Hydrochemical systematics and isotope (δ18O, δD and 3H) variations of aquifer system of southern Bengal Basin: implications for groundwater pollution.

Hydrogeological, hydrochemical and isotopic traits of the groundwater in the Quaternary aquifer system in an urban-periurban locality within and encircling the Kolkata-Howrah twin city in the south Bengal Basin have been synthesised to explain the present- and paleo-hydrological processes, surface and groundwater interaction and mixing dynamics of contamination of groundwater. Rock-weathering, evaporation, ion-exchange and active mineral dissolution are the key processes commanding the groundwater chemistry. Freshwater flushing from the recharge zones had thinned the entrapped sea water which has generated the present-day brackish water by a non-uniform fusion. The best-fit line of the plots of δD and δ18O of groundwater samples displays a slope lower than that of local meteoric water line (LMWL) and global meteoric water line (GMWL) which hints that isotopic constitution of the groundwater of the present area is primarily formed by evaporation before or in the recharging process. A wide range of δ18O values in groundwater suggests that these waters are not blended enough to remove dissimilarities in isotope configuration of recharge water. This also suggests that many groundwaters are a result of mixing of present-day recharge and an older integrant recharged under previously cooler climatic conditions. The groundwater samples are more depleted of oxygen at the shallower level. The depleted samples cluster around the Tolly's nala (canal) where upper aquitard is missing or < 10-m thick. The tritium values range between 0.70 and 15.02 which indicate the occurrence of 'sub-modern', 'a mix of modern and sub-modern water' and 'modern water'. It indicates mingling of isotope-depleted water from the Hugli River by means of Tolly's canal with relatively less-depleted groundwater of Kolkata's late Pleistocene aquifer. The tritium values and Cl/Br ratio of groundwater samples adjoining Tolly's canal and elsewhere refer the direct infiltration of 'modern wastewater and freshwater' which mixes with the 'sub-modern water' in the aquifer system.

Isolable rubidium and caesium derivatives of common organic carbonyl compounds.

Light alkali metal (Li, Na, K) amides have a long history of synthetic utility, but heavier (Rb, Cs) congeners have barely been studied. This study reveals remarkable structurally complex outcomes of reacting AM(HMDS) (AM = Rb, Cs; HMDS = hexamethyldisilazide) with benzaldehyde and acetophenone. Though complicated, reactions give a diversity of eye-catching isolated products, an enolate with a hexagonal prismatic network, two dienolates with distinct extended ladder motifs, and two ß-imino-alkoxides comprising zig-zag chains of metal-oxygen bonds in infinite cages.

Three Oxidative Addition Routes of Alkali Metal Aluminyls to Dihydridoaluminates and Reactivity with CO2.

Three distinct routes are reported to the soluble, dihydridoaluminate compounds, AM[Al(NONDipp )(H)2 ] (AM=Li, Na, K, Rb, Cs; [NONDipp ]2- =[O(SiMe2 NDipp)2 ]2- ; Dipp=2,6-iPr2 C6 H3 ) starting from the alkali metal aluminyls, AM[Al(NONDipp )]. Direct H2 hydrogenation of the heavier analogues (AM=Rb, Cs) produced the first examples of structurally characterized rubidium and caesium dihydridoaluminates, although harsh conditions were required for complete conversion. Using 1,4-cyclohexadiene (1,4-CHD) as an alternative hydrogen source in transfer hydrogenation reactions provided a lower energy pathway to the full series of products for AM=Li-Cs. A further moderation in conditions was noted for the thermal decomposition of the (silyl)(hydrido)aluminates, AM[Al(NONDipp )(H)(SiH2 Ph)]. Probing the reaction of Cs[Al(NONDipp )] with 1,4-CHD provided access to a novel inverse sandwich complex, [{Cs(Et2 O)}2 {Al(NONDipp )(H)}2 (C6 H6 )], containing the 1,4-dialuminated [C6 H6 ]2- dianion and representing the first time that an intermediate in the commonly utilized oxidation process of 1,4-CHD to benzene has been trapped. The synthetic utility of the newly installed Al-H bonds has been demonstrated by their ability to reduce CO2 under mild conditions to form the bis-formate AM[Al(NONDipp )(O2 CH)2 ] compounds, which exhibit a diverse series of eyecatching bimetallacyclic structures.

A novel centroid based sentence classification approach for extractive summarization of COVID-19 news reports.

A COVID-19 news covers subtopics like infections, deaths, the economy, jobs, and more. The proposed method generates a news summary based on the subtopics of a reader's interest. It extracts a centroid having the lexical pattern of the sentences on those subtopics by the frequently used words in them. The centroid is then used as a query in the vector space model (VSM) for sentence classification and extraction, producing a query focused summarization (QFS) of the documents. Three approaches, TF-IDF, word vector averaging, and auto-encoder are experimented to generate sentence embedding that are used in VSM. These embeddings are ranked depending on their similarities with the query embedding. A Novel approach has been introduced to find the value for the similarity parameter using a supervised technique to classify the sentences. Finally, the performance of the method has been assessed in two different ways. All the sentences of the dataset are considered together in the first assessment and in the second, each document wise group of sentences is considered separately using fivefold cross-validation. The proposed method has achieved a minimum of 0.60 to a maximum of 0.63 mean F1 scores with the three sentence encoding approaches on the test dataset.

Alkali Metal Dihydropyridines in Transfer Hydrogenation Catalysis of Imines: Amide Basicity versus Hydride Surrogacy.

Catalytic reduction of a representative set of imines, both aldimines and ketimines, to amines has been studied using transfer hydrogenation from 1,4-dicyclohexadiene. Unusually, this has been achieved using s-block pre-catalysts, namely 1-metallo-2-tert-butyl-1,2-dihydropyridines, 2-tBuC5 H5 NM, M(tBuDHP), where M=Li-Cs. Reactions have been monitored in C6 D6 and tetrahydrofuran-d8 (THF-d8 ). A definite trend is observed in catalyst efficiency with the heavier alkali metal tBuDHPs outperforming the lighter congeners. In general, Cs(tBuDHP) is the optimal pre-catalyst with, in the best cases, reactions producing quantitative yields of amines in minutes at room temperature using 5 mol % catalyst. Supporting the experimental study, Density Functional Theory (DFT) calculations have also been carried out which reveal that Cs has a pathway with a significantly lower rate determining step than the Li congener. In the postulated initiation pathways DHP can act as either a base or as a surrogate hydride.

Hydrocarbon Soluble Alkali-Metal-Aluminium Hydride Surrog[ATES].

A series of group 1 hydrocarbon-soluble donor free aluminates [AM(t BuDHP)(TMP)Al(i Bu)2 ] (AM=Li, Na, K, Rb) have been synthesised by combining an alkali metal dihydropyridyl unit [(2-t BuC5 H5 N)AM)] containing a surrogate hydride (sp3 C-H) with [(i Bu)2 Al(TMP)]. These aluminates have been characterised by X-ray crystallography and NMR spectroscopy. While the lithium aluminate forms a monomer, the heavier alkali metal aluminates exist as polymeric chains propagated by non-covalent interactions between the alkali metal cations and the alkyldihydropyridyl units. Solvates [(THF)Li(t BuDHP)(TMP)Al(i Bu)2 ] and [(TMEDA)Na(t BuDHP)(TMP)Al(i Bu)2 ] have also been crystallographically characterised. Theoretical calculations show how the dispersion forces tend to increase on moving from Li to Rb, as opposed to the electrostatic forces of stabilization, which are orders of magnitude more significant. Having unique structural features, these bimetallic compounds can be considered as starting points for exploring unique reactivity trends as alkali-metal-aluminium hydride surrog[ATES].

Cl/Br mass ratio and water quality index from the Quaternary aquifer of south Bengal Basin in India.

The purpose of this study is to identify suitable areas for the exploitation of groundwater for human consumption by analysing Cl/Br and other indicators of wastewater influence on groundwater quality in and around the twin megacities of Kolkata and Howrah, located on the complex deltaic system of south Bengal Basin in India. About 18%, 56.5%, 28% and 35.5% of the 287 groundwater samples comply with the Indian acceptable limits (ALs) of TDS, Cl, Fe and Mn, respectively. About 29% of the 279 samples analysed have Cl/Br < 268 where the excess Br is derived from organic degradation. About 30% samples have Cl/Br > 308 clustering mainly around a palaeo-channel indicating wastewater contamination. In addition to these, many samples have NO3/Cl > 0.0002 and SO4/Cl > 0.014, indicating that the groundwater is contaminated by wastewater from sewage and septic tank leakage. The concentrations of metals (Fe, Mn) and As are also higher than AL particularly on either side of the palaeo-channel, which may affect the human body functions. The localised recharge of wastewater also contains high SO4. Sulphate reduction in the aquifer sequesters the As in groundwater into the neo-formed arsenopyrite. Based on the estimated water quality index, the groundwater samples have been classified into four types-highly suitable (28.17%), moderately suitable (23.94%), doubtful (15.85%) and unsuitable (32.04%) for drinking purpose. The first two types cover about 62% of the study area. But the highly urbanised area of Kolkata and Howrah city and the industrial areas in the southern fringe of Kolkata city have the latter two types of groundwater. The over-withdrawal of groundwater may expedite the quality deterioration of 'highly and 'moderately suitable water' of the study area into 'doubtful' and 'unsuitable for drinking' categories through mixing mechanism in the aquifer.

Amidomagnesium cations.

We report the synthesis, structure and reactivity of molecular amidomagnesium cations bearing tris{2-(dimethylamino)-ethyl}amine (Me6TREN). Me6TREN binds to the cationic magnesium centre exhibiting κ4 and κ3 coordination modes in [Me6TREN-Mg-N(SiHMe2)2]+ and [Me6TREN-Mg-N(SiMe3)2]+ respectively. [Me6TREN-Mg-N(SiHMe2)2]+ reacts with benzophenone resulting in the insertion of the carbonyl group across ß-SiH bond. The reaction between [Me6TREN-Mg-N(SiMe3)2]+ and CO2 leads to [Me6TREN-Mg-OSiMe3]+, while the reaction with H2O results in [Me6TREN-Mg-OH]22+. Attempts to prepare hydridomagnesium cations from [Me6TREN-Mg-N(SiMe3)2]+ using KH resulted in the precipitation of MgH2 and the isolation of [(Me6TREN)K(THF)3]+.

A disguised hydride in a butylmagnesium cation.

The ability of the ß-CH functionality in a butylmagnesium cation [Me6TREN-Mg-n-Bu]+ to quantitatively reduce benzophenone has been demonstrated. The hydridic nature of the ß-CH functionality is highlighted by its abstraction using B(C6F5)3. ß-CH abstraction over alkylation in [Me6TREN-Mg-n-Bu]+ is dependent on the nature of the incoming electrophile and the polarity of the solvent.