Chemsheets AS006 (Electron arrangement) 19/04/2017 www.CHEMSHEETS.co.uk NMR SPECTROSCOPY © www.chemsheets.co.uk A2 032 17-Jul-12

1H NMR SPECTROSCOPY © www.chemsheets.co.uk A2 032 17-Jul-12

NUCLEAR SPIN Protons and neutrons can be regarded as spinning about their axis. In many atoms these spins are paired against each other and so the nucleus has no overall spin (e.g. 12C). In some atoms (e.g. 1H and 13C) the nucleus has an overall spin. A nucleus that spins generates a magnetic field. The direction of the magnetic field depends which way the nucleus spins. © www.chemsheets.co.uk A2 032 17-Jul-12

Applied magnetic field NUCLEAR SPIN Usually the two possible spin states of the nucleus have the same amount of energy. However, in a magnetic field, the two spin states have different energies. Energy Magnetic field opposed to applied field Applied magnetic field Energy gap corresponds to frequency of radiowaves Magnetic field in same direction as applied field © www.chemsheets.co.uk A2 032 17-Jul-12

butanone © www.chemsheets.co.uk A2 032 17-Jul-12

EQUIVALENT H's In a spectrum, there is one signal for each set of equivalent H atoms. The intensity of each signal being proportional to the number of equivalent H atoms it represents. © www.chemsheets.co.uk A2 032 17-Jul-12

2 sets of equivalent H’s: ratio 6:2 (3:1) © www.chemsheets.co.uk A2 032 17-Jul-12

dimethylethyl propanoate but-2-ene For each of the following compounds, predict the number of signals and the relative intensity of the signals. a) methylpropene propene 2-chloropropane propanone methylamine ethyl propanoate 1,2-dibromopropane dimethylethyl propanoate but-2-ene © www.chemsheets.co.uk A2 032 17-Jul-12

2 signals: ratio 6:2 (3:1) 1 signal 3 signals: ratio 2:1:3 © www.chemsheets.co.uk A2 032 17-Jul-12

3 signals: ratio 2:1:3 3 signals: ratio 3:2:9 © www.chemsheets.co.uk A2 032 17-Jul-12

SOLVENTS & CALIBRATION Samples are dissolved in solvents free of 1H atoms, e.g. CCl4, CDCl3. A small amount of TMS (tetramethylsilane) is added to calibrate the spectrum. It is used because: its signal is away from all the others it only gives one signal it is non-toxic it is inert it has a low boiling point so is easy to remove © www.chemsheets.co.uk A2 032 17-Jul-12

RELATIVE INTENSITY © www.chemsheets.co.uk A2 032 17-Jul-12

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CHEMICAL SHIFT The spectra are recorded on a scale known as the chemical shift (d), which is how much the field is shifted from the field for TMS compared to the field for TMS. d = Field for TMS - Field measured x 106 Field for TMS The d is a measure in parts per million (ppm) of how far the magnetic field required for absorption is shifted away from that for TMS. The d depends on what other atoms/groups are near the H – more electronegative groups gives a greater shift. © www.chemsheets.co.uk A2 032 17-Jul-12

Data from AQA datasheet

f) ethyl methylpropanoate For each of the following compounds, predict the number of signals and the relative intensity of the signals. a) but-1-ene 1,2-dichloropropane propanal d) 2-methylpropan-2-ol e) pentane f) ethyl methylpropanoate © www.chemsheets.co.uk A2 032 17-Jul-12

ethyl methylpropanoate a) but-1-ene 1,2-dichloropropane propanal d) 2-methylpropan-2-ol Pentane ethyl methylpropanoate © www.chemsheets.co.uk A2 032 17-Jul-12

Data from AQA datasheet Number of signals Relative “size” Chemical shift a) propanal b) ethyl methylpropanoate Data from AQA datasheet

Number of H’s next door +1 SPIN-SPIN COUPLING Coupling / Splitting / Multiplicity Number of H’s next door +1 © www.chemsheets.co.uk A2 032 17-Jul-12

n+1 0 H next door singlet (s) 1 H next door doublet (d) 2 H next door triplet (t) 3 H next door quartet (q) more H next door multiplet (m) © www.chemsheets.co.uk A2 032 17-Jul-12

SPIN-SPIN COUPLING signal singlet doublet triplet quartet appearance number of lines 1 2 3 4 number of H’s next door relative size 1:1 1:2:1 1:3:3:1 © www.chemsheets.co.uk A2 032 17-Jul-12

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© www.chemsheets.co.uk A2 032 17-Jul-12

Number of H’s next door +1 But you don’t couple to H’s that are equivalent H’s on O’s © www.chemsheets.co.uk A2 032 17-Jul-12

SUMMARY Number of signals Position of signals Relative intensities Splitting how many different sets of equivalent H atoms there are information about chemical environment of H atom gives ratio of H atoms for peaks how many H atoms on adjacent C atoms © www.chemsheets.co.uk A2 032 17-Jul-12

dimethylethyl propanoate but-2-ene For each of the following compounds, predict the number of signals, the relative intensity of the signals, and the multiplicity of each signal. a) methylpropene propene 2-chloropropane propanone methylamine ethyl propanoate 1,2-dibromopropane dimethylethyl propanoate but-2-ene © www.chemsheets.co.uk A2 032 17-Jul-12

2 signals: ratio 6 : 2 (3 :1) s s 1 signal 3 signals: ratio 2 : 1 : 3 d m d 2 signals: ratio 3 : 2 t q 2 signals: ratio 6 : 1 d m 4 signals: ratio 3 : 2 : 2 : 3 t q q t © www.chemsheets.co.uk A2 032 17-Jul-12

3 signals: ratio 2 : 1 : 3 d m d 3 signals: ratio 3 : 2 : 9 t q s d q © www.chemsheets.co.uk A2 032 17-Jul-12

Number of signals Relative “size” Chemical shift Multiplicity a) 1,2-dichloropropane b) but-1-ene © www.chemsheets.co.uk A2 032 17-Jul-12

13C NMR SPECTROSCOPY © www.chemsheets.co.uk A2 032 17-Jul-12

KEY POINTS 13C NMR spectra are often simpler than 1H NMR spectra. They give a lot of valuable information about the chemical environment of C atoms (e.g. the difference between C atoms in C=O, C-N, CN, C-C, C=C, etc.). There is one signal for each set of equivalent C atoms. There is no coupling (unlike 1H NMR). The size of signal is not relative to the number of equivalent C atoms (unlike H atoms in 1H NMR). As in 1H NMR, the chemical shift (d) is measured relative to TMS. Although deuterated solvents are usually used, there will be a signal for any C atoms in the solvent. © www.chemsheets.co.uk A2 032 17-Jul-12

Data from AQA datasheet

Data from AQA datasheet

Data from AQA datasheet