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Radical cations of aldehydes and ketones
Volume 100, number 2
CHEMICAL
2 September
PHYSICS LETTERS
1983.
COMMENT RADICAL
CATIONS
OF ALDEHYDES
AND KETONES
Martyn CR. SYMONS and Philip J. BOON Department of clremistry, Leicester University, Leicester. LEI 7RH, UK Received 29 June 1983
We confxm the observation made by Snow and williams in the previous letter that the quartet substructure observed in the ESR spectrum of acetaldehyde is due to hyperfiie coupling to a single chlorine nucleus of the solvent, fluorotrichloromethane. Reasons for this reversible interaction,
and its absence for other similar cations are discussed_
We recently published an ESR spectrum, obtained from irradiated solid solutions of acetaldehyde in CFCl,, which we assigned to the acetaldehyde cation [ 11. A large proton coupling of x136 G was
tings were resolved in addition to the splittings due to four equivalent fluorine nuclei_ This was assigned to interaction with a 1g F nucleus of a solvent molecule_ We remain puzzled as to why this n-cation
assigned to the aldehydic proton and a small quartet splitting was assigned to three equivalent methyl protons. Ushida and Shida made the same assignment at about the same time [2]_ Cottpling to matrix nuclei However, the form of this quartet splitting is unusually asymmetric, and difficult to explain in terms of almost isotropic interactions. In our studies on various chloroalkane and bromoalkane cations, however, well-defined hyperfine coupling to a single chorine nucleus of a CFCI, molecule was observed in addition to hyperfine coupling to the solute cation nuclei [3], and this led us to consider the possibility that the quartet splitting in the (MeCHO)+ cation spectrum was actually due to chlorine. Re-examination of the spectra showed that this must be the case, and we confirmed this by studying the spectrum for (CD3CDO)* cations. Snow and Williams have come to the same conclusion [4], with a very thorough study of a variety of deuterated acetaldelydes. We fully agree with
should prefer to exhibit coupling to fluorine rather than chlorine. Snow and Williams suggest that an extra doublet splitting of =:5 G for the acetaldehyde cation adduct might be due to 19F coupling, but we think that this is more likely to be a small splitting between the x and ,I’ features, since there is no reason for assuming axial symmetry, and lgF splitting is not resolved for other adducts of this general type. hlature of the bonding to cltlorine. We have suggested [6] that the matrix interaction involves fonation of a weak u-bond localised between the parent cation and one chorine atom, as shown in I for the acetaldehyde adduct, the u* SOMO being indicated_ -
their conclusions_
Recently
Sevilla
+
F
.c& ‘cl
and his co-workers
[S]
have shown that the cation of methyl formate gives a much stronger interaction with one chlorine nucleus of a CFC13 molecule, and our own work on this cation is in good agreement with theirs [6]. It is interesting to note that in our studies of the C2Fi cation in this matrix 171, well-defined doublet split0 009-2614/83/0000-0000/S
03-00 0 1983 North-Holland
I
For the chloroalkane cation adducts there is almost equal sharing of spin density between the two chlorine atoms [3], and we have shown that for 203
Volume 100, number 2
2 September 1983
CHEMICAL PHYSICS LElTERS
sever4 csamples of such adducts there is a reasonable correlation between the parallel chlorine coupling and the ionization potential of the substrate f6]_ The sign 0ftl~(3~f~~Cl) is unfortu~te~y undeterurincd in these studies. For suclr u* radicaIs it is * xmlly taken as being positive, but Snow and Williams may well be correct in suggesting that it
is negative for the acetaldehyde cation adduct, just just as it is for the halide-ion adducts of alkyl radicals (81. If they are right. and the loss of resolved chlorine coupling at ~140 K is due to motional averaging as they su@est. the isotropic coupling being sery small and not resolved. then their observation that there is no change in spin density on the aldehydic proton on annealing is not significant since they are postuIating that fhe irtteraction is still present. The alternative, that there is a revcrsrble tIrcm~a1 dissociation of the adduct seems to us to be snore probable, and in that case. the insensitivity of the 136 G proton splitting is indeed significant. This is supported by the fact tb.rt the acetoue cation shows no chlorine splitting even at 77 K. when motional averaging is improbable_ The fact that the relatively small steric barrier caused by the second methyl group. and the small decrease
in ionization potentiai combine to prevent adduct formation shows that the bonding is indeed very wedk. If the bond does break. then their argument that A(l I-i) hardly changes strongly supports the contfpt of a very minor interxtion for the acetaldchy de c,rtion. The drastic. irreverstble. change that we [b] and Becker et al. 151 h.rve observed for the methyl formate adduct arises for a different mason. in our view. In this case the initial species is formed from 3 (CFCI,) cation. by formation of a 0% bond to the non-bonding os>*gen orbital. as in I, with -CH;
replxed hy CII,O-_ The larger delocahsation arises hetxuse of the larger ioli~atio!~ potential ofmeth_vl tomrate. l toweve~ _ when Uris bond breaks. the
II
methyl formate cation changes to the more stable a-structure, the SOMO being that indicated in II. This species. despite its similar ionization potential, fails to form an adduct with CFCI,. This nicely illustrates the other factor governing this type of interaction, namely, the extent to which the SOMO is localised, so as to facilitate o-bonding. If the interaction were simply a weak charge transfer between solute cations and matrh molecuies it could still occur for delocalised systems. and also, we woufd expect to observe hyperfine coupling to at least two of the chlorine ligands of CFC$ rather than just one.
References 111 ,II.C_R. Svmons and P_L Boon. Chem. Phys. Letters 89 (19S2) 516. [ 21 E- &hid& and T+Shida, 21st Japanese Symposium on ESR 26 (19521 10. 131 G.W. Eastland. M. H.ryashi, A. Hasegawa. S.P_ hlaj, ?&CR. Sy mom and T_ Wakabayashi, Tetrahedron Letters. submitted for publication. 14) L.D. Snow and 1:. Williams, Chem. Phys. Letters 100 (1983) 198. [S 1 11.Beulier, h;. Plante and h1.D. Sevilla, J. Phys. Chem. 87 (1983) 1648. 161 A. Hasegwa, J. Rideout. G.W. Eastland and M.C.R. Symons. J. Chem. Res., submit?ed for publication. 171 A. W~segawaand M.CR. Symons, J. Chem. SW. Faraday Trans. 179 (1983) 93. 181 XCR. Symons and LG. Smith, 3. Chem. Sot. Perkin Tmns. II (1981) 11 SO.
CHEMICAL
2 September
PHYSICS LETTERS
1983.
COMMENT RADICAL
CATIONS
OF ALDEHYDES
AND KETONES
Martyn CR. SYMONS and Philip J. BOON Department of clremistry, Leicester University, Leicester. LEI 7RH, UK Received 29 June 1983
We confxm the observation made by Snow and williams in the previous letter that the quartet substructure observed in the ESR spectrum of acetaldehyde is due to hyperfiie coupling to a single chlorine nucleus of the solvent, fluorotrichloromethane. Reasons for this reversible interaction,
and its absence for other similar cations are discussed_
We recently published an ESR spectrum, obtained from irradiated solid solutions of acetaldehyde in CFCl,, which we assigned to the acetaldehyde cation [ 11. A large proton coupling of x136 G was
tings were resolved in addition to the splittings due to four equivalent fluorine nuclei_ This was assigned to interaction with a 1g F nucleus of a solvent molecule_ We remain puzzled as to why this n-cation
assigned to the aldehydic proton and a small quartet splitting was assigned to three equivalent methyl protons. Ushida and Shida made the same assignment at about the same time [2]_ Cottpling to matrix nuclei However, the form of this quartet splitting is unusually asymmetric, and difficult to explain in terms of almost isotropic interactions. In our studies on various chloroalkane and bromoalkane cations, however, well-defined hyperfine coupling to a single chorine nucleus of a CFCI, molecule was observed in addition to hyperfine coupling to the solute cation nuclei [3], and this led us to consider the possibility that the quartet splitting in the (MeCHO)+ cation spectrum was actually due to chlorine. Re-examination of the spectra showed that this must be the case, and we confirmed this by studying the spectrum for (CD3CDO)* cations. Snow and Williams have come to the same conclusion [4], with a very thorough study of a variety of deuterated acetaldelydes. We fully agree with
should prefer to exhibit coupling to fluorine rather than chlorine. Snow and Williams suggest that an extra doublet splitting of =:5 G for the acetaldehyde cation adduct might be due to 19F coupling, but we think that this is more likely to be a small splitting between the x and ,I’ features, since there is no reason for assuming axial symmetry, and lgF splitting is not resolved for other adducts of this general type. hlature of the bonding to cltlorine. We have suggested [6] that the matrix interaction involves fonation of a weak u-bond localised between the parent cation and one chorine atom, as shown in I for the acetaldehyde adduct, the u* SOMO being indicated_ -
their conclusions_
Recently
Sevilla
+
F
.c& ‘cl
and his co-workers
[S]
have shown that the cation of methyl formate gives a much stronger interaction with one chlorine nucleus of a CFC13 molecule, and our own work on this cation is in good agreement with theirs [6]. It is interesting to note that in our studies of the C2Fi cation in this matrix 171, well-defined doublet split0 009-2614/83/0000-0000/S
03-00 0 1983 North-Holland
I
For the chloroalkane cation adducts there is almost equal sharing of spin density between the two chlorine atoms [3], and we have shown that for 203
Volume 100, number 2
2 September 1983
CHEMICAL PHYSICS LElTERS
sever4 csamples of such adducts there is a reasonable correlation between the parallel chlorine coupling and the ionization potential of the substrate f6]_ The sign 0ftl~(3~f~~Cl) is unfortu~te~y undeterurincd in these studies. For suclr u* radicaIs it is * xmlly taken as being positive, but Snow and Williams may well be correct in suggesting that it
is negative for the acetaldehyde cation adduct, just just as it is for the halide-ion adducts of alkyl radicals (81. If they are right. and the loss of resolved chlorine coupling at ~140 K is due to motional averaging as they su@est. the isotropic coupling being sery small and not resolved. then their observation that there is no change in spin density on the aldehydic proton on annealing is not significant since they are postuIating that fhe irtteraction is still present. The alternative, that there is a revcrsrble tIrcm~a1 dissociation of the adduct seems to us to be snore probable, and in that case. the insensitivity of the 136 G proton splitting is indeed significant. This is supported by the fact tb.rt the acetoue cation shows no chlorine splitting even at 77 K. when motional averaging is improbable_ The fact that the relatively small steric barrier caused by the second methyl group. and the small decrease
in ionization potentiai combine to prevent adduct formation shows that the bonding is indeed very wedk. If the bond does break. then their argument that A(l I-i) hardly changes strongly supports the contfpt of a very minor interxtion for the acetaldchy de c,rtion. The drastic. irreverstble. change that we [b] and Becker et al. 151 h.rve observed for the methyl formate adduct arises for a different mason. in our view. In this case the initial species is formed from 3 (CFCI,) cation. by formation of a 0% bond to the non-bonding os>*gen orbital. as in I, with -CH;
replxed hy CII,O-_ The larger delocahsation arises hetxuse of the larger ioli~atio!~ potential ofmeth_vl tomrate. l toweve~ _ when Uris bond breaks. the
II
methyl formate cation changes to the more stable a-structure, the SOMO being that indicated in II. This species. despite its similar ionization potential, fails to form an adduct with CFCI,. This nicely illustrates the other factor governing this type of interaction, namely, the extent to which the SOMO is localised, so as to facilitate o-bonding. If the interaction were simply a weak charge transfer between solute cations and matrh molecuies it could still occur for delocalised systems. and also, we woufd expect to observe hyperfine coupling to at least two of the chlorine ligands of CFC$ rather than just one.
References 111 ,II.C_R. Svmons and P_L Boon. Chem. Phys. Letters 89 (19S2) 516. [ 21 E- &hid& and T+Shida, 21st Japanese Symposium on ESR 26 (19521 10. 131 G.W. Eastland. M. H.ryashi, A. Hasegawa. S.P_ hlaj, ?&CR. Sy mom and T_ Wakabayashi, Tetrahedron Letters. submitted for publication. 14) L.D. Snow and 1:. Williams, Chem. Phys. Letters 100 (1983) 198. [S 1 11.Beulier, h;. Plante and h1.D. Sevilla, J. Phys. Chem. 87 (1983) 1648. 161 A. Hasegwa, J. Rideout. G.W. Eastland and M.C.R. Symons. J. Chem. Res., submit?ed for publication. 171 A. W~segawaand M.CR. Symons, J. Chem. SW. Faraday Trans. 179 (1983) 93. 181 XCR. Symons and LG. Smith, 3. Chem. Sot. Perkin Tmns. II (1981) 11 SO.