Structure bridging

This is known as a hydrogen-bridge structure. There are not enough electrons to make all the dotted-line bonds electron-pairs and hence it is an example of an electron-deficient compound. The structure of diborane may be alternatively shown as drawn in... 

Aluminium tetrahydridoborate is a volatile liquid. It is the most volatile aluminium compound known. It is covalent and does not contain ions but has a hydrogen-bridge structure like that of diborane, i.e. each boron atom is attached to the aluminium by two hydrogen bridges ... 

In 1965 he stated, On the other hand, the norbornyl cation does not possess sufficient electrons to provide a pair for all of the bonds required by the proposed bridged structures. One must propose a new bonding concept, not yet established in carbon structures (emphasis added). 

Dieckmann cyclization thiophenes from, 4, 874-875 Thiaannulenes structure, 7, 715 twice-bridged structure, 7, 718 Thiaannulenes, didehydro-structure, 7, 718 Thia[17]annulenes restricted structure, 7, 720... 

The railway lines on bridges are often electrically connected with steel or reinforced steel structures which usually have a very low grounding resistance. In new installations, an electrical separation of the rails from the bridge structure is required according to the grounding resistance of the structure and the type of rail bed. Independent of this, pipelines and metal sheathing of cables are always electrically separated from the structure in order to exclude direct transmission of stray current from the rails in these conductors. 

Figure 16-10 shows a plan of a bridge structure for supplying the Krakatau steel works in Indonesia [20]. The tubular piles are between 25 and 30 m in length. In designing the cathodic protection, a surface area of 4 x 10 m was calculated. The piles... 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

The 2-butyl cation can be observed under stable-ion conditions. The NMR spectrum corresponds to a symmetrical species, which implies either very rapid hydride shift or a symmetrical H-bridged structure. 

A maximum barrier of 2.5kcal/mol can be assigned ftom the NMR data." There have been two extensive MO calculations of the C4H9 species. At the 6-311G /MP4 level of theory, the H-bridged structure was the most stable found and was about 2kcal/mol more... 

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

The stereoselectivity of the radical addition can be explained in terms of a bridged structure similar to that involved in discussion of ionic bromination of alkenes ... 

They have di- -halogen bridged structures and are planar (Figure 3.7). 

Contrary to the simple ab initio methods, semiempirical procedures favour the bridged structure b. The correction of the ab initio results by inclusion of the electron... 

An early process in the cure reaction is protonation of a methylolphenol, followed by loss of a molecule of water to produce a benzylic carbonium ion (see reaction 4.3). This may be followed by reaction with a second phenol to generate a bridged structure, as illustrated in Reaction 4.4. Alternatively the... 

This species is similar to the bromonium ion that is responsible for stereospecific anti addition in the electrophilic mechanism. Further evidence for the existence of such bridged radicals was obtained by addition of Br- to alkenes at 77 K. The ESR spectra of the resulting species were consistent with bridged structures. ... 

Similarly, Cu(II) carboxylates have bridged structures with an uncertain amount of metal-metal bonding, and many Cu(II) complexes, such as I, are prepared by the reaction of the ligand with Cu for investigations of the extent of metal-metal bonding. ... 

Molecular orbital calculations for the parent vinyl cation, Cj H3, were first reported by Hoffmann (161), who used the extended Hiickel method, and more recently by Yonezawa and co-workers (162), who used a semiempirical SCF procedure. Both treated the problem of classical, 172 (R = H), versus bridged structures, 173, but the methods suffered from their inability to account satisfactorily for bond-length changes, and neither discussed the question of linear, 172a, versus bent, 172b, structures. 

The effect of methyl substituents is interesting the classical linear geometry still is preferred over the hydrogen or methyl bridged structure. Furthermore, 3 substitution of a methyl group provides very little or no extra stabilization over the unsubstituted parent system, 172a, R = H. On the other... 

The JV-phosphino-l-azadiene 66 undergoes cycloaddition with DMAD in ether at -20 °C to form the bridged structure 67 <96AG(E)896>. The new compound is thermally unstable and isomerises to the alternative bridged structure 68 at 25 °C. X-ray analysis data have been interpreted to suggest that the isomer 69 co-exists in the solid state. The same ring system is also formed by protonation at the sp carbon atom neighbouring phosphorus in 67 (Scheme 15). 

In contrast to the iminophospheninm cation, 6, described in the previous section, the corresponding phosphaphosphenium (the alternative nomenclature to this species is phosphanediylphosphenium or phosphanetriyiphosphonium), cation 8, shows only a modest intrinsic stability, as suggested by the quantum chemical calculations (see Fig. 1). They predict that a protonated 2 should have a bridged structure [60-66],8a (Scheme 14). 

The investigations indicate that, in agreement with the calculations on the cations, the push-pull substituted diphosphenes tend to form a bridged structure of one ligand the other substituent can easily depart under formation of an ion pair structure. 

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