What is CPD in Chemistry

AK Spiteller -  Analytics - important NMR experiments

Introduction - NMR experiments and how they work

In this small overview the current one- and two-dimensional experiments are presented.
The nucleus excited at the beginning (usually optimally: ¹H) determines the repetition time with its relaxation time. That is the time after the last and before the first pulse. It is made up of the so-called acquisition time aq, which is the FID detection time, and an additional relaxation interval d1.
The gyromagnetic ratio γ of the excited as well as the detected nucleus, together with the field strength, the probe head parameters, the temperature, etc., determines the sensitivity of the experiment. Experiments with ¹H-excitation and detection are therefore ideally suited!

180º pulses (shown as rectangles) applied to the X-core are usually so-called "adiabatic" pulses with field strengths> 8 Tesla, which have a much larger bandwidth than rectangular pulses. CPD stands for Composite Pulse Decoupling, a broadband decoupling of the corresponding core. The standard method for ¹H-decoupling is the WALTZ-16 sequence [J.Magn.Reson. 52,335 (1983)]. The p5m4 pulse sequence [J.Magn.Reson. 124, 474 (1997)] enforced with adiabatic pulses.

So-called B0 gradient techniques are increasingly being used in heteronuclear and increasingly also in homonuclear pulse sequences. As a rule, a first gradient dephased a desired magnetization in order to then rephase it selectively after a polarization transfer. Magnetizations that do not go through this specific polarization transfer remain dephased and therefore undetectable. As with the phase cycles of the individual pulses, the desired coherence transfer path is also selected here. In contrast to this, gradients only rephase the desired magnetization (e.g. ¹³C-bound hydrogen), the undesired thermal energy (e.g. ¹²C-bound hydrogen) remain dephased and do not interfere with the detector.

Unless otherwise stated, the pulses are at 90º and 180º angles, respectively. In the experiments, the desired transverse magnetization (Ix, y or Sx, y) is underlined (dotted).

For each pulse sequence, the current program name is given in the Bruker program library at the end. The experiment, if available, with adiabatic inversion and refocusing pulses is indicated there. If the corresponding pulse program is missing there, it can be displayed in this document and also downloaded!