16.6 16.3 16.3 2.5

trum at 143 K reduces to a simple 1 : 4 I 6 : 4 I 1 quintet pattern with a hyperfine splittingA(4H) of 16.3 G and a g factor (2.0022) close to that of free spin. Also, a 1 : 2 : 1 triplet spectrum with almost identical ESR parameters was similarly generated from ethylene-1,2d2 oxide, proving that the main ESR pattern originates from the hydrogens of the cation. A similar conclusion can be drawn from the results obtained with the CFCl, matrix. Despite the residual substructure which tends to obscure the main pattern for ethylene-Jr4 o-tide, the spectra in fig. 3 demonstrate a clear progression from quintet to triplet to singlet on changing from ethylene-/z4 oxide to ethylene-l J-d7 o-tide to ethylene-d4 oxide, the lineshapes becoming more symmetrical due to the damping of the highly anisotropic substructure by the deuterium hyperfme interaction. Moreover, there is good agreement between the ESR parameters derived from these spectra and those obtained from the high-temperature spectrum of the cation in the CF,CCl, matrix (table l), suggesting that the matrix perturbation in CFC13 has little effect on the spin distribution. It is interesting to note that irrespective of the type of substructure which is present in the anisotropic spectra obtained from ethulenef14 oxide in the two matrices (figs. 2 and 3), these spectra possess an inversion centre indicating little or no g anisotropy. Furthermore, the substructure cannot be attributed to ‘H hyperfine anisotropy since it is present on the centre line of the quintet pattern. We conclude, therefore, that the substructure arises from an anisotropic superhyperfine interaction [8] which is highly specific for the cation-matrix-temperature combination but that the structure of the cation is not 195

Volume 100. number 2

CHEMICAL

PHYSICS LETI-ERS

1 g. 3. Rirar-derivativc ESR spectra of dilute solutions (1-2 molt!;) ofcfltylenc oxide (top). cthylenc-1,2-d, o.xide (crntre). .md cthylenr~~ oxide (bottom) in trichbronuorotlletltane recorded at I-13 K After 2-irradiation (dose. 0.5 &ad) st 77 i;.

significantly perturbed by this matrL_ interaction. As shown in table 1, the radical cation formed from ethylene oside has strikingly different ESR paremeters from those of the trir~le~ly~ex~~ o-tide radical cation_ A similar assignment to an osygen-centred x radical with the ring-closed structure of the parent molecule can therefore be ruled out. On the other hand, the ESR data for CzH30fare readily accommodated by the ring-opened carbon-centred isomer ii which is both isoelectronic (17 valence electrons) and isostructural with the ally1 radical f14], and the atoms being coplanar. Thus, the s factor of 2.0023 and the hyperfine coupling _-l(IH) of 16.4 G are comparable to the parameters of the aIIy1 radical (g = 2_0025_A(eso-2H) = 14.8 G, A(endodH) = 13.9 G [ 14]), considering that a small difference (<3 G) between the couplings for 196

2 September 1983

the exo- and endo-hydrogens would not be resolved in the solid-state spectrum of the cation. Additional strong support for structure II comes from INDO calculations [I 51. Using a standard geometry with all bond angIes equal to 1ZOOand C-O and C-H bond distances of 1.365 and l-1 11 8, respectively, the SOMO of11 is the expected non-bonding 1 A2 (n) orbitat in C2v symmetry. The analogy with the ally1 radical carries over to the spin distribution, a negative spin density (-0.316) in the p, orbital of oxygen being compensated by a positive spin density (0.658) greater than 0.5 in the pn orbital on each carbon_ Finally, these INDO c&ulations predict isotropic couplings of -16.1 and -16.0 G for the exo- and endo-hydrogens in II, in excellent agreement with the experimental value of 16.4 G. As mentioned in section 1, evidence has aIso been obtained for the formation of the ring-opened ethylene oxide cation in the gas phase [12,13]. Although it appears that Beau& and co-workers [12] originally considered this cation to have the symmetrical coplanar (C,,) structure II because of the favourable resonance interaction with 3 conjugated n-electrons. the observation of facile CHZ transfer from Q&Of to a variety of neutraf s&&rates in ion-molecule reactions led Bouma et al. [13] to propose the asymmetric (C,) perpendicular structure represented by the formula CH2=O+--~CH2. This structure was described in more detail in a subsequent ab initio MO study of the 11 possible C2H40f isomers [16], no mention being made, however, of the ~g-owned Cz,, structure_ Using the optimized geometry for the asymmetric structure [16], we find that INDO calculations give a total energy which is 156 kcal mol-l higher than that found for the standard geometry of the CT” structure described earlier. What is of more interest& the present context, however, is the spin distribution for this asymmetric structure. Here, it is reassuring that INDO calculations faithfully reproduced the previous SOMO description {16] and showed that most of the spin density is concentrated in what is approximately an sp3 (o) orbital on the carbon of the perpendicular CHZ group such that there is appreciable ‘H coupiing (18.1 G) only to the two equivalent out-ofplane hydrogens of this methylene group. The asymmetric structure is therefore incompatible with our ESR results showing approximately the same hyperfine coupfing of ail four hydrogen& Hence it must be concluded that the cation produced from ethylene

Volume 100. number 2

CHEMICAL

PHYSICS LETTERS

solid state has the ring-opened symmetric structure II *_ oxide

in the

Acknowledgement This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy (document no. DOE/ER/02968146). * After this paper had been accepted for publication another ESR study of the ethylene oxide radical cation was reported by Symons and Wren [ 17]. Tbesc authors consider the cation to have the ring-closed structure with a “non-bonding” o (at ) SOMO.

References [ 1 ] T. ShidaandT,Kato.Chem.Phys. Letters68 (1979) 106. I?] T. Sbida, Y. Egawa, H_ Kubordera andT. Kato, J, Cbcm. Phys. 73 (1980) 5963. [3] J-T. Wang and 1:. Wihiams, J. Phys. Chcm. 84 (1960) 3156; A. Grimison and G.A. Siiipson, J. Phys. Chem. 72 (1968) 1176.

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1983

[4] H. Kubodera, T. Sitida and K. Shimokosbi, J. Phys. Cbem. 85 (1981) 2583. IS] J.T.Wangand F.Wiims. J. Am.Chem.Soc. 103 (1981) 6994. 161L.D. Snow, J.T. Wang and F. WBiams, J. Am. Chem. Sot. 104 (1982) 2062. 171 M.C.R. Symons and PJ, Boon. Chem, Phys. Letters 89 (1982) 516. Chem. Phys. Letters 100 lS1 L-D. Snow and F. Wiims, (1983) 198. PI D. Becker, K. Plante and M.D. Scviha, Abstracts of Division of Physical Chemistry, 185th Cbcmiccl Society National Meeting, Seattle (1983) no. 30; J. Phys. Chem. 87 (1983) 1648. Anal. Chem. 31 (1959) 82; IlO1 F-W. Mclafferty, J-H_ Beynon. G-R_ Lester and A-E. WlJiarns, J. Pbys_ Cbem. 63 (1959) 186X_ 1111 L-D. Snow and F. WBiams, unpublished work. 1121 R.R. Corderman, P.R. Le Breton, S.E. Buttrill Jr., A.D. Williamson and J.L. Beauchamp, J. Chem. Phys. 65 (1976) 4929_ 1131 IV-J. Bouma, J-K. XfacLeod and L. Radom, J. Chem. Sot. Chem. Commun. (1978) 724. 1131 J-K. Kochi and P.J. Krusic, J. Am. Chem. Sot. 90 (1968) 7157; H--G; Korth. H_ Trill and R_ Sustmann, J_ Am. Chem_ Sot. 103 {1981> 4483. IIS1 J-A. Pop& D-L_ Bevcridge and P-A. Dobosh. J_ Chem. Phys. 47 (1967) 2026. [I61 W-J. Bouma, J-K_ hlacLeod and L. Radom, J. Am, Chem. Sot. lOl(1979) 5540. f171 hf_C_R. Symons and B_\V_Wren, Tetrahedron Letters (1983) 2315.