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A Short History of the Chemical Shift Samuel G. Levine Department of Chemistry, North Carolina State University, Raleigh, NC 27695; levine@#chemdept.chem.ncsu.edu
The distinctive (curious?) term “chemical shift” to designate the location of an NMR signal has a nearly-50-year history. The word “shift” (or a variant) in connection with an NMR signal appeared in papers from several laboratories in the late 1940s and early 1950s. In 1949 W. D. Knight (1) observed “a shift in the nuclear magnetic resonance frequencies from expected values in five metals, Li, Na, Al, Cu, and Ga” in that the resonance frequencies (of the metals) “were higher by tenths of a percent than the resonance frequencies observed in salts of the corresponding metals”. Referring to Knight’s experience with metals, W. C. Dickenson (2), early in 1950 reported that “most unexpectedly, it has been found for 19F the value of the applied magnetic field Ho for nuclear magnetic resonance at a fixed frequency depends on the chemical compound containing the fluorine nucleus.” He later goes on to describe his success in accurately measuring the relative “shifts” in resonance position. During that same period Proctor and Yu (3), working with 14N nuclei and citing both of the above investigators, reported the “surprising observation” that the frequency of 14N resonances “depended strongly on the chemical compound in which it was contained” (e.g. NH4+ as compared with NO3᎑) and they refer to these phenomena as “frequency shifts”. Application of similar terminology to proton spectra appeared in the contemporaneous work of Bloembergen (4,
5) on the fine structure of proton resonance in CuSO4⭈5H2O. This same period also saw the emergence of the two-word term “chemical shift” in the studies by Gutowsky and Hoffman (6, 7 ) on covalent compounds of 19F and 1H. Within two years this group reported on the NMR characteristics of more than 200 organic compounds (7 ). Resonance frequencies were measured with respect to the H2O proton as 0.0 and the term “chemical shift” was used for the first time to designate numerically the separation between the signal and a standard. Thereby was the way paved for the systematic recording of an endless avalanche of NMR data that would soon begin. Literature Cited 1. 2. 3. 4. 5. 6.
Knight, W. D. Phys. Rev. 1949, 76, 1259–1260. Dickenson, W. C. Phys. Rev. 1950, 77, 736–737. Proctor, W. G.; Yu, F. C. Phys. Rev. 1950, 77, 717. Bloembergen, N. Phys. Rev. 1949, 75, 1326. Bloembergen, N. Physica 1950, 16, 95–112. Gutowsky, H. S.; Hoffman, C. J. J. Chem. Phys. 1951, 19, 1259–1267. 7. Meyer, L. H.; Saika, A.; Gutowsky, H. S. J. Am. Chem. Soc. 1953, 75, 4567–4573.
JChemEd.chem.wisc.edu • Vol. 78 No. 1 January 2001 • Journal of Chemical Education
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