The atomic and electronic structure of liquid N-methylformamide as determined from diffraction experiments

Abstract

The structure of liquid N-methylformamide (NMF) has been investigated using synchrotron radiation at 77 keV and 95 keV. The use of high energy photons has several advantages, in this case in particular 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 Å^-1 < Q < 24·0 Å^-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 intra-molecular 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, there is comment on whether the different interaction mechanisms of neutrons and photons can be used to access directly 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.