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
H bonding in exchangeable systems
- Solvent and temp affects peak position of an exchangeable proton on account of H bonding
- H bonding decreases the e- density around the proton and therefore the proton resonate at a lower field
Intermolecular H bonding
- decreased by dilution with a non polar solvent and by an increase in temp
- with polar solvents, the possibility of h bonding with solvent arises
Intramolecular H bonding
- the chemical shift of the acidic proton determines the strength of the bond e.g. the greater the chemical shift, the stronger the H bond
- less affected by the environment that intermolecular bonds
Phenols H bonding= 10-12 ppm
Carb acids H bonding= 10-13 ppm.
Peak width varies from sharp to broad depending on the rate of exchange of the particular acid.
Information obtained from 13C NMR Spectrum
- Number of signals in the spectrum= number of different chemical environments for carbon in the molecule
- The chemical shift of a signal gives information on the electronic environment of that carbon
Carbon chemical shift
- Signal intensity does not correlate with the number of carbons giving rise to that signal.
- Cannot integrate carbon
- Protonated carbons give strong signals, quaternary carbons give weak signals
- A signal arising from 2 carbons will be stronger (approx double) that arising from 1 carbon
Approx 20x proton
Typical Carbon chemical shifts:
- aromatic carbons
- CH-O-R carbons
- aldehyde carbons
- alkane carbons
- alkene carbons
- alkyne carbons
- aromatic protons ~6-8 ppm
- aromatic carbons ~120-160 ppm, 105-160 ppm
- CH-O-R protons ~3-4 ppm
- CH-O-R carbons ~60-80 ppm
- aldehyde protons ~10 ppm
- aldehyde carbons ~200 ppm
- alkane protons ~1 ppm
- alkane carbons ~20 ppm
-alkene carbons ~120-145 ppm
- alkyne protons ~2 ppm
- alkyne carbons ~60-90 ppm
The more highly substituted carbon of a double bond has the greater chemical shift
R-CH3
0=C-CH3
N-CH3
0-CH3
R-CH3 - 5-35 ppm
0=C-CH3 - 20-40 ppm
N-CH3 - 25-45 ppm
0-CH3 - 50- 65 ppm
Generally CH3
13 C NMR chemical shift for carbonyl compounds
- Aldehydes (R-CHO)
- Ketones (R-COCH3)
- Conjugated ketones (R-C=C-COR)
- Carboxylic acids (RCO2H)
- Esters (R-CO2R’)
- Amides (R-CONH2)
- Acid chlorides (R-COCl)
- Aldehydes (R-CHO) - 200ppm
- Ketones (R-COCH3) - 200-220 ppm
- Conjugated ketones (R-C=C-COR) - 185-198 ppm
- Carboxylic acids (RCO2H)- 180 ppm
- Esters (R-CO2R’)- 170 ppm
- Amides (R-CONH2)- 173 ppm
- Acid chlorides (R-COCl) - 168 ppm
Chemical shifts for aromatic compounds:
Calculating aromatic protons
ppm from benzene at 128.5 ppm
S arh- 7.27-d
SFORD
Single Frequency Off Resonance Decoupling
- removes the small 2j and 3J 13C-1H coupling but not larger 1J coupling
- The larger 1J coupling are partially collapsed down to an approx 10-20 Hz residual coupling
- CH3 quartet, CH2 triplet, CH doublet, C singlet
Dept Methods (a more complex 5 pulse sequence)
DEPT-135
- Methyl (-CH3 or quartet) = upward peak
- Methylene (-CH2 or triplet) = downward
- Methine (-CH or doublet) = upward
- Quaternary carbon (singlet) = no peak
DEPT-90
-Methine (-CH or doublet) = upward peak
Comparison with the decoupled spectrum identifies the multiplicity of each carbon signal.
COSY
Correlation Spectroscopy
- A homonuclear experiment
- Shows 1H-1H coupling
- Plots 1H spectrum on each axis
- Diagonal runs through plot
- “Cross peaks” (contours off the diagonal) gives coupling info
HSQC
Heteronuclear Single Quantum Conherence
- Plots 1H spectrum on one axis and 13C spectrum on the other of the same molecule
- Cross peaks show direct (1J) 1H-13C coupling
HMBC
Heteronuclear Multiple Bond Coherence
- Plots 1H spectrum on one axis and 13C on the other
- Cross peaks show long range (2J and 3J) 1H-13C coupling
NOE: Nuclear Overhauser Effect
- A through space phenomenon
- Effect is distance dependent, so only atoms (protons) close in space (within 4-5 angstroms) give an NOE effect
- Small molecules (<1000 Da) in solution generally tumble rapidly and give weak, positive NOEs that grow slowly
- Large molecules (>3000 Da) tumble slowly in solution and give large, negative NOEs that grow quickly
NOEs may not always be observed so either:
-alter solution conditions
-temperature
-solvent viscosity
Or change motional properties
-use rotating frame NOE (ROE) meaurements
NOESY
Nuclear Overhauser Effect Spectroscopy
- A homonuclear experiment
- 2D technique which plots 1H spectrum on each axis
- Maps NOE correlations between protons
- Best suited to large molecules
- Cross peaks show NOEs
- Positive NOEs (rapidly tumbling molecules) have opposite phase to diagonal peaks
- Negative NOEs (slowly tumbling molecules) have same phase as diagonal
- A “phase sensitive” experiment
ROESY
Rotating Frame Nuclear Overhauser Effect Spectroscopy
- A homonuclear experiment
- 2D technique which plots 1H spectrum on each axis
- Maps NOE correlations between protons in the rotating frame
- Cross peaks show NOEs
- Not phase sensitive (everything +)
