Basics of NMR Spectroscopy

NMR Spectroscopy

1. Basics of NMR Spectroscopy Learn Chemistry Think Chemistry Practice Chemistry By Halavath Ramesh, Chemical Science

2. Introduction Nuclear Magnetic Resonance spectroscopy usually abbreviated as NMR( or nmr) spectroscopy. NMR spectroscopy deals with the behaviour of certain nuclei when simultaneously exposed to a magnetic field as well as electromagenetic radiation, unlike vibrational and electronic spectroscopies . Where electrons are promoted from their ground states to higher vibrational and electronic energy levels. Respectively , on exposure to electromagenetic radiation in NMR spectroscopy , transitions occur between nuclear energy levels.

3. Again , in contrast to vibrational and electronic spectroscopies , not all nuclei can be studied by NMR spectroscopy. Only those nuclei which have a non-zero nuclear spin quantum number I give raise to NMR spectra. NMR spectroscopy is a very powerful tool in the structure determination of organic molecules. This is because the two nuclei present in almost all organic compounds, namely proton (1H) and carbon(13C) are both detectable by this method. Besides detection the method also enables us to differentiate between different types of the same nucleus present in the molecules and to quantify them as well. Finally , NMR spectroscopy establishes the connectivity pattern of the nucleus being studied as located in the molecule of interest.

4. Principle of NMR Spectroscopy As mentioned above in the introduction, only those nuclei having I# 0 are NMR active. The proton for which I=1/2 behaves like a small , spinning , bar magnet. When exposed to a magnetic field , the proton orients itself either with the field (which is lower in energy) or against the field (higher in energy). As the proton is spinning , the magnetic field causes it to process or move in a circular path perpendicular to the direction of the field. The processional frequency v is directly proportional to the magnetic field strength H .

5. In order for the proton to absorb energy and thus move from the lower energy state to the higher one, it needs to be exposed to electromagnetic radiation of the correct frequency ῡ . When the processional frequency and the external frequency match, the nucleus is said to be in “ resonance ” . It can now absorb energy and get promoted to the higher energy state. Hence the phenomenon is known as nuclear magnetic resonances (NMR). Two important consequences of this are 1. Saturation 2. Relaxation

6. Saturation: Saturation refers to the condition when, due to excitation from E1 to E2, both energy states are equally populated. If this occurs, no further energy can be absorbed and the system is said to be “ saturated “ and the signal disappears. Fortunately, saturation is offset by relaxation or demotion of the excited nucleus from E2 to E1. One mode of relaxation is for the nucleus to emit the energy as electromagnetic radiation. Two other modes of relaxation , which are radiation less are also important. In this modes, the excess energy is either transferred from one nucleus to another or to the surroundings e.g., solvent. They are referred to as spin-spin and spin-lattice relaxations, respectively.

7. NMR spectroscopy would have little utility in structure determination if all nuclei (e.g. proton) , present in a molecule were to come to resonance at the same frequency. Fortunately, this is not so, as already discussed. The three different sets of protons of 1- bromopropane resonate at three different frequency region. This shift in frequency depends upon the chemical environment in which the protons are located and is, therefore known as the chemical shift. As the frequencies are dependent on the magnetic field strength H 0. Field Low<........... ________> High High<........... _______> Low Frequency

8. NMR Sample Preparation Sample preparation is a very important aspect of NMR spectroscopy. Almost all spectra are recorded in solution and hence the choice of the solvent is also critical. The primary requirements for a suitable solvent are: 1. The sample should be freely soluble in it to give reasonably high concentration. 2. It should be chemically inert with respect to the sample. 3. It should be of low viscosity as viscous solvents give broad spectra. 4. It should not have any absorptions in the regions of interest in the spectrum. 5. The last point, especially for proton samples, is conveniently overcome by using solvents in which protons are replaced by deuterium.

9. NMR-Sample preparation – Quantity For proton NMR spectra of small organic compound ( upto MW 500) anything between 1 and 20mg of sample will be fine. Concentrated solution can be viscous and may result in broad signals , very diute samples could be marked by impurities and solvent peaks. Carbon-13 is present at approximately 1.1% natural abundance. It is intrinsically less sensitive than protons 50-100 mg fine. Solvent height (volume) should be uniform 5 cm or 2 inches 0.5 ml. Sample prepared with too much solvent waste both time and money.

10. Sampling- Preventing contamination from the cap 1. The tube cap may contaminate the sample. 2. Even a new cap releases materials especially when the solvent is Cdcl3 or DMSO-d6. 3. Hence to prevent close the tube with Teflon tape and with the cap. 4. Do not invert the tube but mix with a vortex mixer for a whole minute.

11. The Chemical Shift If all protons were to resonate at the same frequency, NMR would hardly be popular for structure determination. Electro negativity of a substituent is an important factor in determining the chemical shift. Factors that effect chemical shifts: 1. Inductive effect 2. Hybridization of 13C 3. Magnetic anisotropy 4. Mesomeric effect 5. Aromatic ring current effect 6. Steric effect 7. Hydrogen bonding

12. To refresh

13. As the electro negativity of the attached group increases the chemical shift of the attached proton will......................... Increase or decrease Ans : Increase

14. 2. Chemical shift of NMR solvent CDCl3 occurs at ..............ppm in ‘H NMR Spectrum. Ans : 7.26

15. In ‘H NMR spectroscopy the range of chemical shift of aromatic protons occurs between ................................ ANS : 6.5 and 8.7

16. 4 . How many types of chemically equivalent protons are present in toluene ? Ans : 2

17. 5 . Negative peak in DEPT 135 is correspond s to ..........................Carbons Methine Methylene Methyl Both A and B

18. 6. Nuclear magnetic resonance occurs when molecule interacts with which of the following electromagnetic radiation ? Ans : Radiowave