Two-dimensional NMR

Background

Two-dimensional (2D) NMR spectroscopy is often used to provide through-bond (J-coupling) or through-space (NOE) correlation. These correlations, shown as "crosspeaks" in the spectrum, can be analyzed to examine detailed molecular structure. In a 2D experiment, the direct detection dimension is usually 1H while the indirect (transient) dimension could be 1H, 13C, 15N, 31P, or other nuclei. The low sensitivity of the broadband nucleus is overcome by coherence transfer with 1H and encoding their chemical shift in the indirect dimension. The advantage of 2D NMR also lies in its ability to resolve ambiguous or overlapping resonances resulting from one nucleus (i.e 1H) along the indirect dimension of another nucleus (i.e. 13C).

Basic Requirement for 2D Experiments

2D experiments are naturally more complex than typical 1D experiments because more pulses and parameters are involved and good spectrometer stability is required. The following are necessary for good-quality 2D data, among others:

Good suppression of artifacts may also require calibration of certain pulses with your own sample.

While newer spectrometers (such as the Varian 600 and Bruker 800) require no re-cabling, older Varian spectrometers, including our 400 and 500, require a simple recabling for the X channel if the indirect dimension involves X nucleus.

Instrument Capability

NMR500 is equipped with a FTS temperature regulation unit, pulsed field gradients, waveform generators and gradient probes. It is capable of performing most 2D experiments. Data quality is mostly adequate for routine work. The Varian 600 and Bruker 800 MHz machines are newer, each equipped with a cryoprobe with four channel (1H/13C/15N/2H) inputs and PFG. The cryoprobes are designed primarily for proton direct detection and indirect detection of 13C and 15N, ideal for multi-dimensional NMR. Additionally, the 800 cryoprobe also has excellent 13C sensitivity for direct detection. Hardware, vibration control, and sensitivity are much better on these machines. When material is limited or best quality data is desired, the 600 or 800 is recommended.

2D NMR on Varian Systems

General Setting

Setting a 2D experiments on the Varian system usually involves setting the following hardware and additional parameters compared to a 1D experiment, regardless of the experiment:

Spectral width in the indirect dimension (sw1) should be kept just enough to enclose all signals so that good resolution can be achieved wth the same number of ni points.

How a 2D experiment deals with phase detection in the indirect dimension (via phase parameter) is experiment specific, and may have multiple choices for an experiment. The setup also involves setting experiment specific parameters, such as NOESY mixing time, recycle delay etc, although the default setting in the parameter files should be good for most samples. See specific experiment procedures for details.


H. Zhou updated Nov 2010