The conformation of 2,2′-dithiophene in nematic solvents determined by1H-NMR

Abstract

In this paper we report a determination of the structure 2,2′-dithiophene dissolved in different nematic liquid crystal solvents. The 1H-NMR spectra of this molecule dissolved in a 55% Merck ZLI 1132/45% N-(4-ethoxybenzylidene)-4′-n-butylaniline (w/w) mixture are recorded at 300 and 304 K and are analysed. The dipolar coupling constants obtained are used to show that the solute molecule exists in both the s-cis and the s-trans conformations. The s-trans conformer is planar and the absolute value of the dihedral angle Φ between the two thiophene ring planes for the s-cis conformer is found to be (24 ± 1)°. Using the same method of analysis we reinterpreted two additional independent sets of published dipolar coupling constants from 1H-NMR spectra of 2,2′-dithiophene dissolved in Merck Phase IV at room temperature and in 80% N-(4-ethoxy-benzylidene)-4′-n-butylaniline/20% O-carbobutoxy-4-oxybenzoic acid ethoxy phenyl ester at 301 K. These additional studies yield the same results for the structure of 2,2′-dithiophene. We also estimate that in each of the three nematic solvents (61 ± 3)% of the solute 2,2′-dithiophene exists in the s-trans state.