Benjamin M. Gimarc

STRAIN AND RESONANCE ENERGIES IN MAIN-GROUP HOMOATOMIC RINGS AND CLUSTERS

Abstract A summary of recent progress in the estimation of strian and resonance energy parameters that apply to structures composed of representative elements other than carbon, including silicon, nitrogen, phosphorus, oxygen, sulfur and, to a limited extent, arsenic.

Molecular and electronic structures of planar inorganic rings

Over the last 30 years, many inorganic compounds have been prepared which have molecular and electronic structures that are related to those of well-known aromatic hydrocarbons. These planar inorganic rings are com osed of the main-group elements B, N, 0, S, and others, but no carbon at all. !n this paper, the structures of inorganic aromatic rin s will be reviewed Relative stabilities and structures will be rationalized and the possible complexes these rings might form with transition metals will be discussed. and their properties will be compared to those of aromatic a ydrocarbons.

On the aromatic stabilities of thiophene analogues of helicenes

Abstract The conjugated circuits model is used to predict the relative aromatic stabilities of a variety of thia-helicenes, i.e., thiophene analogues of helicenes. All studied thia-helicenes are predicted to be aromatic structures. The parent helicenes are always found to be more aromatic than the corresponding daughter thia-helicenes. The theoretical predictions are supported by modest experimental evidence.

Details of the reaction graphs for intramolecular isomerizations of the carboranes

From proposed mechanisms for framework reorganizations of the carboranes C2Bn-2Hn,n = 5–12, we present reaction graphs in which points or vertices represent individual carborane isomers, while edges or arcs correspond to the various intramolecular rearrangement processes that carry the pair of carbon heteroatoms to different positions within the same polyhedral form. Because they contain both loops and multiple edges, these graphs are actually pseudographs. Loops and multiple edges have chemical significance in several cases. Enantiomeric pairs occur among carborane isomers and among the transition state structures on pathways linking the isomers. For a carborane polyhedral structure withn vertices, each graph hasn(n -1)/2 graph edges. The degree of each graph vertex and the sum of degrees of all graph vertices are independent of the details of the isomerization mechanism. The degree of each vertex is equal to twice the number of rotationally equivalent forms of the corresponding isomer. The total of all vertex degrees is just twice the number of edges orn(n - 1). The degree of each graph vertex is related to the symmetry point group of the structure of the corresponding isomer. Enantiomeric isomer pairs are usually connected in the graph by a single edge and never by more than two edges.

Strain Energies of the Cyclophosphanes

Abstract Strain energies of (PH)n rings, n = 3–8, are obtained from geometry optimized ab initio total energies of (PH)n rings and H(PH)mH chains. These quantities are then compared with strain energies of On, Sn, (NH)n, (SiH2)n, and (CH2)n rings.

Correlations between bond order and bond length in the electron-deficient closo-boranes

Abstract In the closo-boranes, the BB bond distances vary widely, from 1.580 to 1.990 A. These large variations are consistent with the notion of highly delocalized skeletal bonding in electron-deficient polyhedral clusters. In this paper, we show correlations between BB bond distances and four different calculated bond order indices: (1) Wades fractional bond orders; (2) Coulsons bond orders from three-dimensional Huckel MO theory; (3) Mulliken bond populations from extended Huckel MO theory; and (4) Mulliken bond populations from ab initio SCF MO calculations.

Three-dimensional Hückel molecular orbital energy level correlation diagrams for polyhedral rearrangements

Abstract A three-dimensional Huckel method recently developed for cluster compounds has been used to investigate polyhedral rearrangements of organic molecules as well as main-group inorganic clusters. The method starts from information about atomic connectivity and number of cluster electrons, familiar and convenient concepts for chemists. Calculations lead to diagrams that show how molecular orbital (MO) energy levels and second moment scaled total energies correlate between different structural forms. We consider diamond-square-diamond (DSD) framework reorganization mechanisms for pseudo-rotations of the closo-boranes. For B 5 H 5 2− and B 9 H 9 2− , the single DSD mechanism is not allowed by the principle of conservation of orbital symmetry. DSD rearrangements are allowed for the other closo-boranes we studied here, with increasing activation energies, B 8 H 8 2− ∼ B 11 H 11 2− 7 H 7 2− ∼ B 10 H 10 2− 6 H 6 2− , depending on the number of square faces opened during rearrangement. MO correlation diagrams and corresponding total energy curves for various numbers of cluster electrons provide visual rationalizations of observed structural trends with different numbers of cluster electrons. In particular, we discuss framework reorganizations for square to tetrahedron, hexagon to trigonal prism, hexagon to octahedron and cube to square antiprism.

Complexes Involving Inorganic Aromatic Rings

Abstract Inorganic aromatic rings can form complexes with transition metal atoms through sigma bonds, pi bonds, or both. Known and hypothetical examples of such complexes are reviewed and a qualitative molecular orbital model is used to describe and compare chemical bonding between transition metal atoms and the inorganic rings. Analogous organic complexes serve as reference structures.