1 Instrumental Analysis Tutorial 10 Nuclear Magnetic Resonance NMR

Quiz 3 Quiz 3 will take place on Thursday 13 th of December, from 9:00 to 9:30 Am, covering lectures 7, 8 and 9 + their tutorials. Exact Locations for all groups are in the following slide. Bring a pencil and a rubber for the quiz.

Locations LocationGroup Exercise Room – C7.01 T 1 T 2 T 3 T 4 Biotechnology group Exercise Room – C7.02 T 5 T 6 T 7 T 8 T 9 T 10

4 Question # 1 What is the condition the nuclei of certain element should possess in order to be NMR active? Give examples. For two of these examples, indicate the value of the total spin quantum number, I, as well as the number of allowed magnetic spin states. For a nucleus to be NMR active, it must have an odd atomic number or odd mass number (odd no. of proton or odd no. of neutrons). In other word, it should have a net magnetic moment. Example: 1 H:I = ½, the number of magnetic spin states = 2I + 1 = 2 (two states) 14 N:I = 1, the number of magnetic spin states = 2I + 1 = 3 (three states)

5 Question # 2 A nucleus has a spin quantum number of 1/2. How many magnetic energy states does this nucleus have? What is the magnetic quantum number of each? What happen for these states if an external applied field is applied on such a nucleus? For a nucleus of I=1/2, there would be two magnetic energy levels (2I+1). One of these state has a value of m = +1/2 and the other with m = -1/2. In absence of applied magnetic field, these two states have the same energy. Upon applying a field of strength B 0, the spins are lined up parallel to the applied field, either spin aligned (  spin state) or spin opposed (  spin state) to the external field. This means that the two states are no longer of the same energies (splitting of the two states occurs). The stronger the applied field, the greater the difference in energy between the two states. m = +1/2 m = -1/2 Strength of the applied field increases

6 What is the absorption frequency in a 7.1 T magnetic field of a 13 C (Gyromagnetic ratio for 13 C = x10 7 T -1 s -1 ). Question # 3 Calculate the magnetic field (in Tesla ) required to flip the spin of 1 H nuclei in an NMR spectrometer that operates at 360 MHz. (gyromagnetic ratio for 1 H = 2.675x10 8 T -1 s -1 ) Question # 4

7 Explain the differences in the way a continuous-wave and Fourier transform NMR experiment are performed. Question # 5 In the continuous-wave experiment, the strength of the magnetic field is slowly scanned (changed from downfield to upfield) while the frequency of the source is held constant. In some instruments as an alternative, absorption signal is monitored as the frequency of the source is slowly scanned while the strength of the applied magnetic field is kept constant In both cases, if the frequency of the radio-waves matches the Larmor precession frequency of the spinning nuclei (depends on the strength of the applied field), energy can be absorbed causing a flip in the nuclear magnetic dipole (change in the direction of spin).

8 In Fourier transform NMR experiment, nuclei in a very strong magnetic field (constant strength) are subjected periodically to very brief pulses of intense broad band radio-frequency radiation. Question # 5 (continued) Pulse duration or pulse width Interval between pulses Pulse train Each pulse is actually a packet of RF radiation (multiplex)

9 Question # 6 In pulsed or FT-NMR technique, explain briefly what happen before, during and after application of radio-frequency pulse to sample containing hydrogen nuclei. Before irradiation The net nuclear magnetization M 0 is aligned statically along the z axis (M 0 =M z, M xy =0) During the pulse: Under the effect of B 1 (magnetic field caused by the radio-frequency radiation), the net magnetization M 0 is displaced from equilibrium and is flipped toward the xy-plane. The tip angle or the flip angle, , is determined by the power and duration of the electromagnetic irradiation (time for which B 1 is turned on).

10 After irradiation ceases After irradiation ceases, B 1 turns off after the pulse, the magnetic moment M 0 must now rotate in clockwise direction back to presses around the z axis. This motion gives rise to a signal (current) that can be detected by the same coil (along the x axis) that is used to produce the original pulse. As relaxation proceeds, this signal decreases exponentially and approach zero as the magnetic moment reaches the z axis. (Note that the coil acts as a receiver coil or detector that can sense only the magnetic field on the xy plane). This time domain signal is called the free induction decay (FID signal) (FID: free of the influence of radio-frequency field, induced current in the coil and decaying back to equilibrium) Question # 6 (continued) 90 deg pulse  deg pulse X Y Z MoMo z x B1B1 y oo B0B0 x y x M xy y x y 

11 z x M xy y z x y MoMo Free Induction Decay (FID) - One frequency 90 y pulse relaxation Question # 6 (continued) Unlike other spectroscopic technique, in FT-NMR we measure neither the amount of light absorbed (during the pulse) nor the amount of light emitted after the pulse (relaxation process is observed as heat absorbed by the lattice). What we actually measured is the free induction decay (FID) (the NMR signal) that is the current induced in a coil as the nuclei magnetic moment rotate away from the xy plane to the equilibrium position on z axis. Question # 7 What exactly do we measure in pulsed NMR technique, is it the amount of radiation absorbed or emitted or something else?

12 The NMR spectrometer mainly consists of: 1. Superconducting magnet: to produce a stable and homogeneous magnetic field B 0 2.Coil: in which alternating current passes and acts as the source (transmitter (B 1 )) during the application of radio-frequency. Also, the same coil feels the free induction decay signal and acts as receiver (detector) after ceasing the radio-frequency pulse. 3.Sample probe: that holds the sample in a fixed position in the magnetic field, houses the coil or coils that permits excitation and detection of the NMR signal, and contains air turbines to rotate the sample. 3.Powerful computer: for data acquisition and to convert the time domain signal to spectrum (the frequency domain) using Fourier transformation. Question # 8 What are the main components of NMR spectrometer?

13 Note that in FTNMR instrument, one coil (not two as shown in figure) acts as both source and detector at the same time

14 Hardware Superconducting magnet is bathed in liquid helium at temperature of 4 K and has a high field strength (as high as 21 T) and shows very high stability. Sample probe

15 Why are liquid sample spun while being examined in a NMR spectrometer? Question # 9 In order to homogenize the effect of the external magnetic field on different molecules of the sample so that each nucleus feels the same strength of the applied field. Liquid sample or solution Sample probe

16 The index of hydrogen deficiency [Modes of unsaturation] It is a measure of the number of  -bonds and/or a ring a molecule contains. The IHD is determined from an examination of the molecular formula of an unknown substance. C number of carbon atoms (2C+2) maximum number of hydrogen atoms as in alkanes N number of nitrogen atoms X number of halogen atoms Hnumber of hydrogen atoms already presents Each 1 in IHDone double bond or a ring 2 in IHD one triple bond Two double bonds One double bond and a ring Two rings 4 or more high probability of presence of aromatic ring

17 Example Calculate the IHD for each of the following molecules: C 3 H 8 C 7 H 6 OC 3 H 6 O HD of 5 mainly indicates that this compound is aromatic (one ring and three double bonds (4)). In addition, it contains a center of unsaturation outside the aromatic ring. IHD of one mainly indicates that this compound contains on unsaturation center (may be one double bond or one aliphatic ring) For C 3 H 8 For C 7 H 6 O saturated hydrocarbon For C 3 H 6 O