Showing papers by "David A. Case published in 1983"

Human observing: Maintained by stimuli correlated with reinforcement but not extinction.

Abstract: College students received points exchangeable for money (reinforcement) on a variable-time 60-second schedule that alternated randomly with an extinction component. Subjects were informed that responding would not influence either the rate or distribution of reinforcement. Instead, presses on either of two levers (“observing responses”) produced stimuli. In each of four experiments, stimuli positively correlated with reinforcement and/or stimuli uncorrelated with reinforcement were each chosen over stimuli correlated with extinction. These results are consistent with prior results from pigeons in supporting the conditioned-reinforcement hypothesis of observing and in not supporting the uncertainty-reduction hypothesis.

Relativistic effects on molecular hyperfine interactions: Application to XeF and CsO

Abstract: A relativistic theory of molecular hyperfine interactions is presented, based on the use of multiple scattering theory to generate approximate solutions to the Dirac equation. This approach should be especially useful for radicals with significant orbital contributions to the hyperfine tensor. Results for XeF and CsO are in good agreement with experiment, and show that orbital interactions are important for both heavy and light atoms, and that A⊥ for fluorine in XeF is likely to be negative.

Xα multiple scattering calculations on iron(II) porphine

Abstract: Spin‐restricted and spin‐unrestricted Xα multiple scattering calculations are reported for iron porphine with D4h symmetry in the absence of axial ligands. A 3A2g ground state is predicted when the radius of the porphyrin ‘‘hole’’ is less than 2.0 A, whereas a quintet ground state is expected for larger Fe–N distances. A large asymmetry is found in the occupations of the three Fe 4p orbitals. This has important implications for the interpretation of Mossbauer quadrupole coupling constants. Magnetic susceptibility and NMR results can be understood in terms of a model that involves spin‐orbit mixing of an excited 3Eg state with the ground 3A2g state; an essential element in explaining the NMR data is that the proton hyperfine constants are not the same in these two states. Tentative assignments are given for observed absorptions in the optical and near‐infrared regions. It is shown that spin‐unrestricted wave functions are particularly suited for the qualitative interpretation of electronic structure of metallophorphyrins.