Uploaded Nmr Spectroscopy

Spectroscopy It is the study of interaction between matter and energy. When energy is radiant on the matter each matter, substance, atom, molecules or ion etc behave distinctly and in characteristic manner. The matter can absorb energy, it can reflect energy, it can allow the energy to pass through, any two processes or all the three processes can occur. How the matter will behave will also depend upon type of energy. The energy could be light energy, heat energy, magnetic energy etc. The scientific study of all its implication involved with its application in the study of molecular shape and size uses the technique of spectroscopy. The techniques of nuclear magnetic resonance, infrared spectroscopy, and ultraviolet spectroscopy are very useful and most modern. Nuclear Magnetic Resonance Spectroscopy. The nuclei of many elemental isotopes have a characteristic spin. Some isotopes of our interest have half spin. These isotopes are 1H, 13C, 19F and 31P, all of which have I = 1/2. Our discussion of NMR will be limited to these and other having I = 1/2 nuclei. Since these nuclei are charged positively the spinning nuclei act like tiny bar magnet and interact with external magnetic field. In the absence of external field, spins of magnetic nuclei are oriented randomly. In the presence of external magnetic field these nuclei orient themselves specifically. A spinning nucleus can orient so that its own tiny magnetic field is either aligned with or against external magnetic field. The two orientations do not have the same energy. However the difference in the energy is very small and depends upon external magnetic field applied. If these nuclei, under the influence of external magnetic field, are subjected to electromagnetic radiation of proper frequency, energy is absorbed and nuclei of lower energy state “spin- flip” to higher energy state.

When this spin-flip occurs, the magnetic nuclei are in resonance with applied radiation. The exact frequency necessary for resonance depends upon the strength of external magnetic field and the identity of the nuclei. A NMR spectrometer can detect the energy absorption of a nucleus in resonance. In NMR spectroscopy the frequency of electromagnetic radiation is held constant while the magnetic field is varied.

Formation NMR Spectrum When an external magnetic field is applied to a molecule the electrons in the molecule set up their own magnetic field which is in the opposite direction of applied magnetic field. Hence nucleus experience slightly less magnetic field than the applied field. This phenomenon can be called as shielding of nuclei from full effect of external magnetic field. Each specific nucleus in a molecule is in slightly different electronic environment. Each nucleus is shielded to a slightly different extant and the effective magnetic field is different for each nucleus. This difference in the magnetic field can be detected by the NMR spectroscope and distinct NMR signal is available from the instrument. Thus a complete spectrum of all the molecules in a sample and all atoms in the molecule is possible. The instrument will show chart of one atom of a nucleus at a time .i.e. a different chart is available from the instrument for each atom of nucleus. To analyze one molecule of C3H8, two sets of chart will be given out by the NMR spectroscope one for hydrogen and one for carbon. NMR spectrum:

An NMR spectrum is a graph of magnetic field strength absorbed by hydrogen atom (carbon atom also possible) of a given compound at given frequency. The field strength is marked as parts per million (ppm) increases from left to right.

(Although number decreases) and is indicated by “∗”. Left part of the graph is called down field and right part is called up field. There should be a peak at “O” ppm (not present in the graph) which is due to reference compound. The reference compound is tetra-methylsilane which is used to calibrate the instrument.

Characteristics of NMR Spectrum * Each peak represents chemically equivalent hydrogen. * Splitting of peaks is created by neighboring hydrogen. * Equivalent hydrogen has equal chemical shift. * Area under the peak is proportional to number of hydrogen represented by the peak. Number of peaks in the spectrum Each peak represents a different environment for hydrogen atoms in the molecule. In the methylpropanoate spectrum above, there are three peaks because there are three different environments for the hydrogens. Remember that methyl propanoate is CH3CH2COOCH3. The hydrogen in the CH2 group is obviously in a different environment from those in the CH3 groups. The two CH3 groups aren't in the same

environment either. One is attached to a CH2 group, the other to oxygen. In the molecule there are three hydrogen atoms attached to 1st carbon. All the hydrogen atoms attached to 1st carbon are in the same environment hence there is no separate peak for these hydrogen. Similarly there is no separate peak for two hydrogen atoms attached to next carbon and the three hydrogen atoms attached to last carbon Spin splitting of peaks It is noticeable that there are some sub-peaks or peaks have split. This splitting represents neighborhood carbon with hydrogen atom. The amount of splitting tells you about the number of hydrogen attached to the carbon atom or atoms next door. The number of split-peaks in a cluster is one more than the number of hydrogen attached to the next door carbon(s). This is called as n+1 where ‘n’ is number hydrogen that is not chemically equivalent One peak -------- no hydrogen attached to next carbon Doublet ------Triplet carbon

one hydrogen attached to next carbon

--------

Quartet --------

Two hydrogen attached to neighboring Three hydrogen attached

In the spectrum shown above the CH3 group at about 4.1 ppm shows no splitting. It means it does not have any carbon with hydrogen attached to it. At about 2.24 ppm it shows triplet which means it has a CH2 neighbor and lastly at about 1 ppm it shows a quartet which means its neighbor is CH3 Chemical Shift

Chemical shift is the difference in the resonance frequency of the hydrogen in the sample and resonance frequency of reference compound such as tetramethylsilane. Hydrogen which are not differentiable from other hydrogen by way of the location on carbons are called enantiotropic. As explained above they are represented by the same peak on nmr spectrum. The chemical shift of all enantiotropic hydrogen is same. The chemical shift is a constant value irrespective of operating frequency of the instrument. The constant values of chemical can be used to detect the alkyl group, alcohol group and some other group. Chemical shift for R – CH3 is 0.7-0.16 ∗, for -OCH3, ∗ and for R-CH2 2.0-2.9 ∗. Area under the peak Areas under the peak are in the same ratio as the ratio of number of hydrogen atom causing each peak. NMR spectrometers have a device which draws another line on the spectrum called an integrator trace (or integration trace). You can measure the relative areas from this trace. Height of integrated curve is proportional Area under the peak. The greater height of the peak does not mean greater area under the peak but greater height of integrated curve mean greater area under the peak. An instrument known as digital tracer records the number which corresponds to rise in line. The exact number of hydrogen is not given by integral trace or the digital trace. It is the ratio of hydrogen from one peak to another peak is determined.

Electron Shielding

Electron shielding phenomenon as already discussed is the reduction of external magnetic field due magnetic effect of electron surrounding the nucleus. The resonance magnetic field strength increases to cause spin flip due to shielding effect. Thus there is a lateral shift of the peak on the NMR spectrum. Electron withdrawing group or electron donating also effect lateral shift as these group increases or decreases electron density on the hydrogen. Hence lateral shift can also be used to determine the presence of electron withdrawing group and electron donating group although not very accurately. The discussion so far was based on the hydrogen atom. However 13carbon also behave in the same manner. In fact the NMR spectrum properties of hydrogen and 13 carbon are very much similar. It should be noted that all nuclei with odd atomic number or mass will show NMR spectrum. The advantage of NMR technology is that it requires very little energy, a very tiny sample can be used for testing and it is nondestructive testing. ________________________________________________________________________