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B Schimmelpfennig

Publications and source records attributed to B Schimmelpfennig.

At least 19 recordsLinked to original sources
PubMed2001

The mechanism for water exchange in [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-), as studied by quantum chemical methods.

The mechanisms for the exchange of water between [UO(2)(H(2)O)(5)](2+), [UO(2)(oxalate)(2)(H(2)O)](2)(-)(,) and water solvent along dissociative (D), associative (A) and interchange (I) pathways have been investigated with quantum chemical methods. The choice of exchange mechanism is based on the computed activation energy and the geometry of the identified transition states and intermediates. These quantities were calculated both in the gas phase and with a polarizable continuum model for the solvent. There is a significant and predictable difference between the activation energy of the gas phase and solvent models: the energy barrier for the D-mechanism increases in the solvent as compared to the gas phase, while it decreases for the A- and I-mechanisms. The calculated activation energy, Delta U(++), for the water exchange in [UO(2)(H(2)O)(5)](2+) is 74, 19, and 21 kJ/mol, respectively, for the D-, A-, and I-mechanisms in the solvent, as compared to the experimental value Delta H(++) = 26 +/- 1 kJ/mol. This indicates that the D-mechanism for this system can be ruled out. The energy barrier between the intermediates and the transition states is small, indicating a lifetime for the intermediate approximately 10(-10) s, making it very difficult to distinguish between the A- and I-mechanisms experimentally. There is no direct experimental information on the rate and mechanism of water exchange in [UO(2)(oxalate)(2)(H(2)O)](2-) containing two bidentate oxalate ions. The activation energy and the geometry of transition states and intermediates along the D-, A-, and I-pathways were calculated both in the gas phase and in a water solvent model, using a single-point MP2 calculation with the gas phase geometry. The activation energy, Delta U(++), in the solvent for the D-, A-, and I-mechanisms is 56, 12, and 53 kJ/mol, respectively. This indicates that the water exchange follows an associative reaction mechanism. The geometry of the A- and I-transition states for both [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-) indicates that the entering/leaving water molecules are located outside the plane formed by the spectator ligands.

Journal Article↗
PubMed2001

Solvent effects on uranium(VI) fluoride and hydroxide complexes studied by EXAFS and quantum chemistry.

The structures of the complexes UO(2)F(n)(H(2)O)(5-n)(2-n), n = 3-5, have been studied by EXAFS. All have pentagonal bipyramid geometry with U-F of and U-H(2)O distances equal to 2.26 and 2.48 A, respectively. On the other hand the complex UO(2)(OH)(4)(2-) has a square bipyramid geometry both in the solid state and in solution. The structures of hydroxide and fluoride complexes have also been investigated with wave function based and DFT methods in order to explore the possible reasons for the observed structural differences. These studies include models that describe the solvent by using a discrete second coordination sphere, a model with a spherical, or shape-adapted cavity in a conductor-like polarizable continuum medium (CPCM), or a combination of the two. Solvent effects were shown to give the main contribution to the observed structure variations between the uranium(VI) tetrahydroxide and the tetrafluoride complexes. Without a solvent model both UO(2)(OH)(4)(H(2)O)(2-) and UO(2)F(4)(H(2)O)(2-) have the same square bipyramid geometry, with the water molecule located at a distance of more than 4 A from uranium and with a charge distribution that is very near identical in the two complexes. Of the models tested, only the CPCM ones are able to describe the experimentally observed square and pentagonal bipyramid geometry in the tetrahydroxide and tetrafluoride complexes. The geometry and the relative energy of different isomers of UO(2)F(3)(H(2)O)(2-) are very similar, indicating that they are present in comparable amounts in solution. All calculated bond distances are in good agreement with the experimental observations, provided that a proper model of the solvent is used.

Journal Article↗
PubMed1997

Spectroscopy of NbO: Characterization of the Doublet Manifold

Doublet bands of NbO in the near infrared region have been observed in emission with FTS techniques using an electrodeless 2450 MHz discharge as a source. Transitions involving the vibrational levels v = 0-4 of a low-lying a2Delta state of configuration sigma2delta and four additional doublet states (c2Pi, d2Deltai, e2Phi, and f2Pii) have been recorded at high resolution. Furthermore, a 2Sigma state, assigned as b2Sigma-, has been observed through the c2Pi (v = 0) --> b2Sigma- (v = 0, 1) transitions in the 1.6 &mgr;m region. Rotational analyses of the doublet bands have been performed. Most of the excited doublet states lie only slightly above or below the well-known B4Pi state. The density of states is fairly high immediately above B4Pi, and characterizations of some of the states in this region involve difficulties. A currently unanalyzed band, centered at 11 820 cm-1, has been preliminarily attributed to the A4Pi --> X4Sigma- (0, 0) transition. Nuclear hyperfine effects are barely detectable in orbitally degenerate states of the doublet manifold at Doppler-limited resolution, while the b2Sigma- state of configuration sigmadelta2 shows partly resolved magnetic hyperfine structure (b = -0.08191 cm-1). Large-scale all-electron CI calculations have been performed on doublet and quartet manifolds of NbO up to 22 000 cm-1. The calculations have been performed in two steps. In the first step, the spin-orbit effects within electronic states were not considered. An excellent overall agreement with experimental energies was obtained, except for the d2Deltai state. In the second step, all spin-orbit interactions were included. Also these calculations resulted in a good agreement with the experimental observations. The calculations also predict three hitherto unobserved low-lying states: 4Phi at 8545 cm-1, 2Gamma at 8430 cm-1 and 2Sigma+ at 9648 cm-1. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1989

One year follow-up of IOGEL intraocular lenses with ciliary sulcus fixation.

Fifty-one soft hydrogel posterior chamber intraocular lenses (IOLs) were implanted in the ciliary sulcus after planned extracapsular cataract extractions. In nine cases the IOLs were positioned with one haptic in the capsular bag and the other in the ciliary sulcus. During a one-year follow-up, Nd:YAG capsulotomy was performed on five eyes and cystoid macular edema was present in two cases. No other serious complications were encountered. At one year, visual acuity was 20/30 or better in all eyes that did not have preexisting secondary eye problems. To prevent postoperative rotation, decentration, and deformation of the IOL, unnecessary intraocular manipulation and "in-out" positioning of the IOL should be avoided.

Aged↗
PubMed1988

Anterior capsulotomy by means of bipolar radio-frequency endodiathermy.

Anterior capsulotomy, a crucial step in extracapsular implant surgery, is performed by several methods. All must deal with the inherent capriciousness of the tense anterior capsule and so their precision is questionable. An anterior capsulotomy instrument that uses bipolar radio-frequency endodiathermy is described. The modulated high frequency current enables the surgeon to open the anterior capsule more accurately under controlled conditions.

Anterior Chamber↗
PubMed1988

[In vitro studies of the tolerance of hydroxypropyl methylcellulose (HPMC) with regard to the human corneal endothelium].

The compatibility of hydroxypropylmethylcellulose (HPMC) was tested in vitro on 48 human donor corneas. Using a perfusion technique, the function of the corneal endothelium was recorded by stromal thickness measurements under light-microscopic control. The experiments showed that HPMC at a concentration of 0.5% is compatible for 3.5 hours of perfusion in the presence of human corneal endothelium.

Aged↗
PubMed1988

[The tolerance of hydroxypropylmethylcellulose in implantation of posterior chamber lenses].

In 65 patients, the safety of a 2% solution of hydroxypropylmethylcellulose (HPMC) was investigated by comparing HPMC with a balanced salt solution during posterior chamber lens implantation. The controlled open randomized trial consisted of two groups of patients: 1) in 37 patients the surface of the implant was coated with HPMC and the inner wound sealed prior to insertion; 2) in the control group with 28 patients the lens was dipped in the balanced salt solution. There were no statistically significant differences between the two groups as regards any of the characteristics investigated, such as visual acuity, intraocular pressure, corneal thickness, endothelial cell loss, and postoperative irritation within the anterior chamber. There was no rise in mean intraocular pressure in the HPMC group on the first postoperative day, and a rise of 1 mm Hg in the control group. The increase in corneal thickness on the first postoperative day in the HPMC group was 0.07 and in the control group 0.04 mm. At re-examination (50th postoperative day) the mean thickness in both groups was identical to the initial values. The mean endothelial cell loss at re-examination was 18% (+/- 16%). No serious side effects were observed. HPMC may be regarded as a well-tolerated viscoelastic substance.

Aged↗
PubMed1985

[Corneal trephining for penetrating keratoplasty: study of the degree of precision and presentation of a new trephine].

The accuracy of corneal trephination with a Franceschetti trephine was investigated on Duragel contact lenses. A modified 7.3 mm Crock trephine (Brit. J. Ophthal. 62 [1978] 74-80) with a rotating diamond blade fitted was compared with the above-mentioned method. Trephination of Duragel lenses from the convex side yielded diameters either larger or smaller than 7 mm. With one exception trephination from the concave side produced disks smaller than 7 mm. All Duragel disks had an elliptical configuration after trephination with the Franceschetti trephine. Trephination with the modified Crock trephine yielded disks with diameters close to 7.3 mm in all meridians.

Corneal Transplantation↗
PubMed1984

[Topography of age-related size changes in corneal endothelial cells].

Age-related changes in corneal endothelial cell size were investigated in 116 eyes, from newborn to 96 years of age, by light microscopy. The preliminary findings show a regional distribution of cell changes on the endothelial surface. The endothelium of a newborn was homogeneous in cell size and shape. From the third decade of life onward, polymorphism and cell size variations occurred in the central endothelium. In corneas of patients over 50 years old, one-third of the cases often had a geographic arrangement of cells of increased size in the mid-periphery (3.5-4.0 mm paracentrally). Small cells were always predominant at the periphery.

Adolescent↗
PubMed1983

The effect of central retinal lesions on optokinetic nystagmus in the monkey.

In alert monkeys (Macaca mulatta and fascicularis) the effect of central retinal lesions on 'fast' optokinetic responses was investigated during high velocity optokinetic and visual-vestibular conflict stimulation. The 'fast' component of the optokinetic response manifests itself as a rapid rise in the slow-phase eye velocity after light-on, during high velocity optokinetic stimulation; and a sudden drop in eye velocity after light-off. In contrast, the 'velocity storage' component leads only to gradual changes in eye velocity during continuous optokinetic stimulation and after light-off (optokinetic after-nystagmus). Retinal lesions were placed by laser coagulation in and around the fovea. Responses of the normal and lesioned eye were compared. It was found that central lesions up to 12 deg (fovea diameter 6 deg) had only a negligible effect on 'fast' optokinetic responses. With lesions of more than 25-30 deg diameter centered on the fovea definite 'fast' responses could still be obtained, on average reduced to about 50% of the responses of the normal eye. Some monkeys showed initially no 'fast' optokinetic responses and had, therefore, to be excluded from lesion experiments. The results demonstrate that 'fast' optokinetic responses also can be obtained from extrafoveal areas, i.e. areas which are not generally involved in smooth pursuit eye movements. These results are discussed in relation to reports that the 'smooth pursuit' eye movement system is also used to generate 'fast' optokinetic responses.

Animals↗
PubMed1982

Nerve structures in human central corneal epithelium.

Eight corneal buttons obtained after enucleations and keratoplasties were impregnated with gold chloride. In epithelial flat preparations dissected from the stroma, the topography of epithelial nerves was examined by light microscopy. Four main structures constituted the innervation of human central corneal epithelium: (1) Bowman's membrane penetrating stromal nerves; (2) basal epithelial nerve plexus; (3) dendritic cells that are interspersed among the basal plexus and possibly connected with nerve fibres; (4) nerve terminals, originating from the basal plexus and dividing dichotomously in the superficial cell layers.

Cornea↗
PubMed1982

A technique for controlled sensory denervation of the rabbit cornea.

Complications arising from the procedure to produce denervation of the cornea have made experiments on the trophic role of trigeminal nerve difficult. In this study, controlled lesions of the rabbit trigeminal ganglion were produced by radio-frequency thermocoagulation, delivered by a needle electrode inserted through the soft palate of the mouth. The advantages of the procedure were low mortality (less than 5%), permanent destruction of sensory neurons, and absence of spurious ocular effects. Two lesion sizes were reliably produced: 1. The smaller circumscribed lesion resulted in loss of corneal sensitivity, but preserved spontaneous blinking. 2. The larger lesion extended across the ganglion, abolished corneal sensation and spontaneous blink activity.

Animals↗
PubMed1978

Corneal preservation at -4 degrees C in a modified tissue culture medium.

Addition of 6% PVP (polyvinylpyrrolidone, mol wt 40,000) to tissue culture medium No. 199 prevents it from freezing at -4 degrees C. Forty donor corneas stored for five and ten days in this medium had a mean thickness of 0.61 and 0.67 mm, respectively. By means of specular microscopy, intact endothelium was found after these storage periods. Epithelial cell layers were histologically present. Electronmicroscopic studies revealed only slight changes in the subcellular endothelial structures. During perfusion with modified Ringer's solution at +34 degrees C, the thickness of stored corneas decreased.

Cadaver↗
PubMed1978

Cornea guttata in donor material.

Specular microscopy made it possible to detect cornea guttata changes within the regular reflection off the posterior surface of the cornea in donor matrial. Small, roundish lesions were present in mild forms within an otherwise intact endothelium, and in advanced cases, large warts were seen free of endothelial cell coverage.

Aged↗
PubMed1978

[Study on the tissue compatibility of intracorneal horn lenses in experimental animals (author's transl)].

Horn lenses, manufactured from human toenail donor material were implanted intracorneally in 15 rabbits. The thickness of the lenses ranged 0.08 to 0.2 mm, the diameter from 4.5 to 7 mm. 13 lenses were fenestrated, two were not. Within 6 months, growth of corneal tissue into fenestrations of the lenses occurred but no signs of aseptic necrosis were seen histologically. The boundary layer between stroma and horn lens revealed apposition as well as ingrowth of tissue in electron-microscopic studies. Unguinal tissue may be useful in keratoprosthesis on the basis of its permeability to glucose, lack of foreign body-activity and hydrophilia.

Animals↗