A theoretical study of propylene oxide acid-catalyzed hydrolysis was performed by investigation of the S(N)1 and S(N)2-like mechanisms. By using chemometric tools, hierarchical cluster analysis (HCA), and principal component analysis (PCA), the MP2/6-311++G** level of theory was selected from HF, MP2, and DFT as the best method to describe the geometry of the basic skeleton (oxirane). At this level of theory, geometry optimizations, vibrational frequencies, intrinsic reaction coordinate (IRC), and other thermodynamic calculations have shown that the borderline S(N)2 mechanism is more favorable than pure S(N)2 and S(N)1 mechanisms in the gas phase. In the S(N)1 mechanism, the existence of the typical carbocation was not observed, and furthermore, the possibility of epoxide conversion to a protonated aldehyde was indicated, even in the presence of a water molecule (nucleophile). The Chelpg charge distribution of the reactants, steric hindrance, synchronous bond breaking-formation and trajectory angle of nucleophilic attack are discussed for pure and borderline S(N)2 mechanisms. Solvation effect calculations indicate that the pure S(N)2 mechanism is more favorable than borderline S(N)2 and S(N)1 mechanisms. This is discussed in terms of hydrogen bond formation.

A minha amada família. AGRADECIMENTOS Ao amigo de todas as horas, Deus; Aos meus pais (Antônio Carlos e Marlene), irmãos (Carlos Eduardo e Edivaldo) e sobrinhos (João Carlos e Carlos Eduardo Filho) pelo amor, compreensão, paciência e apoio em todos os momentos de minha vida; A minha orientadora, Profª Drª Márcia Miguel Castro Ferreira, pela amizade, apoio e orientação no desenvolvimento deste trabalho; Aos amigos Boaz Galdino, Iracema Camilo, Pe. Adriano e Analice Arruda pela amizade e incentivo;As minhas amigas do pensionato morada da condessa pelo convívio harmonioso e alegre;

Objective To evaluate the optical, chemical, and mechanical properties of feldspathic porcelain after handling with different instrument materials. Material and methods Feldspathic porcelain was manipulated with different spatula compositions: metal spatula (MS), plastic spatula (PS), and glass spatula (GS) for the fabrication of 30 (n = 10) disks. Contrast ratio (CR), translucency parameter (TP), and surface Knoop microhardness (KHN) were measured. The color change (ΔE00) was evaluated using the CIEDE2000 system. Scanning electron microscopy (SEM) and energy‐dispersive spectroscopy (EDS) were used for surface analysis. Data were analyzed using one‐way ANOVA and Tukey test (P < 0.05). Results All groups presented different morphological surfaces with higher presence of Al on the MS. PS group presented lower Al, Si, K than MS, and GS. Higher CR was observed for PS (0.734; P < 0.043), followed by MS (0.696; P < 0.043) and GS (0.65; P < 0.011). The highest TP (13.06) and KHN (386.27) were presented by GS (P < 0.001). MS and PS presented similar KHN results. The higher ΔE00 were found for plastic/metal comparison. Also, the L* values for the MS group (67.49) were lower than the other groups. Conclusion The use of metal spatula promoted higher color alteration during feldspathic porcelain manipulation than did the other materials. Handling with glass instrument promoted higher microhardness than other spatula materials. Clinical significance The effect of the material used for ceramic handling on feldspathic porcelain properties is often ignored. This study shows that the handling spatula material must be carefully chosen to avoid inadvertent changes to the feldspathic porcelain restoration.

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