Electronegativity, Hardness and Atomic number: Mutual relationships explored via novel isoelectronic series methodology

The novel isoelectronic methodology proposed recently, reveals electronic configuration dependent relationship between the hardness and atomic number, and electronegativity and hardness.  In eight out of the first ten isoelectronic series, the hardness measure (I-A)/2 is an excellent linear function of atomic number (Z), and the electronegativity measure, (I+A)/2 is a quadratic function of the hardness measure; Hence, it is inferred that hardness (η) is proportional to atomic number (η α Z) and electronegativity (χ) is proportional to square of hardness (χ α η2). In both the cases, the two inert gas series species are the exceptions where η α Z2 and χ α η, respectively. These relationships are slightly in discord with the previously reported mutual connections. Previous reports do not indicate electronic configuration dependency as well. The linear (I-A)/2 versus Z plots arises as a result of cancellation of Z2 terms, and the slopes of these plots are sensitive indicator of the electron spin pairing and orbital change. A potential use of (I-A)/2 versus Z, and (I+A)/2 versus (I-A)/2 plots in pointing the incorrect ionization potentials and their evaluation has been elaborated by the striking example of the sixth ionization potential of phosphorous. The (I-A)/2 versus Z relations also provides us a new way to obtain hardness values of cationic and anionic atomic species

A unified molecular-wide and electron density based concept of chemical bonding

Chemical bonding is at heart but, not being a quantum mechanical-defined physical property of a system, is a subject of endless and often fruitless debates. Having so many and very different models of chemical bonding without knowing what this really is does not make it easier. There is, however, a general agreement that concentrating electron density (ED) in and delocalizing ED to internuclear region is always associated with minimizing system's energy and synonymous with chemical bonding. Fragment, atomic, localized, delocalized, and interatomic (FALDI)-based density analysis involves entire space occupied by a molecule. From this molecular-wide and density-based methodology, it is possible to quantify localized and delocalized by all atoms ED at any coordinate r, including critical points on Bader's molecular graphs. Each atom and atom-pair contributions of delocalized density are quantified to reveal major players in the all-atom and molecular-wide chemical bonding. Partitioning the total ED to individual molecular or natural orbital's contributions using MO-ED and MO-DI methods, in conjunction with one dimensional (1D) cross section methodology, generates an orbital-based molecular-wide picture. This provides, besides reproducing results from FALDI, qualitative description of orbitals' nature that correlates well with classical understanding of bonding, nonbonding, and antibonding orbitals. A qualitative and quantitative impact of an immediate, distant, or molecular-wide molecular environment on intra- and intermolecular di-atomic, intra- and interfragment interactions is the domain of the Fragment Attributed Molecular System Energy Change (FAMSEC) family of methods. The FALDI, FAMSEC, MO-ED, MO-DI, and 1D cross section methodologies provide consistent and quantifiable physics-based picture of molecular-wide chemical bonding without invoking unicorns, such as a chemical bond.http://wires.wiley.com/compmolsciChemistr

Gold(I)···Lanthanide(III) Bonds in Discrete Heterobimetallic Compounds: A Combined Computational and Topological Study : Inorganic Chemistry

The chemical nature of the ligand-unsupported gold(I)–lanthanide(III) bond in the proposed [LnIII(η5-Cp)2][AuIPh2] (Ln–Au; LnIII = LaIII, EuIII, or LuIII; Cp = cyclopentadienide; Ph = phenyl) models is examined from a theoretical viewpoint. The covalent bond-like Au–Ln distances (Au–La, 2.95 Å; Au–Eu, 2.85 Å; Au–Lu, 2.78 Å) result from a strong interaction between the oppositely charged fragments (ΔEintMP2 > 600 kJ mol–1), including the aforementioned metal–metal bond and additional LnIII–Cipso and C–H···π interactions. The Au–Ln bond has been characterized as a chemical bond rather than a strong metallophilic interaction with the aid of energy decomposition analysis, interaction region indicator, and quantum theory of atoms in molecules topological tools. The chemical nature of the Au–Ln bond cannot be fully ascribed to a covalent or an ionic model; an intermediate situation or a charge shift bond is proposed. The [AuIPh2]− anion has also been identified as a suitable lanthanide(III) emission sensitizer for La–Au and Lu–Au.Peer reviewe

Chemical Concepts in the Era of Computational Chemistry

Present work in the philosophy of chemistry has overlooked a foundational debate among chemists about the proper function of chemical concepts. The debate is fueled by a desire to connect computational models with traditional chemical concepts, and has divided chemists since the origins of quantum chemistry. By analyzing the history of the concepts of electronegativity and the atom in the molecule, I show that there are two camps with conflicting priorities. Theorists who favor rigor seek concepts that neatly summarize important elements of the underlying physical models. Theorists who favor understanding seek concepts that achieve a balance between simplicity and qualitative accuracy. The development of concepts for understanding is shown to involve the use of multiple quantification schemes in order to achieve consistency with other concepts. This practice might appear shortsighted if not for the diverse functionality of the resulting concepts. These concepts can i) help discover new reactions and structures, ii) allow comparison of different models in computational chemistry, and iii) guide chemists to develop more accurate and more interpretable computational models. Finally, it is shown that these conflicting modes of conceptual development have implications for the nature of chemical concepts. Chemists on each side of the debate adopt different positions, explicitly or tacitly, on reduction, pluralism, and the ontology of chemical concepts. Philosophers of chemistry who neglect this debate cannot responsibly interpret chemists’ statements on these issues

Unraveling reaction mechanisms by means of Quantum Chemical Topology Analysis

A chemical reaction can be understood in terms of geometrical changes of the molecular structures and reordering of the electronic densities involved in the process; therefore, identifying structural and electronic density changes taking place along the reaction coordinate renders valuable information on reaction mechanism. Understanding the atomic rearrangements that occur during chemical reactions is of great importance, and this perspective aims to highlight the major developments in quantum chemical topology analysis, based on the combination of electron localization function and catastrophe theory as useful tools in elucidating the bonding and reactivity patterns of molecules. It reveals all the expected, but still ambiguous, elements of electronic structure extensively used by chemists. The chemical bonds determine chemical reactivity, and this technique offers the possibility of their visualization, allowing chemists to understand how atoms bond, how and where bonds are broken/formed along a given reaction pathway at a most fundamental level, and so, better following and understanding the changes in the bond pattern. Their results clearly herald a new era, in which the atomic imaging of chemical bonds will constitute a new method for examining chemical structures and reaction mechanisms. The important feature of this procedure is that in practice the scope of its values is system-independent. In addition, from a practical point of view, it is cheap to calculate and implement because wave functions are the required input, which are easily available from standard calculations. To capture these results two reaction mechanisms: isomerization of C(BH)2 carbene and the thermal cycloheptatriene-norcaradiene isomerizations have been selected, indicating both the generality and utility of this type of analysis

Relation Between Ring Currents and Hydrogenation Enthalpies for Assessing the Degree of Aromaticity

Magnetically induced ring-current strength susceptibilities and nucleus independent chemical shifts (NICS) have been studied for 15 single-ring aromatic, antiaromatic, and nonaromatic molecules. The current densities have been calculated at the density functional theory (DFT), Hartree-Fock (HF) theory, and second-order Moller-Plesset perturbation theory (MP2) levels using the gauge-including magnetically induced current method (GIMIC). The ring-current strength susceptibilities have been obtained by numerical integration of the current density flowing around the molecular ring. The calculated ring-current strength susceptibilities are almost independent of the level of theory. The relative degree of aromaticity deduced from the magnetic properties has been compared with the ones deduced from hydrogenation enthalpies that are considered to be proportional to aromatic stabilization energies (ASE). For the studied single-ring molecules, GIMIC, NICS, and ASE calculations yield similar trends. The study shows that there is a linear correlation between the magnetic and energetic criteria of aromaticity. The largest uncertainty originates from the accuracy of the energy data, because they are much more dependent on the employed computational level than the calculated magnetic properties. Thus, ring-current strength susceptibilities can be used for assessing the degree of aromaticity.Peer reviewe

Information-theoretic approaches to atoms-in-molecules : Hirshfeld family of partitioning schemes

Many population analysis methods are based on the precept that molecules should be built from fragments (typically atoms) that maximally resemble the isolated fragment. The resulting molecular building blocks are intuitive (because they maximally resemble well-understood systems) and transferable (because if two molecular fragments both resemble an isolated fragment, they necessarily resemble each other). Information theory is one way to measure the deviation between molecular fragments and their isolated counterparts, and it is a way that lends itself to interpretation. For example, one can analyze the relative importance of electron transfer and polarization of the fragments. We present key features, advantages, and disadvantages of the information-theoretic approach. We also codify existing information-theoretic partitioning methods in a way, that clarifies the enormous freedom one has within the information-theoretic ansatz

2007 Undergraduate Research Symposium Abstract Book

Abstract book from the 2007 UMM Undergraduate Research Symposium (URS) which celebrates student scholarly achievement and creative activities