Dipole Moments Of Ligands Research Articles

Designing Janus Ligand Shells on PbS Quantum Dots using Ligand-Ligand Cooperativity.

We present a combined experimental and theoretical study of ligand-ligand cooperativity during X-type carboxylate-to-carboxylate ligand exchange reactions on PbS quantum dot surfaces. We find that the ligand dipole moment (varied through changing the substituents on the benzene ring of cinnamic acid derivatives) impacts the ligand-exchange isotherms; in particular, ligands with large electron withdrawing character result in a sharper transition from an oleate-dominated ligand shell to a cinnamate-dominated ligand shell. We developed a two-dimensional lattice model to simulate the ligand-exchange isotherms that accounts for the difference in ligand binding energy as well as ligand-ligand cooperativity. Our model shows that ligands with larger ligand-ligand coupling energy exhibit sharper isotherms indicating an order-disorder phase transition. Finally, we developed an anisotropic Janus ligand shell by taking advantage of the ligand-ligand cooperative ligand exchanges. We monitored the Janus ligand shell using 19F nuclear magnetic resonance, showing that when the ligand-ligand coupling energy falls within the order region of the phase diagram, Janus ligand shells can be constructed.

Li+-ligand binding energies and the effect of ligand fluorination on the binding energies

The Li+-ligand binding energies are computed for seven ligands and their perfluoro analogs using Density Functional Theory. The bonding is mostly electrostatic in origin. Thus the size of the binding energy tends to correlate with the ligand dipole moment, however, the charge-induced dipole contribution can be sufficiently large to affect the dipole-binding energy correlation. The perfluoro species are significantly less strongly bound than their parents, because the electron withdrawing power of the fluorine reduces the ligand dipole moment.

Energy level modification in lead sulfide quantum dot thin films through ligand exchange.

The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto underutilized means of control over the absolute energy levels of QDs within electronic devices. Here, we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the performance optimization of QD optoelectronic devices.

Experimental and Computational Study of Functionality Impact on Sodalite–Zeolitic Imidazolate Frameworks for CO2Separation

This study deals with the enhancement of CO2 uptake by ligand functionalization of zeolitic imidazolate framework (ZIF) materials. The ligand dipole moment could be considered as one of the main criteria for CO2 adsorption enhancement. To verify this hypothesis, an experimental–computational study was performed on an isoreticular ZIF series with sodalite (SOD) topology using published structures (ZIF-8, ZIF-90, and ZIF-Cl) as well as hypothetical structures (ZIF-COOH and ZIF-NO2) designated using DFT calculations. An analysis of structural and adsorptive properties was proposed for these materials used to separate CO2 from CH4, CO, or N2 gas. The accuracy of the calculated results was validated by comparison with our own experimental results. An exponential relationship between the ligand dipole moments and the isosteric heat of adsorption of CO2 was highlighted. Modifying the nature of the linker (dipole moment) allows a 5- to 7-fold improvement in CO2 selectivity for CO2/CH4, CO2/N2, and CO2/CO mixtures.

Quantitative structure–activity relationship (QSAR) for a series of novel cannabinoid derivatives using descriptors derived from semi-empirical quantum-chemical calculations

Recent work implicating the cannabinoid receptors in a wide range of human pathologies has intensified the need for reliable QSAR models for drug discovery and lead optimization. Predicting the ligand selectivity of the cannabinoid CB 1 and CB 2 receptors in the absence of generally accepted models for their structures requires a ligand-based approach, which makes such studies ideally suited for quantum-chemical treatments. We present a QSAR model for ligand–receptor interactions based on quantum-chemical descriptors (an eQSAR) obtained from PM3 semi-empirical calculations for a series of phenyl-substituted cannabinoids based on a ligand with known in vivo activity against glioma [Duntsch, C.; Divi, M. K.; Jones, T.; Zhou, Q.; Krishnamurthy, M.; Boehm, P.; Wood, G.; Sills, A.; Moore. B. M., II. J. Neuro-Oncol., 2006, 77, 143] and a set of structurally similar adamantyl-substituted cannabinoids. A good model for CB 2 inhibition ( R 2 = 0.78 ) has been developed requiring only four explanatory variables derived from semi-empirical results. The role of the ligand dipole moment is discussed and we propose that the CB 2 binding pocket likely possesses a significant electric field. Describing the affinities with respect to the CB 1 receptor was not possible with the current set of ligands and descriptors, although the attempt highlighted some important points regarding the development of QSAR models.

Modeling Complexes of the Uranyl Ion UO2L2n+: Binding Energies, Geometries, and Bonding Analysis

A systematic study has been carried out with density functional theory of UO2L2n+ (n = 0 or 2) complexes. Thirty three ligands have been considered, enabling a large database of binding energies and geometries to be established that can improve our knowledge and understanding of the bonding between the uranyl ion UO22+ and potential ligands or solvent. A statistical study has been performed using the database with the twin aims of predicting the coordination energies of new complexes and revealing the dominant bonding parameters. We have shown that it is possible to predict the coordination energy satisfactorily just from the properties of the isolated ligand. However, the statistical analysis is shown to be physically unsound. To develop our understanding of the nature of the uranyl−ligand bond, the coordination energy has been decomposed into several contributions: electrostatic, repulsion, ligand polarization, uranyl polarization, charge transfer from the ligand to the uranyl ion, and inverse charge transfer. It is clear that the electrostatic term cannot be modeled just by the ligand's dipole moment; a multipole development extending at least to the quadrupole moment is necessary. Our quantitative analysis also shows that the polarization and charge-transfer terms are important and must be included in any force field if numerically and physically reliable results are sought.

Dipole moments of ligands and stark splitting of levels of rare-earth ions

A model for describing polarization and relaxation of ligand ions around impurity defects in crystals is proposed and tested. This approach is based on the introduction of effective electric dipole moments, which are considered to be fundamental parameters of crystals and are determined from experimental data on the energy-level structure of an impurity ion. Calculations performed for rare-earth ions show that their energy spectra depend strongly on the effective electric dipole moments of host-crystal ions surrounding the impurity ions. Parameters of the theory are found. The results of the calculations are consistent with the available experimental data. The possibility of using the considered approach in studies of impurity defects in various crystals (semiconductors and insulators) is discussed.

Molecular Dynamics Simulation Study of the B-States of Solvated Carbon Monoxymyoglobin

Classical molecular dynamics simulation is employed to study the photodissociated B-states of carbon monoxymyoglobin. Four independent 10 ps dynamical trajectories of the fully solvated protein, in a box of 2988 water molecules using periodic boundary conditions, were simulated at 300 K starting from the equilibrium deoxymyoglobin structure. Two prototypical protein conformational substates, defined by the conformation of the distal histidine, were sampled from a 1.5 ns trajectory of solvated, room temperature deoxymyoglobin. The structure and ligand vibrational spectral line shapes and frequency shifts were derived from the simulation trajectories. The simulated structure of the ligand−heme complex shows that the ligand is localized under the C-ring of the heme, which is consistent with the low-temperature X-ray crystal structures of the photolyzed state [Schlichting et al. Nature 1994, 371, 808. Teng; Šrajer; Moffat Nat. Struct. Biol. 1994, 1, 701]. The ligand is found to be parallel to the heme “plane” and confined to a small region displaced from the heme iron in apparent agreement with the low-temperature X-ray crystal structures and room temperature experimental studies [Lim; Jackson; Anfinrud Science 1995, 269, 962]. Stark shifts in the ligand vibrational frequency, induced by the protein electric field in the heme pocket, are estimated using both atomic point charge models with explicit solvent and finite difference Poisson−Boltzmann calculations. Simulated ligand frequency shifts are compared with those of the experimental spectrum as a test of the accuracy of the simulated electric field in the heme pocket. The magnitudes of the induced ligand frequency shifts are in good agreement with experimental results. The largest contribution to the electric field in the heme pocket is due to the heme and proximal histidines not, as it has often been assumed, the distal histidine. The exact value of the protein electric field experienced by the ligand is well correlated with the conformation of the distal His. The dipole induced in the ligand by the protein electric field is found to make a significant contribution to the total ligand dipole moment and is not necessarily aligned with the ligand bond. The proposal that the ligand “docking site” functions to suppress CO binding is examined.