Constant time

Collecting multi-dimensional spectra requires recording chemical shift information via the use of an incremented delay. Since the length of the delay increases over the course of the experiment the signal is recorded at different times for each increment. This allows coupling in the indirect dimension to evolve, which broadens the peaks and decreases resolution. The "constant time" method was developed to eliminate this problem.

Two-dimensional NMR

The development of two dimensional NMR in the 1970s was probably the greatest advance in the history of the technique. The addition of another dimension to NMR spectra expanded the types of information obtainable and made possible the vast array of tailored multi-dimensional experiments available today. The development of two dimensional experiments relied upon the introduction of pulsed NMR and the use of the fourier transform to process the data. To generate two dimensional data a variable delay between two pulses must be used. Incrementing the variable delay allows chemical shift information to be regularly sampled and later processed with the fourier transform to give a second dimension. Read on for more details.

DOSY processing

DOSY experiments can resolve signals from the individual components of a mixture of compounds without physically separating the compounds. This is achieved by using the different rates that molecules diffuse through solution to separate the signals. DOSY data are typically presented as a multi-dimensional NMR spectrum where one of the dimensions corresponds to the diffusion rate. Unlike the frequency dimensions, the diffusion data is not processed with a fourier transform. Instead, non-linear curve fitting is used to extract rates from which a pseudo spectrum is created.

The chemical shift of chloroform

Chloroform is one of the most commonly used solvents in the NMR Facility and many users use the residual solvent peak to reference their spectra. Unfortunately, I have seen a range of values for the chemical shift of chloroform. Its possible that chloroform's chemical shift is temperature dependent and the different values were obtained at different temperatures. To test this, and to determine the correct chemical shift to use, I decided to measure the temperature dependence of the residual chloroform peak in deuterochloroform.

COSY experiments

The COSY experiment was one of the very first 2D NMR experiments developed and still remains an essential tool. The original experiment has several shortcomings, however, and numerous modifications have been proposed. Three of the most useful are the gCOSY, the DQF-COSY, and the CLIP-COSY. This post discusses the problems of the original experiment and how the new versions attempt to solve them.

Probe sensitivity specifications

Despite my best attempts to convince users otherwise, many still want to run 1D ¹³C spectra. Because of the poor sensitivity of ¹³C this is a time consuming, and thus costly, experiment. Sensitivity is mainly determined by the strength of the magnet and the type of probe being used. Since we are currently considering acquiring new equipment, I was curious to see how different probes and fields would impact the time taken to record ¹³C spectra and so collected sensitivity specs for a variety of probes.

T1 noise

T₁ noise is streaks found in the indirect dimension(s) of multidimensional spectra. It is normally found around the most intense peaks in a spectrum and can obscure crosspeaks. How t₁ noise arises, and what can be done to reduce it, are discussed below.