Chemsheets AS006 (Electron arrangement) 10/05/2019 NMR SPECTROSCOPY

1H NMR SPECTROSCOPY

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.

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

butanone

EQUIVALENT H's In a spectrum, there is one signal for each set of equivalent H atoms i.e. those H atoms in the same environment The intensity or area of each signal being proportional to the number of equivalent H atoms it represents.

To identify hydrogen environments we need to look at the molecule:   Draw out the molecule in full. Circle the first environment in one colour Circle the next environment in another colour Continue this process till there are no more hydrogen environments Then work out the number of protons in each hydrogen environment to give you the peak ratio`s (peak areas).

2 sets of equivalent H’s: ratio 6:2 (3:1)

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

2 signals: ratio 6:2 (3:1) 1 signal 3 signals: ratio 2:1:3 2 signals: ratio 3:2 2 signals: ratio 6:1 4 signals: ratio 3:2:2:3

3 signals: ratio 2:1:3 3 signals: ratio 3:2:9 2 signals: ratio 6:2 (3:1)

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

CHEMICAL SHIFT 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.

Number of H’s next door +1 SPIN-SPIN COUPLING Coupling / Splitting / Multiplicity Number of H’s next door +1

Data from AQA datasheet

This is the table that should be used when doing questions on NMR Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2 2.1-2.6

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2 2.1-2.6 quartet

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2 2.1-2.6 quartet

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2 2.1-2.6 quartet

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2 2.1-2.6 quartet

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2 2.1-2.6 quartet 9.0-10.0

2 1 3 Example 1 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 3 0.7-1.2 triplet 2 2.1-2.6 quartet 9.0-10.0

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet 2

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet 2

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet 2

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet 2 2.1-2.6

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet 2 2.1-2.6 multitet

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet 2 2.1-2.6 multitet 3

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet 2 2.1-2.6 multitet 3

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet 2 2.1-2.6 multitet 3

Example 2 3 4 1 2 Hydrogen Environment Number Number of Hydrogens Ratio (peak area) Splitting Pattern 1 6 0.7-1.2 doublet 2 2.1-2.6 multitet 3 3.1-3.9 quartet 4 triplet

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)

SPIN-SPIN COUPLING signal singlet doublet triplet quartet appearance number of lines 1 2 3 4 number of H’s next door

Number of H’s next door +1 But you don’t couple to H’s that are equivalent H’s on O’s

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

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

2 peaks: 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

3 signals: ratio 2 : 1 : 3 d m d 3 signals: ratio 3 : 2 : 9 t q s 2 signals: ratio 6 : 2 (3 :1) d q