3.3 Spectroscopy Flashcards
Unit 2: Organic Chemistry
What are the 4 main techniques to analyse organic compounds?
-Elemental microanalysis
-Mass spectroscopy
-Infared spectroscopy
-Proton nuclear magnetic resonance spectroscopy
What is meant by empirical formula?
This shows the simplest whole number ratio of the element in a molecule
How do you calculate the empirical formula using mass?
-Divide mass by FM to get number of moles
-Find the moles for element (e.g H2O you would times it by 2 as there are 2 hydrogens)
-Find the mass of the element (nxFM)
-Find mass of oxygen (mass of compound-all other elements masses)
-find moles of oxygen (n=m/FM)
-Divide all the moles by smallest n
How do you calculate the empirical formula using percentage product by mass?
-Divide each percentage by FM
-Divide the following number by the smallest one
-That gives the ratio
What is mass spectroscopy used for?
-Determine the GFM of an organic compound
-Determine structural features of a compound
What are the steps of mass spectrometer?
- Vaporisation
- Ionisation
- Acceleration
- Deflection
- Detection
What is vaporisation?
Very high temperatures and a vacuum are used to vaporize the sample quickly, and to prevent interaction with air
What is ionisation?
-The vaporised sample is bombared with high energy electrons, which removes electrons from the molecule
-This produces positive ion fragments-molecular ions
-The parent ion is the ion with one electron removed
-The molecular ion breakdown into smaller positive ion fragments of different masses
What is acceleration?
Parent and fragment ions are accelerated and focussed into an ion stream
What is deflection?
The ion stream passes through a magnetic feild where they are separated according to their mass to charge ratio. Ions with the same charge experience the same force, but their path depends on the mass
What is detection?
Ions with different m/z ratios enter the detector and are counted. A graph of relative abundance against m/z ratio is produced
What information is given by the mass spectrum?
-The peak with the highest m/z is the molecular ion and it gives the GFM of the compound. It can be used to work out the molecular formula
-Peak height presents the relative abundance
What happens what infared radiation is absorbed?
There is enough energy to cause the bonds in the molecules to vibrate but not break. This absorption occurs when the frequency of the IR is the same as the vibrational frequency of a bond or collection of bonds, absorption occurs
What can infared be used to identify?
Certain bonds and functional groups
What is absorbance measured in?
Wavenumber
Where is the fingerprint region on a infared spectra?
Below 1400 cm-1
What information is given by proton nuclear magnetic resonance spectroscopy (1H NMR)
-Provides information on the different chemical environments of the hydrogen atoms
-It can also give information about how many 1H atoms are present in each environment
What is a hydrogen nuclei and how do they behave?
-They are protons
-The nuclei is constantly spinning
What happens when hydrogen nuclei are placed in a strong magnetic field?
-If they are placed in a magnetic field, they behave like tiny magnets
-Some 1H nuclei will align with the magnetic field and the others align against the field
What happens to protons when they are exposed to radio waves?
-When the radio frequency are absorbed by a molecule, the 1H-nuclei can flip from the lower energy alignment to the higher energy alignment. As they fall back down they will emit radiation equal to that which was absorbed and this can be detected by a radio receiver and is plotted on NMR spectrum
How does NMR spectrometer work?
The sample is dissolved in solvents which have their H atoms replaced with “heavy” hydrogen 2H. Strong magnets create the magnetic field to align the protons
What information does NMR spectrum give about organic molecules?
-The hydrogen environment given by chemical shift
-The number of hydrogen atoms in each environment relates to the relative area under the peak
How is the relative area often shown on the peak?
It is often shown as an integration curve on the peak. The height of the integration curve is proportional to the number of 1H atoms in the environments, so a ratio can be determined
What is the difference between low and high resolution proton NMR?
High resolution uses a higher frequency radio than low resolution and the result is a more detailed spectrum
What does high-resolution interaction do to the peaks?
-A high resolution interaction within the carbon atom that neighbor the 1H atom causes the peaks to split into multiplets-clusters of peaks
-The number of sub-peaks in the cluster is one more than the number of 1H atom attached to the neighboring carbon atom- n + 1 rule
