7 [Modern analytical techniques] Flashcards
(18 cards)
What is mass spectrometry?
Mass spectrometry is used to determine the relative molecular mass of organic compounds. Also to determine the structures of them.
Mass spectrum of an element often appears simple with a very small number of vertical lines, called peaks each one representing an isotope of the element.
The mass spectrum of an organic compound can be complex, with a large number of peaks. If all the peaks are included there may be more peaks than atoms in a molecule of a compound
What is the molecular ion peak?
The first thing to do when looking at a mass spectrum of an organic compound is to find the peak furthest to the right. This is the peak with the greatest m/z value (mass to charge ratio). Known as molecular ion peak - the result of the organic molecule losing an electron in the mass spectrometer.
What are the other peaks in mass spectrometry?
The spectrum may show a very small peak just to the right of the molecular ion peak (M+1) peak. This is caused by the presence of a naturally occurring isotope of C13.
The peaks with the smaller m/z values result from fragmentation in the mass spectrometer, rearrangement reactions and the loss of more than one electron. Rearrangement is often unpredictable. The breaking of C-H bonds can occur.
What is fragmentation?
Fragmentation is a very common and can often be used to work out the structure of an organic molecule.
The breaking of C-C bonds in the molecular ion formed from a hydrocarbon 2 species are formed
Another positive ion
A neutral species like a radical
What is an example of fragmentation?
Simple example is the molecular ion of ethane, which can fragment to form a methyl cation and a methyl radical.
(CH3-CH3) > CH3+ + CH3
The ethane molecule is symmetrical and there is only 1 carbon-carbon bond in ethane. The right hand carbon in just as likely to become the positive ion.
Products are identical, so the spectrum does not depend on how the bonds break, the peak will be at m/z = 15
(CH3 - CH3)+. > CH3. + CH3+ (CH2-CH3)
Therefore expect m/z = 15 and m/z = 29 the ethyl cation in its spectrum. These peaks are present, but there are several others.
Radical are not detected in the mass spectrometer.
What’s the possible information that can be from spectrometer?
A complete mass spectrum - the disadvantage of this is a large number of peaks cannot be used to work out the structure and would be distracting. Another possibility is a complete mass spectrum, but with only the m/z values of useful peaks marked.
A simplified mass spectrum showing showing only the peaks that will work out the structure.
A list of the m/z values of useful peaks
What are the characteristics of mass spectrum?
A traditional way to present a mass spectrum to label the vertical axis as relative intensity.
Always from 0 - 100% horizontal axis is labelled m/z. The horizontal axis usually, but starts from zero and continues to just beyond the molecular ion peak.
The base peak represents the ion with highest abundance and is shown with a relative intensity of 100% represents a stable fragment.
What do mass spectra look like for organic compounds containing oxygen?
Oxygen is present in many organic compounds, so need to be familiar with other m/z values. May also come across compounds containing nitrogen and halogens.
What is infrared radiation?
The electromagnetic spectrum of radiation includes infrared radiation. Radiation has a frequency below or less than that of red light.
What happens when molecules absorb infrared radiation?
Importance of infrared radiation is that is absorbed by molecules and causes 2 possible effects, both described vibrations. These effects are:
Stretching - where the bond length increases and decreases
Bending - where the bond angle increases and decreases
What does the amount of energy absorbed by the molecule depend on?
The length of the bond
The strength of the bond
The mass of each atom involved in the bond
The absorption of infrared radiation is linked to changes in the polarity of the molecule, so simple non-polar molecules (H2) do not absorb infrared radiation
What does an infrared spectrum look like?
When a compound is irradiated by infrared radiation, the bonds in the molecule absorb radiation from some part of the spectrum but not from others.
What are the characteristics of axes on the infrared spectrum?
The spectrum is normally shown with the vertical axes labelled transmittance. Shown as a % of 1-100. A value of 100%. A value of 100% transmittance means that 100% if the radiation is transmitted and non is absorbed.
Horizontal axis is labelled as frequency or wavelength or wavenumber. This is the reciprocal of the wavelength represents frequency.
What are absorptions and their intensities?
Much of spectrums consist of an almost horizontal line close to 100% but at specific wavenumbers there are dips and troughs. These are referred to as absorptions. Actual transmittance value is not very important, but intensity is.
What are characteristic absorptions?
For example if there was a strong absorption at 1700cm which is correlated with C=O compound that is responsible for this spectrum could be a carboxylic acid or a ketone.
Missing absorptions are useful if there was no absorptions in the 3300-2500 region compound does not contain an OH group so it suggests it is a ketone not a carboxylic acid
What are the different ways of using infrared spectra?
Different ways understanding infrared spectra could be tested need to become familiar:
Predicting the spectrum of an organic compound
Deducing the functional groups from a list of wavenumbers
Deducing the structure from wavelengths
They all require the use of wavenumber data from a table provided.
What is the fingerpoint region?
The table of wavenumbers starts at 3750 and ends at 1645 even though there are many absorptions in the infrared region between 1500 and 500. Most of the absorptions in this region results from bending vibrations.
This region is sometimes referred to as the fingerprint region because, although individual absorptions are not easily recognised the whole pattern acts like a fingerprint slightly different for similar molecules, ignore this.
How can the spectrum of an organic compound be predicted?
If given the formula of an organic compounds should be able to predict the wavenumber ranges of the compounds infrared spectrum.
