Introduction To Nmr Spectroscopy

NMR is a spectroscopic technique that relies on the magnetic properties of the atomic nucleus. When placed in a strong magnetic field, certain nuclei resonate at a characteristic frequency in the radio frequency range of the electromagnetic spectrum. Slight variations in this resonant frequency give us detailed information about the molecular structure in which the atom resides.

1.1.1 The Classical Model

Many atoms (e.g., XH, 13C, 15N, 31P) behave as if the positively charged nucleus was spinning on an axis (Fig. 1.1). The spinning charge, like an electric current, creates a tiny magnetic field. When placed in a strong external magnetic field, the magnetic nucleus tries to align with it like a compass needle in the earth's magnetic field. Because the nucleus is spinning and has angular momentum, the torque exerted by the external field results in a circular motion called precession, just like a spinning top in the earth's gravitational field. The rate of this precession is proportional to the external magnetic field strength and to the strength of the nuclear magnet:

Vo = yBo/2n where vo is the precession rate (the "Larmor frequency") in hertz, y is the strength of the nuclear magnet (the "magnetogyric ratio"), and Bo is the strength of the external magnetic field. This resonant frequency is in the radio frequency range for strong magnetic fields

NMR Spectroscopy Explained: Simplified Theory, Applications and Examples for Organic Chemistry and Structural Biology, by Neil E Jacobsen Copyright © 2007 John Wiley & Sons, Inc.

Classical

Classical

v = yB0l2n Figure 1.1

and can be measured by applying a radio frequency signal to the sample and varying the frequency until absorbance of energy is detected.

1.1.2 The Quantum Model

This classical view of magnetic resonance, in which the nucleus is treated as a macroscopic object like a billiard ball, is insufficient to explain all aspects of the NMR phenomenon. We must also consider the quantum mechanical picture of the nucleus in a magnetic field. For the most useful nuclei, which are called "spin nuclei, there are two quantum states that can be visualized as having the spin axis pointing "up" or "down" (Fig. 1.2). In the absence of an external magnetic field, these two states have the same energy and at thermal equilibrium exactly one half of a large population of nuclei will be in the "up" state and one half will be in the "down" state. In a magnetic field, however, the "up" state, which is aligned with the magnetic field, is lower in energy than the "down" state, which is opposed to the magnetic field. Because this is a quantum phenomenon, there are no possible states in between. This energy separation or "gap" between the two quantum states is proportional to the strength of the external magnetic field, and increases as the field strength is increased. In a large population of nuclei in thermal equilibrium, slightly more than half will reside in the "up" (lower energy) state and slightly less than half will reside in the "down" (higher energy) state. As in all forms of spectroscopy, it is possible for a nucleus in the lower energy state to absorb a photon of electromagnetic energy and be promoted to the higher energy state. The energy of the photon must exactly match the energy "gap" (AE) between

the two states, and this energy corresponds to a specific frequency of electromagnetic radiation:

AE = hvo = hyBo/2n where h is Planck's constant. The resonant frequency, vo, is in the radio frequency range, identical to the precession frequency (the Larmor frequency) predicted by the classical model.

1.1.3 Useful Nuclei for NMR

The resonant frequencies of some important nuclei are shown below for the magnetic field strength of a typical NMR spectrometer (Varian Gemini-200):

Nucleus

Abundance (%)

Sensitivity

Frequency (MHz)

1H

0 0

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