Test I Flashcards
Three info obtained from 1H NMR spectrum
- Chemical shift
- Multiplicity
- Integral
Proton Chemical shift
The chemical shift of an atom (proton) characterises the electronic environment of that atom
S(ppm)= (v sample- v reference)(Hz)/ v reference (mHz)
- Commonly use TMS
- Greater the electronegativity of the atom, the greater the chemical shift
- More than 1 electronegative atom nearby increase the chemical shift effect
Typical Proton chemical shifts:
-CHn (hydrocarbons)
-CHn-(CO)
-CHn-O-
-C=CH (alkenes)
-C=C(CH3)
-R-CHO (aldehyde)
RN=CH2
-CHn (hydrocarbons) = 0.8-1.5
-CHn-(CO)= 1.9-2.2
-CHn-O- = 3.6-4.1
-C=CH (alkenes) = ~5
-C=C(CH3)= ~2
-R-CHO (aldehyde) = ~10, most down field
RN=CH2 = 6-7 ppm
Typical Proton chemical shifts:
- aryl- H (aromatics)
- CH-Br
- CH-Cl
- CH-F
- CH-N
- OH, NH, COOH
- aryl- H (aromatics) = 6-8
- CH-Br= 2.5- 3.0
- CH-Cl = 3.0-3.5
- CH-F= 4.0-4.5
- CH-N= 2.8-3.8
- OH, NH, COOH= variable
Multiplicity
The number of peaks within a signal, gives structural information
n+1
n=number of adjacent equivalent protons
If there is more than 1 set of adjacent protons, the multiplicity is (n+1) x (n+1)
-if symmetric, half it
Integral
- Intensity of a signal is measured by the area under the curve= Integral
- Ratio of integral = number of protons giving rise to those signals
Coupling
- Measured in Hz, are independent of the field strength of spectrometer
- Equivalent protons do not spin-spin couple
- Proton coupling usually confined to 2 or 3 bond interactions
- Coupling constant, J is usually ≤ 20 Hz
Chemical Shift Rationale
Proximity to an electronegative element or to a π bond affects chemical shift
- A nucleus is shielded by an e- cloud
- Under the influence of a magnetic field, e- will circulate and thus generate their own secondary magnetic field
- The strength of the secondary magnetic field and the spatial orientation of the proton to it affects the chemical shift.
Aromatic compounds
- Protons in benzene resonate at 7.26 ppm due to ‘ring current effect’
- Protons in centre of molecule are in a region of space when the 2° magnetic field generated by the e- opposes the applied field. There is a barrier for those protons coming to resonance
- To overcome this, need to apply more applied field. Higher value of applied foeld, lower chemical shift. Protons are shield from the applied magnetic field by the 2° magnetic field that opposes the applied field
-Protons on prolifery are in a region of space where that 2° magnetic field generated by the e- ring current reinforces the applied field. Same direction as applied field. If it reinforces applied field ,we need a lower value of applied field to bring protons to resonance. Lower value of applied field=higher chemical shift.
Triple Bond
- Found further upfield than electronegativity expected
- Acetylene is linear and the triple bond is symmetrical about the axis, if the axis is aligned with the applied magnetic field, the π electrons can circulate at right angles to the applied field, inducing a field to oppose it
- As the protons lie along the magnetic axis, the induced magnetic lines of force shield the protons and the chemical shift for a triple bond is found further upfield than electronegativity would predict.
- Only a few molecules are aligned with the field at any one time, but the overall average chemical shift is still affected.
Factors affecting coupling constants
1) Vicinal protons (H-C-C-H
-Protons 3 bonds apart
Cis=7-11 Hz
Trans=12-18 Hz
Axial-axial (180°)= 10-13
Axial-equatorial (60°)= 2-5
Equatorial-equatorial= (60°)= 2-5 Hz
Factors affecting coupling constants
2) Geminal Protons (H-C-H
- Methylene groups in a cyclohexane ring= 12-18 Hz
- Methylene groups of a cyclopropane ring = 5 Hz
- Terminal methylene groups= 0-3 Hz
A vinylic system has three different proton environments
-Cis, trans, geminal
Long Range coupling
- common in π systems (seen for >3 bonds)
- Allylic (H-C-C=C-H) coupling usually of the order 0-3 Hz
- Homoallylic coupling (H-C-C=C-C-H) usualyu negligible but maybe up to 1.6 Hz
- W config, for vicinal protons J depends of the dihedral angle between them
- Long range coupling constants are smaller than direct coupling values.
Exchangeable systems
- Functional groups such as -OH, -CO2H, -NH2, -SH contain labile (acidic protons)
- These protons are termed exchangeable
- The appearance of the signal for these protons depends on: Solvent, temp, pH, conc
- If a compound contains a readily exchangeable proton, exposure to an exchangeable solvent (e.g. D2O) will exchange D for H
- The proton spectrum will lack the resonance and coupling observed for the proton before the change
Common exchangeable solvents D20, CD3OD
- Spectra of molecules containing exchangeable protons are often purposefully simplified by shaking the solution with excess D2O
- N-H resonances are broadened by quadrupole.
Exchangeable systems chemical shifts of:
Acetone Chloroform Dimethyl Sulfoxide Methanol Water (D2O)
If water is present, it will peak at a chemical shift position characteristic of the solvent used
Acetone= 2.8 ppm Chloroform= 1.6 Dimethyl Sulfoxide= 3.3 Methanol= 4.8 Water (D2O)= 4.8
Aprotic solvents, the peak is from water
Protic solvents, it arises from HOD as the protons of the water exchange with the solvent dueterium
-solvent
