page_title: William Lipscomb
section_title: Nuclear magnetic resonance and the chemical shift
hierarchical_section_title: William Lipscomb / Scientific studies / Nuclear magnetic resonance and the chemical shift
caption_attribution_description: English: Nuclear Magnetic Resonance NMR spectrum and structure of hexaborane B6H10. NMR Interpretation of hexaborane 12.8 MHz. range (right) A simple example of the chemical shift is the "a" and "b" doublets (pairs of peaks). "A" and "b" are two groups of boron atoms in different electronic environments, which place the "a" doublet to the right of the "b" doublet in the spectrum. The relative peak heights of the "a" and "b" doublets suggest a 1:5 ratio of borons in one environment to borons in another environment. This suggests, combined with the molecular weight of the molecule: 5 boron atoms in one environment ("b") and 1 boron atom in another environment ("a"), which in turn suggests a pyramidal structure, which does appear in the atomic diagram. 40 MHz. range (left) "a" suggests a hydrogen at the apex, which is in the atomic diagram sticking straight up. "b" suggests hydrogens bonded to one boron each, which are in the atomic diagram sticking straight out from the edges. "c" suggests other bridge hydrogens, which is to say hydrogens in the middle of a boron-hydrogen-boron bonding arrangement like a hydrogen bridge between two boron shores, which are in a ring around the atomic diagram. (Williams1959) History X-ray diffraction was needed to be sure of the structure.(Hirshfeld1958) More complex molecules are more challenging, although in some cases common subgroups of atoms produce characteristic spectral patterns, which can be recognized. References (Williams1959) Wiliams, R.E., Gibbins, S.G., and Shapiro, I., J. Chem. Phys, 30, 333 (1959). (Hirshfeld1958) Hirshfeld, F. L., Eriks, K., Dickerson, R. E., Lippert, E. L., and Lipscomb, W. N., "Molecular and Crystal Structure of B6H10,” J. Chem. Phys. 28, 56 (1958).
context_page_description: William Nunn Lipscomb Jr. was a Nobel Prize-winning American inorganic and organic chemist working in nuclear magnetic resonance, theoretical chemistry, boron chemistry, and biochemistry.
context_section_description: In this area Lipscomb proposed that:
  "... progress in structure determination, for new polyborane species and for substituted boranes and carboranes, would be greatly accelerated if the [boron-11] nuclear magnetic resonance spectra, rather than X-ray diffraction, could be used."
  This goal was partially achieved, although X-ray diffraction is still necessary to determine many such atomic structures. The diagram at right shows a typical nuclear magnetic resonance (NMR) spectrum of a borane molecule.
  Lipscomb investigated, "... the carboranes, C₂B₁₀H₁₂, and the sites of electrophilic attack on these compounds using nuclear magnetic resonance (NMR) spectroscopy. This work led to [Lipscomb's publication of a comprehensive] theory of chemical shifts. The calculations provided the first accurate values for the constants that describe the behavior of several types of molecules in magnetic or electric fields."
  Much of this work is summarized in a book by Gareth Eaton and William Lipscomb, NMR Studies of Boron Hydrides and Related Compounds, one of Lipscomb's two books.
caption: NMR spectrum of hexaborane B6H10 showing the interpretation of a spectrum to deduce the molecular structure. (click to read details)
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