Laboration 8a. Relaxation, T 1 -measurement with inversion recovery

Transcription

1 , T 1 -measurement with inversion recovery KR

2 Theory The way the magnetizations returns to equilibrium, relaxation, is a very important concept in NMR, for example, due to the fact that the rate of relaxation can give information about the dynamics of the molecule. Spin relaxation is a magnetic field dependent property and can be divided into two mechanisms, longitudinal relaxation and transverse relaxation. Longitudinal relaxation The part of the magnetization vector, M, that is parallel to the main magnetic field, B 0, is usually called longitudinal magnetization, designated as M z. The process for it to recover to the thermal equilibrium magnetization, M 0, is called longitudinal relaxation, which involves a time constant T 1. Longitudinal relaxation generally comes by interactions between the nucleus of interest and unexcited nuclei in the environment. T 1 is also known as "spin-lattice" relaxation T 1 can be measured by inversion-recovery experiment. Transverse relaxation The part of the magnetization vector M that is perpendicular to the main magnetic field B 0 is usually called transverse magnetization, which can be written as M xy, M T, or. The process for it to decay to zero is called transverse relaxation, which involves a time constant T 2. The transverse relaxation is dominated by interactions between nuclei which are already excited. For this reason, T 2 relaxation is called "transverse" or "spin-spin" relaxation. T 2 can be measured by a Spin-echo experiment. -2-

3 Inversion-recovery The inversion-recovery experiment allows to measure longitudinal or spin-lattice T 1 relaxation times of any nucleus. The basic pulse sequence of the inversion-recovery experiment consists of the following steps: A 180º pulse inverts the magnetization to the -z axis. During the following delay t, relaxation along the longitudinal plane takes place. Magnetization come back to the original equilibrium z-magnetization. A read 90º pulse creates transverse magnetization Acquisition is performed as usual -3-

4 Practical 1. Run a PROTON according to Bruker run manual for 500 MHz NMR", optimize sw and o1p. 2. Check the 1 H 90 o -pulse (lab 3). Experiment setup 3. edc or new and read the parameter set PROTONT1. You will find it when you are in the correct directory. Press the arrow and choose the pathway to Brukers parametersets under Experiment Dirs. /opt/topspin21/exp/stan/nmr/par (Bruker) 4. getprosol (get the preset probe and solvent specific parameters from the prosol list) or if the 1 H 90 o -pulse value need to be changed Set the measured p1 ( 1 H 90 o -pulse) getprosol 1H p1-value pl1-value (get probe and solvent specific parameters and use your p1 value to calculate other pulses) 5. If required, any acquisition parameter can be modified manually or in the AcquPars section, you can see what is valid for the parameters in PulseProg. a. Optimize the values of o1p and sw b. Define a vd-list. Go to Lists in the menu. Click on to the right of the VDList name entry box c. Select t1delay by clicking on it, click Enter the desired delays (in seconds). -4-

5 If any changes save the list in your own picked name and close it. d. Set number of scans to minimum ns=8 and ds=4. e. Set d1 to an accurate value, you find the parameter under the icon. It is very important to use a long recycle delay, usually d1=5*t1( 1 H) (d1=10 s) f. TD(f1) = is the number of experiments = number of delays in VDList. 6. The expected experimental time is displayed with the expt command or 7. Turn the spinner off, T1 experiments should be run non spinning 8. rga find the receiver gain 9. zg start the acquisition Process recorded data 10. Processing parameters can be modified manually or in the ProcPars. Be sure that SI> number of delays in VDList. 11. You have manuals in the Topspin software. Under Help you will find Manuals choose 1D and 2D step-by-step basic. Go to T1 Experiment processing and follow the instructions. 12. Your data points on the T 1 -curve should be automatically picked. To pick them manually, click. To adjust the settings, click. If not all your data points are picked, you may need to increase the number of drift points, and then re-pick. If you have too many points, correct Number of points to the same value as the number of delays in VDList. Input the vdlist file name in this parameters form. Increase the number of drift points in this parameters form if some points are missing after the Start Calculation step. DRIFT Enter the number of drift points. The maximum of a peak will be picked if it lies within DRIFT points of the position specified for the point picking. -5-