B atom

Wlien 2 g > (Eaa BB binary alloy corresponds to an Ismg ferromagnet (J> 0) and the system splits into two phases one rich in A and the other rich in component B below the critical temperature T. On the other hand, when 2s g < (Eaa+ bb > system corresponds to an antiferromagnet the ordered phase below the critical temperature has A and B atoms occupying alternate sites. 

Source Compiled from Parson, M. L. Major, S. Forster, A. R. Appl. Spectrosc. 1983,37, 411-418 Weltz, B. Atomic Absorption Spectrometry, VCH Deerfield Beach, FL, 1985. 

Welz, B. Atomic Absorption Spectroscopy. VGH Deerfield Beach, FL, 1985. 

It has been estimated that using available neutron intensities such as 10 neutrons/(cm -s) concentrations of B from 10—30 lg/g of tumor with a tumor cell to normal cell selectivity of at least five are necessary for BNCT to be practical. Hence the challenge of BNCT ties in the development of practical means for the selective deUvery of approximately 10 B atoms to each tumor cell for effective therapy using short neutron irradiation times. Derivatives of B-enriched /oj o-borane anions and carboranes appear to be especially suitable for BNCT because of their high concentration of B and favorable hydrolytic stabiUties under physiological conditions. 

A/-Trimethoxybora2ines are available from reaction of dichloroboranes and 0-methyl-X,X-his(trimethylsilyl)hydroxylamine (eq. 31). The B-trichloro-bora2iQes undergo substitution reactions at the B atoms to give B-tri(/ f -butoxy)- or B-tri(/ f2 -but5i)-A/-trimethoxybora2iaes (101)... 

If hvQ is small compared with kT, the partition function becomes kT/hvQ. The function kT jh which pre-multiplies the collision number in the uansition state theoty of the bimolecular collision reaction can therefore be described as resulting from vibration of frequency vq along the transition bond between the A and B atoms, and measures the time between each potential n ansition from reactants to product which will only occur provided that die activation energy, AEq is available. 

Solutions normally tend to be random so that one cannot predict which of the sites will be occupied by which atoms (Fig. 2.2c). But if A atoms prefer to have A neighbours, or B atoms prefer B neighbours, the solution can cluster (Fig. 2.2d) and when A atoms prefer B neighbours the solution can order (Fig. 2.2e). 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

The structure requires 160 valence electrons per unit cell computed as follows internal bonding within the 4 icosahedra (4 x 26 = 104) external bonds for the 4 icosahedra (4x12 = 48) bonds shared by the atoms in 2(b) positions (2x4 = 8). However, 50 B atoms have only 150 valence electrons and even with the maximum possible excess of boron in the unit cell (0.75 B) this rises to only 152 electrons. The required extra 8 or 10 electrons are now supplied by 2C or 2N though the detailed description of the bonding is more intricate than this simple numerology implies. 

The thermodynamically most stable polymorph of boron is the /3-rhombohedral modification which has a much more complex structure with 105 B atoms in the unit cell (no 1014.5 pm, a 65.28°). The basic unit can be thought of as a central Bn icosahedron surrounded by an icosahedron of icosahedra this can be visualized as 12 of the B7 units in Fig. 6.1b arranged so that the apex atoms form the central Bn surrounded by 12 radially disposed pentagonal dishes to give the Bg4 unit shown in Fig. 6.3a. The 12 half-icosahedra are then completed by means of 2 complicated Bjo subunits per unit cell,... 

The structures of metal-rich borides can be systematized by the schematic arrangements shown in Fig. 6.6, which illustrates the increasing tendency of B atoms to catenate as their concentration in the boride phase increases the B atoms are often at the centres of trigonal prisms of metal atoms (Fig. 6.7) and the various stoichiometries are accommodated as follows ... 

Figure 6.7 Idealized boron environment in metal-rich borides (see text) (a) isolated B atoms in M3B and M7B3 ...

It will be noted from Fig. 6.6 that structures with isolated B atoms can have widely differing interatomic B - B distances, but all other classes involve appreciable bonding between B atoms, and the B - B distances remain almost invariant despite the extensive variation in the size of the metal atoms. 

The structures of boron-rich borides (e.g. MB4, MBfi, MBio, MB12, MBe6) are even more effectively dominated by inter-B bonding, and the structures comprise three-dimensional networks of B atoms and clusters in which the metal atoms occupy specific voids or otherwise vacant sites. The structures are often exceedingly complicated (for the reasons given in Section 6.2.2) for example, the cubic unit cell of YB e has ao 2344 pm and contains 1584 B and 24 Y atoms the basic structural unit is the 13-icosahedron unit of 156 B atoms found in -rhombohedral B (p. 142) there are 8 such units (1248 B) in the unit cell and the remaining 336 B atoms are statistically distributed in channels formed by the packing of the 13-icosahedron units. 

In this last reaction it is the coordinated B atom at position 9 that is solvolytically cleaved from the cluster. 

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