• Bulletin of the Korean Chemical Society
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• v.17 no.9
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• pp.802-807
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• 1996

Second-order rate constants have been measured spectrophotometrically for the reactions of aryl benzoates (X-C6H4CO2C6H4-Y) with EtO-, Z-C6H4O- and Z-C6H4C(Me)=NO- in absolute ethanol at 25.0 ℃. All the reactions have been performed in the presence of excess 18-crown-6 ether in order to eliminate the catalytic effect shown by alkali metal ion. A good Hammett correlation has been obtained with a large ρ- value (-1.96) when σ- (Z) constant was used for the reaction of p-nitrophenyl benzoate (PNPB) with Z-C6H4O-. Surprisingly, the one for the reaction of PNPB with Z-C6H4C(Me)=NO- gives a small but definitely positive ρ- value (＋0.09). However, for reactions of C6H5CO2C6H4-Y with EtO-, correlation of log k with σ- (Y) constant gives very poor Hammett correlation. A significantly improved linearity has been obtained when σ0 (Y) constant was used, indicating that the leaving group departure is little advanced at the TS of the RDS. For reactions of X-C6H4CO2C6H4-4-NO2 with EtO-, C6H5O- and C6H5C(Me)=NO-, correlations of log k with σ (X) constants for all the three nucleophile systems give good linearity with large positive ρ values, e.g. 2.95, 2.81 and 3.06 for EtO-, C6H5O- and C6H5C(Me)=NO-, respectively. The large ρ values clearly suggest that the present reaction proceeds via a stepwise mechanism in which the formation of the addition intermediate is the RDS.

• Journal of the Korean Chemical Society
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• v.40 no.8
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• pp.557-564
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• 1996

FAB CAD MS/MS(Fast Atom Bombardment Collision Activated Dissociation Mass Spectrometry/Mass Spectrometry) was used to study different degree of bond stability according to the linkage positions of alkali cationized $(Na^+, Li^+, K^+, NH_4^+)$ stereoisomeric and synthetic oligosaccharides. The alkali metal cations were much more stable, requiring over -40 eV of collision energy vs. only -10 eV for the protonated forms. Of the cations, the potassium cationized trisaccharides were more stable than the others. They would not yield fragment ions under the conditions of collision available in triple quadrupole. Other cationized species exhibited decreasing stability in the series $Nap^+>Li^+>NH_4^+$ using 0.8 mTorr argon pressure in the collision cell. Metal cations of the oligosaccharides maintained charge principally on the amino sugar as shown by shift of all the fragment ions containing the amino sugar. The reason for the higher stability of the metal cationized form is the formation of crown ether-like bond around metal cations, N-acetyl group on GlcNAc and oxygens on fucose moiety. Depending on the metal sizes and the conformation of linkage-isomeric region, cationized species gave linkage dependent fragment patterns and exhibited stability in the series 1-6 > 1-4 > 1-3 linkage.

• Bulletin of the Korean Chemical Society
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• v.32 no.7
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• pp.2423-2427
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• 2011

Pseudo-first-order rate constants ($k_{obsd}$) have been measured spectrophotometrically for nucleophilic substitution reactions of 3,4-dinitrophenyl diphenylphosphinothioate 9 with alkali metal ethoxides (EtOM, M = Li, Na, K) in anhydrous ethanol at $25.0{\pm}0.1^{\circ}C$. The plot of $k_{obsd}$ vs. [EtOM] is linear for the reaction of 9 with EtOK. However, the plot curves downwardly for those with EtOLi and EtONa while it curves upwardly for the one with EtOK in the presence of 18-crown-6-ether (18C6). Dissection of $k_{obsd}$ into $k_{EtO^-}$ and $k_{EtOM}$ (i.e., the second-order rate constant for the reaction with dissociated $EtO^-$ and ion-paired EtOM, respectively) has revealed that the reactivity increases in the order $k_{EtOLi}$ < $k_{EtONa}$ < $k_{EtO^-}$ ${\approx}$ $k_{EtOK}$ < $k_{EtOK/18C6}$, indicating that the reaction is inhibited by $Li^+$ and $Na^+$ ions but is catalyzed by 18C6-crowned $K^+$ ion. The reactivity order found for the reactions of 9 contrasts to that reported previously for the corresponding reactions of 1, i.e., $k_{EtOLi}$ > $k_{EtONa}$ > $E_{EtOK}$ > $k_{EtO^-}$ ${\approx}$ $k_{EtOK/18C6}$, indicating that the effect of changing the electrophilic center from P=O to P=S on the role of $M^+$ ions is significant. A four-membered cyclic transition-state has been proposed to account for the $M^+$ ion effects found in this study, e.g., the polarizable sulfur atom of the P=S bond in 9 interacts strongly with the soft 18C6-crowned $K^+$ ion while it interacts weakly with the hard $Li^+$ and $Na^+$ ions.

• Journal of the Korean Chemical Society
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• v.35 no.2
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• pp.119-127
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• 1991

The protonation and the metal ion complexation of 15 to 18 membered diaza crown ether such as 1,12-diaza-3, 4 : 9, 10-dibenzo-5, 8-dioxacyclopentadecane(NtnOenH$_4$), 1,13-diaza-3,4 : 10,11-dibenzo-hydroxy-5,9-dioxacyclohexadecane(NtnOtnH$_4$), 1,13-diaza-3,4 : 10,11-dibenzo-15-hydroxy-5,9-dioxacyclohexadecane(Ntn(OH)OtnH$_4$), 1,15-diaza-3,4 : 12,13-dibenzo-5,8,11-trioxacycloheptadecane (NenOdienH$_4$) and 1,15-diaza-3,4 : 12,13-dibenzo-5,8,11-trioxacyclooctadecane(NtnOdienH$_4$) were studlied by potentiometry and NMR spectroscopy. The protonation constants were used to predict basicity of crown ethers. The sequence of the basicity was NenOdienH$_4$ < Ntn(OH)OtnH$_4$ < NtnOenH$_4$ < NtnOtnH$_4$ < NtnOdienH$_4$. Changes on the basicity were explained in terms of the effects of substituents and the degree of twistness of the macrocyclic ring. The sequence of the complex stabilities were Co(II) < Ni(II) < Cu(II) < Zn(II) for the transition metal complexes and Cd(II) < pb(II) < Hg(II) for the post-transition metal complexes. These changes on the stabilities were dependent on the basicity of the ligand and cavity size of the ring. For the heavy post-transiton metal complexes and Zn(Ⅱ) complex, the former factor was predominent and for the other transition metal complexes, the latter was affected on the stabilities. $^1$H and $^{13}$C-NMR studies for heavy post-transition metal complexes indicated that the nitrogen atom has greater affinity on metal ions than oxygen atom and the planarity of the rings was losed by the complexation with metal ions.

• The Journal of Korean Academy of Prosthodontics
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• v.57 no.2
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• pp.127-133
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• 2019

Purpose: The purpose of present study was to investigate fracture strength and mode of failure of endodontically treated teeth restored with metal cast post-core system, prefabricated fiber post system, and newly introduced polyetherketoneketone (PEKK) post-core system. Materials and methods: A total of 21 mandibular premolar were randomly grouped into 3 groups of 7 each according to the post material. Group A was for metal cast post core; Group B for prefabricated glass fiber post and resin core; and Group C for milled PEKK post cores. All specimens were restored with metal crown. The fracture strength of each specimen was measured by applying a static load of 135-degree to the tooth at 2 mm/min crosshead speed using a universal testing machine. After the fracture strength measurement, the mode of failure was observed. The results were analyzed using Kruscal-Wallis test and post hoc Mann-Whitney U test at confidence interval ${\alpha}=.05$. Results: Fracture resistance of PEKK post core was lower than those of cast metal post and fiber reinforced post with composite resin core. In the aspect of fracture mode most of the root fracture occurred in the metal post core, whereas the post detachment occurred mainly in the fiber reinforced post. In the case of PEKK post core, teeth and post were fractured together. Conclusion: It is necessary to select appropriate materials of post for extensively damaged teeth restoration and clinical application of the PEKK post seems to require more research on improvement of strength.