PBE1PBE/6-311+G(d,p) computations exploring the microsolvation of neutral and zwitterionic glycine are reported. A broad configuration search was performed to identify the lowest energy clusters of glycine with one to seven water molecules. The structures of the clusters are analyzed on the basis of the hydrogen-bonding network established between the water molecules and between water and glycine. Neutral glycine is favored when associated with zero to six water molecules, but with seven water molecules the two structures are isoenergetic.
Accurate and effkient integration of the electron density function over arbitrary regions has been previously achieved by exploiting a separation of variables. Recently, a computer program has been written that calculates p, P p , and V2p in an expeditious fashion, taking advantage of the separation of variables in the electron density function. Accurate integrations of V2p over arbitrary regions can also be accomplished. The structure of the program is suited especially to vector processors. As a result of the efficiencies of these programs, functions of the electron density, such as the density itself, the surrounding electrostatic potential, V p , and V2p have been calculated in three dimensions. Results of calculations for nitrated cubanes are presented illustrating how the effects of the nitro groups are manifested in the electron density and associated properties.The electron distribution of a molecule and its change during chemical reaction are fundamental to understanding molecular structure, chemical reactivity, and molecular interactions. Indeed, the language of chemistry is heavily couched in terms that, either implicitly or explicitly, infer something of the electron density. So, for example, terms such as bonds, lone-pairs, electron-rich, electron-poor, electron-donating, electronwithdrawing, conjugation, polarization, and charge-transfer, to name a few, bring to mind pictures that presume some features of the electron distribution in a molecule. It is the purpose of our investigations to compare these simple and useful pictures with electron distributions obtained from a b initio molecular orbital calculations. In this fashion, a firm theoretical basis for some of the fundmental concepts of chemistry can be established and more precise criteria developed for classifying and, ultimately, predicting chemical phenomenology. Additionally, we hope to develop efficient computational techniques for analyzing electron distrubitions and calculating molecular interactions.Many approaches have been devised for analyzing electron distributions. Bader has developed a topological model that divides the *To whom all correspondence should be addressed. molecule into atomic regions, each of which obeys the same quantum mechanics as the whole molecule.' In the sense that each of the atomic regions is quantum mechanically meaningful and that chemical bonds, rings, and cages can be precisely defined without reference to arbitrary standards, the topological model is superior to other approaches for characterizing electron density distributions. Underlying this approach are the vector v p and the scalar quantity V'p.The vector quantity v p is primarily used for determining atoms in molecules. The surfaces enclosing atoms in molecules are those surfaces through which the flux of v p is zero. Determination of these zero flux surfaces defines a volume of electron density that "belongs" to the enclosed atom. Properties of atoms in molecules such as: atomic charge and higher multipoles, atomic kinetic, potential, and total en...
Nanohoops are macrocycles formed of aromatic rings linked in a 1,4' fashion. Cycloparaphenylenes 1 and nitrogen analogues formed from the building blocks pyridinyl (2), pyrazinyl (3), pyridazinyl (4), and pyrimidinyl (5) are examined at B3LYP/6-31G(d). The nanohoops contain 3-24 repeat units. The strain energy of the nanohoops exponentially decreases with the number of building blocks n, and this strain strongly correlates with the bend angle at the ipso carbons. Nitrogen substitution reduces the o,o' steric interactions between neighboring rings. Nanohoops 3 and 5 have ribbon-like structure with dihedral angles between neighboring rings near zero. Nanohoops 5 are the least strained and, with their ribbon structure, are suggested as synthetic targets for possible interesting bulk properties and structures.
An algorithm for choosing adequate reference systems for ring strain energy by making a modification to the standard homodesmic reaction scheme.
Nucelophilic substitution at selenium is examined using the B3LYP and MP2 methods. Various nucleophiles (HS -, CH 3 S -, HSe -, and CH 3 Se -) and substrates (R 1 SSeR 2 and R 1 SeSeR 2 with R 1 and R 2 ) H or Me) are used to model substitution at selenium in diselenides and selenosulfides. In all cases, the mechanism is additionelimination. A stable hypercoordinate selenium intermediate lies in a well that is 8-14 kcal mol -1 deep. Nucleophilic attack at selenium is both kinetically and thermodynamically more favorable than at sulfur.
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