Reatividade em reações de Diels-Alder: uma prática computacional

A computational quantum chemistry experiment is described of Diels-Alder reactions between 2-cycloenones and cyclopentadiene. The effects of FMO-Frontier Molecular Orbitals (HOMO-LUMO) and of the withdrawing nature of substituents at the C=C bond of cycloenones were evaluated. The calculations were made using HF/STO-3G and B3LYP/6-31+G(d,p) methods. The FMO based indexes are in agreement with the experimentally observed reactivity order. NBO - Natural Bond Orbitals - analysis was used to ascertain the effect of C=C substituents on the dienophile reactivity.

A regra de Bent contextualiza a força da ligação de hidrogênio em clusters trimoleculares

This article presents a theoretical study of the molecular properties of trimolecular clusters of CnHm∙∙∙HCN∙∙∙HX formed by the ϖ∙∙∙H and n∙∙∙H hydrogen bonds. The interaction strengths of these interactions are in line with the variations in s-character, and independently, the red-shift rise whether stronger or weaker bound systems are carried out. This behavior was justified via NBO analysis and supported by Bent´s rule, wherein the greater variations in s-character of X are in good agreement with larger red-shifts and vice-versa. To conclude, the refinement of the supermolecule approach and NBO binding energies also corroborate in this regard.

A INFLUÊNCIA DA LIGAÇÃO DE HIDROGÊNIO EM REAÇÕES QUÍMICAS: REAÇÃO DE PRILESCHAJEW

Theoretical analysis at the BHandHLYP/6-311++G(d,p) level of theory with the support of QTAIM calculations was used to reinvestigate the structure of an intermolecular system formed between ethylene oxide (C2H4O) and formic acid (HCO2H) after the epoxidation known as the Prileschajew reaction. Geometric and infrared vibration results revealed that HO2CH forms a strong hydrogen bond with C2H4O followed by a larger red-shift of the H−O bond. NBO analysis was applied to justify this frequency shift. Finally, QTAIM calculations identified the formation of two hydrogen bonds, namely O···H−O and H···O=C.

Synthèse totale du méthyl ester de la zincophorine et étude DFT du transfert d’hydrogène diastéréosélectif impliquant des intermédiaires radicalaires vicinaux à un ester

Cet ouvrage traite d’une méthodologie pour l’induction de centres stéréogènes au sein des motifs propionates par la chimie des radicaux et de son application à la synthèse totale du méthyl ester de la zincophorine. Cet ionophore, aux propriétés biologiques intéressantes, présente plusieurs défis synthétiques dont une séquence de type polypropionate anti, anti, anti, anti difficilement accessible ainsi qu’un tétrahydropyrane trans trisubstitué. Récemment, l’intérêt renouvelé pour ces composés polyéthers, en tant qu’agents anticancéreux, accentue l’importance de stratégies versatiles permettant l’accès à ces structures ainsi qu’à leurs analogues. Depuis quelques années, notre groupe s’intéresse à la synthèse d’unités propionates acycliques par une séquence réactionnelle contrôlée uniquement par le substrat. La première étape découle d’une aldolisation de Mukaiyama entre un aldéhyde alpha-chiral et un énoxysilane tétrasubstitué portant une liaison carbone-halogène, et où l’issue stéréochimique dépend de la nature monodentate ou bidentate de l’acide de Lewis employé. La seconde réaction-clé implique la formation d’un radical tertiaire, vicinal à un ester, pouvant être réduit diastéréosélectivement en présence d’hydrure d’étain. La première section décrit l’élaboration de motifs tétrahydropyranes trisubstitués et l’induction des centres stéréogènes vicinaux par une réduction radicalaire. Nous avons révélé que l’issue diastéréosélective de la réaction de cyclisation par une iodoéthérification était dictée par le groupement gamma-méthyle des esters alpha,beta-insaturés de départ. Nous avons ensuite démontré que les produits de la réaction radicalaire anti et syn pouvaient être obtenus sélectivement à partir d’un intermédiaire commun, respectivement en prenant appui sur l’effet exocyclique ou endocyclique lors de la réduction. Par une stratégie complémentaire, nous avons révélé que le précurseur radicalaire pouvait également être obtenu par une réaction de cycloéthérification en présence d’un énoxysilane tétrasubstitué. Une étude systématique des substituants du cycle a révélé que certaines relations stéréochimiques conduisaient à une perte de sélectivité au détriment du produit anti. La seconde section concerne l’étude DFT au niveau BHandHLYP/TZVP des intermédiaires radicalaires impliqués lors du transfert d’hydrogène. Par une étude de décomposition de l’énergie d’activation, nous avons été en mesure de rationaliser l’issue diastéréosélective de la réaction sur la base des énergies de distorsion (∆Ed‡) et d’interaction (∆Eint‡) requises pour accéder à la paire d’états de transition pro-anti et pro-syn. De plus, nous avons démontré qu’une analyse NBO permettait de relativiser l’impact des interactions stéréoélectroniques. Par la suite, l’évaluation des intermédiaires radicalaires borinates et aluminates nous a permis de révéler que l’encombrement stérique de la chaîne propionate était la cause principale de la formation sélective des produits anti ou syn lors d’une réduction en présence d’un acide de Lewis. La dernière section décrit l’élaboration de la séquence polypropionate de la zincophorine, et de ses isomères, à partir du fragment tétrahydropyrane substitué. Au cours de notre étude, nous avons identifié que le nombre de sites de chélation potentiels devait être limité à trois lors de l’aldolisation en condition Cram-chélate. De plus, nous avons démontré que les différents motifs acétates sont accessibles sélectivement par l’utilisation d’un énoxysilane encombré. Par ailleurs, nous avons révélé qu’une même séquence réactionnelle pouvait être employée pour la synthèse du fragment C17–C25 de son analogue naturel CP-78,545, et avons complété la plus récente synthèse totale du méthyl ester de la zincophorine.

Spectroscopic and theoretical studies of some N-methoxy-N-methyl-2-[(4 `-substituted) phenylsulfonyl]propanamides

The analysis of the IR carbonyl band of the N-methoxy-N-methyl-2-[(4`-substituted)phenyisulfonyI]propanamides Y-PhSO(2)CH(Me)C(O)N(OMe)Me (Y = OMe 1, Me 2, H 3, Cl 4, NO(2) 5), supported by HF/6-31G(d,p) calculations of 3,, indicated the existence of two gauche conformers (g(1) and g(2)), the g, conformer being the most stable and the least polar one (in gas phase and in solution). Both conformers are present in solution of the non polar solvent (CCl(4)) for 1-5 and in solution of the more polar solvents (CHCl(3)) for 1. 4, 5 and (CH(2)Cl(2)) for 5, while only the g(1) conformer is present in solution of the most polar solvent (CH(3)CN) for 1-5. The g, and g2 conformers correspond to the enantiomeric pairs of diastereomers (diast(1) and diast(2)) whose relative configurations are [C(3)(R)N(R)]/[C(3)(s)N(s)] and [C(3)(R)N(s)]/[C(3)(s)N(R)], respectively. The computed carbonyl frequencies for g(1) (diast(1)) and g(2) (diast(2)) stereoisomers of3 match well the experimental values. The NBO analysis, for 3 shows the important role of the orbital interactions in conformer stabilization and the overall balance of these interactions corroborates that the g, conformer is more stable than the 92 one. The observed abnormal solvent effect on the relative intensities of the carbonyl doublet components is attributed to the molecular crowding in the g2 conformer which hinders its solvation in comparison to the g, conformer (diast(1)). X-ray single crystal analysis performed for 3 shows the existence Of two 92, and g(1b) conformers of diastereomers (diast2, and diast(1b)) whose absolute configurations are [C(3)(R)N(s)] and [C(3)(R)N(R)], respectively. The larger population and. thus, the larger stabilization of the g(2), conformer over the gib form in the crystals may be associated with a larger energy gain deriving from dipole moment coupling in the former conformer along with a series of C-H center dot center dot center dot O electrostatic and hydrogen bond interactions, (C) 2009 Elsevier B.V. All rights reserved.

Spectroscopic and theoretical studies of some N-methoxy-N-methyl-2-[(4 `-substituted) phenylthio]propanamides

The analysis of the IR carbonyl band of the N-methoxy-N-methyl-2-[(4`-substituted)phenylthio]propanamides Y-PhSCH(Me)C(O)N(OMe)Me (Y=OMe 1, Me 2, H 3, Cl 4, NO(2) 5), supported by B3LYP/cc-pVDZ calculations of 3, indicated the existence of two gauche conformers (g(1) and g(2)), the g(1) conformer being the more stable and the less polar one (in gas phase and in solution). Both conformers are present in solution of the polar solvents (CH(2)Cl(2) and CH(3)CN) for 1-5 and in solution of the less polar solvent (CHCl(3)) for 1-4, while only the g(1) conformer is present in solution of non polar solvents (n-C(6)H(14) and CCl(4)) and in solution of CHCl(3) for 5. NBO analysis shows that both the sigma(C-S) -> pi*(C=O) (hyperconjugative) and the pi(C=O) -> sigma*(C-S) orbital interactions contribute almost to the same extent for the stabilization of g(1) and g(2) conformers. The pi*(C=O) -> sigma*(C-S), n(S) -> pi*(C=O) and the n(S) -> pi*(C=O) orbital interactions stabilize more the g(1) conformer than the g(2) one. Moreover, the suitable geometry of the g(1) conformer leads to its stabilization through the LP(O2) -> sigma*(C8-H11) orbital interaction (hydrogen bond) along with the strong O([CO])(delta-) center dot center dot center dot H([O-Ph])(delta+) electrostatic interaction. On the other hand, the appropriate geometry of the g(2) conformer leads to its stabilization by the LP(O22) -> sigma*(C9-H13) orbital interaction (hydrogen bond) along with the weak O([OMe])(delta-) center dot center dot center dot H([o`-Ph])(delta+) electrostatic static interaction. As for the 4`-nitro derivative 5 the ortho-phenyl hydrogen atom becomes more acidic, leading to a stronger O([CO])(delta-) center dot center dot center dot H([o-Ph])(delta+) interaction and, thus, into a larger stabilization of the g(1) conformer in the whole series. This trend is responsible for the unique IR carbonyl band in CHCl(3) solution of 5. The larger occupancy of the pi*(C=O) orbital of the g(1) conformer relative to that of the g(2) conformer, along with the O([CO])(delta-) center dot center dot center dot H([o-Ph])(delta+) electrostatic interaction (hydrogen bond) justifies the lower carbonyl frequency of the g(1) conformer with respect to the g(2) one, in gas phase and in solution. (C) 2008 Elsevier B.V. All rights reserved.

Spectroscopic and theoretical studies of some N,N-diethyl-2-[(4 `-substituted)phenylsulfonyl]acetamides

The analysis of the IR carbonyl band of the N,N-diethyl-2-[(4`-substituted)phenylsulfonyl]acetamides Et(2)NC(O)CH(2)S(O)(2)-C(6)H(4)-Y (Y = OMe 1, Me 2,1-13, Cl 4, Br 5, NO(2) 6) supported by B3LYP/6-31G(d,p) calculations for 3, indicated the existence of three pairs (anti and syn) of cis (c) and gauche (g(1) and g(2)) conformers in the gas phase, being the gauche conformers significantly more stable than the cis ones. The anti geometry is more stable than the syn one, for each pair of cis and gauche conformers. The summing up of the orbital (NBO analysis) and electrostatic interactions justifies quite well the populations and the v(CO) frequencies of the anti and syn pairs of c, g(1) and g(2) conformers. The IR higher carbonyl frequency component whose population is ca. 10%, in CCl(4), may be ascribed to the least stable and most polar cis conformer pair (in the gas phase) and the lower frequency component whose population is ca. 90%, to the summing up of the populations of the two most stable and least polar gauche conformer pairs (g(1) and g(2)) (in the gas phase). The reversal of the cis(c)/gauche (g(1) + g(2)) population ratio observed in chloroform ca. 60% (cis)/40% (gauche) and the occurrence of the most polar cis(c) conformer only, in acetonitrile, strongly suggests the coalescence of the two gauche components in a unique carbonyl band in solution. A further support to this rationalization is given by the single point PCM solvation model performed by HF/6-31G(d,p) method, which showed a progressive increase of the c/(g(1) + g(2)) ratio going from gas to CCl(4), to CHCl(3) and to CH(3)CN. X-ray single crystal analysis of 4 indicates that this compound assumes, in the solid state, the syn-clinal (gauche) conformation with respect to the [O=C-CH(2)-S] moiety, and the most stable anti geometry relative to the [C(O)N(CH(2)CH(3))(2)] fragment. In order to obtain larger energy gain from the crystal packing the molecules of 4 are linked in centrosymmetric dimers through two C-H center dot center dot center dot O interactions (C-H([O-Ph])center dot center dot center dot O([SO2])) forming a step ladder. (C) 2011 Elsevier B.V. All rights reserved.

Spectroscopic and theoretical studies of some 2-ethylsulfinyl-(4 `-substituted)-phenylthioacetates

The analysis of the IR nu(co) bands of the 2-ethylsulfinyl-(4`-substituted)-phenylthioacetates 4`-Y-C(6)H(4)SC(O)CH(2)S(O)Et (Y = NO(2) 1, Cl 2, Br 3, H 4, Me 5, OMe 6) supported by B3LY/6-31G(d,p) calculations along with the NBO analysis for 1.4 and 6 and X-ray analysis for 3, indicated the existence of four gauche (q-g-syn, g(3)-syn. g(1)-atin and q-g(2)-syn) conformers for 1-6 The calculations reproduce quite well the experimental results, i e the computed q-g-syn and g3-syn conformers correspond in the IR spectrum (in solution), to the nu(co) doublet higher frequency component of larger intensity, while the computed grant, conformer correspond to the nu(co) doublet lower frequency component (in solution) NBO analysis showed that the n(s) -> pi(center dot)(c1=o2), no(co) -> sigma(c1-s3), no(co) -> sigma(c1-c4) orbital interactions are the main factors which stabilize the q-g-syn, g(3)-syn, g(1)-anti and q-g(2)-syn conformers for 1, 4 and 6 The no(co) -> sigma(c1-s3) interaction which stabilizes the q-g-syn, g(3)-syn and q-g(2)-syn conformers into a larger extent than the granti conformer, is responsible for the larger tto frequencies of the former conformers relative to the latter one. The q-g-syn, g(3)-syn and q-g(2)-syn conformers are further stabilized sigma(c4-s5) -> pi(co)center dot (strong). pi(co)/sigma(c1-c4,) no(co) -> sigma(c6-H17[Et]) (weak) and pi(co)/sigma(c4-c5) pi(co) (strong) orbital interactions. The q-g-syn conformer is also stabilized by sigma(c4-s5) -> pi(center dot)(co) (strong), pi(co)/sigma(c4-c5).no(co) -> sigma(c6-H17[Et]), pi(C9=C11[ph]) -> sigma(c4-H6x-CH2]) (weak). no((SO)) -> sigma(C11-H23[ph]) (medium) pi(co)/sigma(c4-c5)(strong) orbital interactions. The q-g-syn conformei is further stabilized by the n(S5) O((C))(8-) S((SO))(8+) attractive Coulornbic interaction while the q-g(2)-syn conformer is destabilized by the n55 0,8c-0) repulsive Coulombic interaction. This analysis indicates the following conformer stabilization order. q-g-syn, g(3)-syn > g(1)-anti >> q-g(2)-syn X-ray single crystal analysis of 3 indicates that it assumes in the solid a distorted q-g(2)-syn geometry which is stabilized through almost the same orbital and Coulombic interaction which takes place for the q-g(2)-syn conformer, in the gas, along with dipole moment coupling and a series intermolecular C-HO0 interactions. (C) 2010 Elsevier B V All rights reserved

Conformational preferences for some 2-substituted N-methoxy-N-methylacetamides through spectroscopic and theoretical studies

The analysis of the IR carbonyl band of the 2-substituted N-methoxy-N-methylacetamides Y-CH(2)C(O)-N(OMe)Me (Y = F1, OMe 2, OPh 3, Cl 4), supported by B3LYP/6-311++G(3df, 3pd) calculations along with the NBO analysis for 1-4, indicated the existence of cis-gauche conformers i.e. (c) and (g) for 1 and 3, (c(1), c(2)) and (g(1), g(2)) for 2, and (c) and (g(1), g(2)) for 4. In the gas phase, the g conformer population prevails over the c one, for 1 and 3, the (c(1) + c(2)) population prevails over the (g(1) + g(2)) one for 2, and the (g(1) + g(2)) conformer population is more abundant than (c) one for 4. In n-hexane solution, the cis conformer is more abundant for 1-3. The occurrence of Fermi resonance in the nu(CO) region, in n-hexane, precludes the estimative of relative populations of the (c, g(1), g(2)) conformers for 4. The SCI-PCM calculations agree with the solvent effect on the nu(CO) band component relative intensities for 1-3. NBO analysis showed that the n(N) -> pi.(CO), orbital interaction is the main factor which stabilizes the gauche (g, g(1), g(2)) conformers for 1-4 into a larger extent relative to the cis (c, c(1), c(2)) ones. The n(y) -> pi(.)(Co,) sigma(C-Y) -> pi.(CO,) pi(CO) -> sigma(C-Y) and 7co orbital interactions still contribute, but into a minor extent for the stabilization of the gauche conformers relative to the cis ones. The existence of some pyramidalization at the nitrogen atom of the Weinreb amides 1-4 is responsible for the occurrence of Y(delta)-(4)center dot center dot center dot O(delta)-(9) and Y(delta)-(4)center dot center dot center dot N(delta)-(7) short contacts in the gauche (g, g(1), g(2)) conformers, which originates strong repulsive Coulombic interactions, acting in opposition to the large orbital stabilization of the gauche conformer with respect to the cis one. Therefore, a delicate balance of the Coulombic and orbital interactions seems to be responsible for the observed stabilization of the gauche (g, g(1), g(2)) and cis (c, c(1), c(2)) conformers, both in the gas phase and in the solution for 1-4. However, the cis conformer predominance, in non polar solvents, for the 2-substituted N-methoxy-N-methyl acetamides 1-3, bearing in a first raw (fluorine and oxygen) atoms, is in the opposite direction to the gauche conformer preference for the corresponding 2-substituted N,N-dialkyl-acetamides. (C) 2010 Elsevier B.V. All rights reserved.

Conformational preferences for some 3-(4 '-substituted phenylsulfonyl)-1-methyl-2-piperidones through spectroscopic and theoretical studies

The analysis of the infrared (IR) carbonyl band of some 3-(4'-substituted phenylsulfonyl)-1-methyl-2-piperidones 1-5 bearing as substituents: OMe 1, Me 2, H 3, Cl 4 and NO2 5, supported by B3LY13/6-31G(d,p) calculations along with NBO analysis (for 1, 3 and 5) and X-ray diffraction (for 5), indicated the existence of three stable conformations i.e. quasi-axial (q-ax), syn-clinal (s-cl) and quasi-equatorial (q-eq). In the gas phase, the q-ax conformer is calculated as the most stable (ca. 88%) and the least polar, the s-cl conformer is less stable (ca. 12%) but more polar, and the q-eq conformer is the least stable (ca. 1%) and the most polar of the three conformers evaluated. The sum of the most important orbital interactions from NBO analysis and the trend of the electrostatic interactions accounts for the relative populations as well as for the v(CO) frequencies of the q-ax. s-cl and q-eq conformers calculated in the gas phase. The unique IR v(CO) band in CCl4 may be ascribed to the most stable q-ax conformer. The more intense (60%) high frequency doublet component in CHCl3 may be assigned to the summing up of the least stable q-eq and the less stable s-cl conformers, as their frequencies are almost coincident. The occurrence of only a single v(CO) band in both CH2Cl2 and CH3CN supports the fact that the v(CO) band of the two more polar conformers appear as a single band. Additional support to this rationalization is given by the single point PCM method, which showed a progressive increase of the q-eq + s-cl/q-ax population ratio going from the gas phase to CCl4, to CHCl3, to CH2Cl2 and to CN3CN. X-ray single crystal analysis of 5 indicates that this compound displays a quasi-axial geometry with respect to the [O=C-CH-S] moiety, and that the 2-piperidone ring assumes a slightly distorted half-chair conformation. In the crystal packing, molecules of 5 are arranged into supramolecular layers linked through C-H center dot center dot center dot O interactions along with it pi center dot center dot center dot pi interactions between adjacent benzene rings. (C) 2012 Elsevier B.V. All rights reserved.

The Conformational Analysis Of 2-halocyclooctanones.

The establishment of the most stable structures of eight membered rings is a challenging task to the field of conformational analysis. In this work, a series of 2-halocyclooctanones were synthesized (including fluorine, chlorine, bromine and iodine derivatives) and submitted to conformational studies using a combination of theoretical calculation and infrared spectroscopy. For each compound, four conformations were identified as the most important ones. These conformations are derived from the chair-boat conformation of cyclooctanone. The pseudo-equatorial (with respect to the halogen) conformer is preferred in vacuum and in low polarity solvents for chlorine, bromine and iodine derivatives. For 2-fluorocyclooctanone, the preferred conformation in vacuum is pseudo-axial. In acetonitrile, the pseudo-axial conformer becomes the most stable for the chlorine derivative. According to NBO calculations, the conformational preference is not dictated by electron delocalization, but by classical electrostatic repulsions.

Study of the phenanthroline-Mn-imidazole bonding in Mn(I) triscarbonyl complex: A X-ray and DFT computational analysis

355 nm light irradiation of fac-[Mn(CO)(3)(phen)(imH)](+) (fac-1) produces the mer-1 isomer and a long lived radical which can be efficiently trapped by electron acceptor molecules. EPR experiments shows that when excited, the manganese(I) complex can be readily oxidized by one-electron process to produce Mn(II) and phen(.-). In the present study, DFT calculations have been used to investigated the photochemical isomerization of the parent Mn(I) complex and to characterize the electronic structures of the long lived radical. The theoretical calculations have been performed on both the fac-1 and mer-1 species as well as on their one electron oxidized species fac-1+ and mer-1+ for the lowest spin configurations (S = 1/2) and fac-6 and mer-6 (S = 5/2) for the highest one to characterize these complexes. In particular, we used a charge decomposition analysis (CDA) and a natural bonding orbital (NBO) to have a better understanding of the chemical bonding in terms of the nature of electronic interactions. The observed variations in geometry and bond energies with an increasing oxidation state in the central metal ion are interpreted in terms of changes in the nature of metal-ligand bonding interactions. The X-ray structure of fac-1 is also described. (C) 2011 Elsevier B.V. All rights reserved.

Analysis Of The Dna Fourier Transform-infrared Microspectroscopic Signature Using An All-reflecting Objective.

The Fourier transform-infrared (FT-IR) signature of dry samples of DNA and DNA-polypeptide complexes, as studied by IR microspectroscopy using a diamond attenuated total reflection (ATR) objective, has revealed important discriminatory characteristics relative to the PO2(-) vibrational stretchings. However, DNA IR marks that provide information on the sample's richness in hydrogen bonds have not been resolved in the spectral profiles obtained with this objective. Here we investigated the performance of an all reflecting objective (ARO) for analysis of the FT-IR signal of hydrogen bonds in DNA samples differing in base richness types (salmon testis vs calf thymus). The results obtained using the ARO indicate prominent band peaks at the spectral region representative of the vibration of nitrogenous base hydrogen bonds and of NH and NH2 groups. The band areas at this spectral region differ in agreement with the DNA base richness type when using the ARO. A peak assigned to adenine was more evident in the AT-rich salmon DNA using either the ARO or the ATR objective. It is concluded that, for the discrimination of DNA IR hydrogen bond vibrations associated with varying base type proportions, the use of an ARO is recommended.

Effect Of Platform Connection And Abutment Material On Stress Distribution In Single Anterior Implant-supported Restorations: A Nonlinear 3-dimensional Finite Element Analysis.

Although various abutment connections and materials have recently been introduced, insufficient data exist regarding the effect of stress distribution on their mechanical performance. The purpose of this study was to investigate the effect of different abutment materials and platform connections on stress distribution in single anterior implant-supported restorations with the finite element method. Nine experimental groups were modeled from the combination of 3 platform connections (external hexagon, internal hexagon, and Morse tapered) and 3 abutment materials (titanium, zirconia, and hybrid) as follows: external hexagon-titanium, external hexagon-zirconia, external hexagon-hybrid, internal hexagon-titanium, internal hexagon-zirconia, internal hexagon-hybrid, Morse tapered-titanium, Morse tapered-zirconia, and Morse tapered-hybrid. Finite element models consisted of a 4×13-mm implant, anatomic abutment, and lithium disilicate central incisor crown cemented over the abutment. The 49 N occlusal loading was applied in 6 steps to simulate the incisal guidance. Equivalent von Mises stress (σvM) was used for both the qualitative and quantitative evaluation of the implant and abutment in all the groups and the maximum (σmax) and minimum (σmin) principal stresses for the numerical comparison of the zirconia parts. The highest abutment σvM occurred in the Morse-tapered groups and the lowest in the external hexagon-hybrid, internal hexagon-titanium, and internal hexagon-hybrid groups. The σmax and σmin values were lower in the hybrid groups than in the zirconia groups. The stress distribution concentrated in the abutment-implant interface in all the groups, regardless of the platform connection or abutment material. The platform connection influenced the stress on abutments more than the abutment material. The stress values for implants were similar among different platform connections, but greater stress concentrations were observed in internal connections.

Is Thyroid Stunning Clinically Relevant? A Retrospective Analysis Of 208 Patients.

Current guidelines have advised against the performance of (131)I-iodide diagnostic whole body scintigraphy (dxWBS) to minimize the occurrence of stunning, and to guarantee the efficiency of radioiodine therapy (RIT). The aim of the study was to evaluate the impact of stunning on the efficacy of RIT and disease outcome. This retrospective analysis included 208 patients with differentiated thyroid cancer managed according to a same protocol and followed up for 12-159 months (mean 30 ± 69 months). Patients received RIT in doses ranging from 3,700 to 11,100 MBq (100 mCi to 300 mCi). Post-RIT-whole body scintigraphy images were performed 10 days after RIT in all patients. In addition, images were also performed 24-48 hours after therapy in 22 patients. Outcome was classified as no evidence of disease (NED), stable disease (SD) and progressive disease (PD). Thyroid stunning occurred in 40 patients (19.2%), including 26 patients with NED and 14 patients with SD. A multivariate analysis showed no association between disease outcome and the occurrence of stunning (p = 0.3476). The efficacy of RIT and disease outcome do not seem to be related to thyroid stunning.