Download Lecture Notes on Methods in Magnetic Resonance | CHM 598 and more Study notes Chemistry in PDF only on Docsity!
Lecture
Methods in Magnetic Resonance
We will use the 200 MHz Varian NMR to demonstrate some standard 1D NMR Experiments We have covered the standard 1 pulse 1 H NMR experiment and even shown how to array parameters to create a set of 1D data. For example, a nutation experiment can be done by arraying the pulse width (pw) in a standard 1h NMR experiment.
NUTATION - 1H, WATER Today we will collect a nutation, inversion recovery and spin echo data set for analysis on Thursday. The nutation data was collected with the following parameters:
30 FIDs collected (w/ pw=0,10,20,30…290 μs). An example FID (w/ pw=20) is shown below (type df(3) dscale )
The resulting spectrum (type ds(3) dscale ):
Now using the 2nd or 3rd spectrum to phase ( aph ) and setting the software in absolute intensity mode ( ai ), we can plot all 30 spectra in a horizontal array ( dssh ):
This data looks good and should allow us to determine the π/2 and π times. These will be needed for spin echo and inversion recovery experiments. NOTE: The damping in the sine wave nutation is primarily due to B1 field inhomogeneity and (at long times) spin relaxation. Before leaving, we need to save the data. The FIDs are saved as 1H_Nutation_Water ( svf('1H_Nutation_Water2') and writefid('1H_Nutation_Water2') ) Also, we collected another data set in class that arrayed pw from 0 to 145 in steps of 5 μs. This data was saved as 1H_Nutation_Water
Chm598 - Prof. Yarger Tuesday, February 13, 2007
INVERSION RECOVERY - WATER
Okay, from the nutation you can determine that the π/2 is roughly 30 μs and the π is roughly 60 μs (more accurate values can be obtained from a fit to the Nutation data). So, we will use these estimates to design a spin-echo and inversion recovery experiments to determine T 2 and T 1 , respectively. Next class period will be used to analyze the data collected today. The goal is to determine accurate values for T 1 and T 2 for water.
Okay, lets start with the inversion recovery. This is a π-t-π/2 pulse sequence ( dps ):
Modifying the standard 1H sequence (s2pul), we setup the inversion recovery with the following parameters ( dg arraylist ):
Now, run this arrayed list of 1D 1H NMR experiments ( ga ), ft the data ( ft ) and plot ( dssh ):
Okay, lets save this data, svf('inv_rec_water_chm598_class')
Also, we collected other data sets during class. These were named water_inversion_recovery*
SPIN ECHO - WATER
Now we need to measure the value of transverse relaxation (T 2 ). To do this we will use a standard 2-pulse spin echo NMR experiment (π/2-t-π-t-detect). However, in the s2pul sequence there is no easy way to add a time between the last pulse and detection. Hence, we will collect the entire echo (instead of just the fid). We can use the echo amplitude to versus the delay time between pulses to determine the spin-spin relaxation time (T 2 ). The pulse sequence looks like (type dps ):
The data looks like:
Set d1=5 sec, nt=1 and array d2 from 0.3 to 6 in steps of 0.3 sec.
Save data as spin_echo_chm598_class
We will discuss the data and theory of spin echoes more next week.
To determine T2 from this data set, use the following relation:
M(t) = M (^) o exp(-2t/T 2 )
So, plot log M(t) versus 2t: ln M(t) = ln Mo - 2t/T 2
Also, the decay of an FID goes as: M(t) = Mo exp(-t/T 2 ) (keep in mind that the FID is often dominated by the solvent and the solvent can have a much different T 2 than the spins on the sample.)
And the spectral linewidth is approximated by: δv = 1/πT 2 * (where δv is the half-height linewidth and T 2 *^ is the homogeneous and inhomogeneous components of transverse relaxation (T 2 is the homogeneous part and T 2 +^ is the inhomogeneous part) so,
1/T 2 *^ = 1/T 2 + 1/T 2 +
