The structure and conformations of 2-thiophenecarboxaldehyde obtained from partially averaged dipolar couplings

The proton NMR spectra of samples of 2-thiophenecarboxalde-hyde dissolved in a nematic liquid crystalline solvent, including those from all five singly labelled C-13 isotopomers, have been obtained. These have been analysed to yield sets of partially averaged dipolar couplings which have been used to determine the structure and the relative amounts of the cis and trans forms, which are the two minimum-energy structures generated by rotation about the ring-aldehyde bond. A procedure for applying vibrational corrections to the dipolar couplings in the presence of large amplitude motions is discussed.