Effects of the simultaneous presence of nitro and amine substituents in cubane and some azacubanes

We have carried out an ab initio self-consistent-field molecular orbital study of the structures and bond orders (as measures of relative bond strengths) of nitro and aminonitro derivatives of cubane and some azacubanes. The NO₂ group is found to have, in general, a conformation-dependent effect on relative bond strengths. When the plane of the nitro group is perpendicular to that of an endocyclic bond, that bond is weakened. An aza nitrogen adjacent to C-NO₂, however, eliminates the bond-weakening due to a perpendicular nitro group, except when a C-NH₂ is also adjacent to the C-NO₂. The NH₂ group has a direction-specific bond-weakening effect on one [and only one] endocyclic bond; the particular bond undergoing this weakening is determined by the orientation of the amine nitrogen lone pair. The influences of the NO₂ and NH₂ are essentially independent; when these substituents are on adjacent carbons the effects are reinforcing. Thus, the combination of a perpendicular NO₂ and an NH₂ on adjacent carbons in cubane and azacubanes produces a marked bond weakening. Our findings suggest that strained cage systems that have alternating aza nitrogens and C-NO₂ groups should be further investigated as potential high-energy systems.     

Density Functional Tight-Binding Studies of Carbon Nanotube Structures

A density functional tight-binding self-consistent charge approach has been used to study the structures and elastic properties of nine model carbon nanotubes of different helicities and diameters between 5.5 and 10.8 A. The systems contain from 112 to 268 atoms and were optimized under periodic boundary conditions in the axial direction. Both the carbon networks and the overall tube dimensions were optimized. Most of the C—C bond lengths are slightly lengthened relative to graphene (two-dimensional graphite); the others remain essentially the same or are shorter. There is overall a longitudinal compression of the tube. The strain energy per atom, relative to graphene, varies inversely with the square of the tube radius. The Young's moduli decrease with increasing radius but do not depend upon chirality. The Poisson ratios are nearly constant. The consequences of removing an electron from each system were also investigated. In most instances, the tube dimensions were little affected; in only a few cases is there a change in length or radius (positive or negative) as large as 0.10%. The Young's moduli remain the same as for the neutral systems, but the Poisson ratios tend to increase for metals and semimetals and to decrease for semiconductors.     

Properties of atoms in molecules

Atomic charges calculated by the population analysis method for three types of semi-empirical wave functions have been compared with charges obtained by integrating the corresponding electronic density functions over individual atomic regions. It was found that the two sets of charges compare quite well for CNDO wave functions and for extended-Hückel functions which are in terms of orthogonalized basis orbitals. However only the CNDO charges are reasonably close to those obtained by integrating near-Hartree-Fock electronic density functions.     

The use of bond paths for quantitatively characterizing bond strain

The concept of the bond path as a means for characterizing individual strained bonds is developed in detail and applied to a variety of molecules. A reference path is defined in terms of the superposed electronic densities of the free atoms, and a “bond deviation index” is introduced, for expressing quantitatively the difference between the actual and the reference bond paths. This difference is taken as indicating the degree of strain in the bond. It is shown that bond paths can be significantly affected by not only neighboring bonding electrons, but by lone pairs as well, as in the H₂O and NH₃ molecules.     

An investigation of definitions of the charge on an atom in a molecule

A study has been made of three different procedures used to define the charge on an atom in a molecule. It was found that none of the three is completely satisfactory, although that proposed by Löwdin seemed to give the best results. The effect of certain other factors, such as the nature of the basis set and the inclusion of the Madelung potential, was also investigated.

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Zusammenfassung Drei verschiedene Methoden zur Bestimmung der Ladung eines Atoms in einem Molekül werden untersucht. Keine der drei Methoden kann als völlig zufriedenstellend bezeichnet werden. Die besten Ergebnisse scheint die von Löwdin vorgeschlagene Methode zu liefern. Der Einfluß verschiedener Faktoren wie Art der Basis und Berücksichtigung des Madelung-Potentials werden untersucht.

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Résumé Etude de trois procédés différents pour la définition de la charge sur un atome dans une molécule. Aucun des trois procédés n'est complètement satisfaisant, celui proposé par Löwdin semblant donner cependant les meilleurs résultats. L'effet de certains autres facteurs: nature de la base, introduction du potentiel de Madelung a aussi été étudié.

     

Some potential-energy relationships for isoelectronic atomic series

Relationships between the total energy E, the nuclear-electronic attraction energy V_ne, and the interelectronic repulsion energy V^ee are derived for isoelectronic series of atoms and ions. Using these, it is possible to determine V_ne and V_ee for the members of such series from a knowledge of just their total energies. The method is applied to the ground and first excited states of the two-electron series, and the ground states of the three- and four-electron series, and the results are discussed.     

Molecular surface electrostatic potentials in the analysis of non-hydrogen-bonding noncovalent interactions

Electrostatic potentials computed on molecular surfaces are used to analyse some noncovalent interactions that are not in the category of hydrogen bonding, e.g. “halogen bonding”. The systems examined include halogenated methanes, substituted benzenes,s-tetrazine and l,3-bisphenylurea. The data were obtained byab initio SCF calculations.     

Structures and molecular surface electrostatic potentials of high-density C,N, H systems

Tri-s-triazine and two ditetrazolodiazines are known to have unusually high crystal densities (for unsubstituted C, N, H compounds). We have used a nonlocal density functional procedure to compute the geometries and energies of these and three related molecules, and then calculated the ab initio SCF electrostatic potentials on their molecular surfaces. We attribute the high densities to the relatively small molecular volumes and the strong intermolecular attractions arising from highly varying surface potentials. The energy differences of the two ditetrazoles and their diazide tautomers were computed, as well as for the dinitro derivative of one of the former.     

Properties of atoms in molecules

The electronic charges and the positions of the centers of these charges have been calculated for the atoms of a number of second- and third-row heteronuclear diatomic molecules. For both the oxygen and the fluorine atoms, the charge associated with one of these atoms can be correlated, within a series of molecules containing that atom, with both the orbital energy of the atom's 1s electrons and also with the difference in electronegativities of the atoms that comprise the molecule. The centers of electronic charge are outside of the internuclear regions, except for the positive atoms in the more ionic molecules and in HF.