Lawrence L. Lohr

Relativistically parameterized extended Hückel theory

Abstract A relativistically parameterized extended Huckel molecular orbital method is outlined. One-electron effective hamiltonian matrix elements are related to the corresponding overlap matrix elements calculated in an atomic ilsjm) basis. Different atomic radial functions ar eused for the j = l + 1 2 and j = l 1 2 basis functions for a given n and l. Relativistic and non-relativistic atomic orbital energies are taken from atomic Dirac-Fock and Hartree-Fock calculations. Results are presented for the molecules I2, Br2, IBr, HI, CH4, SiH4, GeH4, SnH4, PbH4, PbH4, (114)H4 and Bi4. It is concluded that our method and its parameterization provide a semi-quantitative description of relativistic effects in chemical bonding.

Analysis of an energy minimization method for locating transition states on potential energy hypersurfaces

Conditions are given for the successful search for a t‘annhon state by an energy minimization method. Proofs For these guIdelines are presented. Advantages of thaw method are discussed, includmg rts use in establishing lower bounds to transibon state energies Comparisons are made with other searchmg methods.

Experimental and ab initio determination of the bending potential of HCP

The emission properties of HCP excited to the A, B, and d electronic states have been studied. Lifetimes and quenching rates have been measured. By spectrally resolving the emission spectrum, the energy of 94 vibrational levels of the ground electronic state have been measured to an accuracy of ≈5 cm−1. These energy levels were fit to experimental accuracy by a rigid bender Hamiltonian thereby determining the bending potential over a range of bending angle from 0 to 100° (0–17 500 cm−1). An ab initio bending potential has been computed for HCP and found to be in excellent agreement with the experimentally fitted one over the range that the experimental data span. This potential predicts that HPC has an energy maximum with respect to the bending coordinate. The bending potential decreases monotonically by about 30 000 cm−1 in going from HPC to HCP.

Paramagnetically induced nuclear magnetic resonance relaxation in solutions containing S>1 ions: A molecular-frame theoretical and physical model

of the interspin vector with the rth principal axis of the zfs tensor. New experimental data are presented for the model S51 complex @trans-Ni~II!~acac!2~H2O!2# (acac5acetylacetonato) in dioxane solvent. The magnetic field dependence of the protonT1 of the axial water ligands has been measured over the range 0.15‐1.5 T, the lower end of which corresponds to the zfs limit. The experimental data have been analyzed using the new analytical theory for the zfs-limit regime in conjunction with spin dynamics simulations in the intermediate regime. Dipolar density power plots are presented as graphical devices which clearly exhibit the physical information in the experiment, and which permit a rapid differentiation of the sensitive and insensitive parameters of theory. The data analysis depends strongly on the zfs parameter uEu and on the electron spin relaxation time t S,z along the zfs-PAS z-axis, but only very weakly on the other parameters of theory. A fit of the data to theory provided the values uEu51.860.1 cm 21 and t S,z58.060.3 ps.

Radial Expansion of 3d Orbitals and the Near Degeneracy of the 4A1g and 4Eg Excited States of Octahedral Mn(II) Complexes

The relationship between the wavefunctions for mostly 3d molecular orbitals and the energies of the nearly degenerate 4A1g and 4Eg excited states of octahedral Mn2+ complexes is investigated from two points of view. First, the expressions relating the energy splitting to the degree of covalency have been extended within the framework of the intermediate neglect of differential overlap (INDO) approximation to include the large but previously neglected two‐center Coulomb integrals and all ligand one‐center Coulomb and exchange integrals. The results resemble those previously obtained in their inability to account for the spectral assignment of the 4Eg state below 4A1g for the MnF64− complex in the salts KMnF3, RbMnF3, and MnF2. Second, the assumption of identical metal‐ion repulsion integrals for the eg and t2g molecular orbitals is discarded in a simple analysis of an all‐electron molecular SCF calculation for the MnF64− complex. While both the eg and t2g orbitals contain a metal‐ion d orbital that is expa...

Nuclear magnetic resonance-paramagnetic relaxation enhancements: Influence of spatial quantization of the electron spin when the zero-field splitting energy is larger than the Zeeman energy

Dissolved paramagnetic ions generally provide an efficient mechanism for the relaxation of nuclear spins in solution, a phenomenon called the nuclear magnetic resonance-paramagnetic relaxation enhancement (NMR-PRE). Metal ions with electron spins S⩾1 exhibit rich NMR relaxation phenomena originating in the properties of the zero-field splitting (zfs) interaction, which vanishes for spin-12 ions but which is nonzero for S⩾1 ions in site symmetry lower than cubic. For S⩾1 ions in the vicinity of the zfs-limit, i.e., at magnetic-field strengths low enough that the zfs energy exceeds the Zeeman energy, the NMR-PRE depends strongly on the detailed structure of the electron spin energy levels as well as on the spatial quantization of the spin motion. It is shown theoretically and experimentally that the NMR-PRE produced by integer spins can be influenced strongly by the small intradoublet zero-field splittings, i.e., the splittings between the components of the non-Kramers doublets, which are produced by noncyl...

A particle-on-a-sphere model for C60

A perimeter model for the x electrons of C, is developed and implemented within the independent particle approximation. Physically, the model gives the energy levels of a particle on a sphere perturbed by the icosahedral potential set up by the 60 carbon atomic cores. Computational techniques are adapted from well known crystal field formalisms. The molecular orbitals are represented by linear combinations of spherical harmonics. One or two adjustable parameters are used to give reasonable agreement with the observed UV-visible spectrum and with semi-empirical and ab initio calculations at a similar level of approximation. A closed-shell ground state, appreciable HOMO-LUMO gap, and electric dipole forbidden HOMO-LUMO transition are predicted. 1. Intmduction Since its discovery in 1985 [ 11, CsO has been the subject of many quantum-mechanical calculations [ 2-91. Techniques ranging from HiIckel theory to rigorous Hartree-Fock SCF computations have predicted the two distinct bond lengths, closed-shell ground state and appreciable HOMO-LUMO gap observed experimentally. In addition, papers which report results at the orbital level are in agreement as to the ordering of the molecular orbitals in terms of energy. These calculations have provided much useful information about Cso but fail to give a simple conceptual basis for understanding the electronic structure of the molecule. Many of the authors have addressed this issue by noting the similarity between the energy levels of the C& R system and those of a particle on a sphere [2-4,7-g 1. We present here an elaboration of this idea, in which the K electrons are treated as independent particles sliding on the surface of a sphere onto which is impressed an icosahedral potential generated by point charges at the carbon atomic positions. The present treatment is restricted to this conceptually simple case and ignores electron correlation. The predictions are therefore limited to results at the orbital level of approximation.

Spin-forbidden electronic excitations in transition metal complexes

A revrew IS presented of recent progress m the detarled understandmg of spm-forbrdden electronic excrtattons in transrtron metal complexes The quantum mechamcal descrrption of the role of spur-orbrt couplmg m such transitions 1s outhned and related to experimental values of the oscrllator strength for the absorption of light These mtenstttes arc shown to be a sensitive measure of certam features m the electronic wave-function Emphasis is placed on the direct coupling of states of drffenng spin by various spin-dependent radiative operators_ For bmuclear and polynuclear complexes there 1s also the posstbrllty of spmforbidden transrtrons occurring vra an exchange-dependent mechanism rather than via spm-orbrt coupling The experimental evidence for this is revrewed and related to recent studies of the couphng between spin excltatlon waves and electronic excltatlon waves In magnetically ordered transrtron metal salts During the past two decades we have seen a great advance m our understanding of the electronic structure of transitron metal complexes 1 Whde a variety of experimental techniques have contributed to this progress, a central role has been played by electronic spec.9 troscopy in the vtsrble and UV spectral regtons Absorption expermients have probably played a greater roie than emrssron studres because the former yield mformatron about more excited states than the latter The electronic transittons responstble for the absorption bands frequently mvolve the rearrangement of electrons wrthm the partially filled d shell of the central ion, although other types, such as &and-to-metal charge transfer transrtrons, may be observed The former type, often called a crystal-field transttion. IS gener-

Relativistically parameterized extended Hückel calculations. 10. Lanthanide trihalides

Abstract The REX relativistically parameterized extended Huckel method is used to study the electronic structure of lanthanide trihalide molecules. All valence orbitals are described in terms of double-zeta Slater functions, with the atomic orbital parameters being determined by a least-squares fitting to published relativistic (Dirac- Fock) radial densities. Comparisons of orbital energies to experimental values are made and various trends are discussed. Ab initio all-electron calculations at the self-consistent field level and as a function of molecular geometry are reported for LaH3, LaF3, and LaCl3. While LaH3 and LaF3 are calculated to be pyramidal, LaCl3 is calculated to be planar.

Electron photodetachment from a Dirac bubble potential. A model for the fullerene negative ion CC60

Abstract A model for the fullerene anion C 60 − is proposed in which the outer electron moves in an attractive deltafunction potential in the shape of a spherical bubble. Exact solutions for this “Dirac bubble potential” were previously worked out. On the basis of this model, the photodetachment cross-section and its angular distribution are computed as functions of photoelectron energy, for the electron in both S-like and P-like bound states. Secondary maxima and minima in the cross section appear at higher energy values, somewhat analogous to the scattering of radiation from a conducting sphere.