Double and Triple Resonance Experiments on 2-Bromo-5-Chlorotoluene: Signs of the Ring and Ring-Methyl Proton Coupling Constants

Abstract

'l'iclcling and decoupling (triple resonance) techniques show that the ortho, meta, and para proton coupling constants have the same sign in 2-bromo-5-chlorotoluene. If the ortho coupling in the ring is positive, then decoupling experiments show that the methyl proton couplings to the ring protons in the ortho and para position to the methyl group are negative, while that to the meta position is positive. These signs are in agreement with the n-r exchange polarization mechanism. INTRODUCTION Long range coupling between the methyl and ring protons in substituted benzenes is lcnown (1-3). Hoffman (1) attributed such coupling in mesitylene to hyperconjugation. Calculations by McConnell (4, 5) invoked a u-a exchange polarization mechanism for long range couplings like the para coupling of ring protons. Acrivos (6) found that the magnitudes of the ring-methyl proton couplings in mesitylene were predicted satisfactorily by the same mechanism, as did Cohen and McLauchlan (3) for 2-carbomethoxy-5,6-dimethylbenzofuran. Theory (5, 6) also predicts the signs of the couplings. While the relative signs of methyl proton couplings with protons on the furan (7) and thiophene rings are known (8, 9), such is not the case for the benzene ring, although there have been a few careful determinations of the magnitudes of these couplings (3, 10).T h e ortho coupling between the protons on the benzene ring is positive (11-13) as are the meta and para couplings in all lcilown cases (14-17). We describe experiments here u~hich determine the sign of the ring-methyl proton couplings and of the meta and para couplings relative to the ortho coupling in 2-bromo-5-chlorotoluene. EXPERIMENTALMeasurements were made on a degassed, 10 mole yo solution of 2-bromo-5-chlorotoluene in carbon disulhde, with tetra~nethylsila~le (TIMS) as internal reference. Decoupling and tickling experiments, some of them involving triple resonance, were performed with a DA-60-1 Varian spectro~neter according t o standard procedures (18)(19)(20)(21)(22). The temperature of the sample was 28.5 "C as determined using an ethylene glycol s a n~p l e .~ RESULTS

Proton Magnetic Resonance Spectrum of Parafluorotoluene. Sigma and Pi Electron Contributions to JpH,F

“…Thus we find that even partial protonation of 2-fluoro-5-methylpyridine does not change J P F p C H 3 . sumption which holds to within experimental error for J~~,~ in benzene derivatives (1)(2)(3)(4)(5), the o electron contribution to JpH3F in fluorobenzene and its derivatives can be discussed as follows. The JpH,F value in fluorobenzene lies between 0.2 and 0.4 Hz depending on solvent and analytical conditions (13,20,21).…”

“…The long-range coupling to the methyl protons in benzene derivatives is remarkably insensitive to substituent effects (1)(2)(3)(4)(5). Coupling between the ring protons shows a characteristic, probably alternating, dependence on the electronegativity of the substituents (6,7).…”

Concerning in 2-methylphenol and its derivatives. A proximate coupling

“…The assignment of H-3 and H-5 follows umambiguously from the magnitudes of their couplings to the methyl protons, always being larger than 6~,y'CH"n toluene derivatives (36)(37)(38). Furthermore, 3~3 4 is larger than 'Jq5, as expected from known substituent effects on vicinal coupling constants in benzene derivatives (39,40).…”

Long-range proton–19F spin–spin coupling interactions in phenyltrifluorosilane

“…The long-range spin-spin coupling oonstants I between ring protons and protons or fluorine nuclei in the sidechain of benzene derivatives I have been investigated in some detail (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). In addition to providing tests of theories of the transmission of spin state information, the couplings are often sensitive to the conformation of the molecule and have been employed in the empirical determination of free energy differences between conformational isomers (17,18).…”