Oriented PMR spectra of acetone and DMSO and molecular orbital studies of ⁴J_HH

Abstract

The 100 MHz proton magnetic resonance spectra of acetone and dimethyl sulfoxide (DMSO) oriented in the nematic phase of 4,4'-di-n-hexyloxyazoxybenzene have been recorded. The spectra have been analyzed as A₃ A₃' spin systems. The four-bond indirect spin couplings between the methyl group protons are found to be 0.55 and 0.40 Hz for acetone and DMSO, respectively, in good agreement with the results obtained from ¹³C satellite studies. The signs and magnitudes of the orientation parameters for acetone have been obtained. The long-range four-bond proton couplings in acetone, isobutene and propane have been studied theoretically employing the finite perturbation SCF-MO method using CNDO/2 and INDO wavefunctions and the Pople-Santry method with extended Huckel theory (EHT) wavefunctions. The finite perturbation theory predicts the conformational dependence of these couplings satisfactorily whereas the Pople-Santry method is much less superior in this respect. The geminal proton couplings are found to be sensitive to the Slater orbital exponents used while the long-range four-bond couplings are not.