Hydrogen -bonded fluids studied by theoretical calculations and NMR experiments: Formamide and formic acid

Many essential materials contain hydrogen bonds such as water, proteins, and alcohols. Despite the chemical and biological importance of these materials, their structure in the liquid state is not certain. One reason is that experimental data from the liquid state are difficult to interpret due to broadened lines in the vibrational spectra and long experimental timescales in nuclear magnetic resonance (NMR). In light of these experimental difficulties, the goal of this thesis is to gain a better understanding of the molecular structure and dynamics of hydrogen-bonded liquids by combining NMR experiments and theoretical calculations.;A specific goal of this work is to study the dynamics and structure of liquid formamide. High-level calculations and NMR chemical shifts provide a method for determining semi-empirical values of deuterium quadrupole coupling constants. These values combined with NMR relaxation experiments yield information about the dynamics of the system in the form of correlation times. The correlation time can then be used to estimate cluster volumes that lead to structural information. These methods are applied to neat formamide and dilute formamide in chloroform. Both studies show that the motion of formamide is anisotropic.;Other NMR parameters, such as spin-spin coupling constants (SSCCs), also provide information about hydrogen bonding and structure. A relatively new approach to the calculation of the Fermi contact term of SSCCs combines finite perturbation theory and density functional theory. This method is used to calculate the SSCCs of formamide using different hybrid density functionals and basis sets, and the calculated values are compared with experimental values. The results show that the SSCCs are very sensitive to the basis set, and basis sets with a flexible description of the core electrons converge more rapidly than standard basis sets.;Another theoretical method for studying hydrogen-bonded liquids, quantum cluster equilibrium ((SCE) theory, uses statistical thermodynamics to calculate cluster populations. This method is applied to formic acid for a large range of temperatures and pressures, and the cluster populations are compared with experimental vapor densities. Three theoretical methods and three basis sets are studied, both with and without counterpoise corrections for a total of 18 distinct theoretical models. The extreme sensitivity of the QCE cluster populations to the calculated energies allows determination of the usefulness of the studied models in describing intermolecular interactions.