Vibrational sum frequency spectroscopic studies and molecular dynamics simulations of water surfaces

Understanding the structure of water has been a topic of much interest for over half a century. To many people outside the field, it would appear surprising that new developments are still being made with regards to earth's most simple and abundant molecular liquid. Yet many questions remain, one of which concerns the structural properties of the water surface. Water surfaces are everywhere, from macroscopic systems such as the earth's oceans and clouds, to microscopic systems such as living cells. Many important physical, chemical and biological processes occur in each of these media, yet it is largely unknown how the structural properties of the water surface aids or influences these processes, or if any correlating relationship can be asserted at all.;A powerful technique used to study the structural properties of the water surface on a molecular level is vibrational sum frequency spectroscopy. This technique probes the OH bonds of water directly, and thus provides information pertaining to the structural features of every detectable water molecule. But herein lies a problem. The phenomenon commonly known as hydrogen bonding creates prolific coupling effects in water that are responsible for many unique and interesting molecular properties. On a spectroscopic basis, these coupling effects provide efficient relaxation pathways for any given solvated water molecule with its immediate environment, resulting in short lifetimes and large natural linewidths. The consequence of this is a profound difficulty in correlating a solvated OH bond vibrational frequency with a particular type of molecular environment.;This dissertation discusses the structural properties of the water surface. In particular, isotopic mixtures of H₂O, HOD and D₂O are studied at the vapor-water interface to assess the impact of hydrogen bonding on the spectral response of water; and mixtures of carbon tetrachloride and 1,2-dichloroethane are applied at the liquid-liquid interface to assess the impact of organic liquid polarity on the spectral response of water. These measurements are studied both experimentally and computationally. As a result of knowing the individual response of each water molecule on a computational basis, the complications arising from hydrogen bonding in water are resolved and demonstrated.