Analytic techniques Flashcards
Formula for IHD where CxHy
IHD = (2x + 2 - 2y) / 2
where each double bond and each ring counts as one IHD and each triple bond counts as two IHD.
How is IHD determined without a formula?
Each double bond and each ring counts as one IHD and each triple bond counts as two IHD.
Oxygen and sulur atoms (O and S) do not aect the IHD.
Halogens (F, Cl, Br, I) are treated like H atoms (i.e. CHCl3 has the same IHD as CH4 ).
For each N, add one to the number of C and one to the number of H (i.e. CH5N has the same IHD as C2H6 ).
How does mass spectrometry work?
When a sample is introduced into the machine the vaporized sample becomes ionized to orm the molecular ion M+ (g). Inside the mass spectrometer some o the molecular ions break down to give ragments, which are also deected by the external magnetic feld and which then show up as peaks on the detector. By looking at the dierence in mass rom the parent peak it is oten possible to identiy particular ragments,
The mass to charge (M/Z) ratio is measured
How does infrared spectroscopy work?
When molecules absorb energy in the inrared region o the spectrum they vibrate (i.e. the bonds stretch and bend). The precise value o the energy they absorb depends on the particular bond and to a lesser extent on the other groups attached to the two atoms orming the bond. When inrared radiation is passed through a sample the spectrum shows the characteristic absorptions, which confrm that a particular bond is present in the molecule.
nrared spectra also possess a fngerprint region. This is a characteristic pattern between about 1400400 cm^-1 , which is specifc to a particular compound
How does proton nuclear magnetic resonance spectroscopy work?
1 H NMR spectroscopy provides inormation on the chemical environment o all the hydrogen atoms in the molecule. The nuclei o hydrogen atoms possess spin and can exist in two possible states o equal energy. I a strong magnetic feld is applied the spin states may align themselves either with the magnetic feld, or against it, and there is a small energy dierence between them. The nuclei can absorb a photon o energy when transerring rom the lower to the higher spin state. The photons energy is very small and occurs in the
radio region o the spectrum. The precise energy dierence depends on the chemical environment o the hydrogen atoms.
The position in the 1 H NMR spectrum where the absorption occurs or each hydrogen atom in the molecule is known as the chemical shit, and is measured in parts per million (ppm). The area under each peak corresponds to the number o hydrogen atoms in that particular environment.
In 1H NMR spectra what does The number of different absorptions (peaks) indiicate?
Each proton in a particular chemical environment absorbs at a particular requency. The number o peaks thus gives inormation as to the number o dierent chemical environments occupied by the protons.
In 1H NMR spectra what does the area under each peak indicate?
The area under each absorption peak is proportional to the number o hydrogen atoms in that particular chemical environment. Normally each area is integrated and the heights o the integrated traces can be used to obtain the ratio o the number o hydrogen atoms in each environment.
In 1H NMR spectra what does the chemical shift indicate?
Because spinning electrons create their own magnetic feld the surrounding electrons o neighbouring atoms can exert a shielding eect. The greater the shielding the lower the requency or the resonance to occur. The chemical shit () o each absorption is measured in parts per million (ppm) relative to a standard. The normal standard is tetramethylsilane (TMS) which is assigned a value o 0 ppm. A table o chemical shits is to be ound in the IB data booklet.
In 1H NMR spectra what do Splitting patterns indicate?
In 1 H NMR spectroscopy the chemical shit o protons within a molecule is slightly altered by protons bonded to adjacent carbon atoms. This spinspin coupling shows up in high resolution 1 H NMR as splitting patterns. I the number o adjacent equivalent protons is equal to n then the peak will be split into (n + 1) peaks
What is the advantage of using TMS as a standard for 1H NMR spectroscopy?
-All the protons are in the same environment so it gives a strong single peak.
-It is not toxic and is very unreactive (so does not interere with the sample) .
-It absorbs upfield well away rom most other protons.
-It is volatile (has a low boiling point) so can easily be removed rom the sample.
Explain splitting patterns in 1H NMR spectra using ratios and methyl groups
Splitting patterns are due to spinspin coupling. For example, i there is one proton (n = 1) adjacent to a methyl group then it will either line up with the magnetic feld or against it. The eect will be that the methyl protons will thus experience one slightly stronger and one slightly weaker external magnetic feld resulting in an equal splitting o the peak. This is known as a doublet (n + 1 = 2).
If there is a -CH2 - group (n = 2) adjacent to a methyl group then there are three possible energy states available.
1. Both proton spins are aligned with the feld
2. One is aligned with the feld and one against it (2 possible combinations)
3. Both are aligned against the feld.
This results in a triplet with peaks in the ratio of 1:2:1.
A methyl group (n = 3) next to a proton will result in the absorption for that proton being split into a quartet with peaks in the ratio 1:3:3:1.
These three H atoms split the absorption due to the single proton into a quartet
How is 1HNMR used in medicine?
NMR is particularly useful in medicine as the energy of the
radio waves involved is completely harmless and there are no known side effects.
1 H NMR is used in body scanning. The whole body of the patient can be placed inside the magnet of a large NMR machine. Protons in water, lipids and carbohydrates give different signals so that an image
of the body can be obtained. This is known as MRI (magnetic resonance imaging). The image can be used to diagnose and monitor conditions, such as cancer, multiple sclerosis and hydrocephalus.
How does X-ray crystallography work?
By measuring the angles and intensities of diffracted beams a three-
dimensional picture o the position of all the atoms within the crystal can be obtained. This definitively gives the distances between all the atoms (bond lengths) and the bond angles and hence the unambiguous molecular or ionic structure. As well as for simple molecules such as ice.
X-ray crystallography is still the main method used to confrm the exact structure o a new compound.
