Line multiplicities in solid-state 13C nuclear magnetic resonance: separation of splittings due to conformational and crystallographic effects in p-alkoxybenzoic acids

Abstract

High-resolution ¹³C n.m.r. spectra of a series of p-alkoxybenzoic acids have been obtained in the solid state. Effects that are observed and are not present in the solution spectra of these compounds are related to the known X-ray data. It is shown that line splittings in the solid-state spectra are due to both conformational and crystallographic effects. Furthermore, it is demonstrated for this homologous series of simple p-alkoxybenzoic acids that if the X-ray structure of only one of the members were known, it would be unjustified to extrapolate the results to the solid-state spectra of the remaining compounds, and thereby draw any meaningful conclusions about their conformations. Caution is therefore still required regarding the assignment of line multiplicities to specific conformational effects in organic molecules on the sole basis of the ¹³C solid-state n.m.r. spectrum.