Boron isotope effect

Fig. 17. Field cooled magnetization measured for increasing temperature at hqH = 2.5 mT on single crystalline YNi2B2C with the two isotopes l0B (solid lines) and 11B (dotted lines), clearly indicating a boron isotope effect...

A remarkable boron isotope effect has been observed for YM2B2C as well as LUM2B2C supporting the classification of these materials as phonon-mediated superconductors (Lawrie and Franck, 1995 Cheon et al., 1999 see Figure 15). The BCS theory predicts for the isotope effect Tc M " where M is the mass of the atoms which substantially participate in the lattice vibrations being relevant for... 

Decomposition of H3B,PF3 by irradiation with a CO2 laser (10.6 pm band) is due to vibrational excitation, and not to net rise in temperature. No boron isotope effect was detected. The rate-determining step seems to be the reaction ... 

Even a boron isotope effect has been determined (B vs B ) for the cleavage of some boronic acids by mercuric chloride (Matte-son et al., 1964). The boron-carbon bonds cleaved with isotope effects of 2-3%. A noteworthy point is the unusual method used to determine the isotope ratios. The method took advantage of the widely differing cross sections of B " and B to neutron bombard-... 

Both sets of results may also be discussed in terms of inductive differences between hydrogen and deuterium (see Halevi, 1963). Brown et al. (1966) jDoint out that both the inductive and steric explanations qualitatively predict isotope effects in the same direction, but that an inductive effect would be expected to operate from the 3 and 4 positions nearly as effectively as from the 2 position . Furthermore, there is no observable isotope effect on the heat of reaction of 2,6-(dimethyl-de)-pyridine with the relatively small molecule diborane A AH = —20 18 cal mol ), but a significant effect is obtained with the larger molecule boron trifluoride AAH = 230 + 150 cal mol ). 

Mulliken, R. S. The isotope effect in band spectra, Part I. Phys. Rev. 25, 119-138 (1925a). Mulliken, R. S. The isotope effect in band spectra, II the spectrum of boron monoxide. Phys. Rev. 25, 259-296 (1925b). 

O Neil JR (1986) Theoretical and experimental aspects of isotopic fractionation. Rev Mineral 16 1-40 Oi T (2000) Calculations of reduced partition function ratios of monomeric and dimeric boric acids and borates by the ab initio molecular orbital theory. J Nuclear Sci Tech 37 166-172 Oi T, Nomura M, Musashi M, Ossaka T, Okamoto M, Kakihana H (1989) Boron isotopic composition of some boron minerals. Geochim Cosmochim Acta 53 3189-3195 Oi T, Yanase S (2001) Calculations of reduced partition function ratios of hydrated monoborate anion by the ab initio molecular orbital theory. J Nuclear Sci Tech 38 429-432 Paneth P (2003) Chlorine kinetic isotope effects on enzymatic dehalogenations. Accounts Chem Res 36 120-126... 

Nakane ei al. (1964) established equilibrium constants of boron isotope exchange between boron trifluoride gas on one side and boron trifluoride methyl fluoride, methyl chloride, isopropyl chloride and t-butyl chloride. The value of the equilibrium constants, which represents the thermodynamic isotope effect, was related to the polarity, stability and catalytic activity of the complexes. 

Spero HJ, Bijma J, Lea DW, Bemis BE (1997) Effect of seawater carbonate concentration on foraminiferal carbon and oxygen isotopes. Nature 390 497-500 Spivack AJ, Edmond JM (1986) Determination of boron isotope ratios by thermal ionization mass spectrometry of the dicesium metaborate cation. Anal Chem 58 31-35 Spivack AJ, Edmond JM (1987) Boron isotope exchange between seawater and the oceanic crust. Geochim Cosmochim Acta 51 1033-1043... 

Zachos J, Pagani M, Sloan L, Thomas E, BiUups K (2001) Trends, rhythms and aberrations in global climate 65 Ma to present. Science 292 686-693 Zeebe RE (1999) An explanation of the effect of seawater carbonate concentration on foraminiferal oxygen isotopes. Geochim Cosmochim Acta 63 2001-2007 Zeebe RE (2005) Stable boron isotope fractionation between dissolved B(OH)3 and BlOH). Geochim Cosmochim Acta 69 2753-2766... 

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. 

T Taturally occurring boron contains 19.8% boron-10 and 80.2% boron-11. The absorption cross section of the natural product for thermal neutrons is 752 barns for pure boron-10 and boron-11, the corresponding values (8) are 3837 and 0.005 barns, respectively. Thus, isotopically pure boron-10 is five times more effective as a neutron shield than natural boron. In view of this difference, it is not surprising that a demand arose, very early in the nuclear era, for separated boron isotopes. 

This Waterloo was unfortunate for the author considers the boron method to be basically sound and potentially extremely powerful. The answer to the benthic enigma may lie in species-to-species differences in the boron isotope vital effect for benthic foraminifera. The measurement method use by Sanyal et al. (1996) required a large number of benthic shells in order to get enough boron to analyze. This created a problem because, as benthics are rare among foraminifera shells, mixed benthics rather than a single species were analyzed. If the boron isotope pH proxy is to... 

The ratio of the boron isotopes and °B is known to depend on ambient pH in the boron incorporated in the carbonate shells of marine foraminifera (Sanyal et ai, 1996 Sanyal et al., 2001). The use of boron isotopes as an indicator for seawater paleo-pH has been extended to the calculation of past CO2 concentrations in ocean surface waters and in the atmosphere (Spivack, 1993 Sanyal et al., 1997 Pearson and Palmer, 1999, 2000 Sanyal and Bijma, 1999 Palmer and Pearson, 2003). Sources of uncertainty in these estimates include the fractionation of boron isotopes during incorporation in carbonate shells, effects of diagenesis, the assumptions needed to calculate CO2 concentrations from pH, and the influence of changing boron isotope ratios in ambient seawater (Lemarchand et al., 2000 Lemarchand et al., 2002). The latter problem is especially serious for estimates based on samples older than the 15 Myr residence time of boron in the oceans. 

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