Nuclear Shielding

BY W. T. RAYNES

1 Introduction

Three alternative procedures suggest themselves for a review on the subject of nuclear magnetic shielding in molecules. First, one could consider the various chemical and physical influences (e.g. the inductive effect, conjugation, magnetic anisotropy) which are believed to be responsible for the differences in shielding between different compounds. These could be discussed in turn, each one being illustrated by examples drawn from the shieldings of all species of magnetic nuclei in a wide range of compounds. Secondly, one could consider the shieldings of a particular nuclear species (e.g. the proton), illustrate the shielding changes that occur along several series of compounds containing that species of nucleus, and interpret these changes in terms of varying contributions from one or more of the above-mentioned phenomena. This would then be repeated in turn for other species of nucleus. The third approach involves selecting a particular compound (e.g. C6H5NO2) and showing how each of the chemical and physical influences makes its own contribution to the shielding of all the nuclei in the compound. This would then be repeated in turn for other compounds.

Of these procedures, it is traditionally the second that has been adopted for dealing with observed shieldings in a wide range of compounds. The third procedure is seldom used, partly because much of the information required (e.g. carbon, nitrogen, and oxygen shieldings) has been unavailable, at least until very recently, and partly because it is obviously unrealistic except when dealing with a group of closely related compounds. Since the present review is 'phenomenon-oriented' rather than 'compound-oriented' the first procedure will, for the most part, be adopted. However, because many workers tend to study the shieldings of only one nuclear species — presumably because of personal interest or limited experimental facilities — a brief survey of recent work in terms of individual species of nuclei has also been included.

This review covers work that appeared in the literature between July 1st 1970 and June 30th 1971. However, since this is the first in the present review series, some important papers appearing in the first half of 1970 will also be discussed. Because of the vast abundance of publications on nuclear shielding, some restrictions have had to be imposed. For instance, no space has been given to experimental aspects, and the details of wave-mechanical calculations have, for the most part, been excluded. The ways in which nuclear shieldings can be changed by intermolecular effects (e.g. dispersion forces, reaction fields) are omitted here but are discussed in Chapter 9. The main concern in this chapter is the interpretation of recent work on observed shieldings of isolated molecules in terms of the various physical and chemical influences which chemists have found to be useful for their understanding. A further omission, requiring the Reporter's apologies, is the discussion of papers in foreign language journals.

Two conventions that are used in this chapter will be stated at this point. For the sake of economy, most numerical values of shieldings and chemical shifts are given without the appellation p.p.m. (parts per million). Secondly, the convention has been adopted that the chemical shift is positive if the nucleus under consideration has a more positive shielding constant than the reference nucleus. The reason for this choice will be clarified in Section 2.

2 Chemical Shift Scales

It is appropriate to commence with a brief discussion of chemical shift scales — a topic which has aroused some controversy. The aim here is not to approve particular choices of reference for particular nuclei but to deal with two general points which need to be stressed. A more detailed review of the definition of chemical shifts has been given by Rummens.

In defining chemical shifts there are two possible starting points, which may be termed 'theoretical' and 'experimental'. The theoretical approach starts with a molecule in a uniform magnetic field B. The field Blocal at a selected point in the vicinity of a molecule is different from B because of the small field B' arising from the induced motions of the electrons. Thus:

Blocal = B – B' (1)

The negative sign is placed here explicitly so that one may refer to the phenomenon as 'magnetic shielding', as any point magnetic dipole (of very small magnitude) placed at the selected point would be shielded from the full effect of B by the influence of the induced electronic motions. We are implying that B' is positive but, of course, this does not exclude the possibility of a negative value of B', in which case the phenomenon is termed 'antishielding'. Provided that the field B is not too large, B' is proportional to B, so that:

B' = σ B (2)

where σ is the magnetic shielding constant. The value of σ obviously depends on the location of the selected point. (It also depends, in general, on the direction of B relative to axes fixed in the molecule. However, no loss of significance for the arguments given below occurs if we imagine the molecule to be tumbling freely in the field with the point-dipole held in a fixed position relative to molecule-fixed axes.) With the negative sign given explicitly in equation (1), it follows that σ is positive when shielding occurs and negative when antishielding occurs.

In practice, one is almost always interested in the value of σ at the site of a particular magnetic nucleus in the molecule. For these special positions σ is called the nuclear (magnetic) shielding constant. These nuclear shielding constants are, of course, fundamental physical properties of a molecule and are, therefore, the quantities which are obtained from quantum-mechanical calculations. From the standpoint of chemical theory, however, it is often the difference between the nuclear shielding constant in the compound of interest and that in some suitable reference compound (σref) with which one is concerned. This quantity δth is the chemical shift and is defined by equation (3). There are three points to be noted here. First, a definition of δth as being σref – σ would be unsatisfactory: as one would say in everyday language, it

δth = σ – σref (3)

would be 'illogical'. Secondly, the choice of reference depends on the particular nucleus in the particular compound of interest. For...