The Influence of Steric Constraints on the Conformational Properties and on the 17O NMR Shielding of ortho-Substituted Perbenzoates

The 17O chemical shifts and band widths for tert-butyl peresters of ortho-substituted benzoic acids have been measured in benzene solution. Two signals are observed, of different line-width, and that at higher field is broader. The two signals shift to lower field with an increase in the number and size of ortho substituents (methyl and tert-butyl groups were chosen), and the signal at lower field is the one more significantly affected. Correlations with a fair degree of linearity are found for each set of chemical shifts with the twist angle of the carbonyl group from the phenyl plane. Twist angles were calculated from molecular mechanics. From the screening constant estimated theoretically at a semi-empirical level with the Karplus–Pople formula and from line-width arguments based on quadrupole relaxation, it is possible to advance the reasonable conclusion that the signal at lower field of these molecules represents two oxygen nuclei [those of the C(O)–O group] and the other the remaining peroxidic oxygen. For the 2,4,6-tri-But derivative which is solid at room temperature and can be obtained in a suitable crystal form, the crystal and molecular structure has been determined by X-ray diffraction. The structure is built up of one crystallographically independent molecule, affected by statistical disorder due to two alternative orientations of all the tert-butyl groups bonded to the ring. The peroxycarboxyl group is planar and nearly perpendicular (80.8°) to the benzene ring plane.