Topic 7: Modern Analytical Techniques I Flashcards
What are the three main functions of a mass spectrometer?
- Create positive ions from a neutral sample
- Separates ions according to their m/z ratio
- Measures the relative abundances of ions and their relative masses; the information being represented as a mass spectrum
How is spectrometry different from spectroscopy?
It is different from spectroscopy because it measures mass rather than the absorption of energy
What occurs during stage 1 of mass spectrometry (ionisation)?
The atom/molecule is isonised by knocking one or more electrons off to give a positive ion (by collision with high energy electrons). This is true even for things which would normally expect to form negative ions or never form ions at all (e.g. argon). A vacuum is necessary as the ions produced in the ionisation chamber need a free run through the machine without hitting air molecules.
What occurs during stage 2 of mass spectrometry (acceleration)?
The ions are accelerated into a finely focused beam. The ions are accelerated so that they have the same kinetic energy.
What occurs during stage 3 of mass spectrometry (deflection)?
Ions are deflected by a magnetic field according to their masses. Deflection occurs on: mass as lighter ions will be more deflected than heavy ones, charge as ions with 2 positive charges are deflected more than those with 1. Ions with the smallest mass/charge ratio are deflected the most and vice versa.
What occurs during stage 4 of mass spectrometry (detection)?
Positive ions hit the negative plate and an electron jumps from the metal on to the ion. That leaves a space amongst the electrons and the electrons in the wire shuffle along to fill it. A flow of electrons in the wire is detected as an electric current. The size of the current is proportional to the number of ions.
How to identify the molecular ion peak on a mass spectrum?
It is the peak with the highest m/z ratio. The m/z value of the M peak is the Mr value for your molecule. For most organic compounds the M peak is the one with the second highest m/z ratio, the small peak on the right is caused by the isotope C13.
Why does the C13 isotope cause the small line (one m/z unit to the right of the main molecular ion peak?
The C13 is a stable isotope of carbon that makes up 1.11% of all carbon atoms. This means that some of the molecular ions have a slightly higher relative molecular mass than that of the normal ion.
What occurs during fragmentation?
When excess energy transferred to the molecular ion makes it unstable, breaking them up into smaller pieces. It breaks into another positive ion and an uncharged free radical. The uncharged free radical won’t produce a line on the mass spectrum.
Define molecular ion peak:
The molecular ion peak indicates the species formed from the molecule by the loss of one electron.
Define fragmentation:
Fragmentation occurs when the molecular ion breaks into smaller pieces: a radical and positive ion.
Define base peak:
The base peak indicates the peak with the greatest abundance.
What are the fragmentation patterns of aldehydes and ketones?
It’s the cleavage of bonds next to the carbonyl group (C=O). Also the position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group.
Describe the fragmentation patterns of chlorine
Chlorine exists as two isotopes,Cl35 and Cl37. A compound containing one chlorine atom will therefore have two molecular ion peaks due to the two different isotopes it can contain. Cl35 = M+ peak and Cl37 = M+2 peak. The ratio of the peak heights is 3:1 as the relative abundance of Cl35 is 3x greater than that of Cl37.
Describe the molecular ion peaks that can occur from a diatomic molecule of chlorine
Cl35 + Cl35 = M+ peak (at m/z 70)
Cl35 + Cl37 = M+2 peak (at m/z 72)
Cl37 + Cl37 = M+4 peak (at m/z 74)
The ratio of heights can be deduced by using the probability of each chlorine atom being Cl35 or Cl37. (Cl35 = 3/4, Cl37 = 1/4)
Describe the molecular ion peaks of halogenoalkanes
Using the molecular ion of C2H5Br for example, there are two peaks. One being the isotope of Br79 and the other Br81 isotope. Also because the two isotopes are of similar abundance, the peaks are similar heights.
Describe the stability of ions in terms of fragmentation
The most stable ions are less likely to fragment and give tall peaks in a mass spectrum. The stability of carbocations is as follows: primary < secondary < tertiary.
How does infrared spectroscopy work?
A pair of atoms joined by a chemical bond is always vibrating. The system behaves like two balls (two atoms) joined by a spring (bond). Stronger bonds vibrate faster (at a higher frequency) and heavier atoms make the bond vibrate slower (at a lower frequency). Every bond has its own unique natural frequency that is in the infrared region of the EM spectrum. When you shine infrared radiation (heat energy) through a sample, the bonds in the sample can absorb energy from the radiation and vibrate more. However, any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond. Therefore, the radiation that emerges from the sample will be the missing frequencies that correspond to the bonds in the sample.
Describe the steps in the infrared spectrometer
- A beam of infrared radiation containing a spread of frequencies is passed through a sample
- The radiation that emerges is missing the frequencies that correspond to the types of bonds found in the sample
- The instrument plots a graph of the intensity of the radiation emerging from the sample, called the transmittance, against the frequency of radiation
- The frequency is expressed as a wavenumber, measured in cm-1
When UV is absorbed by a molecule/atom what is the energy used for and what is the spectroscopy technique called?
- Movement of electrons to higher energy levels
- Ultra-violet/ visible spectroscopy
When Infrared is absorbed by a molecule/atom what is the energy used for and what is the spectroscopy technique called?
- Vibrate bonds
- Infrared spectroscopy
When microwaves is absorbed by a molecule/atom what is the energy used for and what is the spectroscopy technique called?
- rotate molecules
- microwave spectroscopy
When radio waves is absorbed by a molecule/atom what is the energy used for and what is the spectroscopy technique called?
- to change nuclear spin
- NMR (Nuclear magnetic resonance) spectroscopy
What is a dipole moment?
Is when there are two partial opposite charges, separated by a distance. It’s measured in equation: dipole moment = magnitude of charge x distance between the charges
What causes a reading on an IR spectrometer?
If a bond stretches, we are changing the distance between the partial charges and if we change the distance we are changing the dipole moment; this causes an absorption of IR radiation and a peak in the spectrum. Only stretching vibrations that produce a change in the dipole moment will be IR active.
Describe the classification of IR bands
IR bands can be classified as strong, medium or weak depending on their relative intensities in the infrared spectrum. A strong band covers most of the y-axis, a medium band falls to about half of the y-axis, and a weak band falls to about one third or less of the y-axis.
Stretching vibrations depend on:
- the mass of the atoms on either side of the bond
- the bond strength
- the bond length
Why can’t H2 or Cl2 absorb infrared radiation?
Simple, non-polar molecules don’t absorb infrared radiation because they don’t have a dipole. When the molecule stretches due to a vibration there’s no change in the dipole, so there is no absorption of radiation.
Why does a C=C IR active?
There must be a change in dipole moment for the stretch to be IR active. To determine whether or not there will be a change in dipole moment, examine the bond that is stretching and look at the two fragments that it’s connected to. If these two fragments are equivalent, there will be no net change in dipole moment for the symmetrical stretch and it will be IR-inactive.
Define infrared radiation:
Infrared radiation is the part of the EM spectrum with frequencies below that of red light
Define stretching:
Stretching occurs when a bond absorbs infrared radiation and uses it to alter the length of the bond.
Define transmittance:
The transmittance value in an infrared spectrum represents the amount of radiation absorbed at a particular wavenumber.
Define wavenumber:
The wavenumber of an infrared absorption represents the frequency of infrared radiation absorbed by a particular bond in a molecule.
Define intensity:
The intensity of an infrared absorption describes the amount of infrared radiation absorbed
What is the fingerprint region in an infrared spectrum?
It is caused by the vibration of the molecule as a whole; it is unique for every compound, so by comparison with the IR spectrum for compounds in a data bank we can identify a compound; each different compound produces a different pattern of troughs in this part of the spectrum
What are the uses of Infrared spectrometry?
- IR spectroscopy gives information about the presence of absence of key functional groups.
- IR can also be used by a chemist to check to see if a reaction has proceeded as planned. For example, if we wished to convert cyclohexanone to cyclohexanol, a quick comparison of the IR spectra of the cyclohexanone and then of the final product would tell us if we had successfully converted the ketone group to an alcohol.
What do the y-axis and x-axis represent on an IR spectrum?
y-axis = transmittance
x-axis = wavenumber
How to calculate wavenumber?
1/wavelength (in cm) = wavenumber
What does the IR spectrum of a carboxylic acid show?
- Carboxylic acids show a broad absorption between 3200 and 3600cm-1 due to the presence of the O-H bond
- Carboxylic acids also show a strong absorption around 1700cm-1 due to the presence of the C=O bond
What does the IR spectrum of an alcohol show?
Alcohols show a broad absorption between 3200 and 3600cm-1 due to the presence of the O-H bond. The breadth of this signal is a consequence of hydrogen bonding between molecules
What does the IR spectrum of an ester show?
Esters show a strong absorption between 1750 and 1730cm-1 due to the presence of the C=O bond.
How is an O-H bond different in an alcohol than in an acid (as shown on an IR spectrum)
The O-H bond in an alcohol absorbs at a higher wavenumber than in an acid somewhere between 3230-3550cm-1
Why do the major constituents of air not absorb infrared radiation?
In order for a vibration to absorb IR radiation, the molecule must change its dipole moment during the vibration. Homonuclear diatomic molecules like N2 and O2 do not have dipole moments. If the molecules undergoes a stretching motion, where spheres represent the two nuclei, there is no change in the dipole moment during the vibrational motion, therefore N2 and O2 do not absorb infrared radiation.`
Describe how HCl absorbs Infrared radiation
HCl does have a dipole moment. Stretching the HCl bond leads to a change in the dipole moment. If we stretched the bond so far as to break the bond and produce the two original neutral atoms, there would be no dipole moment. Therefore, as we lengthen the bond in HCl, the dipole moment gets smaller. Because the dipole moment of HCl changes during a stretching vibration HCl absorbs IR radiation.
What are peak widths?
Peak widths are related to the strength of the intermolecular bonding. In acids like ethanoic acid, where the strength of the hydrogen bonding is greater, the peak caused by the OH stretch is broad, whereas in alcohols where the H bonding is less significant, the peak caused by the same OH stretch is narrower.
Describe the IR spectrum of alkenes on a mass spectrum
Alkenes show sharp, medium bands corresponding to the C=C bond stretching vibration at about 1600-1700cm-1. Some alkenes might also show a band for the =C-H bond stretch appearing at around 3080cm-1.
