Gradients for coherence selection

In addition to artifact suppression and removing solvent peaks, gradient pulses can be used for coherence selection. Traditionally, this was done by phase cycling but using gradients for coherence selection allows cleaner spectra to be obtained more quickly. In this post the use of gradients to select coherences in heteronuclear experiments is discussed.

Gradients for solvent suppression

The previous post described how a combination of gradients and pulses can be used to help reduce artifacts in NMR spectra. Adding a selective pulse to that sequence enables solvent signals to be excluded. This technique, sometimes known as excitation sculpting, is described below.

Gradients for removing artifacts

The previous post gave an introduction to gradient pulses, describing what they are. In this post, and the next few, I will describe a few applications of gradients. This post will describe how they can be used to reduce artifacts from imperfect pulses, allowing phase cycles to be reduced and experiments to be run more quickly.

Gradients

Gradients have become an essential part of modern NMR spectroscopy. Nearly all probes now have pulsed field gradient capabilities and the vast majority of pulse sequences use gradient pulses. In this post, and those following, I will try and explain what gradients are and how they can be useful.

Sensitivity vs receiver gain

Recently I have noticed many users recording spectra with low values for the receiver gain. The receiver gain is a scaling factor for the FID signal that ensures spectra are not distorted if the signal is too large, or resolution is lost if the signal is too small. Typically the receiver gain is set automatically by the command "rga", but users should pay attention to the value obtained as it is a good indicator of how successful the experiment will be. To demonstrate this I measured sensitivity at different receiver gain values.

15N HMBC sensitivity

The HMBC experiment is normally used to detect ¹H-¹³C correlations, but it can also be used for ¹⁵N. An ¹⁵N HMBC experiment may provide structural information that cannot be obtained from a ¹³C HMBC, and it can also be used to determine the number of nitrogen atoms present in a compound. Recently I was asked, "How does the sensitivity of an ¹⁵N HMBC compare with that of a ¹³C HMBC?", so I decided to try and find out.

Variants of the 1D 13C experiment

1D ¹³C spectra are often used to confirm compound identity. The simplicity of such spectra enable the number and type of carbon atoms present to be quickly evaluated. 2D ¹H-¹³C correlation spectra, while inherently more sensitive and informative, require more work to interpret and may not show signals from all carbon atoms. Consequently, 1D ¹³C spectra still have a place in compound characterization. To increase the sensitivity and information content of 1D ¹³C spectra several variants of the original pulse sequence have been developed. Several of these are discussed below.

Single bond correlations in HMBC spectra

HMBC spectra are designed to show correlations between hydrogens and heteronuclei separated by two or more bonds, however, single bond correlations are often present in HMBC spectra as well. Like the multiple bond correlations in HSQC spectra discussed in the previous post, single bond correlations in HMBC spectra can be confusing if you are not expecting them, but once aware that they may be present they are fairly easy to identify.

Multiple bond correlations in HSQC spectra

¹H-¹³C HSQC spectra are designed to show correlations between carbons and their directly attached protons. Interpretation of all the correlations observed in a HSQC spectrum is an essential step in characterizing a new molecule. It is not uncommon, however, to observe HSQC correlations over more than one bond. At first these long range correlations can be quite confusing, but if one is aware of the possibility of long range correlations being present, they can actually help the analysis.

Assigning a simple compound: ODCB

ODCB (o-dichlorobenzene) is a simple aromatic compound used as a standard for monitoring ¹H resolution. In a ¹H spectrum it produces just two peaks, due to the symmetry of the molecule, but it is not immediately obvious which nuclei cause which peaks.

T1 and T2 relaxation

In NMR, relaxation is the process by which magnetization returns to equilibrium. Relaxation is important because it determines how long to wait between scans, how quickly signals decay, and the width of peaks in a spectrum. In the vector model relaxation is separated into T₁ relaxation along the z-axis and T₂ relaxation in the x-y plane.