Neutron Diffraction Study of Liquid N-Methylformamide Using EPSR Simulation

Abstract

The structure of N-methylformamide (NMF) in liquid state has been investigated using a combination of neutron diffraction measurements augmented with isotopic substitution and empirical potential structure refinement computer simulations. The reference potential used was optimized previously and consisted of Coulomb and 6-12 Lennard-Jones interactions for the atoms. The results show that the three-dimensional model of the liquid structure constructed at the correct atomic number density is consistent with the diffraction experimental data. The liquid structure is orientated by the hydrogen bonds among the molecules. Each NMF molecule is, on average, hydrogen bonded to two others. The findings indicate that dimers and "linear" trimers are very stable species in the liquid bulk. Because of that, the liquid is strongly structured into a chain-like structure. Neighboring chains are stabilized with respect to each other by weak O...H(C) hydrogen bonds. The results are consistent with the known physicochemical properties of the liquid.

Quantum cluster equilibrium model of N-methylformamide–water binary mixtures

“…[32][33][34][41][42][43] The cluster can be considered as a linear trans-trimer, which is stabilized by explicit solvation of two other NMF molecules. These trimers were found to be a very likely model for the liquid phase in several experiments, 33,34,43 molecular simulations, 39,40 and also in an earlier QCE study at the RHF/3-21G level of theory. 38 We cannot make direct comparisons to that QCE study, because at our level of theory, geometry optimizations of linear clusters lead to ring-formation.…”

“…37 An early quantum cluster equilibrium (QCE) study, on the other hand, predicted a mixture of linear trans-dimers, -trimers, -tetramers, and cyclic trans-hexamers, 38 similar to Monte Carlo and molecular dynamics simulations, which suggest linear chains of dimers and trimers. 39,40 Combined ab initio and X-ray scattering experiments show that intramolecular correlations can be equally well described by cyclic cis-trimers, as well as by linear trans-trimers. 33,34 Similar conclusions were drawn from combined nuclear magnetic resonance (NMR) and infrared (IR) spectroscopic investigations.…”

A hybrid neutron diffraction and computer simulation study on the solvation of N-methylformamide in dimethylsulfoxide

“…II , the first action striving to analyze the data using EPSR is to take a suitable reference potential energy function that will be used as the seed to the subsequent structure refinement. A model previously optimized by one of us 27 was used for NMF and the P1 model optimized by Luzar and Chandler 19,20,55 was used for DMSO. The molecular geometries used were those reported previously.…”

“…The molecular geometries used were those reported previously. 19,27 In the simulations, all NMF molecules are the trans conformer, as previously discussed. 27 The potential between atoms α and β was represented by…”

“…25,26 Pure liquid NMF and its aqueous solutions have been the subject of study in recent years, both theoretically and experimentally. [27][28][29][30][31][32][33][34] Figure 1 shows a sketch of both molecules with the dipole moment vector indicated on them.…”