Structure determination and Analysis - Week 11-12 Flashcards
What is spectroscopy?
The interaction of E - M radiation with matter
What is 1 angstrom?
= 10^-8 cm
What is wavenumber proportional to?
Proportional to the energy of the e-m radiation.
What is the main feature of IR spectroscopy?
Molecules aren’t rigid - the bonds within them can move and vibrate.
If the molecules cause a change in the dipole moment of the molecule it is IR active.
What is NMR?
Nuclear Magnetic Resonance
Some nuclei have spin, spin is quantised, a transition from one allowed value to another involves an absorption or emission of energy.
Nuclei with spin generate a magnetic field, increasing the external field causes the difference in allowed energies to increase.
Irradiation of a sample causes flipping of nuclear spins from a lower energy state to a higher energy state as the energy is transferred from the e-m radiation to the spin of the nuclei.
What is mass spectrometry used for?
Determination of Relative Atomic Masses (Ar), Relative Molecular Masses (Mr) and molecular structure
In mass spectrometry what is the weighted average mass?
Weighted average mass = the Relative Atomic Mass (Ar)
How is molar mass and structural information obtained from mass spectrometry?
Analysis of the fragmentation pattern and peak ratios.
How does chromatography work?
by using differences in the affinities of mixture components for two media with which they are able to interact.
One medium - the stationary phase is fixed. The other is a mobile phase where a carrier material moves the mixture over the stationary phase.
What is the stationary phase and mobile phase in paper chromatography?
Stationary phase - Paper ( + adsorbed water)
Mobile phase - pure solvent or solvent mixture
What is the stationary phase and mobile phase in thin layer chromatography (TLC) ?
Stationary phase - silica gel, alumina or cellulose
Mobile phase - pure solvent or solvent mixture
What is the stationary phase and mobile phase in gas chromatography? (GC)
Stationary phase - Microscopic non - volatile liquid or polymer layer on inert support
Mobile phase - Inert carrier gas
What is the stationary phase and mobile phase in column chromatography?
Stationary phase - Silica coated with non volatile liquid or alumina
Mobile phase - pure solvent or solvent mixture
What is the stationary phase and mobile phase in high performance liquid chromatography? (HPLC)
Stationary phase - Silica coated with variety of more or less polar liquids or polymers
Mobile phase - pure solvent or solvent mixture under pressure
What is the equation for Electromagnetic (em) radiation?
E = hc / (lamda)
What are the wavelengths between on in the Infrared (IR) spectrum?
4000 cm^-1 - 450 cm^-1
but 3650 cm^-1 - 1500 cm^-1 is the most useful section as it is where characteristic absorptions by different functional groups occur.
What are the 5 rules for IR absorption?
- bonds must be polar and undergo a change
of dipole moment for motions to be IR
active. The greater the polarity, the stronger
the absorption. - stronger bonds have higher vibrational
frequencies (wavenumbers) - for bonds C-A as the mass of A increases,
the frequency decreases. - Bending motion usually has lower
frequencies than stretching - a change from sp3 to sp hybridisation
causes frequencies to increase
To work out the accurate mass of a molecular formula:
a quick way = to work out what the final digit is in the molecular mass would be for each possible formula and then compare with the known mass
e.g for C6H14N2 it would be 6x0 (for C) 14x8 (for H) and 2x1 (for N) giving 4 as a final digit which agrees with the stated value
What happens to the energy gap in Nuclear magnetic resonance (NMR) ?
As the energy gap increases with the strength of the field experienced by the nuclei.
What would happen in NMR if the field experienced was the same for each nucleus.
All nuclei would produce the same signal
What equation shows the region of the em spectrum
delta E = hv
Why do differences in signal occur in NMR spectroscopy?
Differences in signal arise because of the different extents to which particular nuclei are shielded from the applied field.
What is the standard used in H NMR?
TMS
What is the role of the standard in H NMR?
The standard has zero shielding effect.
The differences between its signal and sample nuclei are recorded and known as chemical shifts. - are the primary way of identifying atoms in different chemical envrionments.
What does a H NMR peak mean if it is downfield?
Downfield -> more deshielded (less shielded), greater frequency required for resonance
What does a H NMR peak mean if its upfield?
More shielded, lower frequency required for resonance
Other than shielding what can effect nuclei?
Small magnetic field variations caused by spin of neighbouring active nuclei cause splitting of the main signals.
Relevant nuclei are coupled and the extent of splitting is given by the coupling constant, J.
Effect is reciprocal so signals from neighbouring nuclei will also be split by interaction with the original H.
What happens to protons within the same group (e.g. bonded to the same atom) in H NMR?
They are magnetically equivalent and do not cause splitting of their own signal.
= (n+1) rule
What are the names for split signals?
Doublets, triplets, quartets or multiplets… etc,
What is the difference between 13C NMR and H NMR?
In a comparable magnetic field the delta E value for 13C is only a quarter of that for 1H.
Signals are much weaker.
There is little chance of two or more 13C nuclei being next to each other in a molecule so coupling between 13C nuclei is not observed.
How do 13C signals normally appear?
as singlets
What region are most chemical shifts in for 13C NMR?
0 - 210 ppm region
What does the number of signals in 13C NMR indicate?
Indicates the number of different types of carbon atom in a molecule.
How do you calculate Rf value in chromatography?
Rf = Distance travelled by solute / distance
travelled by solvent
