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The chemical shift provides the necessary information that permits the structure of a compound to be identified and renders the NMR Spectrum unique to the substance concerned. The theory that explains the chemical shift is complex and outside the remit of this monograph. However, some of the basic principles on which the chemical shift depends needs to be discussed and those interested in the theoretical further details are recommended to read the two books given in References.
The shielding effect that is responsible for the chemical shift has already been defined by equation (4), viz.
H= (HF+HS)(1-α) (4)
Which can be put more simply as.
where (Ho) is the total applied magnetic field
and (α)is called the shielding parameter or screening constant..
If the nucleus has no orbital or spin angular momentum and the electrons can rotate in circles round the direction of the applied field, (α) can be expressed by the following equation,
However, if attempts are made to calculate screening constants for molecular systems the procedure becomes complicated and the screening effects must be classified into different groups as follows.
The diamagnetic effect of the atom. This effect is due to the field resulting from the electron rotating round the atom.
The paramagnetic effect for the atom. Paramagnetic shielding resulting from the presence of close atoms.
3 Contributions from neighbouring atoms. Shielding effects that are transmitted from all neighbouring atoms.
Contributions from inter atomic currents. Currents resulting from π electrons providing inter atomic currents
A hydrogen atom situated in a magnetic field not only experiences the applied field but also the field resulting from the pseudo circular current provided by the electron rotating round the nucleus. As a result the field experienced by the nucleus is slightly less than that from the externally applied magnetic field (i.e. the nucleus is shielded by the field from the rotating electron). This is described as the diamagnetic shielding effect (see 1 above). The diamagnetic shielding effect is depicted in figure 13.
The situation depicted in figure 13 is idealized and in the more general case the circulation of the electron is distorted by the presence of nearby atoms, which ’hinders’ the free circulation of the electron and thus modifies its effect on the magnetic field experienced by the nucleus. This additional effect is defined as the paramagnetic effect (as given in 2 above).
The effect of a secondary field produced by a neighbouring atom X on a given proton is depicted in figure 14. From figure 14 it is seen that the secondary field experienced by the proton under circumstances where the primary field is parallel to it the primary field will be opposed and there will be a shielding effect. Conversely, if the primary field is perpendicular to the bond then the net effective field will be increased and the shielding constant will be reduced.
Consider the situation where the acetylenic bond is lined up with the externally applied magnetic field (i.e. a condition where the diamagnetic susceptibility lies along the carbon-carbon bond axis. This situation is depicted in figure 15.
It is clear that any secondary field resulting from anisotropy from the triple bond linkage will shield the associated protons, In addition, it would follow that nucleus placed perpendicular to the triple bond would cause a deshielding effect.
It has been generally accepted that shielding is always denoted as (+) whereas the process of deshielding is denoted as (-) when denoting those areas that are associated with different structural characteristics. This is achieved by constructing cones around the particular structural feature and by employing the above symbols to indicate the nature of the shielding effect. As an example, the shielding and deshielding zones around the acetylenic and the nitrile bond represented ‘shielding cone’ form is depicted in figure 16.
Contributions from inter atomic currents that result from π electrons providing inter atomic currents are typified by the benzene nucleus. It was suggested by Pauling that the six π electrons of a benzene molecule would precess in a magnetic field in a plane perpendicular to the direction of the field and the angular frequency (ω) would be given by,
The resulting current (I) that could be considered flowing in circle having the same radius as the benzene ring would be given by,
where n is the number of electrons and e is the charge on the electron.
Figure 17. The Ring Current from and Aromatic Ring
The ring current from the aromatic nucleus is depicted in figure 17 and the similarity of the system to that depicted in figure 13 should be noted.
This discussion has only described the basic principles involved in NMR spectroscopy and gives some indication of the complex nature of the theory and the various different magnetic and electromagnetic environments that are involved. The technique has not been extensively used by analysts, or the general practicing chemist. This is largely due to the complexity of the spectroscopic system and the knowledge and experience necessary for the interpretation of NMR spectra. Nevertheless, if reference spectra are available NMR spectroscopy can be used for solute identification and quantitative determinations. However, and for the most part, the application of the technique to problems in chemistry needs to be handled by trained NMR spectroscopists. In addition NMR spectroscopy is not the most common spectroscopic technique used in analytical and general chemistry for practical reasons; an NMR spectroscopy service involves the use of expensive equipment, entails expensive operating costs (general instrument maintenance and, in particular, the maintenance of the low temperature of the superconducting magnet with liquid nitrogen and liquid helium ) and the service of a skilled spectroscopist.
There are many commercial laboratories that offer NMR
spectroscopy services exclusively and so, if required, it is
preferable to send samples to these service laboratories as an
alternative to tolerating the high operating costs of an ‘in
house’ NMR spectroscopy facility. In house NMR spectroscopy
services are found mostly in the research laboratories of
universities and, perhaps, a limited number of industrial research
laboratories. In such environments they are mostly employed for
research into the technique itself and for structure elucidation of
new and hitherto unknown substances and the measurement of some of
their physical properties.
About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.