Enhancing the reactivity of uranium(VI) organoimido complexes with diazoalkanes

Diphenyldiazomethane effects a two-electron oxidation of the uranium(IV) monoimido complex (C5Me5)2U(=N-2,4,6-t-Bu3C6H2) to give the uranium(VI) mixed bis(imido) complex, (C5Me5)2U(=N-2,4,6-t-Bu3C6H2)(=N-N=CPh2), which undergoes a rare cyclometallation reaction upon mild thermolysis to afford a uranium(IV) bis(amide) complex that results from net addition of a C-H bond of an ortho tert-butyl group across the N=U=N core.     

Observation of the nuclear magnetic octupole moment of 133Cs

We describe our high-resolution measurements of the 133Cs 6p (2)P(3/2) state hyperfine structure. An optically narrowed diode laser excites perpendicularly a highly collimated atomic beam. The spectra are calibrated with a stable reference diode laser using a rf locking scheme allowing us to determine the splittings with an accuracy of < or =2 kHz, an order of magnitude better than previous results. The magnetic dipole a, electric quadrupole b, and magnetic octupole c hyperfine coupling constants are determined. The values we obtained are a=50.288 27(23) MHz, b=-0.4934(17) MHz, and c=0.56(7) kHz. This work represents the first observation of the magnetic octupole moment of the cesium nucleus. We carry out atomic-structure calculations and determine the nuclear electric quadrupole moment Q= -3.55(4) mb and nuclear magnetic octupole moment Omega=0.82(10) b x mu(N).     

Chromatographic evaluation of self-immobilized stationary phases for reversed-phase liquid chromatography

The preparation of stationary phases for HPLC using polymers deposited on silica usually includes an immobilization step involving cross-linking by free radicals induced by ionizing radiation or by other radical initiators. The present paper reports changes which occur at ambient temperature in the character of poly(methyloctylsiloxane) deposited on porous silica particles as a function of the time interval between particle loading and column packing. Column performance and retention factors increase with time and these changes are attributed to rearrangement (self-assembly) which result in "self-immobilization" of the polymer molecules on the silica surface.     

An experimental and indo molecular orbital study of variable response of the p-phenyl substituent to electron demand. Relative conformations of two p-arylacetophenones as neutral molecules and +1 molecular ions

The INDO molecular orbital method suggests that the rings of p-phenylacetophenone and 2-fluoro-4′-acetylbiphenyl are more nearly coplanar in the +1 molecular ion than in the neutral molecule. This is indicated by old experimental data for the former and new data for the latter, based on the cleavage to [CH₃CO]⁺.     

An ab initio study of the geometry and rotational barrier of 4-phenylimidazole

The molecular design of several synthetic artificial enzymes, which mimic the action of the serine protease-chymotrypsin, incorporates the phenylimidazole molecular fragment to play the role of the His-57 residue in the native enzyme active site. Study of these artificial enzymes by molecular modeling techniques requires accurate torsional force field parameters for the phenylimidazole interring bond. This, in turn, requires accurate characterization of the barrier to rotation around this bond. Previous semiempirical calculations of this rotational barrier have neglected geometry optimization of the molecule at the points along the rotational pathway. The 4-phenylimidazole rotational barrier (5.6 kcal mol^–1] presented here was obtained by full ab initio geometry optimization at the 3–21G level at each of the points along the rotational pathway.     

Solution of the inverse spectral problem for an impedance with integrable derivative part II

The inverse eigenvalue problem for a Sturm-Liouville equation in impedance form with Dirichlet boundary conditions on a unit interval is solved. The solution of this nonlinear problem requires the investigation of a combined map from the logarithmic derivative of the impedance, assumed to be square integrable, to two sequences of spectral data. The first is the sequence of Dirichlet eigenvalues, shown to be locally bounded with square roots differing from the sequence of integral multiples of π by a square summable sequence. The second sequence has as first term the mean of the logarithmic derivative of the impedance. Each term in the remainder of the sequence is the logarithm of the product of the impedance and the derivative of an eigenfunction evaluated at an endpoint. It is shown that this second sequence is a locally bounded square summable sequence. The combined map is a real analytic isomorphism. A uniqueness result follows from the asymptotics and analyticity shown in the first paper and properties of Riesz bases. The existence is shown by giving a closed form of the solution. It is also shown that the same map solves the inverse Sturm-Liouville problem in impedance form with Neumann boundary conditions. © 1993 John Wiley & Sons, Inc.     

Column design. 3. Theoretical studies of a chiral stationary phase used in column chromatography

Conformational features of a chiral stationary phase used in column chromatography are discussed. The syn forms invoked in chiral recognition models are consistent with MNDO and MM2 calculations. It is speculated that the inherent flexibility of the syn form makes these phases effective templates for analyte binding.