In the Classroom
A Short History of Three Chemical Shifts Shin-ichi Nagaoka Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan;
[email protected] Levine (1) gave a short history of chemical shifts in nuclear magnetic resonance (NMR) in an article in this Journal. However, the term chemical shift is used not only in NMR but also in electron spectroscopy for chemical analysis (ESCA) and Mössbauer spectroscopy (2), which have proven to be useful for chemical studies. Accordingly, a brief review of the history of the three chemical shifts is worthwhile. As mentioned by Levine (1), the phenomenon that came to be called a chemical shift in NMR was first observed in 1950 by two research groups at almost the same time (3, 4). Dickinson (3) reported the dependence of the 19F NMR peak position on chemical compounds, and Proctor and Yu (4) reported the same of the 14N peak position. Each of the groups knew about the other group’s research beforehand through a third party (see the footnotes within refs 3 and 4 ). The term chemical shift soon appeared for the first time in an article on 19F NMR by Gutowsky and Hoffman (5) as part of the title. Lessinger (6) reported in this Journal that the 1H chemical shift of a phenolic hydroxyl group depends on both the kind of solvent and concentration of the solution, which is a good example of the environment affecting the chemical shift in NMR. It should be noted that before the first observation of the chemical shift, the Knight shift in the NMR frequencies of metals was reported in 1949 (7). The energy of an atomic core level in a molecule depends on the chemical environment around the atom, and a shift in the core-level energy due to a specific chemical environment is called a chemical shift in ESCA. For example, the thickness of an oxide layer on aluminum can be determined by using the Al:2p chemical shift (8). According to Siegbahn’s Nobel lecture (9), the first evidence of a chemical shift in ESCA was reported for Cu:1s and 2s core levels by his group at a Swedish national conference held in 1957 (10). The term chemical shift appeared in the text of the conference report. Subsequently, it was discussed in an international conference (11), and an article by Siegbahn’s group on the Cu:1s, 2s, and 2p chemical shifts due to oxidation appeared in 1958 (12). The detailed experimental technique used was given in another article (13). In the early 1920s, before the first observation of any chemical shift, the energy of the X-ray absorption edge for a core level was discovered to depend on the chemical environment of an atom (14–16). This was the first time that the concept of chemical shift to designate the position of a spectral signal had appeared in the literature, though the term chemical shift was not used. The term chemical shift is also used in Mössbauer spectroscopy. The nonzero volume of a nucleus and the electron density due to s electrons within it induce nucleus–electron Coulomb interactions that alter the nuclear energy levels. A shift in the nuclear energy level due to a specific chemical environment is often called a chemical shift in Mössbauer spectroscopy. This phenomenon was first observed for 57Fe bound in Fe2O3 by Kistner and Sunyar in 1960 (17), and they used the phrase “influence of chemical binding” in the title. www.JCE.DivCHED.org
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Armstrong et al. showed that chemical binding clearly affects the 57Fe chemical shift in Mössbauer spectroscopy (18). One month and a half after the first report of this phenomenon, the term chemical shift appeared in the text of an article on 67 Zn Mössbauer spectroscopy by Craig et al. (19). This phenomenon was called an isomeric shift in Mössbauer’s Nobel lecture in 1961 (20). Its official name is now isomer shift (21), but it is also called a chemical shift (2). Why the term chemical shift used for NMR was also applied to ESCA and Mössbauer spectroscopy is not noted in the literature. Because every chemical shift provides information about the electronic environment associated with “chemical” interaction between an atomic site and its surrounding atoms (or groups of atoms), the pioneers may intentionally have employed the same term. However, no general correlation between the NMR and ESCA chemical shifts has been found (22, 23), although linear correlations have been found between ESCA chemical shifts and Mössbauer chemical shifts for compounds of Fe, Sn, and so forth (2, 22). The spatial range of the electronic environment affecting the chemical shift in NMR would be different from those in ESCA and Mössbauer spectroscopy (24). Regrettably, chemical shift has a poor reputation as a technical term. Levine (1) described the term as, “The distinctive (curious?) term ‘chemical shift’...” and noted, “The term chemical shift is shown to have originated in the mistaken assumption that nuclei of a given element will all undergo resonance at the same frequency regardless of their environment.” Walker et al. (25) wrote, “Unfortunately, the phrase ‘chemical shift’ has been used” and claimed that the prefix for the shift should be a word showing the cause of the shift, for example, “isotope shift” as the proper name for effects resulting from the addition of neutrons. However, the chemical environment around an atom of interest influences the electronic environment and hence, leads to spectral shifts. Therefore, using the prefix chemical for the shifts observed in the three forms of spectroscopy is appropriate. Literature Cited 1. Levine, S. G. J. Chem. Educ. 2001, 78, 133. 2. Drago, R. S. Physical Methods for Chemists, 2nd ed.; Saunders College Publishing: Ft. Worth, TX, 1992; Chapters 7, 15, 16. 3. Dickinson, W. C. Phys. Rev. 1950, 77, 736–737. 4. Proctor, W. G.; Yu, F. C. Phys. Rev. 1950, 77, 717. 5. Gutowsky, H. S.; Hoffman, C. J. Phys. Rev. 1950, 80, 110– 111. 6. Lessinger, L. J. Chem. Educ. 1995, 72, 85–87 and references therein. 7. Knight, W. D. Phys. Rev. 1949, 76, 1259–1260. 8. King, D. E.; Swartz, W. E., Jr. J. Chem. Educ. 1987, 64, 981– 983 and references therein. 9. Siegbahn, K. Electron Spectroscopy for Atoms, Molecules and Condensed Matter. In Nobel Lectures, Physics 1981–1990;
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Ekspong, G., Ed.; World Scientific Publishing: Singapore, 1993; pp 63–92. Sokolowski, E.; Nordling, C.; Siegbahn, K. Binding Energies of Internal Shells in Cu, Cr and Zn Studied by Means of a New Precision Method. In Reports from the Conference of the Swedish National Committee for Physics in 1957, June 3–5, 1957, the University of Uppsala; Borelius, G., Rudberg, E., Eds.; Arkiv Fysik 1958, 13, 288–289. Siegbahn, K.; Nordling, C.; Sokolowski, E. Chemical Shifts of Photo- and Auger Electron Lines. In Proceedings of the Rehovoth Conference on Nuclear Structure, September 8–14, 1957, the Weizmann Institute of Science; Lipkin, H. J., Ed.; North-Holland Publishing Co.: Amsterdam, 1958; pp 291–293. Sokolowski, E.; Nordling, C.; Siegbahn, K. Phys. Rev. 1958, 110, 776. Nordling, C.; Sokolowski, E.; Siegbahn, K. Arkiv Fysik 1958, 13, 483–500. Lindgren, I. J. Electron Spectrosc. Relat. Phenom. 2004, 137– 140, 59–71.
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Bergengren, J. Z. Phys. 1920, 3, 247–249. Lindh, A. E. Z. Phys. 1921, 6, 303–310. Kistner, O. C.; Sunyar, A. W. Phys. Rev. Lett. 1960, 4, 412–415. Armstrong, W. H.; Dorflinger, E. E.; Anderson, O. T.; Willeford, B. R., Jr. J. Chem. Educ. 1981, 58, 515–518. Craig, P. P.; Nagle, D. E.; Cochran, D. R. F. Phys. Rev. Lett. 1960, 4, 561–564. Mössbauer, R. L. Recoilless Nuclear Resonance Absorption of Gamma Radiation. In Nobel Lectures, Physics 1942–1962; Elsevier: Amsterdam, 1964; pp 584–601. Herber, R. H. Inorg. Chim. Acta 1974, 8, 10–12. Carlson, T. A. Photoelectron and Auger Spectroscopy; Plenum: New York, 1975; Chapter 5, Section 4.4. Lindberg, B. J. J. Electron Spectrosc. Relat. Phenom. 1974, 5, 149–166. Nagaoka, S.; Nagashima, U.; Ohshita, J. Bull. Chem. Soc. Jpn. 2006, 79, 537–548. Walker, L. R.; Wertheim, G. K.; Jaccarino, V. Phys. Rev. Lett. 1961, 6, 98–101.
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