Interpretation of Light-Quenching Factor Measurements

We observe that the pattern of the quenching factors for scintillation light from various ions, recently studied in $CaWO_4$ in connection with dark matter detectors, can be understood as a saturation phenomenon in which the light output is simply proportional to track length, independent of the ion and its energy. This observation is in accord with the high dE/dx limit of Birks' law. It suggests a simple model for the intrinsic resolution of light detectors for low energy ions, which we briefly discuss.Comment: Seven pages, seven figures, some with colo

What Is Special about Aromatic-Aromatic Interactions? Significant Attraction at Large Horizontal Displacement

High-level ab initio calculations show that the most stable stacking for benzene-cyclohexane is 17% stronger than that for benzene-benzene. However, as these systems are displaced horizontally the benzene-benzene attraction retains its strength. At a displacement of 5.0 Å, the benzene-benzene attraction is still ∼70% of its maximum strength, while benzene-cyclohexane attraction has fallen to ∼40% of its maximum strength. Alternatively, the radius of attraction (>2.0 kcal/mol) for benzene-benzene is 250% larger than that for benzene-cyclohexane. Thus, at relatively large distances aromatic rings can recognize each other, a phenomenon that helps explain their importance in protein folding and supramolecular structures

Supplementary data for article: Malenov, D. P.; Ninkovic, D. B.; Zaric, S. D. Stacking of Metal Chelates with Benzene: Can Dispersion-Corrected DFT Be Used to Calculate Organic-Inorganic Stacking? ChemPhysChem 2015, 16 (4), 761–768. https://doi.org/10.1002/cphc.201402589

Supporting information for: [https://doi.org/10.1002/cphc.201402589]Related to published version: [http://cherry.chem.bg.ac.rs/handle/123456789/1675

Stacking Interactions at Large Horizontal Displacements—Comparison of Various Ring Types

Noncovalent interactions at large distances play important role in molecular recognition processes, early steps of protein folding or design of supramolecular structures. Plateaus or even shallow minima can occur at potential energy curves of stacking interactions between certain ring types. Stacking interactions at large horizontal displacements are also frequently found in crystal structures of various rings. In this work we discuss how the horizontal displacement affects stacking interactions on the basis of interaction energy calculations and energy decomposition analysis performed by Symmetry-Adapted Perturbation Theory (SAPT). We compared aromatic/aromatic to aromatic/aliphatic stacking as well as stacking interactions involving resonance-assisted hydrogen bridged (RAHB) rings, including RAHB/RAHB and RAHB/aromatic interactions. Among RAHB rings we observed different behavior of polar and nonpolar RAHB molecules. A comparison of aromatic/aromatic and aromatic/aliphatic systems provides an insight into fundamental differences in the nature between these two interaction types, while comparing stacking involving RAHB rings we can observe difference between polar and nonpolar RAHB rings

Can the Benzene-Benzene and Water-Water Interactions be Similar?

Benzene and water are quite different by nature, benzene molecule does not have a dipole moment, while water molecule does. Considering these properties of water and benzene molecules, one can expect very different benzene/benzene and water/water interactions. We have analyzed the benzene/benzene and water/water interactions found in crystal structures from CSD and we found that both benzene/benzene and water/water can form antiparallel interactions. Data from crystal structures in CSD shows that most benzene/benzene interactions are stacking interactions with large horizontal displacements, not the geometries that are minima on benzene/benzene potential surface. In these antiparallel interactions, the dipole moment of the C-H bond plays an important role. Also, in water/water interactions, there are a significant number of antiparallel interactions. Antiparallel interactions account for 20% of all attractive water/water contacts in the CSD. These antiparallel interactions result from the interaction of two O-H bonds in which dipoles are in antiparallel orientation. This shows that although these two molecules are very different, they can have similar interactions concerning the local dipole moment. The deciding factor for these two important interactions is antiparallel dipole moments of the O-H and C-H bond

Modification of electrostatic potentials of organometallic compounds as a tool in a design of new class of high energetic materials

Design of new classes of high energetic materials (HEM) with lower sensitivity towards detonation is the ultimate goal of numerous experimental and theoretical studies.[1] One of the most important properties that define the impact sensitivity of HEM molecules is the value of molecular electrostatic potential (MEP) above the central regions of molecular surface. Positive values of MEP are strongly related to high sensitivity of HEM molecules towards detonation.[2] In our previous work, we showed that it is possible to modify MEP of chelate complexes by careful selection of ligands and metal atoms.[3] In this work, we calculated MEPs for series of metallocene molecules and analysed results in the context of their possible detonation properties. Calculations performed at B3LYP/def2TZVP level showed that negative values of MEP above the center of the cyclopentadienyl ligand of ferrocene (-16.55 kcal/mol) were changed to positive values (7.11 kcal/mol) upon the addition of NO2 substituent to cyclopentadienyl ligand. Results of DFT calculations also showed that changing of transition metal atom in metallocene molecule could be used for fine-tuning of electrostatic potential values above the central region of cyclopentadienyl ligands

Benzene and water – different or similar?

Considering the properties of water and benzene molecules, one can expect very different benzene/benzene and water/water interactions. Benzene does not have a dipole moment, while water does. Analysis of the data in the crystal structures in the Cambridge Structural Database (CSD) revealed the most frequent benzene/benzene and water/water geometries. The majority of the benzene/benzene interactions in the crystal structures in the CSD are stacking interactions with large horizontal displacements, and not geometries that are minima on benzene/benzene potential surface. A large number of the water/water contacts in the CSD are hydrogen bonds, 70% of all attractive water/water interactions. In addition, water/water contacts with two water forming antiparallel interactions are 20% of all attractive water/water contacts. In these contacts, the O-H bonds of water molecules are in antiparallel orientation. In benzene/benzene interactions at large horizontal displacements, two C-H bonds also are in the antiparallel orientation. This shows that although the two molecules are different, both of them form antiparallel interactions with a local O-H and C-H dipole moments

Antiparallel Noncovalent Interactions

In spite of being quite different substances, benzene and water can form similar noncovalent interactions. Analysis of the data in the crystal structures in the Cambridge Structural Database (CSD) revealed similarities in benzene/benzene and water/water interactions, since both benzene/benzene and water/water can form antiparallel interactions. The quantum chemical calculations of potential surface of water/water interactions showed that the minimum is hydrogen bond. Analysis of the data in the crystal structures in the Cambridge Structural Database (CSD) revealed antiparallel water/water interactions, in addition to classical hydrogen bonds (1). The geometries of all water/water contacts in the CSD were analyzed and for all contacts interaction energies were calculated at accurate CCSD(T)/CBS level. The results showed that the most frequent water/water contacts are hydrogen bonds; hydrogen bonds are 70% of all attractive water/water interactions. In addition, water/water contacts with antiparallel interactions are 20% of all attractive water/water contacts. In these contacts O-H bonds of water molecules are in antiparallel orientation (Figure). The quantum chemical calculations of potential surface of benzene/benzene interactions showed two minima stacking (parallel displaced) geometry and T-shaped geometry. Analysis of all benzene/benzene contacts in the crystal structures in the CSD revealed the most frequent benzene/benzene geometries (2). Majority of the benzene/benzene interactions in the CSD are stacking interactions with large horizontal displacements, and not geometries that are minima on benzene/benzene potential surface. In benzene/benzene interactions at large horizontal displacements two C-H bonds are in the antiparallel orientation (Figure). In these O-H and C-H antiparallel interactions two dipoles are in antiparallel orientation enabling close contact of positive and negative regions of the dipoles. Symmetry Adapted Perturbation Theory (SAPT) analysis showed that electrostatic is the largest attractive force in the antiparallel interactions. Antiparallel interactions are also possible between O-H and C-H bonds; in the crystal structures from the CSD these interactions are observed as one of the types of water benzene interactions (3)

Role of non-covalent interactions in modification of properties of high energetic materials

У овом раду смо испитивали утицај нековалентних интеракција на електростатичке потенцијале и осетљивост ка детонацији одабраних високоенергетских молекула. Резултати прорачуна рађених на M06/cc-PVDZ нивоу су показали да водоничне везе значајно утичу на вредности електростатичког потенцијала и осетљивост ка детонацији високоенергетских молекула. У случајевима када високоенергетски молекул игра улогу акцептора водоника, вредности електростатичког потенцијала изнад центара високоенергетских молекула се смањују за 20-25%. Ово даје могућност за коришћење водоничног везивања за модификовање осетљивости високоенергетских молекула

Role of hydrogen bonding in modifications of impact sensitivities of high energetic materials: evidence from crystal structures and quantum chemical calculations

The development of new classes of high energetic materials (HEM) with high efficiency and low impact sensitivity is in the focus of numerous experimental and theoretical studies [1]. However, the high efficiency of HEM molecules is usually related to the high sensitivity towards detonation [2]. It is known that the sensitivity of HEM molecules towards detonation depends on many factors, including oxygen balance, energy content, and positive values of electrostatic potential above the central regions of the molecular surface. Analysis of positive values of molecular electrostatic potentials (MEP) showed to be an excellent tool in the assessment of impact sensitivities of high energetic molecules since positive values of MEP above the central regions of molecules are associated with high sensitivity towards detonation of HEM molecules [2]. Here we analysed the influence of hydrogen bonding on the values of the electrostatic potentials of fragments of HEM molecules extracted from crystal structures [3]. Crystal structures of three selected high energetic molecules were extracted from Cambridge Structural Database (CSD) and analysed in terms of non-covalent interactions. Three well-known HEM molecules were selected for the analysis: 1,3,5-Trinitrobenzene (TNB), 2,4,6-Trinitrophenol (TNP), and 2,4,6-Trinitrotoluene (TNT). Geometries of these molecules were used for electrostatic potentials calculations and for the design of model systems for interaction energies calculations. Electrostatic potential maps were calculated for TNB, TNP, and TNT geometries extracted from crystal structures for free molecules and molecules involved in hydrogen bonding. Values of electrostatic potentials above the central regions of molecules were analysed and compared for non-bonded HEM molecules and HEM molecules involved in hydrogen bonding. Analysis of crystal structures showed that selected HEM molecules are involved in three types of hydrogen bonds: O-H…O-N interactions, C-H…O-H interactions, and in the case of TNP molecule O-H…O-H interactions. Analysis of positive values of the electrostatic potentials showed that hydrogen bonds have a significant influence on the values of the electrostatic potential in the central regions of HEM molecules. Calculations performed at M06/cc-PVDZ level showed that in the case when HEM molecules are involved in hydrogen bonding as hydrogen atom donors, positive values of electrostatic potentials in the centres of molecules decreased by 20 – 25%. In the case when HEM molecules were involved in hydrogen bonding as hydrogen atom acceptors, positive values of electrostatic potentials in the centres of HEM molecules increased by 10%. Results presented in this study show that hydrogen bonds could be used as a tool for the modification of positive values of MEP above the central regions of HEM molecules and for the modification of their sensitivities towards detonation. Moderate change of positive electrostatic potential values above the central regions of HEM molecules upon formation of hydrogen bonds provide an opportunity for fine-tuning of sensitivities of HEM molecules towards detonation