![]() Some significant differences are found between results after full quartet more » annihilation and after complete spin projection. The unrestricted Hartree-Fock method itself gives poor results for this property, but is found to yield results of semiquantitative accuracy after a spin projection is performed. Representative sigma radicals (vinyl and ethynyl), nonconjugated pi radicals (methyl and ethyl), and conjugated pi radicals (allyl and cyclohexadienyl) are examined in the unrestricted Hartree-Fock approximation with a wide variety of Gaussian basis sets, ranging from crude minimal sets to a large uncontracted set. « lessĪb initio calculations are reported for the isotropic hyperfine coupling constants arising from contact spin density distributions in hydrocarbon radicals. The calculated proton affinity is also shown to depend on the basis set size however, the acidity trend is well reproduced at all levels of theory except RHF/STO-3G. The theoretical trend in acidity of the terminal and bridging hydroxyls, as determined by the proton affinity, the length of the O-H bond, the charge on the hydroxyl proton, and the O-H stretching force constants and frequencies, is presented and is in agreement with experimental determinations. ![]() The study shows that the structures of the molecules converge toward limiting values at the higher levels of theory. Results are presented for restricted Hartree-Fock (RHF) calculations at levels of theory that range from the minimal RHF/STO-3G to double- more » and triple-zeta basis sets with multiple polarization functions. Also included are H/SiO] and HSiOH that mimic terminal hydroxyl groups that occur at the zeolite surface and defect sites. ![]() The molecules are structural analogs of disiloxane HT-O-TH, and the protonated form HT, Si, Al, B, and P. The structures, force constants, and relative acidities of a series of molecules that mimic the geometries of terminal and bridging hydroxyl groups in various substituted zeolites and clays are calculated by ab initio molecular orbital methods. ![]()
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