Liquid structures of pure N,N-dimethylformamide (DMF) and acetonitrile (AN) as well as of their 1 : 1 molar mixture were investigated by X-ray diffraction at 25 °C. The careful reinvestigation of pure DMF led to a good reproducibility of the previous result. Slight deviations in the structural parameters are discussed and a detailed parameter table is reported. A simple model in which each AN molecule has 2 neighbors at an average distance of 330 pm describes the more ordered first neighbor structure of pure AN. The main interaction forming the structure is of dipole–dipole type, in agreement with previous findings. In the 1 : 1 molar mixture a dipole–dipole model also leads to a satisfactory description of the first neighbor structure. The preferred orientation is that with dipoles in the antiparallel position. A central DMF molecule has 1.75 neighboring AN molecules in average at a distance of 316 pm. The same stands for DMF around AN. No evidence for other kinds of interaction, e.g., weak H-bond type intermolecular interactions could be found in the mixture.
The structure of N,N-dimethylformamide (DMF) was determined at 25 °C by means of X-ray diffraction using a θ-θ type X-ray diffractometer. The least-squares analysis of the reduced intensities results that a DMF molecule has a practically planar skeletal structure in the liquid state. Intramolecular structural parameters within a DMF molecule were determined as follows: C=O; 1.24 Å, N–C(CHO); 1.35 Å, and N–C(CH3); 1.45 Å. The bond lengths are not appreciably different from those found in the gas phase, except for the C=O length, which is slightly longer by 0.04 Å in the liquid state than in the gaseous one. The radial distribution curve of DMF shows that the intermolecular arrangement of DMF molecules is practically fully disordered and no evidence has been found for the cluster formation of DMF molecules in the liquid state. The experimental result for the weak intermolecular interactions between DMF molecules is supported by ab initio MO-SCF calculations.
The liquid structure of dim ethyl sulfoxide (DMSO) was investigated by X-ray diffraction. The intramolecular structure parameters were obtained as follows: S = 0 150(1) pm, C -S 180(1) pm, nonbonding C -0 267(4) pm and C ---C 271 (7) pm. It is suggested that the molecular arrange ment in the liquid state is sim ilar to that in the solid state. It is pointed out that not only the value but also the position of the dipole mom ent vector in the molecule is essential for dipoledipole interm olecular interactions and that the latter factor results in a higher melting point of DMSO than o f N ,N -dim ethylform am ide in spite of their sim ilar dipole moments.
The liquid structure of N-methylformamide (NMF) has been investigated by the X-ray diffraction method and the ab initio MO-SCF method. The structure parameters within a molecule were obtained as follows: C=O : 122(1) pm, C(methyl)–N : 145(2) pm, C(carbonyl)–N : 134(2) pm, N···O : 222(4) pm, C(methyl)···O : 278(6) pm. The intermolecular hydrogen–bonded N···O distance was estimated to be 298(11) pm. A linear and flexible chain structure was proposed for the liquid structure of NMF on the basis of scattered intensity data by the X-ray diffraction method and of interaction energies and geometries of hydrogen-bonded NMF molecules calculated by the ab initio MO calculations.
The structure of liquid formamide is discussed on the basis of X-ray diffraction measurements and MO calculations. It is concluded that liquid formamide has both chain-like and ring-dimer moieties and that the former ones have a longer life time.
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