Chapter 1

The Basics of Modern NMR Spectroscopy

Abstract

The basic principles of NMR spectroscopy, including the key components of NMR spectrometers such as magnets and probes, are described. How to make the best use of your NMR spectrometer through optimizing various instrumental parameters is also presented. Thirty penetrating questions and their well described answers help in understanding why NMR is so different from other spectroscopic techniques and how these fundamental differences make this technique a "work horse" in various fields of molecular sciences.

Keywords

radiofrequency resonance NMR-active nuclei sensitivity and resolution pulse Fourier transform NMR spectroscopy superconducting magnets NMR probes shimming probe tuning deuterium lock NMR tubes

Chapter Outline

1.1 What Is NMR? 1

1.1.1 The Birth of a Signal 3

1.2 Instrumentation 9

1.2.1 The Magnet 10

1.2.2 The Probe 11

1.2.3 Probe Tuning 14

1.2.4 Shimming 17

1.2.5 Deuterium Lock 22

1.2.6 Referencing NMR Spectra 22

1.2.7 NMR Sample Tubes 23

Solutions to Problems 25

References 33

1.1. What is NMR?

Nuclear magnetic resonance (NMR) spectroscopy is the study of molecules by recording the interaction of radiofrequency (Rf) electromagnetic radiations with the nuclei of molecules placed in a strong magnetic field. Zeeman first observed the strange behavior of certain nuclei when subjected to a strong magnetic field at the end of the nineteenth century, but practical use of the so-called "Zeeman effect" was made only in the 1950s when NMR spectrometers became commercially available. Like all other spectroscopic techniques, NMR spectroscopy involves the interaction of the material being examined with electromagnetic radiation. Why do we use the word "electromagnetic radiation"? This is so because each ray of light (or any other type of electromagnetic radiation) can be considered to be a sine wave that is made up of two mutually perpendicular sine waves that are exactly in phase with each other, i.e., their maxima and minima occur at exactly the same point of line. One of these two sine waves represents an oscillatory electric field, while the second wave (that oscillates in a plane perpendicular to the first wave) represents an oscillating magnetic field - hence the term "electromagnetic" radiation. Cosmic rays, which have a very high frequency (and a short wavelength), fall at the highest energy end of the known electromagnetic spectrum and involve frequencies greater than 3 × 1020 Hz. Radiofrequency (Rf) radiation, which is the type of radiation that concerns us in NMR spectroscopy, occurs at the other (the lowest energy) end of the electromagnetic spectrum and involves energies of the order of 100 MHz (1 MHz = 106 Hz). Gamma rays, X-rays, ultraviolet rays, visible light, infrared rays, microwaves and radiofrequency waves all fall between these two extremes. The various types of radiations and the corresponding ranges of wavelength, frequency, and energy are presented in Table 1.1.

Table 1.1

The Electromagnetic Spectrum

Radiation Wavelength (nm) ? Frequency (Hz) ? Energy (kJ mol-1) Cosmic rays <10-3 >3 × 1020 >1.2 × 108 Gamma rays 10-1 to 10-3 3 × 1018 to 3 × l020 1.2 × l06 to 1.2 × l08 X-rays 10 to l0-1 3 × 1016 to 3 × 1018 1.2 × 104 to 1.2 × 106 Far ultraviolet rays 200 to 10 1.5 × 1015 to 3 × 1016 6 × 102 to 1.2 × l04 Ultraviolet rays 380 to 200 8 × 1014 to 1.5 × 1015 3.2 × 102 to 6 × 102 Visible light 780 to 380 4 × 1014 to 8 × l014 1.6 × l02 to 3.2 × 102 Infrared rays 3 × l04 to 780 1013 to 4 × 1014 4 to 1.6 × 102 Far infrared rays 3 × 105 to 3 × 104 1012 to 1013 0.4 to 4 Microwaves 3 × l07 to 3 × 105 1010 to 1012 4 × 10-3 to 0.4 Radiofrequency (Rf) waves 1011 to 3 × 107 106 to 1010 4 × 10-7 to 4 × 10-3 Electromagnetic radiation also exhibits behavior characteristic of particles, in addition to its wave-like character. Each quantum of radiation is called a photon, and each photon possesses a discrete amount of energy, which is directly proportional to the frequency of the electromagnetic radiation. The strength of a chemical bond is typically around 400 kJ mol-1, so that only radiations above the visible region will be capable of breaking bonds. But infrared, microwaves, and radio-frequency radiations will not be able to do so. Let us now consider how electromagnetic radiation can interact with a particle of matter. Quantum mechanics (the field of physics dealing with energy at the atomic level) stipulates that in order for a particle to absorb a photon of electromagnetic radiation, the particle must first exhibit a uniform periodic motion with a frequency that exactly matches the frequency of the absorbed radiation. When these two frequencies exactly match, the electromagnetic fields can "constructively" interfere with the oscillations of the particle. The system is then said to be "in resonance" and absorption of Rf energy can take place. Nuclear magnetic resonance involves the immersion of nuclei in a magnetic field, and then matching the frequency at which they are precessing with electromagnetic radiation of exactly the same frequency so that energy absorption can occur.

1.1.1. The Birth of a Signal

Certain nuclei, such as 1H, 2H, 13C, 15N, and 19F, possess a spin angular momentum and hence a corresponding magnetic moment µ, given by

=?h[I(I+1)]1/22p

(1.1) where h is Planck's constant and ? is the magnetogyric ratio (also called gyromagnetic ratio). When such nuclei are placed in a magnetic field B0, applied along the z-axis, they can adopt one of 2I + 1 quantized orientations, where I is the spin quantum number of the nucleus (Fig. 1.1). Each of these orientations corresponds to a certain energy level:

=-µzB0=-m1?hB02p

(1.2) where m1 is the magnetic quantum number of the nucleus and µz is the magnetic moment. In the lowest energy orientation, the magnetic moment of the nucleus is most closely aligned with the external magnetic field (B0), while in the highest energy orientation it is least closely aligned with the external field. Organic chemists are most frequently concerned with 1H and 13C nuclei, both of which have a spin quantum number (I) of 1/2, and only two quantized orientations are therefore allowed, in which the nuclei are either aligned parallel to the applied field (lower energy orientation) or antiparallel to it (higher energy orientation). The nuclei with only two quantized orientations are called dipolar nuclei. Transitions from the lower energy level to the higher energy level can occur by absorption of radiofrequency radiation of the correct frequency. The energy difference ?E between these energy levels is proportional to the external magnetic field (Fig. 1.2), as defined by the equation ?E = ?hB0/2p. In frequency terms, this energy difference corresponds to

0=?B02p

(1.3) Figure 1.1 Representation of the precession of the magnetic moment about the axis of the applied magnetic field, B0. The magnitude µz, of the vector corresponds to the Boltzmann excess in the lower energy (a) state. Figure 1.2 The energy difference between the two energy states ?E increases with increasing value of the applied magnetic field B0, with a...