Organic NMR 2 Flashcards
How is the integral displayed in H NMR?
As a number on the spectrum
Or
As a vertically rising line which rises in height by an amount proportional to the area under the peak
Saturation effects:
Enough time must be left between pulses to allow nuclei to relax fully and return to the Boltzmann distribution
Digital resolution:
Must have an adequate number of data points
Isotopic satellites:
All C-H signals have 13C satellites. Together these satellites make up to 1.1% of the area of the central peak.
Spinning sidebands:
These appear at +/- the spinning speed in Hz of the spectra. Are worse with poor quality NMR tubes and poorly tuned machines.
Baseline slant:
Spectra can show distortions of the baseline. Processing software can make adjustments for this.
Decoupling:
When decoupling is used peak intensities are distorted by Nuclear Overhauser Effects
What is the spin-spin splitting rule?
2nI+1
The resonance of one proton is influenced by the direction of spin in an adjacent proton. They are said to be coupled and have different chemical shift values
What is the coupling constant?
J
distance between peaks in a multiplet is the coupling constant and is measured in Hz.
It is a measure of how strongly a nucleus is affected by spin states around it.
How can the coupling constant be calculated?
Coupling constant = difference in chemical shifts between peaks of multiplet (ppm) X operating frequency of machine (MHz)
For most aliphatic protons in acyclic systems, the coupling constants are around:
7.5 Hz (6-8Hz)
Two hydrogens on adjacent carbons are described as:
Three bond interactions - 3J
In alkenes coupling constants for cis protons are around:
For trans hydrogens the coupling constants are around:
10Hz
16Hz
Double bond equivalents aka units of unsaturation, are calculated using:
DBE = (2a+2)-b/2 a = Carbon atoms b = hydrogen atoms/ halogens c = oxygen atoms (not used) DBE = (2a+2)-(b-d)/2
Spectra may be second order, this is observed when:
Difference in chemical shift between two atoms of the same nucleus is similar in magnitude to the coupling constant
At what range are Carbonyl Carbons observed?
155-220ppm
At what range are alkene and aromatic carbons observed?
100-175ppm
At what range are carbons affected by electronegative atoms observed?
40-80ppm
At what range are saturated carbons at lowest frequency observed?
8-60ppm
How is a proton decoupled 13C spectra achieved?
Irradiate all of the protons in the molecule, this saturates spins of protons, unable to distinguish between the two spin states - see an average of zero - therefore no coupling
Why can spectra not be integrated reliably when acquired using normal parameters?
NOe is not equal for all carbons
13C typically have poorer signal to noise ratio due to lower abundance of 13C, reduces accuracy of integrals
Carbon have varied and longer relaxation times, so nucleus may have not fully relaxed before another pulse applied
How can a spectra have more accurate integration?
- Record spectrum with nOe enhancement
- Acquire enough scans to give good signal/noise ratio
- Ensure a long enough relaxation delay between pulses
What is the nOe effect?
Carbon nuclei adjust to protons with more spins in excited state by putting more spins in the lower state, increasing the intensity of the carbon signal
What is DEPT?
Distortionless Enhancement by Polarisation Transfer.
Used to find number of protons on carbons, complex set of pulses in H and C channels.
Resulting spectrum contains only signals from protonated carbons, quaternary carbons do not appear.
CH and CH3 point upwards. CH2 point downwards.
What is 2D NMR?
Spectra have 2 axes. Experiment is repeated with a different delay in the pulse sequence each time. Plot of intensity as a function of two frequencies
What is a 1D spectra?
Plot of intensity vs a frequency.
What is a COSY?
Correlation Spectroscopy:
Shows which protons in a molecule are coupled to each other.
Plot the 1D along the F1 and F2 axes to give reference spectra for peaks in 2D spectrum.
Diagonal is 1D H NMR.
Off diagonal peak means coupling.
HSQC is:
Heteronuclear Single Quantum Correlation:
Shows which protons are directly attached to which carbons in a molecule.
H NMR on one axis, C NMR on second axis.
No diagonal peaks
Non protonated carbons do not give rise to signals in spectra
What is HMBC?
Heteronuclear Multiple Bond Correlation:
Correlates chemical shifts of H with C which are separated by 2 or more bonds.
Useful to assign quaternary or carbonyl carbons - no directly bonded protons.
Uses smaller couplings (10Hz)
What is the Nuclear Overhauser Effect Spectroscopy?
Uses NOE
Shows which pairs of nuclei are close together in space.
Similar to COSY.
Assists in determining 3D structure of molecules
Useful for stereochemistry
Magnitude of noesy correlation proportional to inverse sixth power of distance between atoms
