Formamide, liquid structure

To understand how the BF3 affinity depends on self-association, the hydrogen-bonded solvents formamide, methanol and water were also studied as bulk liquids. The results [46] are -AH°(3.10) = 115.32 0.90,109.20 0.44 and 104.40 0.36 kJ mol , respectively. There are two obvious terms in these enthalpies (i) the enthalpy of complexation of BF3 and (ii) the enthalpy due to other interactions and to the reorganization of the liquid structure. It would be necessary to correct for the latter terms to isolate the BF3 affinity of these self-associated compounds. 

From a structural point of view the OPLS results for liquids have also shown to be in accord with available experimental data, including vibrational spectroscopy and diffraction data on, for Instance, formamide, dimethylformamide, methanol, ethanol, 1-propanol, 2-methyl-2-propanol, methane, ethane and neopentane. The hydrogen bonding in alcohols, thiols and amides is well represented by the OPLS potential functions. The average root-mean-square deviation from the X-ray structures of the crystals for four cyclic hexapeptides and a cyclic pentapeptide optimized with the OPLS/AMBER model, was only 0.17 A for the atomic positions and 3% for the unit cell volumes. 

It is worth mentioning that the relative populations of linear and cyclic structures can be shifted by temperature. A recent work which couples experimental measurements with ab initio calculations of quadupole coupling constants determined that rings of six formamide molecules dominate in the liquid state at low temperatures, but are replaced by linear tetramers as T approaches 400° K. 

The situation is more complex in the case of the so-called non-Debye liquids -the protic solvents. Due to their internal structure, these liquids exhibit a complicated dielectric relaxation behavior. This group of solvents comprises alcohols, formamide, propylene carbonate, and some other liquids. One should remember that in the In vs. In Tl analysis (Sec. 3.1.3), the rate constants measured in these solvents deviated from the values measured in aprotic solvents. 

Dimethylformamide (DMF) has a much more complex structure than either acetonitrile or acetone, but it is interesting to compare its infrared spectrum with that of formamide. By replacing the two acidic protons on the amino group by methyl groups, one obtains an aprotic liquid. As can be seen by comparing figs 5.16 and 5.19, the infrared spectrum of DMF is much simpler than that of for-... 

Studies of X-ray diffraction patterns, specific volume, and mechanical properties of PBLG films led us to intuit that some of these solid films cast from certain solvents such as chloroform and methylene chloride retained the local structure of the liquid crystalline phase. On the other hand, conventionally crystalline films of PBLG were obtained from the solvent dimethyl formamide. ... 

This criterion for the existence of order in a given liquid is A AyST > 2, and at 25 °C water has AAyS/R = 7.94, larger than many liquids deemed structured by any criterion (Marcus 1998). However, this quantity is by no means larger than for all structured liquids, see Table 1.5. When the order density in a structured liquid is expressed by division of A Ay5 by V, the molar volume of the liquid, water AAyS/V = 3.23 J cm ) becomes quite similar to 1,2-ethanediol (3.15) and 2-ethanolamine (3.21), having a somewhat lower value than glycerol (4.20), but a higher one than formamide (1.58) and methanol (1.28). 

Contrary to expectations from the open tetrahedral hydrogen-bonded structure of ice, liquid water is a close-packed rather than an open liquid. It shares this property with 1,2-ethanediol, glycerol, and formamide, among common solvents, see Table 1.5. These four liquids have (1 — Vx/V) <0.1, whCTeas most otho" common solvents for ions have larger values of this quantity. Liquids that have large fractions of free volume, i.e., are open , are quite compressible, and there exists a moderate positive correlation of (1 — Vx /1 ) with the isothermal compressibility. Water shares a low isothermal compressibility, kj = 0.457 GPa at 25 °C, with the above named low-openness solvents for which kj < 0.5 GPa , whereas for other common solvents the values range from 0.524 (dimethylsulfoxide) to 1.706 (n-hexane) (Marcus 1998). 

This polymer has high softening temperatures and is insoluble in all but a handful of highly polar liquids with marked hydrogen bonding capacity (e.g. H CO N(CH3)2, dimethyl formamide). Both properties are the direct result of the extreme difficulty of separating the chains and breaking the crystal structure. The polymer is unsuitable for paints, but has ideal properties for a synthetic fibre ( Acrilan ). 

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