11,787,955 research outputs found
A study of the Group 1 metal tetra-aza macrocyclic complexes [M(Me4cyclen)(L)]+ using electronic structure calculations
No full textMetal-cyclen complexes have a number of important applications. However, the coordination chemistry between metal ions and cyclen-based macrocycles is much less well studied compared to their metal ion-crown ether analogues. This work, which makes a contribution to address this imbalance by studying complex ions of the type [M(Me4cyclen)(L)]+, was initiated by results of an experimental study which prepared some Group 1 metal cyclen complexes, namely [Li(Me4cyclen)(H2O)][BArF] and [Na(Me4cyclen)(THF)][BArF] and obtained their X-ray crystal structures [J. M. Dyke, W. Levason, M. E. Light, D. Pugh, G. Reid, H. Bhakhoa, P. Ramasami, and L. Rhyman, Dalton Trans., 2015, 44, 13853]. The lowest [M(Me4cyclen)(L)]+ minimum energy structures (M = Li, Na, K, and L = H2O, THF, DEE, MeOH, DCM) are studied using density functional theory (DFT) calculations. The geometry of each [M(Me4cyclen)(L)]+ structure and, in particular, the conformation of L are found to be mainly governed by steric hindrance which decreases as the size of the ionic radius increases from Li+ → Na+ → K+. Good agreement of computed geometrical parameters of [Li(Me4cyclen)(H2O)]+ and [Na(Me4cyclen)(THF)]+ with the corresponding geometrical parameters derived from the crystal structures [Li(Me4cyclen)(H2O)]+[BArF]− and [Na(Me4cyclen)(THF)]+[BArF]− is obtained. Bonding analysis indicates that the stability of the [M(Me4cyclen)(L)]+ structures originates mainly from ionic interaction between the Me4cyclen/L ligands and the M+ centres. The experimental observation that [M(Me4cyclen)(L)]+[BArF]− complexes could be prepared in crystalline form for M+ = Li+ and Na+, but that experiments aimed at synthesising the corresponding K+, Rb+, and Cs+ complexes failed resulting in formation of [Me4cyclenH][BArF] is investigated using DFT and explicitly correlated calculations, and explained by considering production of [Me4cyclenH]+ by a hydrolysis reaction, involving traces of water, which competes with [M(Me4cyclen)(L)]+ formation. [Me4cyclenH]+ formation dominates for M+ = K+, Rb+, and Cs+ whereas formation of [M(Me4cyclen)(L)]+ is energetically favoured for M+ = Li+ and Na+. The results indicate that the number and type of ligands, play a key role in stabilising the [M(Me4cyclen)]+ complexes and it is hoped that this work will encourage experimentalists to prepare and characterise other [M(Me4cyclen)(L)]+ complexes
A study of the atmospherically relevant reaction between dimethyl sulphide (DMS) and Cl<sub>2</sub> in the absence and presence of water using electronic structure methods
No full textThe thermodynamics and mechanisms of the atmospherically relevant reaction between dimethyl sulphide (DMS) and molecular chlorine (Cl2) were investigated in the absence and presence of a single water molecule, using electronic structure methods. Stationary points on the reaction surfaces were located using density functional theory (DFT) with the M06-2X functional and aug-cc-pVTZ (aVTZ) basis sets. Then single point energy calculations were carried out using the UM06-2X/aVTZ optimised stationary point geometries, with aug-cc-pVnZ basis sets (n = T and Q), using the domain-based local pair natural orbitals coupled cluster [DLPNO-UCCSD(T)] method, to give DLPNO-CCSD(T)/CBS//M06-2X/aVTZ relative energies. The reaction can proceed in three ways depending on the initial van der Waals complex formed i.e. via DMS + Cl2·H2O, DMS·H2O + Cl2, or DMS·Cl2 + H2O. It was found that based on computed equilibrium constants for complex formation and estimated initial concentrations of DMS, Cl2 and H2O in the atmosphere that [DMS·H2O] and [Cl2·H2O] are likely to be much greater than [DMS·Cl2] under atmospheric conditions. It was found that both with and without water the reaction can proceed by two pathways (i) formation of the products CH3SCH2Cl + HCl + (H2O) via a covalently bound intermediate (CH3)2SCl2(H2O) and (ii) formation of the products via a cis-CH3SClCH2:HCl (H2O) intermediate, where (H2O) applies to the with-water case. Although the pathways and mechanisms are similar in the without- and with-water cases, the relative energies of the transition states are significantly lower and the potential energy diagram is much more complex in the with-water case. However, under tropospheric conditions the overall DMS + Cl2 rate coefficient is unlikely to be affected by the presence of water as the concentrations of DMS·H2O and Cl2·H2O are estimated to be much lower than the concentrations of DMS, Cl2 and H2O. This work extends our earlier study of the reaction of DMS with atomic chlorine (Cl) with and without water (L. Rhyman et al., Phys. Chem. Chem. Phys. 2023, 25, 4780-4793).</p
Nickel(II) and copper(II) complexes of allyl 2-(thiophen-2-ylmethylene) hydrazinecarbodithioate: synthesis, X-ray crystal structures, and theoretical study
No full textWe report allyl 2-(thiophen-2-ylmethylene)hydrazine-carbodithioate (HL) and its Ni(II) and Cu(II) complexes, [ML2]. The compounds were fully characterized by elemental analysis, IR, 1H-NMR, UV-Vis, and molar conductivity. The crystal structure analysis indicates that the metal is four-coordinate square planar and that a parallel stacking of the molecular planes is present in the crystals, with stacking distances of 3.642 and 3.676 A for the Ni(II) and Cu(II) complexes, respectively. Gas phase DFT computations indicate that the thione tautomeric form of the free ligand is more stable than the thiol form by 14.52 kJ mol–1. For HL and ML2, comparison between the computed and experimental data shows good agreement
A Study of the Atmospherically Important Reactions between Dimethyl Selenide (DMSe) and Molecular Halogens (X2 = Cl2, Br2, and I2) with ab initio Calculations
No full textThe atmospherically relevant reactions between dimethyl selenide
(DMSe) and the molecular halogens (X2 = Cl2, Br2, and I2) have been studied with
ab initio calculations at the MP2/aug-cc-pVDZ level of theory. Geometry
optimization calculations showed that the reactions proceed from the reagents to
the products (CH3SeCH2X + HX) via three minima, a van der Waals adduct
(DMSe:X2), a covalently bound intermediate (DMSeX2), and a product-like
complex (CH3SeCH2X:HX). The computed potential energy surfaces are used to
predict what molecular species are likely to be observed in spectroscopic
experiments such as gas-phase photoelectron spectroscopy and infrared matrix
isolation spectroscopy. It is concluded that, for the reactions of DMSe with Cl2
and Br2, the covalent intermediate should be seen in spectroscopic experiments,
whereas, in the DMSe + I2 reaction, the van der Waals adduct DMSe:I2 should be
observed. Comparison is made with previous related calculations and experiments
on dimethyl sulfide (DMS) with molecular halogens. The relevance of the results
to atmospheric chemistry is discussed. The DMSeX2 and DMSe:X2 intermediates are likely to be reservoirs of molecular halogens in the atmosphere which will lead on photolysis to ozone depletion
Can cyclen bind alkali metal azides? a DFT study as a precursor to synthesis
No full textCan cyclen (1,4,7,10-tetraazacyclododecane) bind alkali metal azides? This question is addressed by studying the geometric and electronic structures of the alkali metal azide-cyclen [M(cyclen)N3] complexes using density functional theory (DFT). The effects of adding a second cyclen ring to form the sandwich alkali metal azide-cyclen [M(cyclen)2N3] complexes are also investigated. N3? is found to bind to a M+(cyclen) template to give both end-on and side-on structures. In the end-on structures, the terminal nitrogen atom of the azide group (N1) bonds to the metal as well as to a hydrogen atom of the cyclen ring through a hydrogen bond in an end-on configuration to the cyclen ring. In the side-on structures, the N3 unit is bonded (in a side-on configuration to the cyclen ring) to the metal through the terminal nitrogen atom of the azide group (N1), and through the other terminal nitrogen atom (N3) of the azide group by a hydrogen bond to a hydrogen atom of the cyclen ring. For all the alkali metals, the N3-side-on structure is lowest in energy. Addition of a second cyclen unit to [M(cyclen)N3] to form the sandwich compounds [M(cyclen)2N3] causes the bond strength between the metal and the N3 unit to decrease. It is hoped that this computational study will be a precursor to the synthesis and experimental study of these new macrocyclic compounds; structural parameters and infrared spectra were computed, which will assist future experimental work
A study of the atmospherically important reactions of dimethyl sulphide(DMS) with I2 and ICl using matrix isolation spectroscopy and electronic structure calculations
No full textThe reactions of dimethylsulfide (DMS) with molecular iodine (I2) and iodine monochloride (ICl) have been studied by infrared matrix isolation spectroscopy by co-condensation of the reagents in an inert gas matrix. Molecular adducts of DMS + I2 and DMS + ICl have also been prepared using standard synthetic methods. The vapour above each of these adducts trapped in an inert gas matrix gave the same infrared spectrum as that recorded for the corresponding co-condensation reaction. In each case, the infrared spectrum has been interpreted in terms of a van der Waals adduct, DMS : I2 and DMS : ICl, with the aid of infrared spectra computed for their minimum energy structures at the MP2 level. Computed relative energies of minima and transition states on the potential energy surfaces of these reactions were used to understand why they do not proceed further than the reactant complexes DMS : I2 and DMS : ICl. The main findings of this research are compared with results obtained earlier for the DMS + Cl2 and DMS + Br2 reactions, and the atmospheric implications of the conclusions are also considered
A study of the thermodynamics and mechanisms of the atmospherically relevant reaction dimethyl sulphide (DMS) with atomic chlorine (Cl) in the absence and presence of water, using electronic structure methods
No full textThe thermodynamics and mechanisms of the atmospherically relevant reaction dimethyl sulphide (DMS) + atomic chlorine (Cl) were investigated in the absence and presence of a single water molecule, using electronic structure methods. Stationary points on each reaction surface were located using density functional theory (DFT) with the M06-2X functional with aug-cc-pVDZ (aVDZ) and aug-cc-pVTZ (aVTZ) basis sets. Then fixed point calculations were carried out using the UM06-2X/aVTZ optimised stationary point geometries, with aug-cc-pVnZ basis sets (n = T and Q), using the coupled cluster method [CCSD(T)], as well as the domain-based local pair natural orbitals coupled cluster [DLPNO-UCCSD(T)] approach. Four reaction channels are possible, formation of (A) CH3SCH2 + HCl, (B) CH3S + CH3Cl, (C) CH3SCl + CH3, and (C′) CH3S(Cl)CH3. The results show that, in the absence of water, channels A and C′ are the dominant channels. In the presence of water, the calculations show that the reaction mechanisms for A and C formation change significantly. Channel A occurs via submerged TSs and is expected to be rapid. Channel B occurs via TSs which present significant energy barriers indicating that this channel is not significant in the presence of water relative to CH3SCH2 + HCl and DMS·Cl adduct formation, as is the case in the absence of water. Channel C was not considered as it is endothermic in the absence of water. In the presence of water, pathways which proceed via (a) DMS·H2O + Cl, (b) Cl·H2O + DMS and (c) DMS·Cl + H2O were considered. It was found that under tropospheric conditions, reactions via pathway (b) are of minor importance relative to those that proceed via pathways (a) and (c). This study has shown that water changes the mechanisms of the DMS + Cl reactions significantly but the presence of water is not expected to affect the overall reaction rate coefficient under atmospheric conditions as the DMS + Cl reaction has a rate coefficient at room temperature close to the collisional limit.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A study of the atmospherically important reactions between dimethyl selenide (DMSe) and X2(X=Cl,Br and I) with ab initio calculations
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