Spin-Forbidden Branching in the Mechanism of the Intrinsic Haber-Weiss Reaction.

The mechanism of the O2⋅- and H2O2 reaction (Haber-Weiss) under solvent-free conditions has been characterized at the DFT and CCSD(T) level of theory to account for the ease of this reaction in the gas phase and the formation of two different set of products (Blanksby et al., Angew. Chem. Int. Ed. 2007, 46, 4948). The reaction is shown to proceed through an electron-transfer process from the superoxide anion to hydrogen peroxide, along two pathways. While the O3⋅- + H2O products are formed from a spin-allowed reaction (on the doublet surface), the preferred products, O⋅-(H2O)+3O2, are formed through a spin-forbidden reaction as a result of a favorable crossing point between the doublet and quartet surface. Plausible reasons for the preference toward the latter set are given in terms of the characteristics of the minimum energy crossing point (MECP) and the stability of an intermediate formed (after the MECP) in the quartet surface. These unique results show that these two pathways are associated with a bifurcation, yielding spin-dependent products.

Dynamic/Thermochemical Balance Drives Unusual Alkyl/F Exchange Reactions in Siloxides and Analogs.

A recent report has shown that siloxides can undergo an unusual Me/F exchange reaction promoted by NF3 in the gas phase ( Angew. Chem. Int. Ed. 2012, 51, 8632-8635). A more extensive study of this kind of exchange has been carried out using mass spectrometry techniques (FT-ICR), DFT calculations, natural bond orbital (NBO) analysis, and Born-Oppenheimer molecular dynamics simulations (BOMD), using NF3, SO2F2, and CF4 as fluorine donors and evaluating the effect of replacing the Si center by Ge and C. This comprehensive approach shows that NF3 is crucial for the exchange reaction, as SO2F2 forms SO3F(-) via a pentacoordinated channel whereas no reaction is observed for CF4. The uniqueness of NF3 is caused by favorable thermochemical consideration and by dynamic effects that preclude the formation of the ubiquitous Si-F pentacoordinated species. Me3GeO(-) was shown to be as reactive as siloxides toward NF3, whereas C analogs showed no reactions under our experimental conditions. The exchange reaction was also shown to take place for triethylsiloxides. These exchange reactions are examples of reaction systems that avoid the lower energy pathway and are driven by dynamic effects that cannot be explained by the potential energy surface.

Revisiting the concept of the (a)synchronicity of Diels-Alder reactions based on the dynamics of quasiclassical trajectories.

A number of model Diels-Alder (D-A) cycloaddition reactions (H2C=CH2 + cyclopentadiene and H2C=CHX + 1,3-butadiene, with X = H, F, CH3, OH, CN, NH2, and NO) were studied by static (transition state - TS and IRC) and dynamics (quasiclassical trajectories) approaches to establish the (a)synchronous character of the concerted mechanism. The use of static criteria, such as the asymmetry of the TS geometry, for classifying and quantifying the (a)synchronicity of the concerted D-A reaction mechanism is shown to be severely limited and to provide contradictory results and conclusions when compared to the dynamics approach. The time elapsed between the events is shown to be a more reliable and unbiased criterion and all the studied D-A reactions, except for the case of H2C=CHNO, are classified as synchronous, despite the gradual and quite distinct degrees of (a)symmetry of the TS structures.

Dynamic effects dictate the mechanism and selectivity of dehydration-rearrangement reactions of protonated alcohols [Me2 (R)CCH(OH2 )Me](+) (R=Me, Et, iPr) in the gas phase.

The gas-phase dehydration-rearrangement (DR) reactions of protonated alcohols [Me2 (R)CCH(OH2 )Me](+) [R=Me (ME), Et (ET), and iPr (I-PR)] were studied by using static approaches (intrinsic reaction coordinate (IRC), Rice-Ramsperger-Kassel-Marcus theory) and dynamics (quasiclassical trajectory) simulations at the B3LYP/6-31G(d) level of theory. The concerted mechanism involves simultaneous water dissociation and alkyl migration, whereas in the stepwise reaction pathway the dehydration step leads to a secondary carbocation intermediate followed by alkyl migration. Internal rotation (IR) can change the relative position of the migrating alkyl group and the leaving group (water), so distinct products may be obtained: [Me(R)CCH(Me)Me⋅⋅⋅OH2 ](+) and [Me(Me)CCH(R)Me⋅⋅⋅OH2 ](+) . The static approach predicts that these reactions are concerted, with the selectivity towards these different products determined by the proportion of the conformers of the initial protonated alcohols. These selectivities are explained by the DR processes being much faster than IR. These results are in direct contradiction with the dynamics simulations, which indicate a predominantly stepwise mechanism and selectivities that depend on the alkyl groups and dynamics effects. Indeed, despite the lifetimes of the secondary carbocations being short (<0.5 ps), IR can take place and thus provide a rich selectivity. These different selectivities, particularly for ET and I-PR, are amenable to experimental observation and provide evidence for the minor role played by potential-energy surface and the relevance of the dynamics effects (non-IRC pathways, IR) in determining the reaction mechanisms and product distribution (selectivity).

Exploring the intrinsic polar [4+2(+)] cycloaddition reactivity of gaseous carbosulfonium and carboxonium ions.

Gas-phase reactions of model carbosulfonium ions (CH(3)-S(+)=CH(2;) CH(3)CH(2)-S(+)=CH(2) and Ph-S(+)=CH(2)) and an O-analogue carboxonium ion (CH(3)-O(+)=CH(2)) with acyclic (isoprene, 1,3-butadiene, methyl vinyl ketone) and cyclic (1,3-cyclohexadiene, thiophene, furan) conjugated dienes were systematically investigated by pentaquadrupole mass spectrometry. As corroborated by B3LYP/6-311 G(d,p) calculations, the carbosulfonium ions first react at large extents with the dienes forming adducts via simple addition. The nascent adducts, depending on their stability and internal energy, react further via two competitive channels: (1) in reactions with acyclic dienes via cyclization that yields formally [4+2(+)] cycloadducts, or (2) in reactions with the cyclic dienes via dissociation by HSR loss that yields methylenation (net CH(+) transfer) products. In great contrast to its S-analogues, CH(3)-O(+)=CH(2) (as well as C(2)H(5)-O(+)=CH(2) and Ph-O(+)=CH(2) in reactions with isoprene) forms little or no adduct and proton transfer is the dominant reaction channel. Isomerization to more acidic protonated aldehydes in the course of reaction seems to be the most plausible cause of the contrasting reactivity of carboxonium ions. The CH(2)=CH-O(+)=CH(2) ion forms an abundant [4+2(+)] cycloadduct with isoprene, but similar to the behavior of such α,ß-unsaturated carboxonium ions in solution, seems to occur across the C=C bond.

Selectivity and mechanisms driven by reaction dynamics: the case of the gas-phase OH(-) + CH3ONO2 reaction.

Well-established statistical approaches such as transition-state theory based on high-level calculated potential energy profiles are unable to account for the selectivity observed in the gas-phase OH(-) + CH(3)ONO(2) reaction. This reaction can undergo bimolecular nucleophilic displacement at either the carbon center (S(N)2@C) or the nitrogen center (S(N)2@N) as well as a proton abstraction followed by dissociation (E(CO)2) pathway. Direct dynamics simulations yield an S(N)2:E(CO)2 product ratio in close agreement with experiment and show that the lack of reactivity at the nitrogen atom is due to the highly negative electrostatic potential generated by the oxygen atoms in the ONO(2) group that scatters the incoming OH(-). In addition to these dynamical effects, the nonstatistical behavior of these reactions is attributed to the absence of equilibrated reactant complexes and to the large number of recrossings, which might be present in several ion-molecule gas-phase reactions.

Sequential methyl-fluorine exchange reactions of siloxide ions in the gas phase.

Exchange Me for a fluorine: Trimethylsiloxide ions in the presence of NF(3) in the gas phase undergo an unusual and sequential metathesis-type reaction wherein methyl groups are exchanged for fluorine. Theoretical calculations suggest that the reaction proceeds by a three-step internal-nucleophilic-displacement mechanism which features a pentacoordinated siliconate species as a transition state rather than as an intermediate.

Gas-phase nucleophilic and elimination reactions in simple alkyl nitrates.

There has been increasing interest in the gas-phase reactivity of alkyl nitrates because of their well-known applications as explosives and because of their role in atmospheric and in marine processes. This manuscript describes an experimental study by FT-ICR techniques of the gas-phase reactions of OH(-) and F(-) with methyl and ethyl nitrate. For methyl nitrate, the main reaction channel is found to be an elimination process promoted by abstraction of an α proton from the methyl group. Nucleophilic displacement of nitrate anion through an S(N)2 process at the carbon center is also found to be an important reaction channel with methyl nitrate. In ethyl nitrate, formation of NO(3)(-) is greatly enhanced and this is attributed to the ease of an E2-type elimination process promoted by proton abstraction at the ß position of the ethyl group. Theoretical calculations at the MP2/6-311+G(3df,2p)//MP2/6-31+G(d) level of theory are consistent with the relative importance of the reaction channels and suggest that these reactions proceed through a double well potential. The calculations also predict that nucleophilic attack by OH(-) at the nitrogen center (Sn2@N) is energetically the preferred pathway but experiments with (18)OH(-) showed no evidence for this channel. Single-point calculations reveal a strong preference for approach to the carbon center and may explain the lack of reactivity at the nitrogen center. Calculations were also carried out for NH(2)(-) and SH(-) to establish the reactivity pattern to provide a better understanding of environmentally relevant nitrate esters.

The gas-phase alcoholysis of protonated homoleptic alkoxysilanes.

Tetra-alkoxysilanes are common and useful reagents in sol-gel processes and understanding their reactivity is important in the design of new materials. The mechanism of gas-phase reactions that mimic alcoholyis of Si(OMe)(4) (usually known as TMOS) under acidic conditions have been studied by Fourier transform ion cyclotron resonance techniques and density functional calculations at the B3LYP/6-311+G(d,p) level. The proton affinity of TMOS has been estimated at 836.4 kJ mol(-1) and protonation of TMOS gives rise to an ionic species that is best represented as trimethoxysilyl cations associated with a methanol molecule. Protonated TMOS undergoes rapid and sequential substitution of the methoxy groups in the gas-phase upon reaction with alcohols. The calculated energy profile of the reaction indicates that the substitution reaction through an S(N)2 type mechanism may be more favorable than frontal attack at silicon. Furthermore, the sequential substitution reactions are promoted by a mechanism that involves proton shuttle from the most favorable protonation site to the oxygen of the departing group mediated by the neutral reagent molecule.

Gas-phase electrophilic addition promoted by CH(3)S(+)=CH(2) ions on aromatic systems.

The gas-phase methylenation reaction between CH(3)S(+)=CH(2) and alkylbenzenes, aniline, phenol and alkyl phenyl ethers, which yields [M + CH](+) and CH(3)SH, has been studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques and computational chemistry at the DFT level. The methylthiomethyl cation is less reactive than methoxymethyl and, unlike the latter, is unreactive toward benzene. The calculations suggest that reaction with toluene should proceed primarily by addition at the para and ortho positions resulting in a benzyl-type ion. Reaction with aniline-2,3,4,5,6-d(5) reveals that elimination of CH(3)SD is kinetically favored by a factor of 5 over elimination of CH(3)SH. Experiments with C(6)H(6)ND(2) and theoretical calculations suggest that methylenation at the nitrogen atom is energetically favorable and likely, but the observed results may reflect some H/D scrambling, which occurs after attack at a ring position. By comparison, reaction with phenol-2,3,4,5,6-d(5) reveals that methylenation followed by elimination of CH(3)SD is kinetically favored by a factor of 3.8 over elimination of CH(3)SH. For phenol, the theoretical calculations suggest that attack by CH(3)S(+)=CH(2) at the para or ortho position is the only low-energy pathway for methylenation. However, a low-energy pathway for hydrogen scrambling is predicted by the calculations originating from the exit complex, [CH(3)SH(...) CH(2)=C(6)H(4)=OH](+), of reaction at a ring position.

Ion-molecule reactions of [C, H(3), S](+) ions and evidence for long-lived triplet CH(3)S(+).

Gas-phase [C, H(3), S](+) ions obtained by electron impact from (CH(3))(2)S at 14 eV undergo two distinct low-pressure ion-molecule reactions with the parent neutral: proton transfer and charge exchange. The kinetics of these reactions studied by Fourier transform ion cyclotron resonance (FT-ICR) techniques clearly suggests the [C, H(3), S](+) species to be a mixture of isomeric ions. While proton transfer is consistent with reagent ions displaying the CH(2)SH(+) connectivity, the observed charge exchange strongly argues for the presence of thiomethoxy cations, CH(3)S(+), predicted to be stable only in the triplet state. Charge exchange reactions are also observed in the reaction of these same [C, H(3), S](+) ions with benzene, toluene and phenetole. For these substrates, the CH(2)SH(+) ions can promote proton transfer and electrophilic methylene insertion in the aromatic ring with elimination of H(2)S. The results obtained for the different substrates suggest that the fraction of long-lived fraction of thiomethoxy cations obtained at 14 eV by electron ionization of dimethyl sulfide amounts to ~(22 -/+ 4)% of the [C, H(3), S](+) fragments.

A theoretical analysis of the free-energy profile of the different pathways in the alkaline hydrolysis of methyl formate in aqueous solution.

The free-energy profile for the different reaction pathways available to the hydroxide ion and methyl formate in aqueous solution is reported for the first time. The theoretical analysis was carried out by using the cluster-continuum method recently proposed by us for calculating the free energy of solvation of ions. Unlike the gas-phase reaction, our results are consistent with the fact that the reaction occurs mainly by nucleophilic attack of the hydroxide on the carbonyl carbon to yield a tetrahedral intermediate (B(AC)2 mechanism). However, an additional pathway, in which the hydroxide ion acts as a general base and a water molecule coordinated to this ion acts as the nucleophile, is also predicted to be important. The relative importance of these pathways is calculated to be 87 % and 13 %, respectively. The tetrahedral intermediate of the hydrolysis reaction has an estimated lifetime of 10 nanoseconds, and its conjugate acid has a pK(a) of 8.8. This tetrahedral intermediate is predicted to proceed to products by two pathways: elimination of methoxide ion (84 %) and by water catalyzed elimination of methanol (16 %). The less common reaction pathway, which involves attack of the hydroxide ion on the formyl hydrogen (decarbonylation mechanism) and leads to water, carbon monoxide, and methanol, is calculated to be only 3 kcal mol(-1) less favorable than the B(AC)2 mechanism. By comparison, direct attack of the hydroxide ion on the methyl group (B(AL)2 or S(N)2 mechanism) leading to an acyl-oxygen bond cleavage has a very high free energy of activation and is not expected to be important. The theoretically observed activation free energy at 298.15 K is calculated to be 15.5 kcal mol(-1), in excellent agreement with the experimentally measured value of 15.3 kcal mol(-1). This present model allows for a clear distinction between contributions due to solvation and those due to intrinsic (gas-phase) effects and proves to yield results in very good agreement with available experimental data.

Ciência e tecnologia no Brasil: a capacitaçäo brasileira para a pesquisa científica e tecnológica - v.3 / Science and technology in Brazil: the brazilian capacitation to the technological and scientific research - v.3

Estuda a situaçäo da pesquisa e capacitaçäo de recursos humanos na área da saúde, no Brasil. Analisa as seguintes áreas: médica, odontológica, enfermagem, educaçäo física e terapia ocupacional. (NMPM)