Reversal of order of chemical shifts. Differential solvent effects in the hydrogen shifts of toluene, m‐xylene and o‐xylene as determined by 320 MHz tritium NMR spectroscopy

Abstract

Chemical shifts for the aromatic tritons of toluene decrease in the order meta > para > ortho for pure toluene, but in the order meta>ortho>para for dilute solutions of toluene in carbon tetrachloride, chloroform, cyclohexane and dimethyl sulphoxide; m-xylene shifts are 5>4,6>2 in the pnre state and 5>2>4,6 in carbon tetrachloride and dimethyl snlphoxide; exylene shifts are 4,5 > 3,6 in the pnre state and reversed in carbon tetrachloride.We report the reversal of the order of the chemical shifts for the ortho and para tritons within toluene on the addition of a variety of solvents to the pure hydrocarbon. This observation rationalizes a previously discrepancy in the literature for the proton chemical shifts of toluene, since proton and triton shifts are essentially e q~i v a l e n t .~ Similar reversals occur in the 2-and 4,6-tritons of rn-xylene and the aromatic tritons of o-xylene.Recent measurement of the 3H NMR spectrum of tritiated toluene at 64 MHz revealed that the order of the chemical shifts of the aromatic tritons was meta > para > ortho.' This assignment was subsequently shown2 to correlate with the chemical shifts of the aromatic protons of toluene as determined by 'H NMR spectroscopy of 99% deuteriated material. It was noted2 that the order of the ortho and para shifts agreed with an earlier analysis4 of the 'H spectrum of toluene, but was in disagreement with the order calculated from another earlier analysis' of the 200MHz 'H spectrum of 25% toluene in carbon tetrachloride, viz. meta > ortho > para.This anomaly led to a further 3H NMR study of pure toluene at 64MHz where the order m e t a ) para > ortho was confirmed,6 but it was also noted that with 25% toluene solutions in carbon tetrachloride the ortho and para peaks were closer than 0.02ppm and their resolution was not achieved. This result6 raised the possibility of a reconciliation of the 200MHz 'H data with the 3H and more recent 'H data on the basis of a solvent effect.* Author to whom correspondence should be addressed. Wiley Heyden Ltd, 1984The availability of a high-field 3H NMR spectrometer has now permitted careful measurement of the influence of solvent on the triton chemical shifts, confirming that the addition of all non-aromatic solvents tested does indeed change the chemical shift order from para > ortho in pure toluene to ortho > para in dilute solutions (Fig. 1). Table 1 lists the chemical shifts obtained for various concentrations of toluene on a Bruker CXP-300 spectrometer operating at 320 MHz with broad band proton decoupling.

1H nuclear magnetic resonance spectral parameters of toluene. Implications for conformational analysis and isotope shifts

“…The ring proton shifts, measured at 100 MHz in CCL, were extrapolated to infinite dilution (2), as were the 3H nmr shifts (3). From these results it is clear that measurement at 2.0 mol% as in Table 1 yields a maximum difference of less than 0.002 ppm between these shifts and those at infinite dilution.…”

“…The latter no doubt account for the fact that in neat toluene the ortho protons resonate to high field of the para proton, whereas in other solvents, including that in Table 1, the para protons are most shielded (2,3).…”

“…Based on somewhat complex referencing procedures (3,26), it was concluded that aromatic tritons resonate downfield from aromatic protons. The A values in Table 2 appear to confirm this conclusion because the 'H nmr shift of benzene in the appropriate solvent is used to calculate the A values (the shift of in [,HI-benzene relative to the shift of 'H in benzene is apparently unavailable).…”

“…[3], where the indicated errors come from the 95% For personal use only. figure. 'No evidence for finite magnitudes of these couplings existed in the spectrum, but the couplings were allowed to vary in the iterations.…”

“…The order of the chemical shifts of the ring protons is solvent dependent and is of interest from the viewpoint of the methyl group as an electron donor (2) , and in the use of 3H nmr spectroscopy (3). The coupling constants are of use in, for example, the analysis of I3C nmr spectra (4)(5)(6) and in the theory of spin-spin coupling mechanisms (7 -9).…”