Computational and experimental assessment of efficient dye adsorption method from aqueous effluents by halloysite and palygorskite clay minerals.

The removal of dyes from effluents of textile industries represents a technological challenge, due to their significant environmental impact. The application of halloysite (Hal) and palygorskite (Pal) clay minerals as adsorbents for the removal of Congo red (CR) and methylene blue (MB) was evaluated in this work. The materials were applied both in natural and acid-treated forms, and characterized by XRD, XPS, SEM-EDS, FTIR, and N2 adsorption-desorption isotherm techniques to identify their properties and main active sites. The adsorbents showed potential to remove CR (> 98%) and MB (> 85%) within 180 min, using 0.3 g adsorbent and initial dye concentration of 250 mg L-1. Semi-empirical quantum mechanical calculations (SQM) confirmed the interaction mechanism between dyes and the adsorbents via chemisorption (- 69.0 kcal mol-1 < Eads < - 28.8 kcal mol-1), which was further observed experimentally due to the high fit of adsorption data to pseudo-second order kinetic model (R2 > 0.99) and Langmuir isotherm (R2 > 0.98). The use of Pal and Hal to remove dyes was proven to be economically and environmentally viable for industrial application.

Theoretical and Experimental Investigation of Acidity of the Glutamate Receptor Antagonist 6,7-Dinitro-1,4-dihydroquinoxaline-2,3-dione and Its Possible Implication in GluA2 Binding.

The acidity of organic compounds is highly relevant to understanding several biological processes. Although the relevance and challenges in estimating pKa values of organic acids is recognized by several reported works in the literature, there is a lack in determining the acidity of amides. This paper presents an experimental/theoretical combined investigation on the acid dissociation of the compound 6,7-dinitro-1,4-dihydroquinoxaline-2,3-dione (DNQX), a well-established antagonist of ionotropic glutamate receptor GluA2. DNQX was synthesized, and its two acidic constants were determined by UV-vis spectroscopy. The experimental pKa of 6.99 ± 0.02 and 10.57 ± 0.01 indicate that DNQX mainly exists as an anionic form (DNQXA1) in physiological media, which was also confirmed by 1H NMR analysis. Five computational methods were applied for estimating the theoretical pKa values of DNQX, including B3LYP, M06-2X, ωB97XD, and CBS-QB3, which were able to provide reasonable estimates for pKa associated with DNQX. Molecular dynamics studies have demonstrated that DNQXA1' binds more effectively to the pocket of the GluA2 than neutral DNQX, and this fact is coherent to the interactions between amidic oxygens and Arg845 being the main interactions of this host-guest system. Moreover, interaction of GluA2 with endogenous glutamate is stronger than that with DNQXA1, which is in agreement with literature. To the best of our knowledge, we report herein an unprecedented approach involving acidity of the antagonist DNQX, as well as the possible implications in binding to GluA2.

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.

Dynamical Bifurcation in Gas-Phase XH- + CH3 Y SN 2 Reactions: The Role of Energy Flow and Redistribution in Avoiding the Minimum Energy Path.

The gas-phase reactions of XH- (X=O, S) + CH3 Y (Y=F, Cl, Br) span nearly the whole range of SN 2 pathways, and show an intrinsic reaction coordinate (IRC) (minimum energy path) with a deep well owing to the CH3 XH⋅⋅⋅Y- (or CH3 S- ⋅⋅⋅HF) hydrogen-bonded postreaction complex. MP2 quasiclassical-type direct dynamics starting at the [HX⋅⋅⋅CH3 ⋅⋅⋅Y]- transition-state (TS) structure reveal distinct mechanistic behaviors. Trajectories that yield the separated CH3 XH+Y- (or CH3 S- +HF) products directly are non-IRC, whereas those that sample the CH3 XH⋅⋅⋅Y- (or CH3 S- ⋅⋅⋅HF) complex are IRC. The IRCIRC/non-IRC ratios of 90:10, 40:60, 25:75, 2:98, 0:100, and 0:100 are obtained for (X, Y)=(S, F), (O, F), (S, Cl), (S, Br), (O, Cl), and (O, Br), respectively. The properties of the energy profiles after the TS cannot provide a rationalization of these results. Analysis of the energy flow in dynamics shows that the trajectories cross a dynamical bifurcation, and that the inability to follow the minimum energy path arises from long vibration periods of the X-C⋅⋅⋅Y bending mode. The partition of the available energy to the products into vibrational, rotational, and translational energies reveals that if the vibrational contribution is more than 80 %, non-IRC behavior dominates, unless the relative fraction of the rotational and translational components is similar, in which case a richer dynamical mechanism is shown, with an IRC/non-IRC ratio that correlates to this relative fraction.

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.

Assessment of density-functionals for describing the X(-) + CH3ONO2 gas-phase reactions with X = F, OH, CH2CN.

The energetics of the ECO2, SN2@C and SN2@N channels of X(-) + CH3ONO2 (X = F, OH, CH2CN) gas-phase reactions were computed using the CCSD(T)/CBS method. This benchmark extends a previous study with X = OH [M. A. F. de Souza et al., J. Am. Chem. Soc., 2012, 134, 19004] and was used to ascertain the accuracy and robustness of nineteen density-functionals for describing these potential energy profiles (PEP) as well as the kinetic product distributions obtained from RRKM calculations. Assessments were based on the mean unsigned error (MUE), the mean signed error (MSE), the #best : #worst (BW) criterion and the statistical confidence interval (CI) for the MSE. In general, double-hybrid (DH) functionals perform better than the range-separated ones, and both are better than the global-hybrid functionals. Based on the MUE and CI criteria the B2GPPLYP, B2PLYP, M08-SO, BMK, ωB97X-D, CAM-B3LYP, M06, M08-HX, ωB97X and B97-K functionals show the best performance in the description of these PEPs. Within this set, the B2GPPLYP functional is the most accurate and robust. The RRKM results indicate that the DHs are the best for describing the selectivities of these reactions. Compared to CCSD(T), the B2PLYP method has a relative error of only ca. 1% for the selectivity and the accuracy to provide the correct conclusion concerning the nonstatistical behavior of these reactions.

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).

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.

CASSCF and multireference CI with singles and doubles study of low-lying valence and Rydberg states of 2H-tetrazole.

Complete active space self-consistent field (CASSCF) and multireference CI with singles and doubles (MR-CISD) calculations [including extensivity corrections, at MR-CISD+Q and multireference averaged quadratic coupled cluster (MR-AQCC) levels] have been performed to characterize the low-lying valence and the Rydberg states of 2H-tetrazole. The highest level results (MR-AQCC/d'-aug'-cc-pVDZ) indicate the following ordering of the valence singlet excited states: S(1) (n-pi*), 6.06 eV; S(2) (n-pi*), 6.55 eV; S(3) (pi-pi*), 6.55 eV. The MR-CISD+Q/d'-aug'-cc-pVDZ results indicate the same ordering, but at slight higher energies: 6.16, 6.68, and 6.69 eV, respectively. According to our MR-CISD+Q/d'-aug'-cc-pVDZ results, the next two states are Rydberg states, at 7.69 eV (pi-3s) and 7.89 eV (n-3s). The calculated energies of these two states, as well as their proximity, are consistent with the conclusion reached by Palmer and Beveridge (Chem Phys 1987, 111, 249) that the first band of the photoelectron spectrum of 2H-tetrazole is likely to be associated to the first two ionizations processes (of pi and N lone pair electrons), at energies close to 11.3 eV.

Controlling bichromatic emission in scattering gain media.

It is experimentally shown that the competition between the two lasing modes of bichromatic emission in a dye solution with nanoparticle scatterers within the diffuse regime can be externally controlled by varying only the excitation beam diameter and the radiation detector position. It is established that this feature is a consequence of the changes in the reabsorption process strengths between monomer and dimer dye aggregates. The controllable switching of these lasing modes could open the real possibility of implementing previously suggested applications for this effect as optical switches.