  The structure of liquid N-methylformamide (NMF) has been investigated using
synchrotron radiation at 77 and 95 keV. The use of high energy photons has
several advantages, in this case especially the large accessible momentum
transfer range, the low absorption and the direct comparability with neutron
diffraction. The range of momentum transfer covered is 0.6 \AA$^{-1} <$ Q
$<$24.0 \AA$^{-1}$. Neutron diffraction data on the same sample in the same
momentum transfer range have been published previously. In that study two
differently isotope - substituted species were investigated. In order to
compare neutron and photon diffraction data properly Reverse Monte Carlo (RMC-)
simulations have been performed. Some modifications had to be added to the
standard RMC- code introducing different constraints for inter- and
intramolecular distances as these distances partly overlap in liquid NMF. RMC-
simulations having only the neutron data as input were carried out in order to
test the quality of the X-ray data. The photon structure factor calculated from
the RMC- configurations is found to agree well with the present experimental
data, while it deviates considerably from earlier X-ray work using low energy
photons (17 keV). Finally we discuss whether the different interaction
mechanisms of neutrons and photons can be used to directly access the
electronic structure in the liquid. Evidence is presented that the elastic self
scattering part of liquid NMF is changed with respect to the independent atom
approximation. This modification can be accounted for by a simple charged atoms
model.

Chemical Physics;Chemical Physics