Isotope shift boron

A number of other models were considered and tested (for example, direct B—H bonding). The most significant test was the IR vibrational spectrum, where a sharp absorption band at 1875 cm-1 was found, corresponding to the Si—H stretch mode softened by the proximity of the B-atom. Had the hydrogen been bonded to boron, a sharp absorption band at 2560 cm-1 would have been expected. Also, Johnson (1985) showed that deuteration produced the expected isotopic shift. The most definitive and elegant proof of the correctness of the Si-H-B bonding model was provided by Watkins and coworkers (1990), on the basis of a parametric vibrational interaction between the isotopes D and 10B. 

The spectra of a number of triatomic radicals with 15 valence electrons are now known for elements of the first period they are B02, N3, and NCO. In addition, the isoelectronic ions C02+ and N20+ have been investigated. The spectrum of B02 was observed by Johns41 in the flash photolysis of a mixture of BCI3 and 02, but later was found to be identical to the green bands observed in most flames and arcs containing boron.42 Isotope shifts (B1X-B10) and alternate missing lines in the spectrum immediately identify the carrier as... 

Isotopes in interstellar gas With the aid of the Hubble Space Telescope it has been possible for the first time to measure the boron isotopic ratio within diffuse clouds of the Milky Way. The interstellar ultraviolet radiation renders B ionized (B+) in the diffuse clouds therefore its spectrum is similar to that of the element Be, but at shorter wavelengths. The strongest resonance line lies in the ultraviolet, visible to Hubble spectrometers. The smaller mass of the 10B isotope shifts its line by 0.013 A (about 0.001%) toward longer wavelengths. Very detailed analysis of the line pair has shown in several clouds that today s interstellar abundance ratio is UB/ 10B = 3.4 0.7, which is consistent with the solar ratio 4.05. For the first time one can conclude that the solar ratio is not an abnormal one, but is shared by interstellar gas at a value larger than the ratio 2.5 that is produced by cosmic-ray collisions in the interstellar gas. Another source of11B is needed. 

In astrophysics much interest has in recent years been focussed on boron. Although its cosmic abundance is extremely low, it plays an important role in testing models of Big Bang Nucleosynthesis [10]. Optical spectroscopy is the only method for establishing B abundances in stellar objects, and thus a good knowledge of energy structure, transition probabilities, hyperfine structure and isotope shifts is needed [11]. 

Isotopic shifts ( B/ B) in the IR and Raman spectra of BPO4 enabled distinctions to be made between vB-0 and vP-O modes, based on a model of a PO4 tetrahedron surrounded by four boron atoms. Characteristic BOs " modes were seen in the IR spectrum of Tm -doped Na3La2(B03)3. The IR spectrum of [B2Si206]oo can be interpreted in terms of Si206 units connected via trigonal planar-coordinated boron atoms (vBO 1310 cm", vSiO 1070 cm" and 5Si-0-B 670 cm" ). ... 

Raman spectra of natural and isotope-enriched boron phosphide single-crystal wafers indicate one strong band at 828.6 cm and 846.2 cm, respectively (Fig. 10) (55). The former is in good agreement with 829 cm" for BP powder (56) and a strong infrared absorption band at 12.1 4 The frequency shift can be described by the change in mass of the isotopes ... 

The problem was solved quantitatively by the decomposition of the IR optical stretching mode of the three-atom chain by model calculations taking the possible compositions and the frequency shift depending on the mass distribution in natural and isotope-enriched boron carbide into account (57). The determined concentrations of B12 and BnC icosahedra and C—B—C and C—B—B chains are shown in Fig. 26. Other chain compositions can be excluded. Toward the boron-rich limit of the homogeneity range, an increasing number of unit cells without chains arise. Two alternative models are compatible with the optical spectra completely chain-free unit cells and unit cells in which single B atoms saturate the outer bonds of the equatorial atoms of the adjacent icosahedra. Theoretical calculations of reaction kinetics based on the second version (132) satisfactorily confirm the results in Fig. 26. 

Isotopic substitution may help us to obtain rotation constants. It also allows us to define atom positions within a molecule precisely (the effect of change of mass is small if the substituted atom is close to the center of mass, and zero if it is at the center of mass, for example, the rotation constants of BCI3 are unaffected by the boron isotopic mass), and may confirm the presence of a particular type of atom in the molecnle. This latter point is of particular value in studying unstable species where elemental analysis is impracticable. For example, in order to prove that a species under investigation is an oxide we may observe a shift in the position of rotational lines when 0 replaces 0. 

Tickling experiments on and B in BH4 and BF4 have been reported. (372) They show that the primary isotope effect upon the boron chemical shift is only 0-11 0-03 ppm, which is negligible for most practical purposes. 

The multiple-quantum (MQ)/MAS NMR is one of the 2D NMR methods, which is capable of averaging out the second-order quadrupolar interaction in nuclei with spin > 1/2 such as H, "B, O, etc. The "B MQ/ MAS NMR measurements on boron as contained in silyl-carborane hybrid Si-based polymer networks considered here. The molded samples are cut into small pieces to insert them into a 4-mm NMR rotor and spun at 12 kHz in a MAS probe. The observation frequency of the "B nucleus (spin number I = 3/2 and isotope natural abundance = 80.42%) is 96.3 MHz. Excitation of both the echo (—3Q) and anti echo (+3Q) coherences is achieved by using a three-pulse sequence with a zero quantum filter (z-filter). The widths of the first, second, and third pulses are 3.0 4.1 ps, 1.1-1.6 ps, and 19-28 ps, respectively. The z-filter is 20 ps. The recycle delay time is 6-15 s and the data point of FI (vertical) axis is 64 and for each the number of scans is 144. Then, the total measurement time is 15-38 h. The phase cycling used in this experiment consists of 12 phases. Boron phosphate (BPO4 3 = 0 ppm) is used as an external standard for "B. The chemical shift value of BPO4 is —3.60 ppm from BF3 O(C2H5)2 which is used as a standard reference in " B NMR in the liquid state. The transmitter frequency of " B is set on peak of BPO4 for a trustworthy chemical shift after Fourier transform." " ... 

Boron compounds, inducting boronic acid derivatives, can be conveniently analyzed by NMR spectroscopy [352]. Of the two isotopes, B is the most abundant (80%) and possesses properties that are more attractive towards NMR. These attributes include its lower resonance frequency, spin state (3/2) and its quadrupole moment, a wide range of chemical shifts, and its higher magnetic receptivity (16% of H). When analyzing boronic adds in non-hydroxylic solvents by NMR spectroscopy, it is often necessary to add a small amount of deuterated water (e.g. one or two drops) to the sam-... 

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