Analysis of Nuclear Magnetic Resonance Spectra of Protons in Predominantly Isotactic Polystyrene.

摘要

NMR chemical shifts have been calculated for methine and aromatic protons of the central >CHC6H5 group in the all-meso m8 and in the nonads rrm6, mrrm5, m2rrm4, and m3rrm3 containing a single racemic triad, each nonrad being situated within an otherwise isotactic (meso) polystyrene chain. The magnetic shieldings by phenyl groups that are first and second neighbors along the chain were computed according to their distances and orientations relative to the resonant proton H* in each conformation of the chain using the ring current representation of the pi electrons or, alternatively, the magnetic anisotropy of the phenyl group and the McConnell equation. The resulting chemical shifts were averaged over all conformations on the basis of the rotational isomeric state scheme appropriate to polystyrene. Shielding by solvent (tetrachloroethane) was treated empirically by taking averages over the occurrence of phenyl groups in direct apposition to H*, whereby solvent is excluded from H*. The calculations for the methine proton are in excellent agreement with the experimental 300-MHz spectra, for selectively deuterated, partially epimerized (3%) isotactic polystyrene. The calculations match the observed spectra both in the order of occurrence of the resonances for the various nonads and, approximately, in the magnitudes of the spacings between them. Analogous treatment of the chemical shifts for the aromatic protons in ortho and meta positions is indecisive owing to the extraordinary sensitivity of the shielding to torsional angles in the chain backbone.