Spectroscopic Techniques

Introduction

In n.m.r. a radio frequency is applied to the substance of interest together with a progressively increasing magnetic field. When the magnetic field is such that the separation of the energy levels it induces corresponds to the energy of a photon of the applied radiation there will be absorption of the radiation and therefore a signal. Nuclei in chemical compounds are not ‘bare’, and the molecular environment of a nucleus influences the spacing of the energy levels and hence the magnitude of the field at which a signal will occur. This is called chemical shift and is of the order of parts per million of the applied field.

Not all atomic nuclei respond in that way to a magnetic field. Those which do have the property of spin, and this requires an odd number of nucleons (protons and neutrons). Each nucleon has spin ½ so a single unpaired one causes a nucleus to have spin ½. Such details for nuclei which feature in this article are in table 1.

1 H comprises one proton and the total spin is ½. 2 H, the deuterium nucleus, comprises an unpaired proton and an unpaired neutron, each with spin ½ giving total spin 1. Each is therefore responsive to a magnetic field. Even so, in n.m.r. in which 1 H is of interest D 2 O can be used as solvent without any interference as the fields to which 2 H responds are so different in magnitude from those to which 1 H responds.

Solid state n.m.r. and the ‘magic angle’

1 H n.m.r. (also called proton n.m.r.) was well established in organic chemistry by about 1970. In such applications the material of interest is got into solution (D 2 O being a common solvent as already noted) for n.m.r. analysis. Developments did follow in solid state n.m.r. which for less straightforward applications than routine characterisation of a single organic compound is likely to give more useful information. For example in soil analysis by n.m.r., which features later in this article, solid state n.m.r. on a sample of soil would intuitively be preferable to solvent extraction of chemicals from the soil and n.m.r. analysis of those. That being said, interactions of chemical groups within a solid with each other and effects of such interaction on the signals is a difficulty with solid state n.m.r. Such interactions are of course distinct from chemical shift which is intramolecular and they tend to lead to broad and indistinct signals which are difficult to assign to particular structural groups within the solid. Whereas the methyl group CH 3 – is very easily recognised in the 1 H n.m.r. spectrum of acetaldehyde (CH 3 CHO), a methyl group in a coal structure will not give as unequivocal a signal because of the interactions described.

The nature of the interactions which lead to blurring of n.m.r. signals with solid samples is such that the orientation of the sample with respect to the magnetic field can be used to control them. Imagine a right angle triangle having one side 1 length unit and another √2 length units. It follows from Pythagoras’ theorem that the hypotenuse is of length √3 units and that the angle between the shorter side and the hypotenuse is:

This is the ‘magic angle’ at which for a solid sample the interactions leading to imprecision of n.m.r. signals will be at their lowest to the enhancement of clarity of the recorded spectrum. A sample for analysis will therefore be placed at that angle with respect to the field and rapidly rotated: this is magnetic angle spin (MAS) n.m.r. MAS n.m.r. is a widely used and very powerful technique. Soil was mentioned previously as one of the substances within the scope of this journal to which n.m.r. can be applied. This is taken up in the next section.

Relaxation times

When resonating because of application of the magnetic field and of the radio frequency the nucleons have more energy than previously, and their return to the initial energy state after resonance is called relaxation. This is another facet of n.m.r. application. The relaxation occurs in two stages separated by about two orders of magnitude in duration. In liquid samples, the first relaxation process is over a period of the order 10 nanoseconds and this is followed by further relaxation over a period of the order of 1 microsecond. In the n.m.r. spectrum each is detectable as broadening of a signal on its descent from peak to baseline.

Applied fields are of the order of Tesla (T), and broadening due to the more rapid relaxation process is of the order 10 -4 T and that of the slower relaxation process of the order of 10 -6 T. Note that the former is an order of magnitude higher than chemical shifts which are routinely measured where they are of interest. Resonance times are therefore easily measurable from the broadening. If the broadening in magnetic units is re-expressed in energy terms an approximate expression for the relaxation time (symbol t) is:

where Δ (J) is the broadening so expressed and h is Planck’s constant (6.626 × 10 -34 Js). Resonance times complement chemical shift in applications of n.m.r. Broadly speaking, chemical shift is the basis of analytical chemistry applications. The more advanced applications which are based on relaxation times include Magnetic Resonance Imaging (MRI), a very widely used technique in medicine a brief coverage of which is given later in this piece.

Examples of application

Nuclear magnetic resonance has been extensively applied to the characterisation of soils. The organic – ‘humic’ – component of soil, which accounts for most of its mass, can be studied by MAS 13 C n.m.r. In this way, proportions of the carbon in different structures such as aliphatic and aromatic can be determined. 15 N n.m.r., also using the ‘magic angle’ principle, has been carried out on soils. This enables such nitrogen containing groups as amine and amide amongst others to be identified within a sample of soil.

One would not however perform a 15 N n.m.r. analysis on untreated soil. This is because the level of nitrogen in soil is so low that the presence of small amounts of paramagnetic substances including iron will interfere with if not totally obscure the signal. 15 N n.m..r. on soils therefore requires that the organic material is separated from the inorganic by dissolution of the latter in hydrofluoric acid. This is believed to cause no loss of originality of the organic structure and might in fact be carried out for 13 C n.m.r. analysis of a soil where interference from paramagnetic substances is not impossible.

Phosphorous in solid can be studied by 31 P n.m.r., and phosphorous (which belongs to the same group in the periodic table as nitrogen) is often ‘complexed’ in soil having bonds both to carbon and to another inorganic, most probably calcium or aluminium. Similarly aluminium in soil can be studied by reason of 27 Al. Both 1 H and 2 H n.m.r. have been used to resolve the water present in soil at low temperatures into ice and liquid components 1 .

The former is solid state giving signals having the characteristics previously described and their distinction from those due to liquid water is straightforward. In air fluoro compounds can be quantitatively assessed by n.m.r., the 19 F nucleus having spin ½ as noted in the table. Chlorofluorocarbons (CFCs) in the atmosphere, originating from refrigeration and air conditioning, cause damage to the ozone layer so continuous monitoring of such substances in the atmosphere is necessary e.g. 2 . Polyaromatic hydrocarbons (PAH) are a particularly hazardous air pollutant.

They originate from incomplete combustion and are the precursor to soot. Structures of particular polyaromatic hydrocarbons can be studied by 1 H and/or 13 C n.m.r., e.g. 3

Magnetic Resonance Imaging (MRI)

On July 3rd 1977 the first MRI examination of a live person was conducted in the USA by Dr. Raymond Damadian and co-workers. It was at least as significant an event in the history and development of medicine as the first medical application of X-rays about 80 years earlier. The principle is different relaxation times for resonating hydrogen atoms according to their environment, that is, the nature of the tissue in which they are present. In the human brain white matter and grey matter are distinguishable by 1 H n.m.r. relaxation times. When such relaxation times from multiple determinations are statistically processed a difference attributable to gender is evident. The value of such information in the identification of neurological illnesses is of course clear.

MRI in the diagnosis of cancer has as its basis the difference in 1 H relaxation times between cancerous cells and healthy ones. This is now very widely practiced and the American Cancer Society has recommended that women with a hereditary propensity to breast cancer should have an annual MRI examination in addition to a mammogram.

A genealogy of Nobel Laureates

Nuclear magnetic resonance is first and foremost a phenomenon in physics. Its discoverers F. Bloch (Stanford University) and E.M. Purcell (Harvard University) shared the 1952 Nobel Prize for Physics. The 1991 Nobel Prize for Chemistry went to Richard R. Ernst of Switzerland. The official citation invoked his ‘contributions to the development of the methodology of high resolution nuclear magnetic resonance (NMR) spectroscopy’. In 2002 a Nobel Prize for Chemistry came the way of K. Wüthrich of Switzerland ‘for his development of nuclear magnetic resonance spectroscopy for determining the three-dimensional structure of biological macromolecules in solution’. The 2003 Nobel Prize for Physiology or Medicine was awarded to P. Lauterbar (USA) and P. Mansfield (UK) for work on MRI. Nuclear magnetic resonance has therefore been the basis of Nobel awards in three disciplines. That gives a perspective on its importance, and also on the futility of attempting anything more than the briefest sketch in an article on the scale of this one.

Concluding remarks

A reader of this article not previously having encountered n.m.r. will have gained basic information on the phenomenon and on selected Spectroscopy applications. There will be those amongst the readers of AWE International who have hands-on experience of using an n.m.r. spectrometer. It is to be expected that the research literature in many disciplines will continue to feature n.m.r. as more applications are developed. There remains very wide scope for such development.

References

1 http://www.ncbi.nlm.nih.gov/pubmed/15543746

2 http://www.spectroscopynow.com/coi/cda/detail.cda?chId=5&id=19125&type =Feature&page=1

3 Rongbao L., Zengmin S. and Li B. ‘Structural analysis of polycyclic aromatic hydrocarbons derived from petroleum and coal by 13C and 1H-n.m.r. spectroscopy’ Fuel 67 565-569 (1988)

4 Atkins P.W. ‘Physical Chemistry’ Oxford University Press, any available edition.

Published: 01st Jun 2009 in AWE International