First observation of different diffusion coefficients for two conformers in a neat liquid

Self-diffusion coefficients were studied for the highly polar liquid N-meth ylformamide at pressures up to 200 MPa between the melting pressure curves and 420 K by the spin-echo method. N-Methylformamide exists as a mixture of two conformers in the neat liquid, These conformers have large differences at lower temperatures in their dynamic and structural properties. The self -diffusion coefficient of the cis-conformer being 17% lower than that of th e trans-conformer at the same T and p, This is the first observation of suc h an effect. The experimental study is supported by Monte Carlo (MC) calcul ations which show that the first neighbors around a cis conformer are arran ged differently than in an all trans liquid. The difference leads in the si mulations to a much lower dielectric constant for the trans-cis mixture and might also explain the retardation of diffusion for the cis conformer.