Mass spectrometry Mass of the molecule IR infra red spectroscopy Functional groups in the molecule UV Vis spectroscopy conjugation conjugated double bonds NMR nuclear magnetic resonance H or C carbon and hydrogen framework Lecture 11 Monday February 2 2015 3 47 PM Isolated from Cinchona tree IR UV and NMR Electromagnetic waves For export Page 1 Lecture 11 Monday February 2 2015 3 49 PM Nuclear magnetic resonance or NMR In external magnetic field nuclei orient in such a way that the magnetic field associated with them are either parallel or antiparallel with the external magnetic field The two possible orientation are not the same Parallel is more stable by a bit So more than half will be parallel and the rest will be antiparallel Overall there will be small excess of those that are parallel We can make those switch to the antiparallel orientation by supplying the photon with the energy that is the same as the energy difference between the two orientation That photon will be in the radiofrequency of the electromagnetic field Those nuclei will absorb the photon and will get excited to a higher energy level and we can record this absorption in the form of the spectrum So we can see that at certain frequencies our sample has absorbed photons Stronger the magnetic field higher the frequency we need For export Page 2 Lecture 11 Monday February 2 2015 3 51 PM And we get a proton NMR spectrum like the one below What information does the spectrum contain z Number of signals Position of the signals Chemical shift Shape of the signals Splitting Surface area under the signal Integral 1 2 3 4 For export Page 3 Lecture 11 Monday February 2 2015 3 55 PM Number of signals Each set of chemically unique protons will give a signal in NMR Two H are chemically equivalent if by replacing either one with another group D I get the same molecule Ha and Hb are diastereotopic and in principle should each give a separate signal in an NMR spectrum Ha and Hb are enantiotopic and give one signal in NMR For export Page 4 Lecture 11 Monday February 2 2015 8 25 PM Position of signals Position of the signal chemical shift local magnetic field Chemical shift Electrons around protons create a magnetic field that is opposing the external magnetic field of the instrument We say that electrons shield the proton More electrons around a nucleus more shielding and therefore lower the frequency at which the electrons will absorb remember the frequency of the absorbed photon is directly related to the energy difference of the two states which is directly related to the strength of the magnetic field the nucleus feels Beff Bo Be From all this is comes that chemical shift is indicative of the electron density around a specific proton in a molecule Smal l differences in electron densities lead to different chemical shifts For export Page 5