The Spin Echo And The Attached Proton Test

A major theme of this book is developing a "toolbox" of nuclear magnetic resonance (NMR) pulse sequence building blocks. So far we have described a number of fundamental tools of NMR experiments: pulses, continuous-wave (CW) low-power irradiation, and decoupling sequences such as waltz-16. In this chapter we will see the value of a delay, a simple waiting period of precise duration without any pulses, in building up a more complex "building block." The spin echo is a combination of pulses and delays that carries out a specific function: It allows us to control the type of changes that occur (due to chemical shifts only, J coupling only, or neither) during a precise period of time. We can "plug in" this module anywhere we want in a complex pulse sequence to achieve these predictable effects. Once we have used the vector model to understand what happens during a spin echo, we will not have to go through this analysis again because we will see that the overall effect is predictable in a simple way. Eventually, we will add other fundamental tools (e.g., selective pulses, pulsed field gradients, and spin locks) and continue combining these into more pulse sequence building blocks. As we learn how to predict the effects of these building blocks, eventually even the most complex and advanced NMR experiments can be pulled apart into a series of modules that we understand completely.

In this chapter, we will look at a one-dimensional technique for 1H-decoupled 13 C spectra that uses the phase of the 13C signal (positive or negative peaks) as a way to encode information about the number of protons attached to a carbon (Cq, CH, CH2, or CH3). We saw that a fully coupled spectrum gives this information, but the sensitivity is very low, and even with a simple molecule like sucrose, the overlapping multiplets are difficult to sort out. By designing experiments that modulate the sign of a single 1H-decoupled carbon peak (positive or negative), we can get this information without overlap, because the peak remains a singlet and does not increase its horizontal "footprint". This "editing" of the 13C spectrum according to the number of attached protons (Cq, CH,

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

CH2, or CH3) is achieved with a ubiquitous building block of NMR pulse sequences: the spin echo.

To understand these new building blocks, we need to look in detail at the motion of the net magnetization vector during a delay as it precesses in the x-y plane. This motion is called "evolution" because the net magnetization changes with time or "evolves" as it rotates.

0 0

Post a comment