Stable carbocations. 257. One-bond carbon-13-carbon-13 NMR coupling constants in substituted benzoyl cations (original) (raw)

1984, Journal of the American Chemical Society

Figure 4. Plot of AJCiw- ,-arbony, in 0-protonated benzaldehydes vs. those in 0-protonated acetophenones. that there is a linear free energy relationship between AJ values of two similar system. A plot of AJci,-c,b,, values in substituted 0-protonated benzaldehydes against the corresponding values in 0-protonated acetophenones is also linear (Figure 4) (r = 1 .O). These linear plots of AJ values indicate that the substituent effect on Jcc values (SCC) in systems t h a t are closely related in their electron demand is very similar. Jcc values can, a s shown in the present study, be valuable in the study of carbocationic systems. W e are continuing our further study on 13C-13C coupling in such electron-deficient systems. Experimental Section All acetophenones and benzaldehydes used were commercially available samples with 299% purity. Regular 13C NMR and I3C satellite spectra were recorded at 50.3 MHz with use of a Varian XL-200 su-perconducting NMR spectrometer equipped with a variable-temperature broad-band probe. The spectra of neutral acetophenones and benzaldehydes were obtained at room temperature in CDCI, solvent (-30% solution). The 0-protonated carboxonium ions were prepared by slow addition of the corresponding acetophenone or benzaldehyde to FSO3H at ca.-40 ' C (-25% solution), and the spectra were recorded in FSO3H at-35 'C. The pulse sequence used for the I3C satellite spectra, based on Freeman et aI.,l7 is 90' (x)-+-180°Cy)-r900(x)-A-90'(~)-Acq.(+), where T N (2n + 1)/4Jcc, A is a very short delay (-10 f i s) needed to reset the radio-frequency phase during which time double quantum COherance evolves,and $J and + are the phase of the last 90' "read" pulse and the receiver reference phase, respectively. Optimum setting of T for direct coupling is when n = 0 and thus is set at 4.5 ms (corresponding to a Jcc value of-55 Hz). The repetition rate of this sequence is-10 s, and reasonable S/N was achieved in 6-8 h of acquisition. The coupling constants can be directly measured from the "satellite" spectra." The accuracy of the coupling constants in a neutral compound are 10.2 Hz while those in the protonated carboxonium ions are 1 0. 3 Hz. Some of the coupling constants could not be measured accurately due to overlap of peaks or due to broadening because of slow rotation around the Cipo-Ccsrbnyl bond (in the carboxonium ions).