Information Theory, the Shape Function, and the Hirshfeld Atom

Following the work of Nalewajski and Parr, there has been a surge of interest in the use of information theory to describe chemical bonding and chemical reactions. However, the measure of “information” used by Nalewajski and Parr is not any of the usual conventional entropies, chiefly because the electron density is not normalized to one. The consequences of this are discussed, and a solution is constructed using the shape function and an “entropy of mixing” term. The same revision, however, cannot be made when if the Tsallis entropy, instead of the Shannon form, is used. This serves to emphasize that the Hirshfeld atom is a very specific result, associated only with logarithmic measures of information. A less specific derivation due to Nalewajski provides one resolution to this quandary; this derivation is analyzed in detail.     

Infrared and Mössbauer Spectral Studies on Some Tin(IV) Complexes: A Qualitative Correlitation with the Hard and Soft Acid-Base Theory

The principle of the hard and soft acid-base theory (HSAB) is that binding of hard acids with hard bases and soft acids with soft bases is preferred.¹ This implies that the maximum bond strength in a series of acid-base complexes should occur, all other factors being equal, where the degree of hardness or softness of the acid and base are most nearly matched. To test this hypothesis, we prepared a number of tin(IV) complexes of varying softness with the soft base 2,5-dithiahexane (CH₃SCH₂CH₂OCH₃, DTH) and the hard base 1,2-dimethoxyethane (CH₃OCH₂CH₂OCH₃, DME). The acids employed were SnCl₄, CH₃,SnCl₃, SnBr₄, and SnI₄. The complexes thus obtained were studied by infrared and Mcssbauer techniques in order to determine which, if any, of the complexes exhibited spectral details which could be correlated with HSAB.     

Using the spin-resolved electronic direct correlation function to estimate the correlation energy of the spin-polarized uniform electron gas

We report a simple ansatz that includes spin-polarization in two different models for the exchange-correlation hole. Both models were originally conceived for the non-polarized uniform electron gas. One of them is based on the electronic direct correlation function, a novel approach to build the long-range behavior of the exchange-correlation hole. This ansatz is shown to work quite well considering the limitations of the original models. Interestingly, the hole based on the direct correlation function provides better results in general. This suggests that it is better to model the electronic direct correlation function than it is to model the hole directly.     

The unconstrained local hardness: an intriguing quantity, beset by problems

Developing a mathematical approach to the local hard/soft acid/base principle requires an unambiguous definition for the local hardness. One such quantity, which has aroused significant interest in recent years, is the unconstrained local hardness. Key identities are derived for the unconstrained local hardness, δμ/δρ(r). Several identities are presented which allow one to determine the unconstrained local hardness either explicitly using the hardness kernel and the inverse-linear response function, or implicitly by solving a system of linear equations. One result of this analysis is that the problem of determining the unconstrained local hardness is infinitely ill-conditioned because arbitrarily small changes in electron density can cause enormous changes in the chemical potential. This is manifest in the exponential divergence of the unconstrained local hardness as one moves away from the system. This suggests that one should be very careful when using the unconstrained local hardness for chemical interpretation.     

Should negative electron affinities be used for evaluating the chemical hardness?

Despite recent advances in computing negative electron affinities using density-functional theory, it is an open issue as to whether it is appropriate to use negative electron affinities, instead of zero electron affinity, to compute the chemical hardness of atoms and molecules with metastable anions. We seek to answer this question using the accepted empirical rules linking the chemical hardness to the atomic size and the polarizability; we also propose a new correlation with the C6 London dispersion coefficient. For chemical reactivity in the gas phase, it seems to make no difference whether negative, or zero, electron affinities are used for systems with metastable anions. For reactions in solution the evidence that is presently available is insufficient to establish a preference. In addressing this issue, we noted that electron affinity data from which atomic chemical hardness values are computed are out of date; an update to Pearson's classic 1988 table [Inorg. Chem., 1988, 27, 734-740] is thus provided.     

Variational second order density matrix study of F3-: importance of subspace constraints for size-consistency

Variational second order density matrix theory under "two-positivity" constraints tends to dissociate molecules into unphysical fractionally charged products with too low energies. We aim to construct a qualitatively correct potential energy surface for F(3)(-) by applying subspace energy constraints on mono- and diatomic subspaces of the molecular basis space. Monoatomic subspace constraints do not guarantee correct dissociation: the constraints are thus geometry dependent. Furthermore, the number of subspace constraints needed for correct dissociation does not grow linearly with the number of atoms. The subspace constraints do impose correct chemical properties in the dissociation limit and size-consistency, but the structure of the resulting second order density matrix method does not exactly correspond to a system of noninteracting units.     

Organometallic Polymers. XXVII. Radical-Initiated Polymerization and Copolymerization of π-(2, 4-Hexadien-1-yl Acrylate) tricarbonyliron

The novel monomer, π-(2, 4-hexadiene- l-yl acrylate) tricarbonyliron (HATI), has been prepared by two routes. It was homopolymerized and copolymerized with acrylonitrile, vinyl acetate, styrene, and methyl acrylate in benzene solutions. In all cases azobisisobutyronitrile was the initiator. The relative reactivity ratios, where HATI is defined as M₁, were determined: r₁ = 0.34, r₂ = 0.74, M₂ = acrylonitrile; r₁ = 2.0, r₂ = 0.05, M₂ = 0.74, M₂ = acrylonitrile; r₁ = 2.0, r₂ = 0.05, M₂ = vinyl acetate; r₁ = 0.26, r₂ = 1.81, M₂ = styrene; and r₁ = 0.30, r₂ = 0.74, M₂ = methyl acrylate. The homo-and copolymers had high values of T_g. When polymerizations are carried out at high concentrations, a very high molecular weight tail is observed in HATI hompolymerizations and in HATI-methyl acrylate copolymerizations. The polymers were characterized by IR, gel permeation chromatography, viscosity, and differential scanning calorimetry studies. Finally, thermal decompositions carried out in air resulted in decomposition of the Fe(CO)₃ group, producing Fe₂O₃ as a fine powder. Thermal decomposition under nitrogen (in solution and on solids ground into KBr pellets) resulted in slow destruction of the Fe(CO)₃ groups but the resulting polymer mass was insoluble, and the question of what form the iron exists in (Fe metal, oxides, carbides, etc.) has not been answered.     

Drug release by pH‐responsive molecular tweezers: Atomistic details from molecular modeling

pH‐responsive molecular tweezers have been proposed as an approach for targeting drug‐delivery to tumors, which tend to have a lower pH than normal cells. We performed a computational study of a pH‐responsive molecular tweezer using ab initio quantum chemistry in the gas‐phase and molecular dynamics (MD) simulations in solution. The binding free energy in solution was calculated using steered MD. We observe, in atomistic detail, the pH‐induced conformational switch of the tweezer and the resulting release of the drug molecule. Even when the tweezer opens, the drug molecule remains near a hydrophobic arm of the molecular tweezer. Drug release cannot occur, it seems, unless the tweezer is in a hydrophobic environment with low pH. © 2014 Wiley Periodicals, Inc.