X-ray scattering and density-functional theory calculations to study the presence of hydrogen-bonded clusters in liquid N-methylacetamide

Abstract

A structural investigation of liquid N-methylacetamide (NMA) is performed by x-ray scattering and density functional theory (DFT). Experimental data are analyzed to yield the total structure function SM(Q) and the pair correlation function g(r). The DFT calculations, using the standard triple zeta valence basis set augmented by a diffuse function for carbon, nitrogen and oxygen atoms, are performed on the one hand to study the structure and stability of the two possible conformers cis and trans. On the other hand, they are meant to examine some possible clusters which may describe the intermolecular arrangement in liquid NMA. Among two series of dimers and trimers associations, the spectra are particularly interpreted in terms of: Trans NMA dimers and trimers which resemble the short-range crystal structure, mixed cis and trans trimers and cis cyclic trimers. The H-bonding parameters and the intermolecular energy for each model are described.

Local Order in Fully Deuterated Liquid N-Methylacetamide (C3D7NO) As Studied by Neutron Diffraction and Density-Functional Theory Calculations

“…Similar curves corresponding to X-ray investigations 13 are reported in Figure 4D,F. Two important points should be addressed.…”

“…Similar curves as deduced from our previous X-ray study 13 are also drawn in Figure 3C,D for comparison. One can notice that the theoretical curves reproduce the experimental ones well at low r-range.…”

“…Figure 4A shows respectively the Fourier transformations g L HB (r) trans and g L HB (r) cis of the D M HB (Q) trans and D M HB (Q) cis functions as deduced by considering, respectively, the trans (solid line) and cis (dotted line) isomers. Similar curves based on X-ray scattering data 13 are also reported in Figure 4B. It is important to underline that in our fitting procedure of neutron data, the N‚‚‚O and D‚‚‚O distances are deduced from DFT calculation, the DW parameters µ N‚‚‚O are considered from our previous X-ray study, 13 and only the µ D‚‚‚O ones are fitted here.…”

“…Similar curves based on X-ray scattering data 13 are also reported in Figure 4B. It is important to underline that in our fitting procedure of neutron data, the N‚‚‚O and D‚‚‚O distances are deduced from DFT calculation, the DW parameters µ N‚‚‚O are considered from our previous X-ray study, 13 and only the µ D‚‚‚O ones are fitted here. The hydrogen bond parameters are quoted in Table 1.…”

“…By comparing parts A and B of Figure 4, we can clearly notice the contribution of the X‚‚‚D interaction in the D M HB (Q) function. The observed peaks respectively at 2.95 and 2.85 Å in g L HB (r) trans and g L HB (r) cis , assigned to the N‚ ‚‚O interactions, are more manifest in the X-ray curves, 13 whereas neutron results show two more pronounced peaks at 1.95 Å, corresponding to D‚‚‚O interactions. It is interesting to underline, here, the complementarity between the information that can be extracted from X-ray and neutron scattering.…”