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 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 their conclusions_ Recently Sevilla and his co-workers