Strong Laser Field-Driven Coupled Electron-Nuclear Dynamics: Quantum vs Classical Description.

We have performed a coupled electron-nuclear dynamics study of H2+ molecular ions under the influence of an intense few-cycle 4.5 fs laser pulse with an intensity of 4 × 1014 W/cm2 and a central wavelength of 750 nm. Both quantum and classical dynamical methods are employed in the exact similar initial conditions with the aim of head-to-head comparison of two methodologies. A competition between ionization and dissociation channel is explained under the framework of quantum and classical dynamics. The origin of the electron localization phenomena is elucidated by observing the molecular and electronic wave packet evolution pattern. By probing with different carrier envelope phase (CEP) values of the ultrashort pulse, the possibility of electron localization on either of the two nuclei is investigated. The effects of initial vibrational states on final dissociation and ionization probabilities for several CEP values are studied in detail. Finally, asymmetries in the dissociation probabilities are calculated and mutually compared for both quantum and classical dynamical methodologies, whereas Franck-Condon averaging over the initial vibrational states is carried out in order to mimic the existing experimental conditions.

Metal-free sp2 -C7-H Borylation of Tryptophan Containing Peptides and Late-stage Modification.

The discovery of milder and robust strategies to enable the introduction of organoboronates in peptides remains conspicuously underdeveloped. Herein, we demonstrate an efficient method for the site-selective sp2 -C7-H borylation of tryptophan under metal-free condition using BBr3 directed by pivaloyl group. The versatility of this approach is that gram scale synthesis and C7-borylated N-Phth-Trp(N-Piv)(C7-BPin)-OMe was modified into various C7-substituted derivatives. Moreover, the strategy enables for the peptide elongation and late-stage borylation of peptides, natural product Brevianamide F and drug Oglufanide.

Bicyclic (alkyl)(amino)carbene (BICAAC) in a dual role: activation of primary amides and CO2 towards catalytic N-methylation.

Herein, we report the first catalytic methylation of primary amides using CO2 as a C1 source. A bicyclic (alkyl)(amino)carbene (BICAAC) exhibits dual role by activating both primary amide and CO2 to carry out this catalytic transformation which enables the formation of a new C-N bond in the presence of pinacolborane. This protocol was applicable to a wide range of substrate scopes, including aromatic, heteroaromatic, and aliphatic amides. We successfully used this procedure in the diversification of drug and bioactive molecules. Moreover, this method was explored for isotope labelling using 13CO2 for a few biologically important molecules. A detailed study of the mechanism was carried out with the help of spectroscopic studies and DFT calculations.

Theoretical Chemistry Symposium 2021.

The editorial paints a brief history of the conference - Theoretical Chemistry Symposium, organized by eminent theoretical and computational chemists from India.

Dissociative ionization of the H2 molecule under a strong elliptically polarized laser field: carrier-envelope phase and orientation effect.

A coupled electron-nuclear dynamical study is performed to investigate the sub-cycle dissociation and ionization of the H2 molecule in a strong 750 nm 4.5 fs elliptically polarized laser pulse. A quasi-classical method is employed in which additional momentum-dependent potentials are added to the molecular Hamiltonian to account for the non-classical effects. The effect of molecular orientation with respect to the laser polarization plane on the probabilities of different dynamical channels and proton energy spectra has been examined. We demonstrate the 2D-control of proton anisotropy by manipulating the carrier-envelope phase of the pulse. We demonstrate that the quasi-classical method can capture the carrier-envelope phase effects in the dissociative ionization of the H2 molecule. Our results indicate that the classical models provide an efficient approach to study the mechanistic insights of strong-field molecular dynamics.

Effect of temperature on CO oxidation over Pt(111) in two-dimensional confinement.

Confined catalysis between a two-dimensional (2D) cover and metal surfaces has provided a unique environment with enhanced activity compared to uncovered metal surfaces. Within this 2D confinement, weakened adsorption and lowered activation energies were observed using surface science experiments and density functional theory (DFT) calculations. Computationally, the role of electronic and mechanical factors responsible for the improved activity was deduced only from static DFT calculations. This demands a detailed investigation on the dynamics of reactions under 2D confinement, including temperature effects. In this work, we study CO oxidation on a 2D graphene covered Pt(111) surface at 90 and 593 K using DFT-based ab initio molecular dynamics simulations starting from the transition state configuration. We show that CO oxidation in the presence of a graphene cover is substantially enhanced (2.3 times) at 90 K. Our findings suggest that 2D confined spaces can be used to enhance the activity of chemical reactions, especially at low temperatures.

Mode selective chemistry for the dissociation of methane on efficient Ni/Pt-bimetallic alloy catalysts.

The mode selectivity of methane dissociation is studied on three different Ni/Pt-bimetallic alloy surfaces using a fully quantum approach based on reaction path Hamiltonian. Dissociative sticking probability depends on the composition of alloying metals, excited vibrational mode, and symmetry of the reaction path about the plane perpendicular to the catalyst surface containing the carbon atom and two hydrogen atoms. Our calculations show that symmetry of the minimum energy reaction path depends on the surface alloy composition. In the transition state, the dissociating C-H bond elongates significantly for the dissociation of methane on these alloy systems. A significant decrease in the frequency of the symmetric stretching mode and the two bending modes near the transition state is observed on all the alloy surfaces. Under the vibrational adiabatic limit, excitation of these softened modes enhanced the dissociation probability compared to the ground vibrational state. The reaction probability values decrease abruptly at the incident energies less than the zero-point energy corrected barrier height. With the inclusion of non-adiabatic vibrational coupling terms, reaction probability in the low incident energy region increases to a greater extent, and mode selective behavior also becomes different from that observed within the vibrational adiabatic limit. Symmetric stretching mode displayed the highest reactivity on all the alloy surfaces. Overall, Ni8/Pt(111) is found to be the most reactive toward the methane dissociation.

Oxygen-Induced Dissociation of a Single Water Molecule in Confined 2-D Layers: A Semiempirical Study.

Semiempirical quantum mechanical methods provide a middle ground to molecule-surface interactions between computationally demanding full ab initio quantum chemistry calculations and force-field calculations. In the present study, the PM7 semiempirical method is used to evaluate the adsorption energy values of X@h-BN monolayer [X=O, OH, and H2 O], followed by a mechanistic study of oxygen-induced water dissociation on a free-standing h-BN monolayer. Based on oxygen adsorption configurations, two reaction pathways for water dissociation are studied that yield two distinct configurations of double OH-functionalized h-BN monolayer. The effect of a graphene cover layer on these proposed mechanistic pathways is then investigated by placing the graphene cover layer on the top of the h-BN monolayer and continuously tuning the separation (dGr/h-BN ) between these two layers.

Bicyclic (alkyl)(amino)carbene (BICAAC) as a metal-free catalyst for reduction of nitriles to amines.

Bicyclic (alkyl)(amino)carbene (BICAAC) is introduced as a metal-free catalyst for the reduction of various nitriles to the corresponding amine hydrochloride salts in the presence of pinacolborane. Mechanistic investigations combining experiments and DFT calculations suggest a B-H addition to the carbene center, which acts as a carrier of the hydride source.

Spatial and temporal variation of the ambient noise environment of the Sikkim Himalaya.

Ambient noise characteristics are perused to assess the station performance of 27 newly constructed broadband seismic stations across Sikkim Himalaya and adjoining Himalayan foreland basin, installed to study the seismogenesis and subsurface structure of the region. Power spectral densities obtained at each station, compared against the global noise limits, reveal that observed vertical component noise levels are within the defined global limits. However, the horizontal components marginally overshoot the limits due to the tilt effect. Ambient noise conditions significantly vary with different installation techniques, analysis revealing that seismic sensors buried directly in the ground have reduced long-period noise in comparison to pier installations. Tectonic settings and anthropogenic activities are also noted to cause a significant rise across short-period and microseism noise spectrum, varying spatially and temporally across the region. Day-time records higher cultural noise than night-time, while the microseism noise dominates during the monsoonal season. An assessment of the effect of the nationwide lockdown imposed due to COVID-19 pandemic revealed a significant decrease in the short-period noise levels at stations installed across the foreland basin marked with higher anthropogenic activity. Our study summarizes the overall ambient noise patterns, validating the stability and performance of the seismic stations across the Sikkim Himalayas.

Schematic Design of Metal-Free NHC-Mediated Sequestering and Complete Conversion of SO2 to Thiocarbonyl S-Oxide Derivatives at Room Temperature.

The sequestering and complete conversion of SO2 to valuable chemicals in a metal-free pathway is highly demanded. The recent success of SO2 fixation by N-heterocyclic carbenes instigated further studies in this regard. Previous reports were confined within the carbene-SO2 reaction mechanism and the stability of oxathiirane S-oxide derivatives. The complete conversion of captured SO2 to precious chemicals was not studied. The present inquisition has accomplished the scarcity of the earlier studies. It is observed that in the presence of an excess amount of carbene, the registered SO2 is converted to the ketone derivative and thiocarbonyl S-oxide derivative. An electronic level investigation of these reactions is carried out. From the change of the molecular orbitals along the reaction path, it is concluded that the reaction between the oxathiirane S-oxide derivative and carbene follows a frog's hunting mechanism.

4,5-Dioxo-imidazolinium Cation-Promoted α-Selective Dehydrative Glycosylation of 2-Deoxy- and 2,6-Dideoxy Sugars.

Herein, we report the great potential of the 4,5-dioxo-imidazolinium cation activation strategy for dehydrative glycosylation reactions employing the readily available and economical geminal dichloroimidazolidinediones (DCIDs) that promotes the glycosylation between 2-deoxy- and 2,6-dideoxy-sugar hemiacetals with various acceptors in good yields and high α-selectivity. This research not only provides a mild and efficient alternative approach for stereoselective dehydrative glycosylation but also extends the dichloroimidazolidinedione as a novel promoter in the field of glycoscience.

Biochar-Supported TiO2-Based Nanocomposites for the Photocatalytic Degradation of Sulfamethoxazole in Water-A Review.

Sulfamethoxazole (SMX) is a frequently used antibiotic for the treatment of urinary tract, respiratory, and intestinal infections and as a supplement in livestock or fishery farming to boost production. The release of SMX into the environment can lead to the development of antibiotic resistance among the microbial community, which can lead to frequent clinical infections. SMX removal from water is usually done through advanced treatment processes, such as adsorption, photocatalytic oxidation, and biodegradation. Among them, the advanced oxidation process using TiO2 and its composites is being widely used. TiO2 is a widely used photocatalyst; however, it has certain limitations, such as low visible light response and quick recombination of e-/h+ pairs. Integrating the biochar with TiO2 nanoparticles can overcome such limitations. The biochar-supported TiO2 composites showed a significant increase in the photocatalytic activities in the UV-visible range, which resulted in a substantial increase in the degradation of SMX in water. The present review has critically reviewed the methods of biochar TiO2 composite synthesis, the effect of biochar integration with the TiO2 on its physicochemical properties, and the chemical pathways through which the biochar/TiO2 composite degrades the SMX in water or aqueous solution. The degradation of SMX using photocatalysis can be considered a useful model, and the research studies presented in this review will allow extending this area of research on other types of similar pharmaceuticals or pollutants in general in the future.

Competitive Reactivity of SO2 and NO2 with N-Heterocyclic Carbene: A Mechanistic Study.

Recent DFT based molecular engineering to obtain stable oxathiirane S-oxide derivatives evokes the recommencement of the use of carbenes for the sequestering of SO2, which has been kept separate so far. Carbene is one of the key chemicals for the sequestering of various premier greenhouse gases like CO2, CO, N2O, etc. In this respect, a comparative study of the reactivity of carbenes with variant greenhouse gases is highly demanding. The present investigation is engrossed in the comparative reactivity of SO2 and NO2 with carbenes. All three selected carbenes are highly susceptible to SO2 and NO2. Through an immaculate mechanistic study, we are able to corroborate that the end product of the carbene-SO2 reaction is an adduct which has a preferable structure having a six-membered ring with hydrogen bonding instead of ketone and SO with higher thermodynamic stability than the corresponding oxathiirane S-oxide derivative. Carbene reacts with NO2 to form a stable carbene N, N-dioxide derivative which forms vibrationally excited oxaziridine N-oxide which rapidly dissociates to form a ketone derivative. The formation of carbene S, S-dioxide and carbene N, N-dioxide is a barrierless process. The dissociation of oxaziridene N-oxide is also a barrierless process.

Tris(pentafluorophenyl)borane-Catalyzed Stereoselective C-Glycosylation of Indoles with Glycosyl Trichloroacetimidates: Access to 3-Indolyl-C-glycosides.

An efficient and highly regioselective method for the synthesis of 3-indolyl-C-glycosides has been developed through coupling of glycosyl trichloroacetimidates with a wide range of substituted indoles in the presence of catalytic amounts of B(C6F5)3 within a few minutes. This methodology has a wide scope of substrates under mild reaction conditions and provides exclusively ß-stereoselective 3-indolyl-C-glycosides in 64-87% yields.

Controlling the Ultrafast Dynamics of HD+ by the Carrier-Envelope Phases of an Ultrashort Laser Pulse: A Quasi-Classical Dynamics Study.

A theoretical study on the coupled electron-nuclear dynamics of HD+ molecular ions under ultrashort, intense laser pulses is performed by employing a well-established quasi-classical model. The influence of the laser carrier-envelope phase on various channel (H + D+, D + H+, and H+ + D+) probabilities is investigated at different laser field intensities. The carrier-envelope phase is found to govern the dissociation (H + D+ and D + H+) and Coulomb explosion (H+ + D+) channel probabilities. The kinetic energy release distributions of the fragments are also found to be sensitive to the carrier-envelope phase of the laser pulse. Our results are in agreement with the previously reported quantum dynamics studies and experiments.

Controlling Chemical Reactions with Laser Pulses.

One of the holy grails of contemporary science is to understand and manipulate chemical reactions to obtain desired products preferentially. To achieve this goal, chemists traditionally choose the correct starting materials and reaction conditions, but it often lacks selectivity and efficiency. A promising alternative is to design laser control schemes and apply them to guide and control chemical reactions. This mini-review attempts to provide theoretical insight into the laser-induced control of chemical reactions by highlighting some recent achievements, discussing the present challenges, and shedding some light on future prospects.

Chiral auxiliaries: Usefullness in stereoselective glycosylation reactions and their synthetic applications.

Oligosaccharides play a very important role in biological system and structure-activity relationships that is why it has a lot of application to medicinal chemistry and development of polysaccharide conjugate vaccines. The stereoselective introduction of a glycosidic linkage presents the principal challenge for biological importance oligosaccharide synthesis. The main aim of this review is to described the importance of chiral auxiliary and neibhouring group participation for the stereoselective 1,2-cis glycosidic bonds formation and their application in complex oligosaccharide synthesis.Numerous 1,2-cis-linked oligosaccharides and glyconjugates are naturally found in the compounds of blood group, human milk, antigens of bacterial lipopolysaccharide etc.that predominantly increased it's importance in this field.

3D characterisation and quantification of an offshore freshened groundwater system in the Canterbury Bight.

Although offshore freshened groundwater (OFG) systems have been documented in numerous continental margins worldwide, their geometry, controls and emplacement dynamics remain poorly constrained. Here we integrate controlled-source electromagnetic, seismic reflection and borehole data with hydrological modelling to quantitatively characterise a previously unknown OFG system near Canterbury, New Zealand. The OFG system consists of one main, and two smaller, low salinity groundwater bodies. The main body extends up to 60 km from the coast and a seawater depth of 110 m. We attribute along-shelf variability in salinity to permeability heterogeneity due to permeable conduits and normal faults, and to recharge from rivers during sea level lowstands. A meteoric origin of the OFG and active groundwater migration from onshore are inferred. However, modelling results suggest that the majority of the OFG was emplaced via topographically-driven flow during sea level lowstands in the last 300 ka. Global volumetric estimates of OFG will be significantly revised if active margins, with steep coastal topographies like the Canterbury margin, are considered.

The strategic use of para-quinone methides to access synthetically challenging and chemoselective α,α'-diarylmethyl N-glycosides from unprotected carbohydrate amines.

Reported herein is a practical route to access synthetically challenging and chemoselective α,α'-diarylmethyl N-glycosides via Sc(OTf)3-catalyzed 1,6-conjugate addition of amino sugars with para-quinone methides (p-QMs). The reactions proceed smoothly without a base and under mild reaction conditions with a broad substrate scope and moderate to good yields.