Boron molecule

It must be emphasized that the formulation of three-center T-bonds provides a qualitative (not merely incremental) improvement in the accuracy of the natural Lewis-structure description of diborane. Because a three-center orbital is intrinsically more mathematically flexible than a two-center orbital, the description of any molecule is seemingly improved by employing three-center in place of two-center NBOs. However, for most non-boron molecules this improvement would be quite negligible (e.g., less than 0.1% for ethane, whose two-center Lewis-structure... 

Some speculation is inevitable in proposing any photochemical mechanism and some liberty has been taken in this regard in the present discussion. Areas of boron photochemistry are indicated where proposed mechanisms are controversial and where further experimental work is necessary. For example, very little is known about triplet states of boron molecules. Some of these questions may be answered in the future by non-photochemical techniques... 

An analogous description of each of the higher boranes can be produced by selecting not the traditional pyramids favored by ACS, but rather those solid geometry models which heuristically have a concept of "regularity" and, when that is not attainable, of "semi-regularity". For example, exactly as the tetra-boron molecule minimized Coulomb repulsion... 

Organic boron molecules with multidentate Lewis acidic sites such as (2) and (3) bind strongly with fluoride ion so as to form a pyramidalized chelate (4) (Scheme 3.32). The incorporation of the anion into a medium-sized cyclic structure releases ring strain via the pyramidalization of planar boron centers. It is interesting to see the binding energies of... 

Carborane plastics are produced from the boron molecule (a boron hydride of composition B10FI14) by replacing the four terminal hydrogens with two carbon atoms. The monomer polymerizes at high heat and pressure to form rubbery solids, which will withstand temperatures above 315°C but the commercial plastics are usually copolymers with vinyls, sihcones, or other plastics. Carboranes have been used as specialty adhesives in the aerospace industry. 

Boric acid is a weak acid used in eye washes and to clean glass. Boron is also a natural insecticide, since many insects caimot metabolize the boron molecule. Boric acid when mixed with equal parts white flour and powdered sugar makes a tempting insect bait for cockroaches. It causes internal blockage and death, while being safe for animals and children. 

Boron is Is lp. The electronic configuration of B2 depends on the relative positioning of the Experimental measurements indicate that the boron molecule has ttvo unpaired electrons in the level. Thus the electronic configuration of B2 is (cTs ) (cr ) (7r/)(tr/), giving one net tr bond. The bond length of B2 is 1.59 A. The bond energy of B2 is 69 kcal/mole. 

In the last decade, boron carrier development has taken two directions small boron molecules and boron-conjugated biological vehicles. Unlike approaches using pharmaceuticals, boron carriers... 

The use of liposomes as boron delivery vehicles for BNCT has been extensively studied, since liposomes can transport large amounts of boronated molecules, both hydrophobic and hydrophilic in nature, selectively into tumor cells. Three methods for incorporation of boron into liposomes have been reported incorporation of hydrophobic molecules into lipid bilayers, entrapment of hydrophilic molecules into the liposomes aqueous core, and the use of boronated lipids. Recently, PEG-based liposomes (DPPC/cholesterol/DSPC-PEG2000) have been used as effective carriers of boronated compounds within tumor tissues. Intraveuously injected B-PEG-liposomes on human pancreatic carcinoma xenografts in nude mice inhibited tumor growth after thermal neutron irradiation. The cationic liposome formulation (DOTAP/DOPC/DOPE) was used to deliver ortho-carboranyl P-lactoside (LCOB) aud l-methyl-ort/j< -carboranyl-2-hexylthioporphyrazine (H2PZ-COB) within DHD/K12/TRb (DHD) rat colon carcinoma and B16-F10 murine melanoma cell lines. Results from these studies show that a thirty-fold increase on the cellular uptake of B was obtained for the boronated compounds in the liposome formulation as compared with that of the BPA-fructose complex. 

Write an orbital wave function without antisymmetrization or normalization for the diatomic boron molecule in its ground level. 

Boron achieves a covalency of three by sharing its three outer electrons, for example BFj (p. 153). By accepting an electron pair from a donor molecule or ion, boron can achieve a noble gas configuration whilst increasing its covalency to four, for example H3N->BCl3. K BF4. This is the maximum for boron and the second quantum level is now complete these 4-coordinate species are tetrahedral (p. 38). 

In this case the covalency of boron is brought up to four because the donor molecule supplies the necessary electrons. The adduct formed, trimethylamine-borane, is a stable white solid. Other compounds of a similar kind are known, all derived from the simple structure H3N -> BH3. This compound is isoelectronic with ethane, i.e. it contains the same number of electrons and has the same shape. 

These molecules exist in the solid halides, explaining the low melting points of these halides, and also in the vapour phase at temperatures not too far above the boiling point. At higher temperatures, however, dissociation into trigonal planar monomers, analogous to the boron halides, occurs. 

The monomers are electron pair acceptors, and donor molecules are often able to split the dimeric halide molecules to form adducts thus, whilst the dimeric halides persist in solvents such as benzene, donor solvents such as pyridine and ether appear to contain monomers since adduct formation occurs. Aluminium halides, with the one exception of the fluoride, resemble the corresponding boron halides in that they are readily hydrolysed by water. 

Both boron and aluminium chlorides can be prepared by the direct combination of the elements. Boron trichloride can also be prepared by passing chlorine gas over a strongly heated mixture of boron trioxide and carbon. Like boron trifluoride, this is a covalent compound and a gas at ordinary temperature and pressure (boiling point 285 K). It reacts vigorously with water, the mechanism probably involving initial co-ordination of a water molecule (p, 152). and hydrochloric acid is obtained ... 

Boron nitride is chemically unreactive, and can be melted at 3000 K by heating under pressure. It is a covalent compound, but the lack of volatility is due to the formation of giant molecules as in graphite or diamond (p. 163). The bond B—N is isoelectronic with C—C. 

The covalently bonded oxygen atom still has two lone pairs of electrons and can act as an electron pair donor. It rarely donates both pairs (to achieve 4-coordination) and usually only one donor bond is formed. A water molecule, for example, can donate to a proton, forming H30, and diethyl ether can donate to an acceptor such as boron trifluoride ... 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Only reaction 1 provides a direct pathway to this chiral molecule the intermediate 2-methyl-butanal may be silylated and reacted with formaldehyde in the presence of the boronated tartaric ester described on page 61. The enantiomeric excess may, however, be low. 

Boron trifluoride is a trigonal planar molecule There are six electrons two for each B—F bond associated with the valence shell of boron These three bonded pairs are farthest apart when they are coplanar with F—B—F bond angles of 120°... 

Step 1 A molecule of borane (BH3) attacks the alkene Electrons flow from the 7C orbital of the alkene to the 2p orbital of boron A 7C complex is formed... 

The names of addition compounds are formed by connecting the names of individual compounds by a dash (—) and indicating the numbers of molecules in the name by Arabic numerals separated by the solidus (diagonal slash). All molecules are cited in order of increasing number those having the same number are cited in alphabetic order. However, boron compounds and water are always cited last and in that order. 

Figure 8.16 shows the B l spectmm of the B5FI9 molecule. The boron atoms are situated at the comers of a square pyramid. There are four B-FI-B bridging hydrogen atoms and... 

One of the first applications of this technique was to the enrichment of and "B isotopes, present as 18.7 and 81.3 per cent, respectively, in natural abundance. Boron trichloride, BCI3, dissociates when irradiated with a pulsed CO2 laser in the 3g vibrational band at 958 cm (vj is an e vibration of the planar, D j, molecule). One of the products of dissociation was detected by reaction with O2 to form BO which then produced chemiluminescence (emission of radiation as a result of energy gained by chemical reaction) in the visible region due to A U — fluorescence. Irradiation in the 3g band of BCls or "BCI3 resulted in °BO or BO chemiluminescence. The fluorescence of °BO is easily resolved from that of "BO. 

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