Nelson H. Morgon

Molecular Variations in Aromatic Cosolutes: Critical Role in the Rheology of Cationic Wormlike Micelles

Wormlike micelles formed by the addition to cetyltrimethylammonium bromide (CTAB) of a range of aromatic cosolutes with small molecular variations in their structure were systematically studied. Phenol and derivatives of benzoate and cinnamate were used, and the resulting mixtures were studied by oscillatory, steady-shear rheology, and the microstructure was probed by small-angle neutron scattering. The lengthening of the micelles and their entanglement result in remarkable viscoelastic properties, making rheology a useful tool to assess the effect of structural variations of the cosolutes on wormlike micelle formation. For a fixed concentration of CTAB and cosolute (200 mmol L(-1)), the relaxation time decreases in the following order: phenol > cinnamate> o-hydroxycinnamate > salicylate > o-methoxycinnamate > benzoate > o-methoxybenzoate. The variations in viscoelastic response are rationalized by using Mulliken population analysis to map out the electronic density of the cosolutes and quantify the barrier to rotation of specific groups on the aromatics. We find that the ability of the group attached to the aromatic ring to rotate is crucial in determining the packing of the cosolute at the micellar interface and thus critically impacts the micellar growth and, in turn, the rheological response. These results enable us for the first time to propose design rules for the self-assembly of the surfactants and cosolutes resulting in the formation of wormlike micelles with the cationic surfactant CTAB.

Descrições estruturais cristalinas de zeólitos

Crystalline structures of zeolites can be studied using different representations: the internal symmetry obtained by X-Ray or neutron diffraction crystallography techniques or a systematic analysis of the basic structural units which can be arranged to build the geometries of each kind of zeolite. In this work the basic concepts of three building units, SBU (Secondary Building Units), SSU (Structural SubUnits) and PBU (Periodic Building Units) are presented. The properties of the resulting crystalline structures are discussed (pores, cavities, channels), describing the influence of each one of these properties in processes of physical-chemical interest. Representative case studies of known zeolite crystalline structures are also discussed in terms of their space group classification.

Polar [4+2(+)] diels-alder cycloaddition to nitrilium and immonium ions in the gas phase: Applications of multiple stage mass spectrometry in a pentaquadrupole instrument.

Multiple stage MS2 and MS3 mass spectrometric experiments, performed using a pentaquadrupole instrument, are employed to explore the gas-phase ion-molecule chemistry of several nitrilium [R-C≡N+-H (1), R-C≡N+-CH3 (2), and H-C≡N+-C2H5 (3)] as well as immonium ions RR1C=N+R2R3 (4) with the neutral diene isoprene. Polar [4+2+] Diels-Alder cycloaddition is observed for nitrilium ions when the energy gap between the lowest unoccupied molecular orbital (LUMO) of the ion and the highest occupied molecular orbital (HOMO) of the isoprene is small and the competing proton transfer reaction is endothermic. Thus, C-protonated methyl isonitrile H-C≡N+-CH3 (2a) and its higher homolog H-C≡N+-C2H5 (3a) form abundant [4+2+] cycloadducts with isoprene, but several protonated nitriles 1 do not; instead they show exothermic proton transfer as the main ion-molecule reaction. Replacement of the methyne hydrogen in 2a by a methyl, ethyl, or phenyl group (2b–d) raises the LUMO-HOMO gap, which greatly decreases the total yield of ion-molecule products and precludes cycloaddition. On the other hand, the electron-withdrawing acetyl and bromine substituents in 2e and 2f substantially lower the LUMO energy of the ions and cycloaddition reaction occurs readily. The simplest member of the immonium ion series, CH2=NH2+ (4a), reacts readily by cycloaddition, whereas alkyl substitution on either the carbon or nitrogen (4b–f) dramatically lowers the overall reactivity, which substantially decreases or even precludes cycloaddition. In strong contrast, the N-phenyl (4g) and N-acetyl (4h) ions and the N-vinyl-substituted immonium ion, N-protonated 2-aza-butadiene (4i), react extensively with isoprene, mainly by [4+2+] cycloaddition. However, the isomeric C-vinyl-substituted ion (4j) displays only modest reactivity in both the proton-transfer and the cycloaddition channels.Collision-induced dissociation (CID) of the cycloadducts performed by on-line MS3 experiments demonstrates that they are covalently bound and supports their assignments as cycloaddition products. Retro Diels-Alder fragmentation is a major process for cycloadducts of both the immonium and the nitrilium ions, but other fragmentation processes also are observed. The cycloadduct of 4a with butadiene displays CID fragmentation identical to that of the authentic ion produced by protonation of 1,2,3,6-tetrahydropyridine, which thus strengthens the [4+2+] cycloaddition proposal. AM1 calculations also support the formation of the [4+2+] cycloadducts, which are shown in several cases to be much more stable than the products of simple addition, that is, the ring-open isomers.

Structure-drift time relationships in ion mobility mass spectrometry

Ion mobility spectrometry (IMS) separates ions while they travel through a buffer gas under the influence of an electrical field. The separation is affected by mass and charge but most particularly by shape (collision cross section). When coupled to MS, IMS-MS offers therefore a powerful tool for structural elucidation and isomer separation. Systematic studies aimed to compare and quantitate the effects of structural changes on drift time such as length and ramification of carbon chain, unsaturation, geometrical isomerism (cis/trans isomers for instance), cyclization and ring size are, however, scarce. Herein we used traveling wave ion mobility mass spectrometry (TWIM-MS) to systematically evaluate the relationship between structure and drift time. For that, a series of deprotonated carboxylic acids were used as model ions with a carboxylate “charge tag” for gas phase MS manipulation. Carboxylic acids showed a near linear correlation between the increase of carbon number and the increase of collision cross section (CCS). The number of double bonds changes slightly the CCS of unsaturated acids. No differences in drift time and no significant differences in CCS of cis- and trans-double bond of oleic and elaidic acids were observed. Cyclization considerably reduces the CCS. In cyclic carboxylic acids, the increase of double bonds and aromatization significantly reduces the CCS and the drift times. The use of a more polarizable drift gas, CO2, improved in some cases the separation, as for biomarker isomers of steranoic acids. The β-isomer (cis-decaline) has smaller CCS and therefore displayed lower drift time compared to the α-isomer (trans-decaline). Structural changes revealed by calculations were correlated with trends in drift times.

Implementation of pseudopotential in the G3 theory for molecules containing first-, second-, and non-transition third-row atoms.

Compact effective pseudopotential (CEP) is adapted in the G3 theory providing a theoretical alternative referred to as G3CEP for calculations involving the first-, second-, and non-transition third-row elements. These modifications tried to preserve as much as possible the original characteristics of G3. G3CEP was used in the study of 247 enthalpies of formation, 22 atomization energies, 104 ionization potentials, 63 electron affinities, and 10 proton affinities, resulting in the calculation of 446 species for the first-, second-, and third-row atoms. The final average total absolute deviation was of 1.29 kcal mol(-1) against 1.16 kcal mol(-1) from all-electron G3 for the same calculations. The CPU time has been reduced by 7% to 56%, depending on the size of the molecules and the type of atoms considered.

Gas-phase nucleophilic reactions of Ge(OCH3)4: experimental and computational characterization of pentacoordinated Ge anions

Abstract The gas-phase ion/molecule reactions of F − and CH 3 O − with Ge(OCH 3 ) 4 have been investigated by Fourier transform ion cyclotron mass spectrometry. Both nucleophiles react preferentially by an addition mechanism to yield XGe(OCH 3 ) 4 − (X = F, OCH 3 ) complexes that are identified as typical pentacoordinated Ge species. Pentacoordinated Ge adducts formed with excess internal energy can undergo elimination of formaldehyde to yield HGe(OCH 3 ) 4 − , or further elimination processes that result in the formation of germyl anions like Ge(OCH 3 ) 3 − . Other minor product ions are also observed which can be attributed to the intermediacy of a pentacoordinated adduct. Dissociation of the XGe(OCH 3 ) 4 − anions induced by infrared multiphoton excitation leads to sequential losses of formaldehyde and gives rise to different germyl anions like Ge(OCH 3 ) 3 − , HGe(OCH 3 ) 2 − , and H 2 GeOCH 3 − . The XGe(OCH 3 ) 4 − and germyl anions react readily with BF 3 through successive methoxide-fluoride exchange and this reaction provides a gas-phase synthetic pathway for multiply fluorinated Ge anions. Ab initio calculations performed on model pentacoordinated species F n +1 Ge(OH) 4− n − (n = 0–4) reveal that addition of a fluoride ion on hydroxygermanes occurs preferentially in the apical position of a trigonal bipyramid. The fluoride affinity of the prototype molecule Ge(OH) 4 is calculated to be 60.9 kcal mol −1 , and fluoride affinity increases monotonically with increasing fluorine substitution. The fluoride affinity of GeF 4 is calculated to be 79 kcal mol −1 . Similar calculations also predict an unusually high hydride affinity (60 kcal mol −1 ) for Ge(OH) 4 with the hydride occupying an equatorial position.

Hypobromous acid, a powerful endogenous electrophile: Experimental and theoretical studies

Hypobromous acid (HOBr) is an inorganic acid produced by the oxidation of the bromide anion (Br(-)). The blood plasma level of Br(-) is more than 1,000-fold lower than that of chloride anion (Cl(-)). Consequently, the endogenous production of HOBr is also lower compared to hypochlorous acid (HOCl). Nevertheless, there is much evidence of the deleterious effects of HOBr. From these data, we hypothesized that the reactivity of HOBr could be better associated with its electrophilic strength. Our hypothesis was confirmed, since HOBr was significantly more reactive than HOCl when the oxidability of the studied compounds was not relevant. For instance: anisole (HOBr, k2=2.3×10(2)M(-1)s(-1), HOCl non-reactive); dansylglycine (HOBr, k2=7.3×10(6)M(-1)s(-1), HOCl, 5.2×10(2)M(-1)s(-1)); salicylic acid (HOBr, k2=4.0×10(4)M(-1)s(-1), non-reactive); 3-hydroxybenzoic acid (HOBr, k2=5.9×10(4)M(-1)s(-1), HOCl, k2=1.1×10(1)M(-1)s(-1)); uridine (HOBr, k2=1.3×10(3)M(-1)s(-1), HOCl non-reactive). The compounds 4-bromoanisole and 5-bromouridine were identified as the products of the reactions between HOBr and anisole or uridine, respectively, i.e. typical products of electrophilic substitutions. Together, these results show that, rather than an oxidant, HOBr is a powerful electrophilic reactant. This chemical property was theoretically confirmed by measuring the positive Mulliken and ChelpG charges upon bromine and chlorine. In conclusion, the high electrophilicity of HOBr could be behind its well-established deleterious effects. We propose that HOBr is the most powerful endogenous electrophile.

A universal basis set to be used along with pseudopotentials

Abstract Universal basis sets from all-electron calculations can be adapted to be used along with pseudopotentials by the generator coordinate method (GCM). Calculations carriedout with Hay and Wadt or Steven, Bash and Krauss pseudopontentials along with a universal basis set are in agreement with all-electron calculations and also with improved basis sets specially developed to be used with pseudopotential. The support provided by GCM allows a simple procedure to enlarge or reduce the size of the universal basis set as well as a systematic procedure to include polarization functions.

A theoretical study of isomeric C6H4Br− ions

Abstract The structural stability of bromide o -benzyne complexes was studied. A new procedure was applied to adapt a basis set to be used with the Hay and Wadt pseudopotential for the Br atom. Calculations at the MP2, MP3 and MP4 levels established two stable structural isomers for the C 6 H 4 Br − ion: (a) a bromphenide anion and (b) an isomer with a loose hydrogen-bonded bromide-benzyne complex where the Br − ion is straddling the ortho and meta hydrogen of the ring. At the MP4 level the barrier between the two isomers is below the typical energies associated with the loose complexes formed as intermediates in ion/molecule reactions.

Separation of glycosidic catiomers by TWIM-MS using CO2 as a drift gas

Traveling wave ion mobility mass spectrometry (TWIM-MS) is shown to be able to separate and characterize several isomeric forms of diterpene glycosides stevioside (Stv) and rebaudioside A (RebA) that are cationized by Na(+) and K(+) at different sites. Determination and characterization of these coexisting isomeric species, herein termed catiomers, arising from cationization at different and highly competitive coordinating sites, is particularly challenging for glycosides. To achieve this goal, the advantage of using CO2 as a more massive and polarizable drift gas, over N2, was demonstrated. Post-TWIM-MS/MS experiments were used to confirm the separation. Optimization of the possible geometries and cross-sectional calculations for mobility peak assignments were also performed.