24/03/2016 Draw the displayed formula of ethanol What would we see in an IR and in the mass spec for ethanol? 24/03/2016 Key words • Odd • Radio • CDCl3 • Spin • TMS • Magnetic field 24/03/2016 So what is NMR? • A sample of the compound is spun in a magnetic field • hydrogen atoms in different environments respond differently to the field •each different environment of hydrogen produces a signal in a different position •the area under each peak / signal is proportional to the number of hydrogens •The signal can be split according to how many H’s are on adjacent atoms 24/03/2016 Look at your displayed formula for ethanol Predict how many peaks you would see in the 1H nmr 24/03/2016 NMR of ethanol 24/03/2016 24/03/2016 Low resolution and high resolution • the signal produced indicates the number of protons on adjacent carbon atoms • low resolution nmr gives 1 peak for each environmentally different group of protons What do you notice about the height of each peak? LOW RESOLUTION SPECTRUM OF 1BROMOPROPANE 24/03/2016 Draw displayed formula for the following and sketch the predicted nmr you would see a) propan-1-ol, b) propan-2-ol, c) propanone d) propanal Can we distinguish between any of these compounds based on low resolution nmr alone? 24/03/2016 Technical bits and pieces 24/03/2016 Tetramethylsilane (CH3)4Si PROVIDES THE REFERENCE SIGNAL The molecule contains four methyl groups attached to a silicon atom in a tetrahedral arrangement. All the hydrogen atoms are chemically equivalent. 24/03/2016 What other features should our reference have? • non-toxic liquid - SAFE TO USE • inert - DOESN’T REACT WITH COMPOUND BEING ANALYSED • has a low boiling point - CAN BE DISTILLED OFF AND USED AGAIN • all the hydrogen atoms are chemically equivalent PRODUCES A SINGLE PEAK 24/03/2016 What other features should our reference have? • twelve hydrogens so it produces an intense peak DON’T NEED TO USE MUCH • signal is outside the range shown by most protons WON’T OBSCURE MAIN SIGNALS • given the chemical shift of = 0 • the position of all other signals is measured relative to TMS 24/03/2016 Chemical shift 24/03/2016 Chemical shift • each proton type is said to be chemically shifted relative to a (our) standard (usually TMS) • the delta () scale is widely used as a means of reporting chemical shifts = Observed chemical shift (Hz) x 106 Spectrometer frequency (Hz) ppm (parts per million) the chemical shift of a proton is constant under the same conditions (solvent, temperature) 24/03/2016 Approximate chemical shifts H - CX RO H CHO -COOH 13 12 11 - C-H TM S -C=CH10 9 8 7 6 5 DOWNFIELD ‘deshielding’ The actual values depend on the environment 4 3 2 1 0 24/03/2016 24/03/2016 Summary An nmr spectrum provides several types of information :number of signal groups tells you the number of different proton environments Multiplicity tells you how many protons are on adjacent atoms peak area (integration) tell you the number of protons in each environment chemical shift tell you the general environment of the protons 24/03/2016 Draw displayed formula for the following and sketch the predicted nmr you would see a) propan-1-ol, b) propan-2-ol, c) propanone d) propanal Can we distinguish between any of these compounds based on low resolution nmr alone? 24/03/2016 What is high resolution NMR? 24/03/2016 High resolution gives more complex signals doublets, triplets, quartets, multiplets HIGH RESOLUTION SPECTRUM OF 1BROMOPROPANE The broad peaks are split into sharper signals The splitting pattern depends on the number of hydrogen atoms on adjacent atoms 24/03/2016 Challenge – can you draw the HIGH resolution NMR for a) propan-1-ol, b) propan-2-ol, c) propanone d) propanal?