Browsing by Author "García-Río, Luís"
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- Cooperative assembly of discrete stacked aggregates driven by supramolecular host-guest complexationPublication . Basílio, Nuno; Piñeiro, Ángel; Silva, José P. da; García-Río, Luísp-Sulfonatocalix[4]arene (SC4) interacts with the aromatic dye crystal violet (CV) to form complexes with stoichiometries ranging from SC4:CV = 1:1 up to 1:5 both in solution and in the gas phase. While the 1:1 complex is of the inclusion type, as frequently observed for other guests, in the higher-order complexes the CV molecules interact with SC4 in a peripheral manner. The formation of such complexes is driven by ionic interactions established between the dye and the calixarene and by CV-CV stacking interactions. The application of an advanced fitting procedure made possible a quantitative analysis of the UV-vis data and allowed the determination of the stepwise binding constants. This unprecedented approach provides evidence that the formation of the highest-order complexes occurs through a cooperative mechanism. Moreover, the development of a quantitative analytical model enables the possibility of using this type of system for water-soluble sensing assays, as is also exemplified in the present work. © 2013 American Chemical Society.
- Cucurbit[7]uril: surfactant host-guest complexes in equilibrium with micellar aggregatesPublication . Pessêgo, Márcia; Basilio, Nuno; Moreira, José; García-Río, LuísIn order to compare the formation of host–guest complexes between b-cyclodextrin (b-CD) or cucurbit[7]uril (CB7) and cationic surfactants we studied the hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC). The selected surfactants allowed the length of the hydrocarbon chain to be varied between 6 and 18 carbon atoms. Contrary to the expected behaviour, the values of the binding constants between CB7 and surfactants are independent of the alkyl chain length of the surfactant. In the case of b-CD, however, a clear dependence of the binding constant on the hydrophobic character of the surfactant was observed. The values obtained with CB7 are significantly higher than those obtained with b-CD and these differences are explained to be a consequence of electrostatic interactions of the surfactants with the portals of CB7. It was found that a small percentage of uncomplexed CB7 was in equilibrium with the cationic micelles and this percentage increased on increasing the hydrophobic character of the surfactant.
- Differences in cucurbit[7]uril: surfactant complexation promoted by the cationic head groupPublication . Pessêgo, Márcia; Silva, José P. da; Moreira, José; García-Río, LuísThe formation of host/guest complexes between cucurbit[7]uril (CB7) and tetradecyltrialkylammonium bromide surfactants with trimethyl (C14TMA+), triethyl (C14TEA+), and tripropyl (C14TPA+) head groups was studied. Complexation was studied by ESI-MS, isothermal titration calorimetry (ITC), and NMR spectroscopy. The structure of the complexes is strongly dependent on the surfactant head group. Both 1:1 and 2:1 complexes are formed with all cationic surfactants. The magnitude of the K1:1 binding constants decreases with the alkyl length of the head group, whereas that of the K2:1 binding constants is independent of the nature of the surfactant head group. Distinct structures were found for the 1:1 and 2:1 complexes. C14TMA+ and C14TPA+ surfactants form 1:1 inclusion complexes that comprise the head group close to the CB7 portal and 2:1 external complexes with both host molecules near the head group. The 1:1 complexes between C14TEA+ and CB7 showed that the head group was centered inside the CB7 cavity, whereas in the 2:1 complex the second molecule of CB7 was located at the end of the main alkyl chain.
- Electrostatic repulsion between cucurbit[7]urils can be overcome in [3]pseudorotaxane without adding saltsPublication . Pessêgo, Márcia; Moreira, José; Costa, Ana M. Rosa da; Corrochano, P.; Poblete, F. J.; García-Río, LuísThe host−guest chemistry betweencucurbit[7]uril (CB7) and a series of bolaform (Bn) surfactants with different chain lengths, n = 12−22, was the target of our study. [3]Pseudorotaxanes are formed when the alkyl chain of the bolaform has more than 14 carbon atoms. In these cases, two CB7 molecules can be accommodated between the two head groups of the bolaform without addition of electrolytes to the medium. In the case of a bolaform with 12 carbon atoms, the electrostatic repulsion between the carbonyl groups of the CB7 molecules avoids the threading of a second CB7 molecule yielding a mixed structure formed by a [2]pseudorotaxane and an external host−guest complex. The assembly behavior was investigated using NMR spectroscopy, isothermal titration calorimetry (ITC), and kinetic measurements.
- Equilibrium constants and protonation site for N-methylbenzenesulfonamidesPublication . Moreira, José; Costa, Ana M. Rosa da; García-Río, Luís; Pessêgo, MárciaThe protonation equilibria of four substituted N-methylbenzenesulfonamides, X-MBS: X = 4-MeO (3a), 4-Me (3b), 4-Cl (3c) and 4-NO2 (3d), in aqueous sulfuric acid were studied at 25 °C by UV–vis spectroscopy. As expected, the values for the acidity constants are highly dependent on the electron-donor character of the substituent (the pKBH+ values are −3.5 ± 0.2, −4.2 ± 0.2, −5.2 ± 0.3 and −6.0 ± 0.3 for 3a, 3b, 3c and 3d, respectively). The solvation parameter m* is always higher than 0.5 and points to a decrease in the importance of solvation on the cation stabilization as the electron-donor character of the substituent increases. Hammett plots of the equilibrium constants showed a better correlation with the σ+ substituent parameter than with σ, which indicates that the initial protonation site is the oxygen atom of the sulfonyl group.
- Evidence of higher complexes between cucurbit[7]uril and cationic surfactantsPublication . Pessêgo, Márcia; Moreira, José; García-Río, LuísThe host–guest assembly of CB7 with a series of alkyl(trime-ACHTUNGTRENUNGthyl)ammonium (CnTA+) surfactants of different chain lengths (n=6–18) has been studied. The complexation behaviour was investigated by NMR spectroscopy, isothermal titration calorimetry and kinetics measurements. The combined results of these techniques provided evidence for the formation of 1:1 inclusion and 2:1 external complexes in the cases of CnTA+ with n=12–18. The binding constants for the 1:1 complexes are independent of the alkyl chain length of the surfactant, whereas a relationship between K2:1 and the chain length of the surfactant was found for the 2:1 complexes.
- Kinetic study of an autocatalytic reaction: nitrosation of formamidine disulfidePublication . Francisco, Vitor; García-Río, Luís; Moreira, José; Stedman, GeoffreyThe reaction kinetics for the acid nitrosation of formamidine disulfide (FDS) show an autocatalytic behavior that arises from the fact that the thiocyanate ion formed as a product acts as a powerful catalyst for the nitrosation reaction. In the presence of added nucleophiles the suppression of the autocatalytic route results from competition for the nitrous acid between the added halides and the thiocyanate anion, which is formed as a reaction product. Analysis of the kinetic data enabled extraction of the bimolecular rate constants, kNO+ = (3.2 1.8) 1010 M 1 s 1; kNOSCN = (2.1 0.2) 105 M 1 s 1; kNOBr = (9.4 0.2) 106 M 1 s1 and kNOCl = (4.0 0.2) 107 M 1 s 1, for the pathways catalyzed by SCN , Br and Cl , respectively. Kinetic results are consistent with the attack on the nitrosating agent as the rate limiting step, i.e., the nitrosation of FDS behaves in a similar manner to the nitrosation of an amine. Rather different behavior is found for other substrates with an imino moiety adjacent to an amino nitrogen, such as the guanidines, which react by a mechanism in which the rate limiting step is the reorganization of the nitrosated substrate.
- Mechanism for basic hydrolysis of N-nitrosoguanidines in aqueous solutionPublication . Moreira, José; Leis, J. R.; Norberto, F.; García-Río, Luís; Araujo, E.; Ribeiro, LauraA kinetic study was carried out on the hydrolysis of two N-nitrosoguanidines, 1-nitroso-1-methyl- 3-tolylsulfonylguanidine (TSGNO) and 1-nitroso-1-methyl-3-benzoylguanidine (BCGNO). We observed an absence of buffer catalysis using H2PO4 -/HPO4 2-, H3BO3/H2BO3 -, and HCO3 -/CO3 2- regulators and a complex dependency of the rate constant on the pH. We discovered the existence of three simultaneous reaction paths: spontaneous decomposition of the neutral form of the N-nitrosoguanidine, decomposition of the monoanion, and decomposition through the form of the dianion. The analysis of the kinetic data has allowed us to obtain the acidity constant for the formation of the monoanion of the N-nitrosoguanidine, with values of pKa I ) 11.5. The reaction rate for the process through the monoanion, k2, decreases as the acidity increases. The application of the principle of nonperfect synchronization shows that the basicity and reactivity do not correlate when there exists a possibility of stabilization of the negative charge by resonance. This behavior is consistent with the mechanism E1cB whereby the stabler the negative charge, the slower the elimination reaction. When dealing with the case of the elimination through the neutral form we observe that the reaction rate increases together with the capacity of stabilization of the positive charge on the nitrogen atom adjacent to the imino group. For the reaction through the dianion we used a maximum value of k3 ) 1010 s-1 to estimate the value of pKa II for the formation of the dianion of the N-nitrosoguanidine, obtaining values of pKa II < 24.
- Nitrosation of amines in non-aqueous solvents: difference between N-N=O and O-N=O Nitroso donorsPublication . García-Río, Luís; Leis, J. R.; Moreira, José; Serantes, D.A kinetic study has been carried out on nitroso group transfer from substituted N-methyl-N-nitroso-benzenesulfonamides to different secondary amines: pyrrolidine, piperidine, Nmethylpiperazine,and morpholine in cyclohexane. The observed pseudo-first-order rate constant kobs shows a linear and quadratic dependency on the amine concentration with the existence of a primary kinetic isotope effect. Experiments carried out at different temperatures show Arrhenius-type behavior. Addition of isopropylamine(iPrNH2) to the reaction medium produces an increase in kobs. In the presence of a high iPrNH2 concentrations the influence of secondary amine concentration on kobs shows the disappearance of the quadratic dependency of kobs on the secondary amine. These results would be compatible with an addition-elimination mechanism, similar to that observed for the aminolysis of thealkyl nitrites in apolar solvents. The observed behavior in the presence of 18-crown-6 is very different, however. Addition of crown ether catalyses the reaction of aminolysis of alkyl nitrites insofar as it does not alter the rate of the nitroso group transfer from N-nitrososulfonamides. This behavior has been interpreted in terms of a concerted reaction mechanism through cyclical transition states, with four or six centers, involving one or two molecules of secondary amine. Addition of iPrNH2 to the reaction medium causes the appearance of a third reaction path, which emerges through a mixed transition state formed by a molecule of a secondary amine and a molecule of isopropylamine.
- Nitroso group transfer from N-nitrososulfonamides to thiolate ions. Intrinsic reactivityPublication . Moreira, José; García-Río, Luís; Leis, J. R.; Adam, C.The nitroso group transfer from N-nitrososulfonamides to thiolate ions was studied. Based on the results, the reaction rate is strongly dependent on the nature of the leaving group (αlg≈−1.30)(αlg≈−1.30), but virtually independent of the basicity of the thiol (βnuc≈0.10)(βnuc≈0.10). This dependence is ascribed to the presence of a nucleophile desolvation equilibrium (βd)(βd) that is followed by the attack of the thiolate ion on the nitroso group (βnuc′) via a concerted mechanism. The equilibrium constants for the loss of a nitroso group from a nitrosothiol and an N-nitrososulfonamide were used to obtain the equilibrium constants for the different reactions involved. By using rate–equilibrium correlations, the parameters αlgnorm, βdβd, and βnuc′norm were obtained.