Proton-proton nuclear overhauser effect as a probe for symmetry-preserving relaxation in oriented molecules

Abstract

Difference nuclear Overhauser effect (NOE) experiments on oriented acetone and 2,1, 3-benzoselenadiazole, containing A₃A'₃ and AA'BB' spin systems, respectively, show that in the case of the former, NOE transfer is restricted to within a given irreducible representation while in the case of the latter, transfer from symmetric to antisymmetric representation does occur. Two possible mechanisms of relaxation in such systems, the intramolecular dipole-dipole and the external random field, were investigated and it was seen that in general the dipole-dipole mechanism does not restrict NOE transfer to a given representation. However, special geometry could restrict NOE transfer to transitions of the same representation. On the other hand a fully correlated external random field does restrict NOE transfer to one representation. Detailed theoretical calculations of the NOE in AA'BB' show good fit with the experiment for ωτ_m ∼ 2.0.