Tuesday, July 29, 2008
Resolution of Overlapping Signals Based on T1's
When students are asked what defines an NMR signal, they will most often say: the chemical shift, the coupling pattern, and the line width. One parameter which is often overlooked is the T1 relaxation time. The T1 is the time constant for the build up of magnetization along the magnetic field direction (z axis) when a sample is first placed in a strong magnetic field or after a pulse has been applied. It governs how long a spin system takes to come to equilibrium. T1's are measured with a 180-tau-90 inversion recovery sequence as a function of tau. For each resonance in an inversion recovery spectrum, there is a value of tau for which the signal will be nulled. If two signals with different relaxation properties overlap one another, tau values can be found which will null each of the signals individually and thus reveal the other signal. An example of this is shown in the figure below where the overlapping 1H signals of the methylene protons one either side of the carbonyl group of 3-heptanone are examined (i.e. those in the 2 and 4 positions). The black trace in the bottom panel is the 1H spectrum of the overlapping signals. The blue trace in the middle panel is an inversion recovery spectrum (phase corrected by 180 degrees) where a 4.1 second delay was employed. This delay nulls the protons on the 4 position revealing the quartet in the 2 position. The red trace in the top panel is an inversion recovery spectrum where a 5.5 second delay was employed. This delay nulls the protons on the 2 position revealing the triplet in the 4 position.