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PubMedDate not supplied

Solid state 13C NMR of 30-crown-10 ether and 30-crown-10.4H2O.

The 13C NMR solution spectra of 30-crown-10 ether and its tetrahydrate show only one resonance at all accessible temperatures. In contrast, the solid state 13C NMR spectrum of the 30-crown-10.4H2O shows two resonances in the ratio of 4:1, separated by 1.2 ppm. In the case of 30-crown-10 itself, six resolvable 13C resonances in the ratio of 4:1:1:2:1:1 are observed in the solid with an overall chemical shift dispersion of 5 ppm. The remarkably different spectral behavior of these two systems in the solid state is discussed in terms of the torsional environments of the crystallographically unique carbons and the results of GIAO calculations of isotropic 13C shieldings for simpler model compounds. Results of dipolar dephased 13C CPMAS spectra indicate that 30-crown-10 does not undergo a large amplitude molecular motion, in contrast to earlier results for 18-crown-6. Only a small amount of residual intensity is found in the dipolar dephased spectrum of 30-crown-10.4H2O, indicating that it also is relatively rigid in the solid.

Carbon Isotopes↗
PubMed2005

Inclusion of a Cu2+ ion by a large-cavity crown ether dibenzo-24-crown-8 through supramolecular interactions.

A supramolecular copper-aqua-crown ether complex, [Cu(II)(H2O)4(dibenzo-24-crown-8]2+ (1c) is stabilized with a Lindqvist-type polyoxometalate anion, [Mo(VI)6O19]2- (1a), in an ion-pair compound [Cu(II)(H2O)4(dibenzo-24-crown-8][Mo(VI6O19] identical with [1c][1a] identical with 1. In the crystal, 1c and 1a assemble to a chainlike structure in which each polyoxoanion 1a is sandwiched by two 1c cations. 1c is a structurally characterized dibenzo-24-crown-8 (a larger-cavity crown ether) supramolecular complex that shows encapsulation of a small cation at the center of its internal cavity, and compound 1 represents a unique example of a first-row transition metal-crown ether inclusion complex that interacts with a polyoxometalate anion.

Journal Article↗
PubMed1997

Synthesis, Reactivity, and Structure of [H(13)O(6)][PtCl(5)(H(4)O(2))].2(18-cr-6): A Crown Ether Complex of a Pentachloroaquaplatinic Acid with an [H(13)O(6)](+) Cation in a Cage of Three Crown Ether Molecules.

An aqueous solution of H(2)[PtCl(6)].6 H(2)O reacts with crown ether 18-crown-6 to give [H(13)O(6)][PtCl(5)(H(4)O(2))]. 2(18-cr-6)(1) (1) and (H(3)O)(2)[PtCl(6)].2(18-cr-6) (2a). In the presence of HCl, only 2a is formed; in an analogous manner, (H(3)O)(2)[PtCl(6)].2(15-cr-5).2 H(2)O (2b) and (H(3)O)(2)[PtCl(6)].2(DCH-18-cr-6)(1) (2c) were obtained. 1 is gradually decomposed in water to give [PtCl(4)(H(2)O)(2)].(18-cr-6).2H(2)O (3). 1-3 were characterized by microanalysis and IR and NMR spectroscopy ((1)H, (13)C, (195)Pt). The X-ray structure analysis of 1 (orange needles; space group P2(1)2(1)2(1), a = 7.938(1) Å, b = 15.691(2) Å, c = 34.861(4) Å; Z = 4) shows an anionic entity [PtCl(5)(H(4)O(2))](-) and a cationic entity [H(13)O(6)](+) which are separated by a crown ether molecule 18-cr-6. The [H(13)O(6)](+) cations exhibit the structure [H(3)O(H(4)O(2))(2)(H(2)O)](+) and are also separated by crown ether molecules in such a way that the crystal is threaded by chains built up of 18-cr-6/[H(13)O(6)](+) units at which the 18-cr-6/[PtCl(5)(H(4)O(2))](-) moieties are fixed as lateral branches. Thus, the cations [H(13)O(6)](+) are embedded in three crown ether molecules. All terminal hydrogens are involved in hydrogen bridges to oxygen atoms of the crown ether molecules. The O(w).O(cr) distances are distinctly longer in the mean than the O(w).O(w) distances within the [H(13)O(6)](+) moiety.

Journal Article↗
PubMed2003

Shoot growth and crown development: effect of crown position in three-dimensional simulations.

Trees have been increasingly considered as modular organisms, with individual shoots forming autonomous units that respond semi-independently to their surrounding environment. However, there is evidence for fairly strict hormonal control of tree crown development. Studies on the hydraulic architecture of trees suggest a closer functional connection between shoots and crown development than is postulated by the theory of branch autonomy. We studied how shoot growth pattern influences growth and crown architecture in young Scots pine trees simulated by the LIGNUM model assuming that (a) the growth of a shoot mainly depends on its light climate and (b) the growth of a shoot is influenced by its position within the crown. We determined shoot position within the crown based on a recently developed vigor index. The vigor index compares the relative axis cross-sectional area from the base of the tree to each shoot and gives a value of 1 to the pathway of the greatest cross-sectional area. All other shoots attain values between 0 and 1 depending on their cross-sectional areas and the cross-sectional areas of the branches leading there from the main axis. The shoot light climate is characterized by annually intercepted photosynthetically active radiation. We compared the results from simulations (a) and (b) against an independent data set. The addition of a within-shoot position index (the vigor index) to our simulation (simulation b) resulted in a more realistic tree form than that obtained with simulation (a) alone. We discuss the functional significance of the results as well as the possibilities of using an index of shoot position in simulations of crown architecture.

Models, Biological↗
PubMed1992

Comparison of the fracture strengths of ceramometal crowns versus several all-ceramic crowns.

Fracture resistance to forces applied to the incisal edges of four types of anterior crowns was tested. Ceramometal crowns fractured at significantly higher values (720 psi) than the all-ceramic crowns (approximately 360 psi). No significant difference was found among the fracture values of the Dicor crowns, the aluminous porcelain jackets, and the crowns fabricated from Dicor veneered with aluminous body and incisal porcelain.

Aluminum Oxide↗
PubMed1992

Dimensional accuracy of castable apatite ceramic crowns: the influence of heat treatment on dimensional changes and distortion of crowns.

Castable apatite ceramic crowns are subjected to heat treatment during crystallizing and during coloring after casting. The dimensional accuracy of the heat-treated crowns was investigated with respect to its influence on dimensional changes in castable apatite ceramic crowns and on distortion of the interior of the crowns, using two investment techniques and a three-coordinate measuring machine. Dimensional changes were not influenced by the number of coloring firings. Distortion of the interior of crowns was slightly outward at the gingival plane. Shrinkage because of crystallizing was approximately 20 microns and the shrinkage of the gingival plane was equal to that of the occlusal plane.

Apatites↗
PubMed1994

Fracture strength of full-contoured ceramic crowns and porcelain-veneered crowns of ceramic copings.

This is vitro study compared the fracture strength of Dicor ceramic crowns with a 1 mm axial wall thickness and a 2 mm occlusal thickness with Dicor copings veneered with Vitadur-N or Dicor-Plus porcelain to create similar contours. Tooth preparations for complete crowns were made on human molars with a 10-degree total convergence and a 1 mm shoulder. The artificial crowns were internally etched, given an application of silane, and cemented with a dual-cured resinous cement. After thermocycling, each sample was loaded to failure with a vertical load on the occlusal surface. There were no significant differences in fracture strength between full-contour Dicor crowns and crowns of copings veneered with either Vitadur-N or Dicor-Plus porcelain.

Analysis of Variance↗
PubMed1999

Titanium full crown casting: thermal expansion of investments and crown accuracy.

OBJECTIVES: The aim of the present study was to establish a statistical model between thermal expansion values of investments and dimensional accuracy of cast titanium full crowns. METHODS: Thermal expansion (TE) measurements were made on cylindrical specimens (5O x 20 mm) dried to constant mass and full crown wax patterns were prepared from metal dies having the same design. Castings were made using a one-chamber high-pressure casting machine. Dimensional accuracy of the cast crowns was expressed by the discrepancy (micron) between the wax pattern and a casting prepared from the same die. A simple least squares linear regression analysis was made on data accumulated over the last five years. RESULTS: A high correlation coefficient (R = 0.87) was found between dimensional accuracy (Y) and thermal expansion value (X), and the accuracy was predicted by a formula Y = 1040.50X - 1004.45 microns. For zero discrepancy, a thermal expansion of 0.97% was required. SIGNIFICANCE: The dimensional accuracy expressed as the discrepancy between wax pattern and casting is easy to understand and less confusing than the traditional percentage value. The crown accuracy was highly predictable (R = 0.87) from the thermal expansion value of investments determined under controlled laboratory measuring conditions. New investments being released for titanium casting can be under constant modification. For this, regular monitoring of their TE behaviour is essential and use of the same batch is preferable for expansion measurement and crown accuracy assessment.

Analysis of Variance↗
PubMed2003

Marginal adaptation and fracture resistance of adhesively luted glass fibre-composite reinforced molar crowns with different inner crown surfaces.

OBJECTIVES: This study compared the influence of different inner crown surfaces on the fracture resistance and marginal adaptation of adhesively fixed glass fibre-reinforced molar crowns. MATERIALS AND METHODS: Vectris/Targis crowns were constructed with an inner framework of glass fibres (directly on the tooth) or an inner veneering composite layer between the fibre-framework and the tooth-substance. Both groups were sandblasted inside using Al(2)O(3); 50 microm grain size (200 kPa, 20 s) and silane coated. A control group had the inner fibre framework, but was neither sandblasted nor silane coated. The crowns were adhesively cemented on extracted human teeth, and thermally cycled and mechanically loaded (TCML: 6000 x 5 degrees C/55 degrees C; 1.2 x 10(6) x 50 N, 1.66 Hz). The marginal adaptation before and after TCML was evaluated and the fracture resistance was investigated using a Zwick universal testing machine. RESULTS: After TCML the proportion of 'perfect margin' of the control group decreased significantly at the interface crown/cement. For the variations with an inner fibre framework or inner composite layer the marginal adaptation or fracture resistance did not decrease significantly after ageing. The fracture resistance values were control: 1509N+/-486; inner fibre framework: 1896N+/-342; inner composite layer: 1754N+/-340. CONCLUSIONS: In the case of the investigated fibre framework and veneering composite, the inner surface of glass fibre-reinforced molar crowns can be covered with a composite layer or with a glass fibre framework. Both methods achieve comparable high fracture strengths and reliable marginal adaptation.

Aluminum Oxide↗
PubMed2002

Crowns and other extra-coronal restorations: preparations for full veneer crowns.

Preparations for full veneer crowns is the eighth in the series on crowns and other extra-coronal restorations. Whilst handpiece skills are important, many other factors combine to ensure provision of a satisfactory full veneer crown (also termed 'full coverage crown'). Our aim in writing this article is to consider the principles which influence crown preparation, seasoned with clinical advice our undergraduate and postgraduate students have found useful.

Composite Resins↗
PubMedDate not supplied

A telescopic crown concept for the restoration of the partially edentulous arch: the Marburg double crown system.

PURPOSE: This article describes the concept of the Marburg double crown system (MDC system) in the treatment of partially edentulous patients. Long-term success is assessed by a review of patient records. MATERIALS AND METHODS: Double crowns with clearance fit are used to retain tooth-, mucosa-, and implant-supported removable partial dentures (RPD). To achieve retention, an additional attachment, the TC-SNAP system, is used. All metal components are fabricated in a single cobalt-chromium-molybdenum alloy; the framework (including outer crowns) is cast in one piece. Because of the framework's rigidity, the RPD can be constructed without major and minor connectors. The denture base adjacent to the abutments is fabricated using a perioprotective design that is similar to fixed partial dentures. One hundred eleven dentures, of which 49 (44%) were fabricated for patients with intraoral defects, were evaluated by reviewing patient records. RESULTS: The mean age of the patients at the time of insertion was 57.5 years (+/- 12.3). The mean number of double crowns per denture was 3.5 +/- 2.1 (range, 1 to 9). The probability that a patient will have lost all abutment teeth 10 years after insertion of the denture is 4%. The probability that a patient will have kept all teeth that abut the denture 5 years after insertion is 87%; 10 years after insertion the probability is 80%. CONCLUSION: The Marburg double crown system is a versatile and successful means of achieving the long-term restoration of the partially edentulous jaw. Insertion and removal of the denture and routine oral hygiene are easy to perform, even for patients with limited manual dexterity. As a full-arch reconstruction, the MDC system enables easy adjustment, modification, and relining with low follow-up costs.

Adolescent↗
PubMed2004

[Restoring ceramo-metal tooth by using self-made crown post-core in order to re-utilize the broken crown of the base tooth].

OBJECTIVE: To explore a new method and its clinical significance of restorative ceramo-metal by using a self-made crown post-core to reutilize the broken crown. METHODS: 18 teeth were restored with ceramo-metal tooth by using self-made crown post-core in order to preserve the old broken base tooth. The patients' own feelings and clinical effect were observed. RESULTS: Followed up for 1-3 years, 83.3% (15 teeth) of the patients were successful. Among the group one was failed because of bad condition of post and caused split of tooth root. The second failure was caused by uneven force by coherent paste uneven. The third failure was caused by tight biting force and caused the tooth broken. CONCLUSION: Restoring ceramo-metal tooth by using self-made crown post-core in order to re-utilize the broken crown of the base tooth is convenient and low cost and is suitable for basic hospitals.

Crowns↗
PubMed2001

Syntheses, vibrational spectra, and theoretical studies of the adamantanoid Sn(4)Ch(10)(4-) (Ch = Se, Te) anions: X-ray crystal structures of [18-crown-6-K](4)[Sn4Se10]*5en and [18-crown-6-K](4)[Sn4Te10]*3en*2THF.

The salts [18-crown-6-K](4)[Sn(4)Se(10)].5en and [18-crown-6-K](4)[Sn(4)Te(10)].3en.2THF were isolated upon addition of THF to the ethylenediamine (en) extracts of the alloys KSn(0.90)Se(1.93) and K(4)Sn(4)Te(10) that had been extracted in the presence of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane). The Sn(4)Te(10)(4-) anion has been structurally characterized for the first time by a single-crystal X-ray diffraction study of [18-crown-6-K](4)[Sn(4)Te(10)].3en.2THF: P2(1)/n, a = 22.420(5) A, b = 19.570(4) A, c = 24.680(5) A, beta = 96.90(3)(o), Z = 4, and R(1) = 0.0468 at -183 degrees C. In addition to Si(4)Te(10)(4-) and Ge(4)Te(10)(4-), the Sn(4)Te(10)(4-) anion represents the only other known group 14 adamantanoid telluride. The X-ray crystal structure determination of the related [18-crown-6-K](4)[Sn(4)Se(10)].5en salt has also been determined: P2(1)/n, a = 22.003(2) A, b = 18.966(2) A, c = 24.393(2) A, beta = 97.548(8)(o), Z = 4, and R(1) = 0.0843 at -123 degrees C. The anion geometries are of the adamantanoid type where the Sn(IV) atoms occupy the bridgehead positions and the chalcogen atoms occupy the bridging and terminal sites. The energy minimized geometries of Sn(4)Ch(10)(4-) have also been determined using density functional theory (DFT). Mayer bond order analyses, Mayer valencies, and empirical bond valencies indicate that the terminal Sn-Ch bonds have significant multiple bond character, with the terminal Sn-Se bond having more multiple bond character than the terminal Sn-Te bond. The vibrational frequencies of the Sn(4)Se(10)(4-) and Sn(4)Te(10)(4-) anions have been calculated using DFT methods, allowing the Raman spectrum of Sn(4)Se(10)(4-) to be fully assigned.

Journal Article↗
PubMed2006

Crown bowl: metallocyclophane by self-assembly of 4'-pyridylmethyl-armed 12-crown-4 ethers with Ag+ ions.

New 3'-pyridylmethyl- and 4'-pyridylmethyl-armed monoaza-12-crown-4 ethers were prepared by the reductive amination of monoaza-12-crown-4 with the appropriate pyridinecarbaldehyde in the presence of NaBH(OAc)3. X-ray crystallography, cold electrospray ionization mass spectrometry, and 1H NMR titration experiments show that Ag+ complexes with 3'-pyridylmethyl- and 4'-pyridylmethyl-armed monoaza-12-crown-4 ethers are dimetallo[3.3]metacyclophane and trimetallo[3.3.3]paracyclophane, respectively (crown bowl). The structure of the metallocyclophanes can be controlled by the positions of the N atoms in the pyridine side arms and the ring size of the crown moiety.

Journal Article↗
PubMed1999

Unusual Me-O Bond Cleavage in a Metalated Crown-Ether: X-ray Molecular Structure of (5-Methoxy-4,6-dimethyl-1,3-xylylene-2-one)-15 Crown-4 Complex of Pentamethylcyclopentadienyl Iridium.

The reaction of [(C(5)Me(5))Ir(Solvent)(3)][BF(4)](2) (1) with (2,5-dimethoxy-4,6-dimethyl-1,3-xylylene)-15 crown-4 (2) affords the metalated crown-ether complex [(eta(5)-C(5)Me(5))Ir(eta(6)-C(18)H(28)O(6))][BF(4)](2) (3) in 88% yield. Complex 3 undergoes a facile Me-O bond cleavage to give the related semiquinone form of the metalated crown-ether [(eta(5)-C(5)Me(5))Ir(eta(5)-C(17)H(25)O(6))][BF(4)] (4). A single-crystal X-ray structure determination of complex 4 is reported. Complex 4 crystallizes in the monoclinic space group P2(1)/m with a = 8.187(5) Å, b = 17.193(4) Å, and c = 10.900(3) Å, alpha = 90 degrees, beta = 109.68(1) degrees, gamma = 90 degrees, and Z = 2. The structure provides us with valuable information about the nature of the eta(5)-semiquinone form of the metalated crown-ether and reveals that, surprisingly, the Me-O unit close to the crown chain is the one that undergoes hydrolysis. A rationale consistent with the experimental results is advanced.

Journal Article↗
PubMed2000

Synthesis and Photochromism of Crowned Spirobenzothiapyran: Facilitated Photoisomerization by Cooperative Complexation of Crown Ether and Thiophenolate Moieties with Metal Ions.

Spirobenzothiapyrans bearing monoaza-12-crown-4, -15-crown-5, -18-crown-6, and oligooxyethylene moieties were synthesized, and their photochromism was examined in the presence of cations in acetonitrile. The cation complexation by their crown ether moieties cannot induce thermal isomerization to their corresponding colored merocyanine form, unlike the corresponding spirobenzopyran derivatives. The UV-light-induced isomerization was, however, facilitated by the cation complexation of the crown ether moieties and the affinity of the merocyanine thiophenolate anion to metal ions, especially in the presence of Li(+) and Ag(+). The presence of Ag(+) brought about the most remarkable effect in the facilitation of photoisomerization of the spirobenzothiapyrans and the thermal stability of the colored merocyanine form mainly due to the powerful interaction of the thiophenolate anion with the soft metal ion.

Journal Article↗
PubMed1984

Crown area as an indicator of changing crown size in human pre-natal teeth.

This study deals with correlations of tooth size in human primary teeth. Traditionally, mesiodistal or buccolingual measurements have been used to express crown size. However, in this study, crown area was used as a reference parameter. Graphic reconstructions of 38 histologically prepared human fetuses with Crown-Rump Length (CRL) between 54 and 280 mm were used to obtain linear and area measurements. Based on clinical records and physical examinations, these fetuses were considered typical-for-age, or "normal". Correlation matrices indicated high levels of concordance among all developing deciduous tooth crowns and arch measurements. It was concluded that crown area is a statistically better trait to use in comparison of teeth rather than the traditional mesiodistal measurement.

Body Height↗