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.