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At least 19 recordsLinked to original sources
PubMed2004

Possible role of relativistic effects in the plasticity of the coordination geometry of cadmium(II). A voltammetric study of the stability of the complexes of cadmium(II) with 12-crown-4,15-crown-5 and 18-crown-6 in aqueous solution and the structures of [Cd(benzo-18-crown-6)(NCS)(2)] and [K(18-crown-6)][Cd(SCN)(3)].

A differential pulse voltammetric study of complexes of Cd(II) and Pb(II) with crown ethers is reported. Measured log K(1) values for Cd(II) with 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane), 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane), and 12-crown-4 (1,4,7,10-tetraoxacyclododecane) are respectively 2.53 (+/-0.06), 1.97 (+/-0.07), and 1.72 (+/-0.08) and for Pb(II) with 18-crown-6 is 4.17 (+/-0.03), all at 25 degrees C in 0.1 M LiNO(3). Cd(II) is smaller than is usually associated with strong bonding with crown ethers. The high log K(1) values for Cd(2+) with crown ethers found here are discussed in terms of distortion of Cd(II) by relativistic effects. The resulting plasticity of the coordination geometry of the Cd(II) ion allows it to meet the metal ion size requirements of all the crown ethers, allowing high log K(1) values to occur. Crystal structures for [Cd(bz-18-crown-6)(SCN)(2)] (1) (bz-18-crown-6 = benzo-1,4,7,10,13,16-hexaoxacyclooctadecane) and [K(18-crown-6)][Cd(SCN)(3)] (2) are reported. 1 was triclinic, space group P1, a = 8.5413(2), b = 10.0389(2), and c = 13.4644(2) A, alpha = 94.424(1), beta = 102.286(1), and gamma = 93.236(1) degrees, Z = 2, and final R = 0.023. 2 was orthorhombic, space group Cmc2(1), a = 14.7309(3), b = 15.1647(3), and c = 10.6154(2) A, Z = 4, and final R = 0.020. In 1, the Cd occupies the cavity of the bz-18-crown-6 with long average Cd-O bond lengths of 2.65 A and is N-bonded to the thiocyanates with short average Cd-N bonds of 2.12 A. In [Cd(bz-18-crown-6)(SCN)(2)], the linear coordination involving the Cd and the two N-bonded thiocyanate groups in 1 is discussed in terms of the role of relativistic effects in the tendency to linear coordination geometry in group 12 metal ions. In 2 Cd forms a polymeric structure involving thiocyanate bridges between Cd atoms and K(+) occupies the cavity of the crown ether. 2 highlights the fact that cadmium is almost never S-bonded to thiocyanate except in bridging thiocyanates.

Journal Article↗
PubMedDate not supplied

[Gdansk combined crown with occlusal rest. 1. Clinical and laboratory preparation of combined crown with occlusal rest on the basis of cast crown].

The combined crown used since many years in the Prosthetic Laboratory, Institute of Stomatology, Medical Academy in Gdańsk contains an occlusal rest basis of a cast crown (the metal part is of a gold alloy or chromium-nickel steel, with veneer of acrylateor Colorstat) provides, owing to a special preparation of the abutment tooth and laboratory procedure, full stabilization of the crown, protection of parodontium and good maintenance of the veneer which plays a cosmetic role in this type of crown. The combined crowns with occlusal rest are particularly suitable for supporting bridgeworks. They may be applied onto devitalized or live teeth since they require less extensive grinding of teeth.

Crowns↗
PubMed2003

Stabilization of the P(CF3)(2)(-) and P(C6F5)(2)(-) ions by coordination to pentacarbonyl tungsten: structures of [18-crown-6-K]P(CF3)2, [18-crown-6-K][W[P(CF3)2](CO)5], and [18-crown-6-K][[W(CO)5]2[mu-P(C6F5)2]].THF.

The stabilization of the P(CF(3))(2)(-) ion by intermediary coordination to the very weak Lewis acid acetone gives access to single crystals of [18-crown-6-K]P(CF(3))(2). The X-ray single crystal analysis exhibits nearly isolated P(CF(3))(2)(-) ions with an unusually short P-C distance of 184(1) pm, which can be explained by negative hyperconjugation and is also found by quantum chemical hybrid DFT calculation. Coordination of the P(CF(3))(2)(-) ion to pentacarbonyl tungsten has only a minor effect on electronic and geometric properties of the P(CF(3))(2) moiety, while a strong increase in thermal stability of the dissolved species is achieved. The hitherto unknown P(C(6)F(5))(2)(-) ion is stabilized by coordination to pentacarbonyl tungsten and isolated as a stable 18-crown-6 potassium salt, [18-crown-6-K][W[P(C(6)F(5))(2)](CO)(5)], which is fully characterized. The tungstate, [W[P(C(6)F(5))(2)](CO)(5)](-), decomposes slowly in solution, while coordination of the phosphorus atom to a second pentacarbonyl tungsten moiety results in an enhanced thermal stability in solution. The single-crystal X-ray analysis of [18-crown-6-K][[W(CO)(5)](2)[mu-P(C(6)F(5))(2)]].THF exhibits a very tight arrangement of the two C(6)F(5) and two W(CO)(5) groups around the central phosphorus atom. NMR spectroscopic investigations of the [[W(CO)(5)](2)[mu-P(C(6)F(5))(2)]](-) ion exhibit a hindered rotation of both the C(6)F(5) and W(CO)(5) groups in solution.

Journal Article↗
PubMed2001

Hydration to benzo-15-crown-5, benzo-18-crown-6 and the benzo-18-crown-6-potassium ion complex in the low-polar organic solvents.

The coextraction of water with benzo-15-crown-5 (B1SC5), benzo-18-crown-6 (B18C6) and the B18C6-K+ complex into seven low-polar solvents, i.e., carbon tetrachloride (CTC), chloroform (CF), dichloromethane (DCM), 1,2-dichloroethane (1,2-DCE), benzene (BZ), chlorobenzene (CB) and o-dichlorobenzene (o-DCB), has been investigated. The mean hydration number, nH2O, of these solutes in the water-saturated organic solvents was determined. There is a trend that the nH2O values for any solutes increase with increasing the water concentration in the solvents. Those of B18C6 and B15C5 converge at almost 0.8 for B18C6 and 0.4 - 0.5 for B15C5 in the solvents with the relatively high water concentration, i.e., CF, 1,2-DCE, DCM, and nitorobenzene (NB). The nH2O value of B15C5 is about one-half of that of B18C6 for a given organic solvent. The dominant species of the B18C6-K+ complex in these solvents is non-hydrated. From these results, the hydration equilibrium constants, KH2O, in the organic solvents were estimated.

Journal Article↗
PubMed2005

Hexafluoroantimony(V) salts of the cationic Ti(IV) fluoride non metallocene complexes [TiF3(MeCN)3]+ and [TiF2L]2+ (L = 15-Crown-5 and 18-Crown-6). Preparation, characterization and thermodynamic stability.

The cationic titanium fluoride containing complexes [fac-TiF3(MeCN)3][SbF6].MeCN (1), [trans-TiF2(15-Crown-5)][SbF6]2(2) and [trans-TiF2(18-Crown-6)][SbF6]2(2), were prepared by the reaction of TiF4, the molecular ligand and SbF5 in MeCN. Complexes 1-3 were characterized by X-ray single crystal analysis, elemental analysis, IR, NMR and mass spectroscopy. Titanium tetrafluoride reacts with the SbF5 in SO2 with the formation of fac-[TiF3(SO2)3]+, detected by 19F NMR. Application of the volume-based approach to thermodynamics (VBT) offers a means, for the first time, of exploring the energetics surrounding these materials and in the thermodynamic section a discussion of this new approach is provided. It emerges that the basis of the thermodynamic driving force of formation of [TiF3L3][SbF6](s) salts, that enforces the unfavourable [DeltaH degrees =+ 237 (+/-20) kJ mol(-1)] fluoride ion transfer from the Lewis acid TiF4(s) to SbF5(l) to give the hypothetical [TiF3]+[SbF6]-(s), is the higher Ti-L (L = ligand) bond energy in the cationic complexes [TiF3L3]+ as compared to that in the molecular adducts TiF4L2(s) and SbF5L(s) so giving rise to larger enthalpies of complexation of [TiF3]+(g) by 3L(g) compared to those for complexation of TiF4(g) by 2L(g) and SbF5(g) by 1L(g). Formation of the trans-[TiF2(15-Crown-5)]2+ and trans-[TiF2(18-Crown-6)]2+ is accounted for the stabilization of [TiF2]2+ cation by the five donor acceptor Ti-O contacts and the accompanying positive charge delocalization. Cationic titanium(IV) complexes fac-[TiF3MeCN)3-nLn]+(n= 0-3) and cis-[TiF318-Crown-6)]+, trans-[TiF2(Crown)]2+(Crown = 15-Crown-5 and 18-Crown-6) were obtained in MeCN solution by the reaction of fac-[TiF3(MeCN)3]+ and L = Et2, THF, H2 or crown ethers. Complexes fac-[TiF3(MeCN)3-nLn][SbF6] L = Et2, THF, H2O, crown ethers are unstable in MeCN solution and slowly decompose giving molecular complexes cis-TiF4L2, cis-TiF4(Crown), SbF5L, titanium oxofluoride and alkoxide complexes. The structure of the fac-[TiF3(MeCN)3]+ is similar to the fac-[TiCl3(MeCN)3]+ and the complexes trans-[TiF2L]2+ L = 15-Crown-5, 18-Crown-6 have very similar geometries to that of trans-[TiCl2(15-Crown-5)]+ showing that the essential features of coordination are the same for the cationic titanium chloride and fluoride complexes with MeCN and 15-Crown-5, 18-Crown-6.

Journal Article↗
PubMed2006

Coordination of crown structure, leaf plasticity and carbon gain within the crowns of three winter-deciduous mature trees.

We examined the vertical profiles of leaf characteristics within the crowns of two late-successional (Fagus crenata Blume and Fagus japonica Maxim.) and one early-successional tree species (Betula grossa Sieb. et Zucc.) in a Japanese forest. We also assessed the contributions of the leaves in each crown layer to whole-crown instantaneous carbon gain at midday. Carbon gain was estimated from the relationship between electron transport and photosynthetic rates. We hypothesized that more irradiance can penetrate into the middle of the crown if the upper crown layers have steep leaf inclination angles. We found that such a crown has a high whole-crown carbon gain, even if leaf traits do not change greatly with decreasing crown height. Leaf area indices (LAIs) of the two Fagus trees (5.26-5.52) were higher than the LAI of the B. grossa tree (4.50) and the leaves of the F. crenata tree were more concentrated in the top crown layers than were leaves of the other trees. Whole-crown carbon gain per unit ground area (micromol m(-2) ground s(-1)) at midday on fine days in summer was 16.3 for F. crenata, 11.0 for F. japonica, and 20.4 for B. grossa. In all study trees, leaf dry mass (LMA) and leaf nitrogen content (N) per unit area decreased with decreasing height in the crown, but leaf N per unit mass increased. Variations (plasticity) between the uppermost and lowermost crown layers in LMA, leaf N, the ratio of chlorophyll to N and the ratio of chlorophyll a to b were smaller for F. japonica and B. grossa than for F. crenata. The light extinction coefficients in the crowns were lower for the F. japonica and B. grossa trees than for the F. crenata tree. The leaf carbon isotope ratio (delta(13)C) was higher for F. japonica and B. grossa than for F. crenata, especially in the mid-crown. These results suggest that, in crowns with low leaf plasticity but steep leaf inclination angles, such as those of F. japonica and B. grossa trees, irradiance can penetrate into the middle of the crowns, thereby enhancing whole-crown carbon gain.

Betula↗
PubMed2000

Effect of crown lengthening and ferrule placement on static load failure of cemented cast post-cores and crowns.

STATEMENT OF PROBLEM: Restoration of mandibular second premolars with completely missing clinical crowns in the Kennedy Class I and II arches is costly and the risk of failure is high. Should the dentist choose crown-lengthening to allow the addition of a ferrule, despite incurring the disadvantage of an increased crown/root ratio? PURPOSE: This in vitro study determined the combined effect of crown lengthening and placement of a ferrule on the failure resistance to static load of decoronated and restored mandibular second premolar analog teeth. MATERIAL AND METHODS: An extracted mandibular second premolar of average dimensions was selected to form molded composite root analogs and metal crowns. Ten specimens prepared with no-ferrule root and negligible axial wall lengths and 10 specimens with ferrule root, apically repositioned finish lines, were made. Cast gold alloy post and cores and complete crowns were made and cemented. Each root was placed in bone-simulating resin located 3 mm from the finish line. Crown lengthened/ferrule and no-crown lengthened/no-ferrule groups were equally subdivided, whereby 5 crowns were compressively loaded on the buccal cusp tip and 5 were compressively loaded on the mesial marginal ridge. A testing machine applied force until failure occurred. Failure loads were examined with 2-way ANOVA. Significant differences were accepted at P </=.05. RESULTS: Mean failure loads for the crown lengthened/ferrule and no-crown lengthened/no-ferrule groups were as follows: buccal load, 0.61 kN (SD +/- 0.11) and 0.83 kN (SD +/- 0.08); mesial load, 0.70 kN (SD +/- 0.08) and 1.00 kN (SD +/- 0.15), respectively. Crown lengthened/ferrule and no-crown lengthened/no-ferrule groups were significantly different (P <.001). CONCLUSION: The combination of simulated surgical crown-lengthening and more apical crown margin placement to provide a 2-mm crown ferrule on a decoronated mandibular second premolar analog resulted in a reduction of static load failure for the restored analog tooth.

Analysis of Variance↗
PubMed2002

Comparison of the fracture strengths of metal-ceramic crowns and three ceromer crowns.

STATEMENT OF PROBLEM: The fracture strength of a number of new ceromer systems for the fabrication of anterior fixed partial dentures is unknown. PURPOSE: The purpose of this study was to compare the fracture strengths of metal-ceramic crowns and 3 types of ceromer crowns in a simulated anterior tooth preparation. MATERIAL AND METHODS: A resin maxillary central incisor analog was prepared with a 5-degree convergence angle and a 90-degree, 1-mm shoulder. The incisal edge was reduced by 2 mm, and the axiogingival and axioincisal line angles were rounded. A mold of this tooth was made with vinyl polysiloxane impression material. Ten wax patterns were made from the mold and cast in a nickel-chromium alloy for the fabrication of metal dies, from which 10 metal-ceramic crowns were fabricated. Ten crowns for each of 3 ceromer systems (Artglass, Sculpture, and Targis) were fabricated from the same metal die. Each crown group was prepared, polymerized, air-particle abraded, and finished in accordance with the specific manufacturer's instructions. All 4 types of finished crowns were luted to the dies and embedded in polymethyl methacrylate resin blocks. Their fracture resistance was tested in a universal testing machine. The load was directed at the incisolingual line angle, at 130 degrees to the long axis of each specimen, until catastrophic failure occurred. A 7-mm-diameter rod was used to load the artificial crowns, with the center of the rod in contact with the crown surfaces. Analysis of variance and Tukey's multiple comparisons test (P<.05) were applied to the data. RESULTS: Metal-ceramic crowns fractured at significantly higher values than ceromer crowns (1317 vs 602 N, respectively) (P<.05). No significant difference was found among the fracture values of Artglass, Sculpture, and Targis crowns. CONCLUSION: Within the limitations of this study, failure load values for the 3 ceromer crowns tested, although significantly lower than the values for metal-ceramic crowns, still exceeded normal occlusal forces.

Analysis of Variance↗
PubMed1989

[Retention in the conic telescope crown system. The influence of the inner crown form].

In a study of retention of the conic telescope crown system, the influence of the morphological condition of the inner crown on retention was examined. The usefulness of a milling machine with a polishing disk, a newly developed polishing system, was also evaluated. The results were as follows. 1) In polishing the axial surface of the inner crown of the conic telescope crown system, the milling machine with a polishing disk facilitated specular finishing without causing undercutting in the region from the occlusal surface to the dental cervix. 2) In producing the outer crown of the conic telescope crown system using Deguvest HFG, a phosphate-investing material, the retentive force of the crown tended to increase as the concentration of the solution for specific use was decreased to 50%, 30% and 10%, and reached appropriate levels of 500-1,000 g with the 50% solution. 3) When the height of the inner crown was changed to 2 mm, 4 mm and 6 mm, no significant difference was found in the retentive force, the mean values being 516 g, 836 g and 1,003 g under loading of 10 kg, 20 kg and 30 kg, respectively. 4) The retentive force tended to increase with increased width of the dental cervix of the inner crown: it was 745 g, 1,067 g and 1,376 g at 6 mm, 8 mm and 10 mm of cervical diameter, respectively, under a load of 30 kg. 5) The retentive force was significantly changed with varying loads (99% reliability), and tended to increase with increased load. 6) There was a highly significant correlation between retentive force and sinking of the inner crown. 7) When a tensile load was placed 3 mm lateral to the central axis of a test piece, the retentive force tended to increase when the vertical diameter of the inner crown cervix was larger than the horizontal diameter, while it decreased when the vertical diameter was shorter than the horizontal diameter, as compared with central axis loading.

Crowns↗
PubMed2004

Field-dependent effect of crown ether (18-crown-6) on ionic conductance of alpha-hemolysin channels.

Closing linear poly(ethylene glycol) (PEG) into a circular "crown" dramatically changes its dynamics in the alpha-hemolysin channel. In the electrically neutral crown ether (C2H4O)6, six ethylene oxide monomers are linked into a circle that gives the molecule ion-complexing capacity and increases its rigidity. As with linear PEG, addition of the crown to the membrane-bathing solution decreases the ionic conductance of the channel and generates additional conductance noise. However, in contrast to linear PEG, both the conductance reduction (reporting on crown partitioning into the channel pore) and the noise (reporting on crown dynamics in the pore) now depend on voltage strongly and nonmonotonically. Within the whole frequency range accessible in channel reconstitution experiments, the noise power spectrum is "white", showing that crown exchange between the channel and the bulk solution is fast. Analyzing these data in the framework of a Markovian two-state model, we are able to characterize the process quantitatively. We show that the lifetime of the crown in the channel reaches its maximum (a few microseconds) at about the same voltage (approximately 100 mV, negative from the side of protein addition) where the crown's reduction of the channel conductance is most pronounced. Our interpretation is that, because of its rigidity, the crown feels an effective steric barrier in the narrowest part of the channel pore. This barrier together with crown-ion complexing and resultant interaction with the applied field leads to behavior usually associated with voltage-dependent binding in the channel pore.

Computer Simulation↗
PubMed2005

Dibenzotetraaza crown ethers: a new family of crown ethers based on o-phenylenediamine.

Dibenzotetraaza (DBTA) crown ethers possess two o-phenylenediamine moieties. They are homologues of dibenzo crown ether phase-transfer catalysts and were prepared from the condensation of benzimidazoles with oligo(ethyleneglycol) dichlorides and oligo(ethyleneglycol) ditosylates. Compounds with ring sizes ranging from 18-crown-6 to 42-crown-14 were prepared. In addition, various altered benzimidizoles were used to produce DBTA crown ethers with modified substituents and ether bridges, as well as benzimidazolidine crown ethers. The synthetic approach presented here proved to be a convenient route to a new family of crown ethers with overall yields of up to 48% based on the benzimidazole. Yields for the ring-closing step were generally high, ranging from 51% to 94%, without the need for high-dilution conditions. Reaction of the DBTA crown ethers with alkyl and benzyl halides was found to be a facile way to obtain the corresponding tetra(N-organyl) compounds. Picrate extraction studies were carried out to determine phase-transfer catalytic capabilities. Extraction efficiencies for alkali-metal ions were lower than those for dibenzo-18-crown-6. Efficiencies were higher for other metal ions, with some selectivity for Pb(2+). Tetra(N-methyl) DBTA-18-crown-6 generally exhibited higher extraction efficiencies than its N-H analogue, but the selectivity was lower.

Journal Article↗
PubMed1990

[The method of esthetic crown restoration with composite resin jacket crown in primary molars].

The term "esthetics" has recently been also used in the dental field, and a field called esthetic dentistry is increasingly being noted. The number of not only adult but also pediatric patients who visit for treatment aiming at esthetic recovery is being increased. Inpedodontics, composite resin of the coronal color is generally used in the restoration of deciduous incisors. However, the method using metal crowns for the deciduous teeth is used for the deciduous molars at present. We applied a composite resin jacket crown to the deciduous molar in a way similar to that of esthetic crown restoration for the anterior teeth. The surgical procedure before crown preparation varied slightly according to the presence or absence of pulpal treatment of vital teeth and with non-vital teeth, but the application was performed as follows: 1) Desensitization of pulp, pulpal treatment and core construction. 2) Preparation of crown. 3) Selection, trial set and occlusal equilibration of a metal crown for the deciduous tooth. 4) Precision impression with a silicone impression material. 5) Removal of the metal crown for the deciduous tooth from the impression material. 6) Making of an under-cut to the abutment tooth on the buccal lingual side. 7) Filling of the impression with chemical polymerization resin. 8) Application of pressure in the oral cavity. 9) Adjustment of edge and crown forms. Thus, the preparation method for the composite resin jacket crown was relatively simple. Since this surgery, the patient has been followed up for 1 year and 6 months, and no specifically troublesome points have been observed clinically. The patient and her parents are satisfied with the results.(ABSTRACT TRUNCATED AT 250 WORDS)

Child↗
PubMed2006

A theoretical investigation on the Wurster's crown analogue of 18-crown-6.

An ab initio, quantum mechanical study of the Wurster's crown analogue of 18-crown-6 and its interactions with the alkali metal cations are presented. This study explores methods for accurately treating large, electron-rich species while providing an understanding of the molecular behavior of a representative member of this class of crowns. The molecular geometries, binding energies, and binding enthalpies are evaluated with methods similar to those reported for the analysis of 18-crown-6 and its alkali metal complexes to facilitate direct comparison. Hybrid density functional methods are applied to gauge the effects of electron correlation on the geometries of the electron-rich phenylenediamine moiety present in the Wurster's crowns. While the structure of the crown ether backbone is largely unperturbed by the incorporation of the redox active functionality, the alkali metal binding enthalpies are uniformly stronger for the Wurster's crown complexes, adding 1.8 to 5.1 kcal/mol to the strength of the interaction, depending on cation type. The additional strength, due to the exchange of an oxygen donor atom in the crown ether backbone by a nitrogen donor supplied by the redox group, is tightly coupled to the rotation of the dimethylaminophenyl group with respect to the plane of the macrocycle. Gas-phase selectivities favor the more highly charge-dense cations, while the explicit addition of only a few waters of hydration in the calculations recovers the selectivities expected in solution. The alkali metal binding affinity to the singly oxidized Wurster's crown is significantly diminished, while it is completely eliminated for the doubly oxidized ligand.

Journal Article↗
PubMed2006

Newly synthesized tetraoxa-diaza crown ether derivatives versus commercialized crown ethers in the separation of positional isomers with capillary electrophoresis.

Three new tetraoxa-diaza derivatives of 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane (R-1, R-2 and R-3) and three commercially available crown ethers, 18-crown-6 (18C6), (+)-18-crown-6-tetracarboxilic acid (18C6H4) and 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane, were investigated to separate the positional isomers of aminophenol, aminobenzoic acid and aminocresol. The running electrolyte, in which the crown ethers were dissolved, was a 50 mM Tris solution adjusted to pH 2.0 with hydrochloric acid. Using 50 mM H3PO4 buffer, whose pH was adjusted to 2.0 with Tris, or only hydrochloric acid solution with the same pH, did not allow good separations for the tested components. The effect of the crown ether concentration on the separation of the 11 positional isomers was studied in the concentration range of 10-50 mM. The best separations were achieved using the 18C6 and the 18C6H4 crown ethers: 9 isomers out of 11 could be separated within one run. The m- and p-aminophenol isomers could not be separated under the investigated experimental conditions. The newly synthesized tetraoxa-diaza crown ether derivatives were only found suitable for the separation of aminobenzoic acid positional isomers. The macrocyclic ring of the tetraoxa-diaza crown ethers was not able to form a stable inclusion complex with the tested positional isomers. Consequently, the aminophenol and aminocresol isomers were not separated, the isomers migrated with the same or very similar velocities.

Crown Ethers↗