Carboranes computations

Ab initio computations, particularly at MP2 and (the computationally less intensive) B3LYP levels of theory, have been largely responsible for the rapid progress in carborane structural research, permitting the determination of the molecular geometry of a novel carborane from solution-state NMR data where X-ray diffraction could not be used (such as those in trace amounts or for which suitable crystals could not be obtained). Earlier... 

Ab initio computations have so far proved to be reliable structural tools for neutral carboranes and their anions. They have been used to predict the carborane geometries for six-vertex, eight-vertex, and 11-vertex nido-carboranes,67-69 and for 5-12 vertex r/aro-carboranes.70,71 However, carborane anions in the presence of a group I metal cation may form ion-pair anions with one cation tightly bound to the carborane anion (see Chapter 3.03).72,73 Even computations on a salt with a carborane anion in a non-coordinating solvent would need to include the cation for computed shifts that tally with observed NMR data.74... 

Two carborane cations, zV/ -2,3,4,5,6-McsC5BBr+ and the tentatively proposed arachno-N ci WX I71, were reported. The identity and pyramidal structure of the pentacarbaborane cation was determined by X-ray crystallography.112 The small arachno-carborane was detected on the basis of ab initio computations and 11B NMR data in the mixture from reaction of the 2-propyl cation with diborane.113 Two neutral adducts based on the pentacarbaborane cation, ///V/9-2,3,4,5,6-McsCsBBKCI2 (R = Cl or SiClj), were made from MesCsSiMe3 or (MesCs Si with B2CI4, respectively.114 The BCI3 adduct was structurally characterized by X-ray crystallography. 

Building on and extending earlier studies, Schleyer, Najafian, et al. [57] employed computed geometric, energetic, and magnetic properties to quantify the aromaticity of the closo boranes B H 2 (6 < n < 12), and their isoelectronic counterparts, the CB iH and C2B 2H carboranes, and the NB H azaboranes [34]. All possible heteroatom placements were considered. The most stable iso-... 

As discussed in the introduction (Section 3.2.1), derivatives of the diboracyclo-propane 1C are non-classical organoboranes having 8 SE, and according to the 2n+2 SE rule may be classified as the simplest doso-carboranes of the series CH(BH) H ( = 2). Compounds IB and 1C have been computed [5] to be 17.5 and 47.6 kcal mol-1 lower in energy, respectively, than the classical diboracyclopropane 1A. They are 2e aromatics and possess planar-tetracoordinate centers in IB this unusual geometry is found at the carbon and one boron atom, in 1C at both boron atoms (Scheme 3.2-2). 

Several schools of thought then arose. Members of one school treated each structure as a separate case, more or less succumbing to the thesis of an almost infinite variety of structural parameters. A second school took the view that although carborane structures were complex in nature, such structures could eventually be categorized or collated by improved theoretical treatments which could be expected to become ever more accurate because of constantly improving computer systems. A third school considered the complexity of carborane structures to be not as severe as had appeared at first view, arguing that when a sufficient number of structures would eventually be determined, the fundamental structural precepts would become decipherable and, consequently, amenable to a simplistic empirical organizational format. 

Table XIV contains data for computationally derived second-order hyperpolarizabilities, principally by Marder, Ratner, and their co-workers, and more recently by Humphrey et al. The focus of these studies has been similar to that of experimental investigations, namely ferrocenyl, metal carbonyl, and metal acetylide complexes, and very recently carboranes. Calculated values were all obtained using ZINDO, the methodology for which is described in Section II,E.

Contemporary Boron Chemistry contains 80 reports in nine chapters. Clearly, since much research is interdisciplinary in nature, our decision to include a report in one particular section rather than another was sometimes an arbitrary one. With this caveat in mind, the sections are as follows Applications to Polyolefin Catalysis Materials and Polymers Medicinal Applications Cluster Synthesis Carboranes Metallaboranes Metallaheteroboranes Organic and Inorganic Chemistry of Mono- and Di-boron Systems and Theoretical and Computational Studies. 

In contrast to the limitations noted above in the value of localized bond treatments for closo carboranes, MO treatments and computations based thereon provide really useful insight into their bonding. An early illustration of their value for closo deltahedral borane dianions B 11 was provided in 1955 by... 

In 1977 we reported a method based on graph theory for study of the skeletal bonding topology in polyhedral boranes, carboranes, and metal clusters Q). Subsequent work has shown this method to be very effective In relating electron count to cluster shape for diverse metal clusters using a minimum of computation. Discrete metal clusters treated effectively by this method Include post-transition metal clusters (, ) > osmium carbonyl clusters (O, gold clusters, platinum carbonyl clusters (J., 7 ) > and... 

The interest in high-temperature elastomeric materials stems from the high demand for such materials for application in advanced technologies, particularly the aerospace, defense, and computer industries. Such materials are expected to have long-term thermal, thermo-oxidative and hydrolytic stability at and above 300 to 350 °C and to also have the ability to maintain pronounced flexibility to well below ambient temperature. The elastomeric crossUnked network polymers of carboran-ylenesiloxanes and silarylene-siloxanes described in this chapter possess similar properties and are therefore ideal candidates for a wide variety of engineering applications under unusual service conditions. In addition, these network polymers also have the utility in other applications that may or may not directly relate to their... 

Chiral bromohydrin derivatives reacted under acidic conditions with very high stereoselectivity (essentially stereospecific). This points to a mechanism involving a chiral bromonium ion as the reactive intermediate. The protonation of chloroethane by the carborane superacid H(CHBuClii) proceeds via a shared-proton intermediate that decays by HCl loss to form the carbocation-like ethyl carborane. This reacts with a second EtCl to form the Et2Cl+ cation. The transannular electrophilic bromination of a polycyclic system with two C=C in close proximity was studied by computational methods. The initial bromonium was found to rearrange into more stable carbocations through reaction with the nearby carbon—carbon double bond. 

Buhl M, Gauss J, Hofmann M, Schleyer PVR (1993) Decisive electron correlation effects on computed B and C NMR chemical shifts. Application of the G1AO-MP2 method to boranes and carboranes. J Am Chem Soc 115 12385-12390... 

C. Gaussian ab Initio Computations on Deltahedral Boranes and Carboranes... 

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