tailieunhanh - PROCEEDINGS OF THE INTERNATIONAL SCHOOL OF PHYSICS "ENRICO FERMI"_2

In many respects, the science of materials has only fully utilized two of its three fundamental tools —the variables of temperature and chemical composition. Pressure, the third fundamental variable altering materials, is in many ways the most remarkable, as it spans some 60 orders of magnitude in the universe. Yet, its true potential for exploring the nature of materials was for years unfulfilled for a number of reasons: the accessible pressure-temperature conditions were too modest to cause significant changes in many materials, samples under high pressure could not be subjected to thorough analyses, or theory was not sufficiently well developed to understand or predict the variety of phenomena. | This page intentionally left blank Hydrogen bonding at high pressure J. s. Loveday Department of Physics and Astronomy and Centre for Science at Extreme Conditions The University of Edinburgh - Mayfield Rd Edinburgh EH9 3JZ Scotland UK 1. Introduction The properties of the hydrogen bond are applicable to a wide range of fields. They play a crucial role in many areas of biology the base pairings in DNA are the result of H-bonds the behaviour of water and other H-bonded solvents are crucial in chemistry H-bonds and their directional nature are responsible for the structural versatility of ice giving rise to at least eleven phases below 2 GPa hydrogen bonding plays an important role in determining the dehydration properties of hydrous minerals implicated as a possible cause of deep-focus earthquakes 1 and since the outer planets and their satellites contain large quantities of ice ammonia and methane the properties of these systems are crucial to planetary modelling. This ubiquity provides a very powerful motivation to understand the microscopic behaviour of hydrogen bonding including the relationships between bonding strength atomic species and bond geometry 2 . 2. - Definitions Figure 1 shows a schematic of a hydrogen bond. Atom A is covalently bonded to a hydrogen which hydrogen bonds to atom B. Atom A is referred to as the donor and B the acceptor. The criteria which determine if a particular contact is a hydrogen bond are somewhat subjective but consist of a combination of geometric and vibrational properties. The principal criterion is that the H B distance is less than the sum of the van der Waals radii of H and B taking the value for H to be 1Â 3 . In addition there is an expectation that the A-H stretch vibrational mode should soften and that Societá Italians di Fisica 357 358 J. s. Loveday Atom A 8 Atom B Acceptor Atom A Donor Atom B Acceptor 6- Fig. 1. - A schematic diagram of long upper and short lower H-bonds. the A-H B libration mode should stiffen. For