Molecular mechanism of the transformation of oxidized lignin to N-substituted aromatics.

The cleavage of C-C bonds in oxidized lignin model compounds is a highly effective methodology for achieving lignin depolymerization, as well the generation of N-substituted aromatics. Here, density functional theory calculations were performed to understand the mechanism of the transformation of an oxidized lignin model compound (ligninox) and hydroxylamine hydrochloride to N-substituted aromatics. The reaction was proposed to proceed via an energetically viable mechanism featuring the initial production of HOAc acting as proton bridge. According to our calculations, Z-type oxime is the major intermediate of the reaction, with an energy barrier of 22.9 kcal mol-1, owing to the weak interactions between methoxy and oximino groups being stronger than that of E-type oxime. Additionally, the hydroxy addition is the rate-determining step, with an energy barrier of 27.0 kcal mol-1. Moreover, the huge net energy change of Beckmann and abnormal Beckmann rearrangements is the main overall thermodynamic driving force for producing N-substituted aromatics from oximes. The theoretical results have provided a clear picture of how ligninox transforms into N-substituted aromatics and are expected to provide valuable theoretical guidance for lignin depolymerization.

Evolution of time-delay signature of polarized chaos in a unidirectional-coupling VCSEL system with variable-polarization optical injection.

The evolution characteristics of the time-delay signature (TDS) of polarized chaos is systematically investigated in a unidirectional-coupling vertical-cavity surface-emitting (VCSEL) scheme with variable-polarization optical injection (VPOI), by means of the time series, optical spectra, power spectra, and autocorrelation function. In this scheme, the polarized chaos with TDS from a master VCSEL (M-VCSEL) with the external cavity is unidirectionally injected into another solitary slave VCSEL (S-VCSEL) through VPOI. The numerical results show that the VPOI can exert significant influence on the TDS characteristics of polarized chaos in the S-VCSEL. Under special injection parameters, as the polarization injection angle (θp) of VPOI varies, the TDS evolution for the X polarization component can exhibit almost an opposite evolutionary pattern to that for the Y polarization component in the S-VCSEL. A series of TDSs is mapped in the θp plane, and the injection strength and frequency detuning have been simulated to thoroughly elaborate the TDS evolution of polarization-resolved chaos in the S-VCSEL. Moreover, the optimal parameter spaces with effectively suppressed TDS are also determined for each polarization component in those maps.

Time-delay signature suppression of polarization-resolved wideband chaos in VCSELs with dual-path chaotic optical injections.

The combining investigation on the time-delay signature (TDS) and chaos bandwidth have been theoretically investigated in a vertical-cavity surface-emitting laser (VCSEL) system with dual-path chaotic optical injections. In this scheme, the polarized chaos with the TDS from an external-cavity master VCSEL is routed into two different paths and then unidirectionally injected into another solitary slave VCSEL. With the aid of the autocorrelation function and the effective bandwidth calculation, the TDS and bandwidth of polarized chaos from the chaotic system are quantitatively evaluated. The results show that, in such a dual-path chaotic optical-injection system, the high-quality polarized chaos with the successful TDS suppression and chaotic bandwidth enhancement can be achieved in wider parameter regions in contrast with the case for the single-path chaotic optical injection. Further research also finds that the injected time-delay difference between two injection paths is desired to mismatch the feedback time delay, which is conducive to suppressing TDS and expanding bandwidth of polarized chaos. Besides, the better chaotic quality with low TDS and wide bandwidth can be expected by choosing the appropriate injection strengths of two injection paths.

How and Why a Protic Ionic Liquid Efficiently Catalyzes Chemical Fixation of CO2 to Quinazoline-2,4-(1H,3H)-diones: Electrostatically Controlled Reactivity.

A density functional theory study has been conducted to gain insight into the intriguing experimental observations on the synthesis of quinazoline-2,4-(1H,3H)-diones from 2-aminobenzonitriles reacting with CO2 catalyzed by protic ionic liquids (ILs). We explored the molecular mechanism of the titled reaction, as well as the origin and catalytic nature of different ILs toward the reaction in detail. The calculated energetically viable mechanism involves CO2 attack, intramolecular rearrangement, and intramolecular cyclization stages. This mechanism features the initial polarization of the C≡N triple bond with the assistance of the real catalytic species, [HDBU+][TFECOO-], where the cation [HDBU+] acts as Brønsted acid and the anion [TFECOO-], the adduct of anion [TFE-] and CO2, acts as a nucleophile. The calculated results present the electrostatically controlled character of the reaction, where the reactivity relies on the electrostatic interaction of the IL cation with the anion. The reactivity can be controlled and regulated by the basicity of the deprotonated counterpart of the IL cation as well as the CO2 adsorption ability of the IL anion. The best catalytic performance of [HDBU+][TFE-] is attributed to its strongest basicity of the deprotonated counterpart of [HDBU+] and its most efficient CO2 adsorption property of [TFE-]. These theoretical results are expected to provide guidance for designing efficient IL-based catalysts in preparing quinazoline-2,4-(1H,3H)-diones by reacting 2-aminobenzonitriles with CO2.

Reaction mechanism of organoselenium-catalyzed syn-dichlorination of alkenes: a DFT study.

A new method for the syn-dichlorination of alkenes at room temperature has been proposed by Denmark et al. The method uses diselenide (PhSeSePh) as the precatalyst, benzyltriethylammonium chloride (BnEt3NCl) as the source of chlorine, and an N-fluoropyridinium salt as the oxidant to recover the catalyst. This approach has achieved exquisite diastereocontrol on a number of alkene substrates. In this paper, we report the results of DFT calculations we performed to study the mechanism of this reaction. We were able to identify a reasonable reaction path, including the intermediate and transition-state structures. The results also indicate that PhSeCl3, rather than PhSeCl, is the active catalyst.

New Insight into the Formation Mechanism of Imidazolium-Based Ionic Liquids from N-Alkyl Imidazoles and Halogenated Hydrocarbons: A Polar Microenvironment Induced and Autopromoted Process.

To illustrate the formation mechanism of imidazolium-based ionic liquids (ILs) from N-alkyl imidazoles and halogenated hydrocarbons, density functional theory calculations have been carried out on a representative system, the reaction of N-methyl imidazole with chloroethane to form 1-ethyl-3-methyl imidazolium chloride ([Emim]Cl) IL. The reaction is shown to proceed via an SN2 transition state with a free energy barrier of 34.4 kcal/mol in the gas phase and 27.6 kcal/mol in toluene solvent. The reaction can be remarkably promoted by the presence of ionic products and water molecules. The calculated barriers in toluene are 22.0, 21.7, and 19.9 kcal/mol in the presence of 1-3 ionic pairs of [Emim]Cl and 23.5, 21.3, and 19.4 kcal/mol in the presence of 1-3 water molecules, respectively. These ionic pairs and water molecules do not participate directly in the reaction but provide a polar environment that favors stabilizing the transition state with large charge separation. Hence, we propose that the synthesis of imidazolium-based ILs from N-alkyl imidazoles and halogenated hydrocarbons is an autopromoted process and a polar microenvironment induced reaction, and the existence of water molecules (a highly polar solvent) in the reaction may be mainly responsible for the initiation of reaction.

Theoretical study of the reaction of chitosan monomer with 2,3-epoxypropyl-trimethyl quaternary ammonium chloride catalyzed by an imidazolium-based ionic liquid.

The molecular mechanism of the graft reaction of 2,3-epoxypropyl-trimethyl quaternary ammonium chloride with chitosan monomer was investigated by performing density functional theory (DFT) calculations. The calculated results show that the -NH2 group of chitosan monomer is more reactive than its -OH and -CH2OH groups, and the graft reaction on the -NH2 group is calculated to be exothermic by 20.5kcal/mol with a free energy barrier of 42.6kcal/mol. The reaction cannot benefit from the presence of the intruded water molecule, but can be considerably assisted by 1-allyl-3-methylimidazolium chloride ([Amim]Cl) ionic liquid. The reaction catalyzed by the ion-pair is calculated to be exothermic by 36.5kcal/mol and the barrier is reduced to 29.3kcal/mol, which are further corrected to 28.0 and 29.1kcal/mol by considering the solvent effect of [Amim]Cl ionic liquid. Calculated results verified the experimental finding that imidazolium-based ionic liquids can promote the reaction of chitosan with epoxy compounds.

A simple and convenient method to synthesize N-[(2-hydroxyl)-propyl-3-trimethylammonium] chitosan chloride in an ionic liquid.

N-[(2-Hydroxyl)-propyl-3-trimethyl ammonium] chitosan chloride (HTCC) was synthesized through nucleophilic substitution of 2,3-epoxypropyltrimethyl ammonium chloride (EPTAC) onto chitosan using ionic liquid of 1-allyl-3-methylimidazole chloride (AmimCl) as a homogeneous and green reaction media. The chemical structure of HTCC was confirmed by FTIR, (1)H NMR and (13)C NMR. The FTIR peak intensity of amino group at 1595 cm(-1) decreased and that of [Formula: see text] at 1475 cm(-1) increased with the increase of reaction time, confirming the substitution of EPTAC on CS. The degree of substitutions (DS) were calculated from the integral area of (1)H NMR, and the optimum reaction condition was obtained, namely, reaction time of 8h, temperature of 80°C and [Formula: see text] of 3/1. The degree of crystallinity and thermal properties of HTCC were characterized by XRD, TG, DSC, and DMA methods. Data from XRD, TG, DSC and DMA show that the degree of crystallinity, thermal stability, as well as glass transition temperature of HTCC decreased with the increase of DS. The reaction mechanism of chitosan with EPTAC in AmimCl was elucidated by performing density functional theory (DFT) calculations.