Effects of temperature and pressure on hydrogen bonds in water and in formamide

Effects of temperature and pressure on the hydrogen-bonding liquid structure of water and formamide are discussed on the basis of X-ray diffraction and NMR data measured at high temperatures and pressures. In the discussion of the water structure at high temperatures and high pressures, it is suggested that density can be a unique parameter to describe the water structure. Supercritical water has hydrogen bonds even at a low density of 0.7 g dm(-3). The distance between adjacent water molecules is kept practically unchanged at about 292 pro over a wide density region of ca. 0.95 g dm(-3) through 0.7 g dm(-3), while the number of nearest neighbor water molecules decreases monotonously from 4.4 under ambient* condition to 1. 7 at the density of 0.7 g mol(-3) at high temperature and pressure. On the other hand, in formamide, whose structure had been concluded to consist of a mixture of the head-and-tail-type-linear and ring-dimer structures, hydrogen bonds are broken with the elevation of temperature but are enhanced with pressure to form the more compact ring-dimer structure rather than the linear one. (C) 2003 Elsevier Science B.V. All rights reserved.