1,721,026 research outputs found
Electronic Structure and Reactivity of Dioxygen-Platinum Complexes: an ab Initio MO-LCAO Study
No full textAb initio Hartree Fock calculations have been carried out on the complex L2Pt(η2-O2) and its derivatives L2Pt-(OOH)X and L2Pt(OOH)(XY) obtained by reaction with HX (HX = H2O, HCl, HOOCH, NH3) and HXYH (HXYH = oxalic acid, o-catechol, hydrazine, hydroxylamine, ethylendiamine, o-phenylendiamine). In addition also the reaction L2Pt(OOH)X + HX → H2O2 + L2PtX2 leading to the formation of hydrogen peroxide has been studied. The theoretical investigation gave, for all considered species, optimum complex geometry, charge distribution and reaction energies being evaluated by means of a correlation energy density functional. The ability of the HX reagents to form hydrogen peroxide is discussed in terms of absolute acidity and coordinating capability of the X- groups. This allows interpretation of the more peculiar aspects of the chemical activation of dioxygen that leads to the coordinated hydroperoxide species or hydrogen peroxide
RECENT ADVANCES IN COMPUTATIONAL CHEMISTRY - VOL. 1 (RECENT ADVANCES IN DENSITY FUNCTIONAL METHODS) PART III
No full textElectronic and geometrical structures of nitroso ligands coordinated to transition-metal atoms. A nonempirical theoretical study of bis(phosphino)(nitrosotrifluoromethane)platinum and bis(phosphine)(nitrosomethane)platinum
No full textThe modifications of the molecular geometry and electronic structure of CF3NO and CH3NO caused by coordination to a Pt(0) center have been studied by means of nonempirical MO-LCAO theoretical calculations on the model complexes Pt(PH3)2(RNO). The η2 coordination mode of RNO, in which the N-O group acts as a side-on σ-donor and π-acceptor, is found to be more favorable than the η1 mode, in which only the N atom directly interacts with the metal center. The bonding characteristics of the RNO ligands are compared with those of the isoelectronic dioxygen ligand by investigating the electronic structure of the Pt(PH3)2(η2O2) complex
Tailoring Transition Metal Complexes for Nonlinear Optics Applications. 2. A Theoretical Investigation of the Second-Order Nonlinear Optical Properties of M(CO)5L Complexes (M = Cr, W; L = Py, PyCHO, Pyz, PyzBF3, BPE, BPEBF3)
No full textThe authors report an ab initio study of 2nd-order nonlinear optical (NLO) properties and absorption electronic spectra of push-pull transition metal chromophores [M(CO)5L] (M = Cr, W; L = pyridine (Py), 4-formyl-pyridine (PyCHO), pyrazine (Pyz), trans-1,2-bis(4-pyridyl)ethylene (BPE)). Pyz and BPE are considered either with 1 N atom free or interacting with the strong acceptor BF3. All of the mol. properties were calcd. using 2 different and methodol. independent approaches: the time dependent and coupled perturbed d. functional theories (TDDFT and CPDFT) and the sum-over-states (SOS) approach, where the excited states are obtained via the single CI (SCI) ab initio method. DFT results are in acceptable agreement with the exptl. energy values of electronic transitions (with the exception of chromophores with the large greek p-delocalization, like BPE); SCI calcns. overestimate excitation energies and produce an inversion in the order of dM greek pi*L and dM greek p*CO transitions. The SCI-SOS approach gives 1st-order hyperpolarizabilities, basically in agreement as trend and values with the expts. and seems to be a tool generally suitable for the evaluation of these properties also for transition metal complexes. However, the 1st-order hyperpolarizabilities computed using the CPDFT approach are consistently overestimated in comparison with the exptl. results, esp. in the case of a ligand with large greek p-delocalization. Also the 2-level approxn. taking into account only the lowest energy charge transfer excitation (e.g., dM greek p*L) is not applicable to chromophores with the extended greek p-delocalized ligand (BPE) coordinated to a transition metal, due to significant contributions originating from intraligand greek pL greek p*L transitions. This study reports a detailed anal. and comparison of electronic NLO effects of transition metal complexes computed with DFT and ab initio SCI-SOS methodol
The physico-chemical behaviour of tetraazadiene complexes.
No full textThe 1H nmr spectra at variable temperature for the series of tetraazadiene complexes M(NO) (PPh3)n (N4R2) (M = Rh, Ir; n = 1,2), Pt(L) (PPh3)(N4R2) (L = CO, PPh3;R = p-CH3C6H4SO2) have been studied. From the simulated 1H nmr spectra of the iridium complex (n = 1) the activation parameters for an isomerization process which takes place in solution have been calculated. Studies on the chemical reactivity of the tetraazadiene ligand bound to a transition metal have shown that the organic moiety is susceptible to attack only by mineral acids
Tailoring transition metal complexes for non linear optics applications - A theoretical investigation of the electronic structure of M(CO)(x)ClyL complexes (M = Cr, W, Re, Ru, Os, Rh, Ir; L = Pyz, PyzBF(3), BPE, BPEBF3)
No full textDensity functional theory (DFT) investigations of title organometallic compounds led to identification of electronic features which determine the magnitude and the orientation of the dipole moment. The analysis of our theoretical results shows that the presence of a strong acidic BF3 group, or more generally, the substitution of ligands in the M(CO)xCl yL complexes can affect the orientation of the dipole moments, out of the direction of the first charge transfer (CT) excitation and, as a consequence, the magnitude of the vector part of the second order hyperpolarizabilities
An ab initio MO-LCAO investigation of the electronic structure of organic hydroperoxides and platinum(II) hydroperoxo complexes: a contribution to the knowledge of the mechanism of olefin epoxidation
No full textA possible new process of activation of the OOH group in the mechanism of ethylene epoxidation catalysed by Pt(II) diphosphine complexes has been investigated by ab initio MO-LCAO calculations. The electronic and geometric features of YOOH species (Y = H, CH3, t-But, CF3, CH3CO, (PH3)2Pt(CF3), (PH3)2PtCl) have been evaluated and compared. Coordination of the OOH group to platinum induces an inversion of the polarity of the O-O bond when compared to any organic hydroperoxide; parallelly it favours the isomerisation of the OOH group from a hydroperoxo to an oxywater-like structure. This latter effect could be an important factor in favouring the reaction of the platinum coordinated OOH group with ethylene to form ethylene oxide. In fact, calculations on the energetics of the interaction between H2O2 and C2H4 have shown that isomerisation of HOOH to H2OO oxywater structure is the rate determining step for the epoxidation process
Forced unbinding of GPR17 ligands from wild-type and R255I mutant receptor models
No full textBackground:
GPR17 is a “hybrid” GPCR that responds to two unrelated ligand families, extracellular nucleotides and cysteinyl-LTs. Its in vivo blockade reduces progression of cerebral ischemic damage, highlighting GPR17 as a novel therapeutic target for ischemia.
Methods:
To explore the binding mode of the ”purinergic” and “leukotrienic” components of the receptor, we study the binding and the forced unbinding of two GPR17 ligands, the endogenous agonist UDP and the receptor antagonist pranlukast, from the wild-type and a mutant (R255I) models of GPR17, by means of docking and molecular dynamics simulations (MD) techniques .
Results:
MD suggests that GPR17 nucleotides binding pocket is enclosed between the helical bundle and covered by EL2. The driving interaction involves R255 and the phosphate moieties of nucleotides. This hypothesis was also supported by steered MD experiments. These showed that the energy required to unbind UDP was higher for the WT receptor than for R255I.
MD showed three possible binding sites for pranlukast. In one of its preferential docking conformation pranlukast places tetrazole group close to R255 and phenyl rings into a subpocket highly conserved among GPCRs. Pulling forces developed to break polar and aromatic interactions of pranlukast were comparable. No significant differences between wild-type and mutant receptors were found for the unbinding of pranlukast from receptor.
Conclusion:
MD suggest a crucial role for R255 in nucleotides binding for GPR17. Aromatic interactions were likely to play a predominant role in recognition of the leukotrienic ligand pranlukast, suggesting that two different binding sites are present in GPR17
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
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