Theoretical Hartree-Fock Investigation of the Structure and Associated Hyperfine Properties of Hydrogenic Impurities in Fullerenes and Alkali Halides

Three different sets of systems have been investigated with the ab initio real-space Hartree-Fock procedure: C _{60}H(Mu), C_{61}H_2, and hydrogenic impurities in KBr. The properties investigated include the geometry, electronic structure and hyperfine properties, including isotropic and anisotropic magnetic hyperfine constants as well as the nuclear quadrupole coupling constant and associated asymmetry parameter. In C_ {60}H(Mu), three different bonding sites for hydrogen or its lighter impurity, muonium (Mu) have been studied: exohedral, at the center and endohedral. Calculations performed on single molecules show the exohedral site to be the most likely bonding site from energy considerations, followed by the central site. The magnetic hyperfine constants correspond the experimentally-observed low- and high-frequency signals. Further studies involving more than one molecule should be performed to verify the exohedral site is the correct one. There exist two isomers of C_{61}H_2: the 6-6 and 5 -6 isomers. Calculations of the geometrical structures of both isomers are in general agreement with previous calculations. The ^2H nuclear quadrupole coupling constants were studied at the HF/STO-3G level. The results for the 5-6 isomer are in accord with experimental NQR studies. Studies on the smaller, related systems methane and cyclopropane show that use of extended basis sets and electron correlation are needed to further explain the experimental results. Hydrogen and muonium impurities were investigated in order to explain the both the magnetic hyperfine constants and the isotopic differences between those constants. Convergence with respect to basis set was shown. The lattice was found to relax little in the presence of the hydrogenic impurity. Electron correlation was found to reduce the hyperfine constant, and preliminary investigations including three-dimensional zero-point vibrational averaging can explain the isotopic difference. Further studies including larger basis sets, higher orders of perturbation, as well as the V^{N-1} potential for the determination of virtual orbitals should be performed to examine their effects on the calculation of the electronic structure of this system.